Ipedex Instrumentation Training Modules
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Ipedex Instrumentation Training Modules...
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TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
TABLE OF CONTENT INSTRUMENT MODULES VOLUME 1 MODULE No. 1 - INSTRUMENTATION 1 Unit 1 Unit 2 Unit 3 Unit 4 Unit 5 Unit 6
Introduction Pressure Measurement The Pressure Transmitter Flow Measurement Measurement of Level Practical Tasks
VOLUME 2 MODULE No. 2 - INSTRUMENTATION 2 Unit 1 Unit 2 Unit 3 Unit 4 Unit 5
Measurement of Temperature Temperature Transmitter The Controller Valves and Actuators Practical Tasks
MODULE No. 3 - INSTRUMENTATION 3 Unit 1 Unit 2 Unit 3 Unit 4
Converters and Positioners Recorders Indicators and Combined Units Hazardous Areas and Intrinsic Safety
MODULE No. 4 - INDUSTRIAL ELECTRONICS 1 Unit 1 Unit 2 Unit 3 Unit 4 Unit 5 Unit 6 Unit 7 Unit 8
Page 1
The Electrical Circuit Series and Parallel Circuits Electromagnetic Principles Basic Electrostatics and the Capacitor The Inductor, Capacitor and DC AC Principles Common Electrical Symbols Practical Tasks
MODULE No. 5 - INDUSTRIAL ELECTRONICS 2 Unit 1 Unit 2 Unit 3 Unit 4 Unit 5
Basic Semiconductor Theory Diode Application The Controlled Diode Transistor Practical Tasks
VOLUME 3 MODULE No. 6 - INDUSTRIAL ELECTRONICS 3 Unit 1 Unit 2 Unit 3 Unit 4 Unit 5 Unit 6 Unit 7 Unit 8 Unit 9
Digital Mathematics Introduction to Digital System Logic Gates, Flip-Flops, Counters and Registers Memories and Clocks Multiplexers, Decoders and Displays Digital / Analog & Analog / Digital Converters The Computer Introduction to Data Transmission Practical Tasks
MODULE No. 7 - INSTRUMENT WORKSHOP Unit 1
Practical Tasks
MODULE No. 8 - P&ID’s Unit 1 Unit 2 Unit 3
General Symbols Reading P&ID Practical Tasks
MODULE No. 9 - CONTROL SYSTEMS 1 Unit 1 Unit 2
Introduction Practical Tasks
MODULE No. 10 - PROCESS CONTROL FUNDAMENTAL Unit 1 Unit 2 Unit 3 Unit 4 Unit 5 Unit 6
Basic Control Theory Tuning a Controller Introduction to DCS and PLC Honeywell TDC 3000 DCS Foxboro IA DCS Practical Tasks
MODULE No. 11 - INSTRUMENT CRAFT PRACTICE
Page 2
Unit 1 Unit 2 Unit 3 Unit 4 Unit 5
Workshop Safety and Tools Care Basic Hand Tools Tubing Systems Crimping Practical Tasks
MODULE No. 12 - INTRODUCTION TO PLC Unit 1
Page 3
PLC Fundamentals
TRAINING MANUAL INSTRUMENTATION
MODULE No. 1 INSTRUMENTATION 1
July 1999- Rev.0
TRAINING MANUAL INSTRUMENTATION
MODULE No. 1 INSTRUMENTATION 1 UNIT No. 1 INTRODUCTION
July 1999- Rev.0
UNITS IN THIS COURSE UNIT 1
INTRODUCTION TO INSTRUMENTATION
UNIT 2
PRESSURE MEASUREMENT
UNIT 3
THE PRESSURE TRANSMITTER
UNIT 4
FLOW MEASUREMENT
UNIT 5
MEASUREMENT OF LEVEL
UNIT 6
PRACTICAL TASKS
Module No. 1: Instrumentation 1
Unit No. 1 - Introduction
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 1/9
Unit No. 1 - Introduction
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para
Page
1.0
COURSE OBJECTIVE
3
1.1
INTRODUCTION
4
1.2
INSTRUMENT WORK
4
1.3
BASIC DEFINITIONS
5
1.4
PROCESS CONTROL
7
1.4.1
Open Loop (Manual Control)
7
1.4.2
Closed Loop (Automatic Control)
8
CONCLUSION
9
Module No. 1: Instrumentation 1
1.5
July 1999- Rev.0
Page 2/9
Unit No. 1 - Introduction
TRAINING MANUAL INSTRUMENTATION
1.0
COURSE OBJECTIVE The student will be able to •
Explain in general terms the duties of an instrument technician.
•
Define the following terms used in instrumentation. Instrument
⇒
Instrumentation
⇒
Process and process variable.
⇒
Controller
⇒
Correcting unit.
⇒
Transmitter and transmission signal
⇒
Process loop and plant
⇒
Indicate and record.
Explain in general terms the purpose of instrumentation to obtain automatic control.
Module No. 1: Instrumentation 1
•
⇒
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Page 3/9
Unit No. 1 - Introduction
TRAINING MANUAL INSTRUMENTATION
1.1
INTRODUCTION The aim of this unit is to introduce the subject of instrumentation, the duties of the instrument technician and what is meant by instrumentation and control.
1.2
INSTRUMENT WORK The duties of an instrument technician fall into five main areas. •
Repair and calibration of instruments which measure physical quantities, for example pressure, level, gas concentration, acidity, etc.
•
Repair and calibration of instruments that indicate and record the value of a physical property. For example, temperature (thermometer), pressure (gauges), chart recorders, etc.
•
Repair and calibration of the final control element, for example a control valve, electric heater, thermostat, etc.
•
Repair and calibration of a complete control system, for example the control of a gas turbine, steam plant, etc.
•
Carry out preventive maintenance programs.
The instruments in use are very varied, depending on how old the installation is. They may be air (pneumatic), liquid (hydraulic) or electric / electronic in operation. The way the information is shown or recorded may be simple, like a clock or thermometer. It may use the latest information technology to display information on a personal computer screen (video display unit).
Module No. 1: Instrumentation 1
The aim of this course is to introduce all the above topics. Real working instrument systems will have to be learnt in the plant after this course is over.
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Page 4/9
Module No. 1: Instrumentation 1
Unit No. 1 - Introduction
TRAINING MANUAL INSTRUMENTATION
1.3
BASIC DEFINITIONS Instrumentation uses a lot of words which need to be explained. Before we can talk about instrumentation and process control you need to understand the following words. •
Instrument
-
Any device for measuring, indicating, controlling, recording and adjusting a physical or chemical property e.g. flow, pressure, acidity, weight, gas concentration, etc.
•
Instrumentation
-
A complete set of instruments used to control a process, e.g. refining, oil/gas production, LNG, LPG, etc.
•
Indicator
-
A device which shows a measured value to the operator.
•
Recorder
-
A device which continuously records measurements, either electronically or on an ink chart. It is used to show production figures, etc.
•
Process Loop
-
A group of instruments used to control a single process variable e.g. pressure, flow, level, etc.
•
Process
-
The operator's word for a manufacturing unit e.g., refining, liquefying gas, etc.
•
Measured Variable or Process Variable (MV)
-
The value of the property being controlled by a single process loop e.g. pressure flow, or Process Variable level, etc.
•
Desired Value or Set Point (S P)
-
The value required by the operator.
•
Error Signal (ES)
-
The difference between the measured variable and the set point should be zero for good control.
•
Controller
-
A device, either pneumatic or electrical / electronic, which adjusts the error signal to zero.
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•
Correcting Unit (Final Control Element)
-
A device which works on the command of the controller. It is used to adjust the measured value to obtain a zero error signal, e.g. control valve, etc.
•
Transmission
-
A method of standardising signals sent from various parts of the plant.
•
Transmitter
-
A device which takes a measurement and changes it into a standard signal.
•
Transducer
-
A device which changes one form of energy to another; particularly from electrical to pneumatic.
Module No. 1: Instrumentation 1
Unit No. 1 - Introduction
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 6/9
Unit No. 1 - Introduction
TRAINING MANUAL INSTRUMENTATION
1.4
PROCESS CONTROL
1.4.1
Open Loop (Manual Control)
Figure 1-1 The Open Loop
Figure 1-1 shows what is called OPEN LOOP or MANUAL control. The process is temperature control. The indicator is a thermometer. The correcting unit is the gas control valve. The controller is the operator who uses his own judgement to keep the water temperature constant.
Module No. 1: Instrumentation 1
Manual control has its uses as it is cheap to install and maintain, and simple to operate. However, it is very seldom used in industry because: •
The operator must remain in position at all times.
•
It cannot be used if the operator is placed in a dangerous area.
•
The process changes faster than the operator can react.
•
A mistake by the operator can have dangerous results.
These problems are avoided by using automatic control (closed loop). The job of the instrument technician is to make sure that this type of control operates correctly. Modern household appliances now use automatic control to make work easier. For example: •
Refrigerators and water heaters use automatic temperature control.
•
Washing machines use automatic heating and water control.
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Unit No. 1 - Introduction
TRAINING MANUAL INSTRUMENTATION
1.4.2
Closed Loop (Automatic Control)
Figure 1-2 Simple Automatic Control.
Figure 1-2 shows a simple automatic controller. The boiler now has the loop closed and no operator is required. The following items are added. The temperature transmitter (T.T) which measures (senses) the temperature of the hot water and changes it to a standard signal. A signal line from the transmitter to the controller, the signal may be either pneumatic or electrical. A controller which keeps the temperature of the hot water at a position set by the operator (set point) The controller adjusts the correcting unit (automatic control valve) using an output signal line similar to the input line from the transmitter.
Module No. 1: Instrumentation 1
The controller may provide alarm signals to alert the operator if the system fails. It may also shut off the gas if the water starts to boil.
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TRAINING MANUAL INSTRUMENTATION
CONCLUSION This unit has introduced instrumentation and control. The following units will explain in detail how control loops are made and operated. We will start with the measurement (sensing) and transmitting unit. There are many process variables in the petroleum industry but most of these variables fall into four main groups: pressure, flow, level, and temperature. We will look at these groups. Most of the other process variables (e.g., density, gas concentration, acidity, etc.) will be explained when needed during a specialist analyser course.
Module No. 1: Instrumentation 1
Unit No. 1 - Introduction
1.5
July 1999- Rev.0
Page 9/9
UNITS IN THIS COURSE UNIT 1
INTRODUCTION TO INSTRUMENTATION
UNIT 2
PRESSURE MEASUREMENT
UNIT 3
THE PRESSURE TRANSMITTER
UNIT 4
FLOW MEASUREMENT
UNIT 5
MEASUREMENT OF LEVEL
UNIT 6
PRACTICAL TASKS
Module No. 1: Instrumentation 1
Unit No. 2 - Pressure measurement
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 1/22
Unit No. 2 - Pressure measurement
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para 2.0
COURSE OBJECTIVE
3
2.1
INTRODUCTION
4
2.2
PRESSURE
4
2.2.1
Pressure and Liquids
4
2.2.2
Pressure and Gases
5
2.2.3
Pressure Units
5
2.2.4
Absolute, Gauge and Atmospheric Pressure
9
2.2.5
Example 1
10
2.3
Module No. 1: Instrumentation 1
Page
PRESSURE MEASURING DEVICES
11
2.3.1
The Manometer
11
2.3.2
The Well Manometer
12
2.3.3
The Inclined Limb Manometer
12
2.3.4
The Bourdon Tube Pressure Gauge
13
2.3.5
Special Adaptation of the Bourdon Tube.
14
2.4
BELLOWS
16
2.5
DIAPHRAGMS AND CAPSULES
17
2.5.1
Diaphragms
17
2.5.2
Capsules
18
2.6
ELECTRICAL METHODS
19
2.6.1
The Piezo Electric Effect.
19
2.6.2
The Capacitive Cell
20
2.6.3
The Strain Gauge
21
2.6.4
July 1999- Rev.0
Vibrating (Resonant) Wire
22
Page 2/22
2.0
COURSE OBJECTIVE The student will be able to •
Define pressure
•
Explain the action of pressure on liquids and gases.
•
Explain the terms ⇒
Absolute pressure
⇒
Gauge pressure
⇒
Vacuum pressure
⇒
Differential pressure
•
State the standard pressure units used in the petrochemical industry.
•
Carry out simple pressure unit conversions using standard tables.
•
Draw and explain the operation of the following pressure sensors ⇒
Manometer
⇒
Bourdon tube
⇒
Bellows
⇒
Diaphragm and capsules.
⇒
Strain gauge
⇒
Capacitance element
⇒
Vibrating (resonant) wire
Module No. 1: Instrumentation 1
Unit No. 2 - Pressure measurement
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 3/22
Unit No. 2 - Pressure measurement
TRAINING MANUAL INSTRUMENTATION
2.1
INTRODUCTION The aim of this unit is to define pressure and describe the common devices used to measure it.
2.2
PRESSURE
2.2.1 Pressure and Liquids PRESSURE (P) is defined as the FORCE (F) applied divided by AREA
Module No. 1: Instrumentation 1
Figure 2-1 Pressure on a Liquid
Figure 2-1 shows a force (F) applied to a piston pressing on a liquid in a cylinder. The liquid is considered INCOMPRESSIBLE and the pressure of the liquid on the-walls of the cylinder is the same in all directions. This gives the formula:
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Unit No. 2 - Pressure measurement
TRAINING MANUAL INSTRUMENTATION
2.2.2
Pressure and Gases
Figure 2-2 Pressure on a Gas.
Figure 2-2 shows a force (F) applied to a piston pressing on gas in a cylinder. The gas is COMPRESSIBLE. The volume of the gas will decrease until the pressure of the gas on the walls of the cylinder equals the pressure applied by the piston.
2.2.3 Pressure Units
Module No. 1: Instrumentation 1
There is no agreed standard for pressure measurement in the petrochemical industry. Some companies use IMPERIAL UNITS (USA), some use INTERNATIONAL STANDARD METRIC UNITS (ISO) and some use both. The instrument technician must understand both systems and be able to change from one to another.
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Unit No. 2 - Pressure measurement
TRAINING MANUAL INSTRUMENTATION
CONVERSION 1 psi = 6895 Pa The Pascal is a very small unit so the KILOPASCAL (kPa) is often used. A bigger unit is the bar. This is the most common ISO unit. CONVERSION 100 kPa = 1 bar Note :
On very old installations the kilogram per centimetre square is still used. For all general purposes. 1 kg/cm2 = 1 bar.
Very small pressures are measured using the height of a column of liquid. The liquids used most are water (H2O) and mercury (Hg). The ISO unit for this type of measurement is 1 mm height of mercury. This is called the torr. CONVERSION VALUES: 1 inch H20
0.03613 psi
1 mm H20
249.1 Pa
=
0.002491 bar
0.004122 psi 9.8907 Pa
=
0.000098907 bar
1 inch Hg
0.4912 psi
3386 Pa
=
0.03386 bar
1 mm Hg
0.01934 psi
133.3 Pa
=
0.001333 bar
Module No. 1: Instrumentation 1
An instrument workshop usually has a conversion table for easy reference. You can photocopy the conversion table on the next page and use it for easy reference.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Pressure measurement
PRESSURE CONVERSION TABLE
Module No. 1: Instrumentation 1
C0LUMN
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Page 7/22
TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Pressure measurement
Using the conversion table The rows (across) and columns (up and down) are complementary, so that you can convert bar to psi or psi to bar. The bar row gives 14.50 in the psi column The psi row gives 0.0689 in the bar column The following examples are given to show how the table is used. Examples (a) Convert 50 psi to bar 1 psi = 0.0689 bar 50 psi = 50 x 0.0689 = 3.445 50 psi = 3.445 bar
(b) Convert 120 kPa to psi 1 Pa = 1.450 x 10-4 120 kPa = 1.450 x 10-4 X 120 x 103 17.5 120 kPa
17.5 psi
(c) Convert 150 mmHg to bar 1 mmHg = 1.333 x 10-:3 150 mmHg = 150 x 1.333 x 10*3 = 0. 2000
Module No. 1: Instrumentation 1
150 mmHg = 0.2 bar
(d) Convert 50 in H20 to psi 1 in H20 = 0.03613 50 in H20 = 50 x 0.03613 = 1.8065 50 inH20 = 1.8065 psi
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Pressure measurement
2.2.4
Absolute, Gauge and Atmospheric Pressure The price of oil or gas depends on the quantity (mass) of the product. The quantity of oil or gas in a given volume depends on the pressure. For this measurement, absolute pressure must be used. Absolute Pressure This is the pressure above a total vacuum (there are no particles of matter in a total vacuum) Gauge Pressure This is the pressure measured by a gauge. The pressure above the pressure of the surrounding atmosphere. Atmospheric Pressure The pressure of the air all around you. This is not constant it depends on things like the weather and the altitude of the plant. The equation linking the above pressures together is: Absolute pressure = Gauge pressure + Atmospheric pressure (A. P)
(G. P)
(Atmos)
Because atmospheric pressure can vary a STANDARD ATMOSPHERIC pressure has to be used. This is normally 1.013 Bar or 14.70 psi.
Module No. 1: Instrumentation 1
Gauge pressure is written as psig. Absolute pressure is written as psia.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Pressure measurement
2.2.5
Example 1 Question
:
A pressure gauge indicates 11.4 psi. Find the absolute pressure if the atmospheric pressure is 14.65 psi. Solution: AP = GP + ATMOS AP = 11.4 + 14.65 = 26.05 Absolute pressure = 26.05 psi Example 2 Question A pressure gauge reads 3.5 psi vacuum. Find the absolute pressure if the atmospheric pressure is 14.73 psi. Solution: Vacuum pressures are normally given as gauge pressures below atmospheric pressure (zero gauge). That is a negative gauge pressure. AP = - GP + ATMOS AP = - 3.5 + 14.73 = 11 .23 3.5 psi vacuum pressure = 11.23 psi absolute pressure
Module No. 1: Instrumentation 1
A simple diagram is shown below as an example.
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Unit No. 2 - Pressure measurement
TRAINING MANUAL INSTRUMENTATION
2.3
PRESSURE MEASURING DEVICES
2.3.1
The Manometer The manometer is a simple device to measure small amounts of pressure. It consists of a glass tube of a fixed diameter. It is bent into a U shape with (vertical) sides. The sides are next to a scale. The manometer is filled with a liquid e g water or mercury
Figure 2-3 The Manometer
Figure 2-3 shows the construction -of the manometer and its uses. •
Absolute Pressure One side is sealed with a vacuum above the liquid in the manometer. An unknown pressure is applied to the other side (limb) which forces the liquid down. The difference in height (H) of the liquid column will give the unknown absolute pressure. For example, a 9 inch difference, with water as the liquid, will give an absolute pressure of 9 x 249.1 = 2242 Pa.
Module No. 1: Instrumentation 1
•
Gauge Pressure One side is left open to the atmosphere. An unknown pressure is applied to the other side. This pressure will force the liquid in the tube down. The height of the liquid gives the gauge pressure of the unknown pressure.
•
Differential Pressure If unknown pressures are applied to both sides the difference in level (H) will give the difference (differential) between the two in absolute pressure.
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TRAINING MANUAL INSTRUMENTATION
The Well Manometer
Unit No. 2 - Pressure measurement
2.3.2
Figure 2-4 Well Manometer
The well manometer (see Figure 2-4) is like the U tube manometer. It is used to measure very low pressures. The pressure is measured in inches of water (H20). This measurement is divided by the ratio of the areas A and B. This gives the unknown pressure. A single limb manometer in a workshop usually has its scale already calibrated to allow for the ratio of the areas A and B. So, no calculations are required.
Module No. 1: Instrumentation 1
2.3.3
The Inclined Limb Manometer
Figure 2-5 The Inclined Limb Manometer
The inclined limb manometer (see Figure 2-5) is another device for measuring very low pressures. The unknown pressure is applied to the well and the single limb is tilted. This makes the scale longer so the pressure can be measured more accurately. The actual pressure is the height, H. No calculations are required as the scale is set by the manufacturer to give accurate readings.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Pressure measurement
2.3.4
The Bourdon Tube Pressure Gauge The Bourdon tube gauge is the most common pressure indicator in the petrochemical industry. It shows the pressure in a clear, simple way.
Figure 2-6 above shows a typical Bourdon gauge. It consists of the following parts: •
The Bourdon tube itself. This is a metal tube shaped like a "C". It has an oval cross sectional area. It is sealed at one end. The sealed end is free to move.
•
A linkage and pinion to turn the pointer.
•
A scale to indicate the pressure.
Module No. 1: Instrumentation 1
Operation When a pressure is applied to the inside of the tube it will try to straighten. The closed end (the tip) will move and the linkage moves the pinion which moves the pointer. The movement of the pointer shows how much pressure is applied to the Bourdon tube. Bourdon gauges come in all shapes and sizes and can measure from about 0-15 psig (0-1 bar) to 0-10,000 psig (0-700 bar) depending on the stiffness of the material used.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Pressure measurement
Materials In the low ranges the Bourdon tube is made from copper alloys In the medium ranges it is made from mild steel or stainless steel. In the high ranges it is made from high tension steel. Calibration adjustments The calibration adjustments on a Bourdon gauge depend on the manufacturer. However, some basic rules are:
2.3.5
•
Zero adjustment is done either by moving the pointer to zero on the scale or by moving the scale so that zero is under the pointer.
•
Span adjustment (the maximum reading of the scale) is set by adjusting the linkage in the geared sector and pinion.
•
Linearity is adjusted by changing the length of the linkage (not often added on modern gauges) Remember to follow the manufacturer's instructions when you calibrate a gauge.
Special Adaptation of the Bourdon Tube.
Module No. 1: Instrumentation 1
The Spiral Bourdon Tube.
Figure 2-7 Spiral Bourdon Tube
Figure 2-7 shows a spiral Bourdon tube (Foxboro). It is used to indicate low pressures. When pressure is applied the spiral unwinds and the free end moves to indicate the pressure. The construction and calibration of this type of gauge depends on the manufacturer. The handbook must be used.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Pressure measurement
The Helical Bourdon Tube
Figure 2-8 Helical Bourdon Tube.
Module No. 1: Instrumentation 1
Figure 2-8 shows a helical Bourdon tube (Foxboro). This is usually used to indicate high pressures. When pressure is applied the helix unwinds and the free end moves to indicate the pressure applied. The actual construction and calibration depend on the manufacturer and the manual must be used.
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TRAINING MANUAL INSTRUMENTATION
BELLOWS Bellows are tubes with thin walls, made of brass, stainless steel, etc. The thin walls are corrugated. This improves their ability to expand and contract. When pressure is applied (either to the outside or the inside), the corrugated walls expand or contract. This movement is used to indicate pressure. Bellows units are used- in various ways. The three most common methods are shown below:
Module No. 1: Instrumentation 1
Unit No. 2 - Pressure measurement
2.4
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Pressure measurement
An example of a differential bellows unit (Foxboro) is shown in Figure 2-9. The air is pumped out of one bellows (evacuated). This makes a vacuum so that when a pressure is applied to the other bellows unit absolute pressure is measured.
Figure 2-9 Differential Bellows Unit (Foxboro)
2.5
DIAPHRAGMS AND CAPSULES
2.5.1
Diaphragms
Module No. 1: Instrumentation 1
A diaphragm is a stiff corrugated disc which is flexible under pressure. A single diaphragm is often used as a seal to protect a gauge from corrosive liquids. A typical example is given in Figure 2- The Schaffer gauge.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Pressure measurement
Diaphragms are also used to make high pressure bellows (a diaphragm stack). An example is shown in Figure 2-11.
Figure 2-11 A Diaphragm Stack
2.5.2
Capsules
Module No. 1: Instrumentation 1
Capsules are made of two diaphragms welded onto a metal ring and filled with a fluid. Different mechanical and electrical methods are used to show the differential pressure across the capsule. Figure 2- shows a Foxboro capsule used in a pneumatic differential pressure transmitter.
Figure 2-12 The Capsule (Foxboro)
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TRAINING MANUAL INSTRUMENTATION
ELECTRICAL METHODS The old mechanical methods of detecting pressure are slowly being replaced by electrical methods. Electrical methods are more accurate and cheaper. The following notes give a simple explanation of the principle involved.
2.6.1
The Piezo Electric Effect.
Module No. 1: Instrumentation 1
Unit No. 2 - Pressure measurement
2.6
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Pressure measurement
2.6.2
The Capacitive Cell This method is used by Rosemount in their sensing capsules. A cut away section of this capsule is shown in Figure 2-13.
Module No. 1: Instrumentation 1
Figure 2-13 The Capacitive Cell
When a differential pressure is applied across the capsule the silicone oil will be pressurised more on one side than the other. The sensing diaphragm will move away from one fixed capacitor plate and nearer to the other. The difference in capacitance between A and B, and B and C, is measured by an electronic amplifier. This measurement shows the differential pressure.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Pressure measurement
2.6.3
The Strain Gauge The strain gauge is a resistor which has been deposited onto a flexible bar. As the bar is bent the resistor will change in length and thus its resistance. The changes in resistance are detected by a Wheatstone bridge and electronically changed to a pressure signal. A strain gauge is shown in Figure 2-14. This method is used by Honeywell in their electrical transmitters.
Module No. 1: Instrumentation 1
Figure 2-14 Simplified Strain Gauge Capsule
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Pressure measurement
2.6.4
Vibrating (Resonant) Wire The vibrating wire is the operating method used by Foxboro in their pressure transmitters. Figure 2-15 shows the basic construction. Operation:
Figure 2-15 The Vibrating Wire
Module No. 1: Instrumentation 1
The frequency of vibration of a wire depends on its tension. The tension of the vibrating wire is changed by the pressure applied to the diaphragm. The electronics unit has a driving coil (D.C) to make the wire resonate and a sensing coil (S.C) to measure the resonant frequency. This changes the pressure applied to the diaphragm into an electrical signal output.
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UNITS IN THIS COURSE UNIT 1
INTRODUCTION TO INSTRUMENTATION
UNIT 2
PRESSURE MEASUREMENT
UNIT 3
THE PRESSURE TRANSMITTER
UNIT 4
FLOW MEASUREMENT
UNIT 5
MEASUREMENT OF LEVEL
UNIT 6
PRACTICAL TASKS
Module No. 1: Instrumentation 1
Unit No. 3 - The pressure transmitter
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 1/15
TABLE OF CONTENTS Para
Page
3.0
COURSE OBJECTIVE
3
3.1
INTRODUCTION
4
3.2
THE FLAPPER-NOZZLE
4
3.3
THE PNEUMATIC RELAY
6
3.4
THE PNEUMATIC TRANSMITTER
7
3.5
THE PNEUMATIC SIGNAL LOOP
11
3.6
THE AIR PRESSURE REGULATOR
12
3.7
THE ELECTRICAL PRESSURE TRANSMITTER
14
Module No. 1: Instrumentation 1
Unit No. 3 - The pressure transmitter
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 2/15
3.0
COURSE OBJECTIVE The student will be able to: •
Explain the operation of a simple flapper nozzle.
•
Explain the function of a pneumatic relay.
•
Using a diagram, name the parts of a typical pneumatic transmitter.
•
Explain the need for the feedback (positioning) bellows.
•
State the standard signals produced by a pneumatic transmitter.
•
Draw the layout of a typical electrical transmission loop.
•
State the standard signals produced by an electrical transmitter
Module No. 1: Instrumentation 1
Unit No. 3 - The pressure transmitter
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Unit No. 3 - The pressure transmitter
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3.1
INTRODUCTION The previous unit (Unit 2) explained the basic devices used to measure and indicate pressure. This unit will describe the methods used to convert the pressure measurement to an instrument signal for the controller.
3.2
THE FLAPPER-NOZZLE The flapper-nozzle is the primary device for all pneumatic instruments which convert a measurement to a pneumatic signal. Figure 3-1 shows the layout of the device.
Figure 3-1 The Flapper-Nozzle Operation:
Module No. 1: Instrumentation 1
The air supply input (20 psi (1 .4 bad) passes through a restrictor (small hole). It then goes out of the nozzle or down the air signal output line. If the flapper is placed against the nozzle, no air can escape through it. So, the air signal output shows full pressure. If the flapper is pulled away from the nozzle, most of the air flows out of the nozzle, so the air signal output pressure is very small. The back pressure output signal depends on how near the flapper is to the nozzle. A simple graph of the output pressure (P) against flapper distance (X) is shown below.
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The graph is linear (straight) over the distance A B. This reflects only a few millimetres of travel of the flapper. This part of the curve is used to convert a change in a measured value connected to the flapper into an output signal. The restrictor increases the speed of operation. The small volume (V) can change pressure quickly before the air supply can pass through the small hole in the restrictor. The change in output pressure due to flapper movement is very small. It must be enlarged (amplified) using a device called a PNEUMATIC RELAY.
Module No. 1: Instrumentation 1
Unit No. 3 - The pressure transmitter
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Unit No. 3 - The pressure transmitter
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3.3
THE PNEUMATIC RELAY Different manufacturers make pneumatic relays in different ways. However, they all work on the same principle. A simplified explanation of this device is given in Figure 3-2.
Figure 3-2 The Pneumatic Relay
Operation The output pressure from the flapper-nozzle goes to the top of the diaphragm. The diaphragm Moves down against the controlling spring and opens the ball valve. The air supply now enters the area under the diaphragm and goes into the output.
Module No. 1: Instrumentation 1
At some point, the pressure from the air supply under the diaphragm will equal the pressure above. The diaphragm moves up and the ball valve closes and they hold momentarily at that pressure. If the flapper-nozzle pressure increases, the ball valve will open and hold momentarily at the new higher output pressure. If the pressure on the diaphragm decreases, the ball valve stays closed and the output signal falls as air escapes through the vent. When the output pressure has fallen enough the ball valve opens again to maintain the output at the new lower pressure. This kind of relay is called a continuous bleed device because it controls the output signal pressure by slowly venting the air supply all the time. The standard amplified signal from the relay is: (a)
3 -15 psi
imperial
(b)
0.2 -1 bar
ISO.
Remember that these standards are not the same. The control system can work on (a) or (b). It must never work on a mixture of the two standards.
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Unit No. 3 - The pressure transmitter
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3.4
THE PNEUMATIC TRANSMITTER There are many different types of pneumatic pressure transmitters. However, they are not used much nowadays. One of the few types still in use is the Foxboro type 11 . This is' shown below as an example of the pneumatic transmitter.
Figure 3-3 The Pneumatic Pressure Transmitter
Figure 3-3 shows the basic design of a Foxboro pressure transmitter.
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Operation: • The process pressure to be measured and transmitted as a standard signal is applied to a diaphragm capsule. •
The pressure moves the capsule. This movement is applied to one end of a force bar pivoted about the diaphragm seal.
•
The force bar moves the flexible connector. The connector pulls the flapper to and from the nozzle.
•
The back pressure from the nozzle is amplified by the relay. This gives the standard output signal.
The system is not stable. The flapper will go either full on or full off. So a feedback bellows is added. The output signal goes to the bellows. The bellows applies a force to the range rod in opposition to the force bar. The system balances to give an output signal which depends on the position of the range wheel.
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Unit No. 3 - The pressure transmitter
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Unit No. 3 - The pressure transmitter
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Figure 3-4 Balancing action
Figure 3-4 shows the balancing action of the transmitter. The movement of the flapper is the pressure applied times B over A (B:A). The feedback movement is the output pressure times the ratio D:C. The ratio B:A is fixed but D:C can be changed by the range wheel. If the D:C ratio is large the feedback is small. This makes the range larger.
Module No. 1: Instrumentation 1
Calibration Adjustments •
When there is no pressure (zero gauge), the zero spring, which sets the force on the range rod, is adjusted for a 3 psi or 0.2 bar output signal.
•
With the maximum pressure (range) applied, the range wheel is adjusted to give 15 psi or 1 bar output signal.
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Note
Module No. 1: Instrumentation 1
Unit No. 3 - The pressure transmitter
TRAINING MANUAL INSTRUMENTATION
Figure 3-3 shows a gauge pressure transmitter. This transmitter can easily be adapted to measure differential pressure. This is done by adding an extra input to the diaphragm capsule as shown below. The force bar now moves according to the differential pressure applied. Remember the device must be connected correctly to the high and low pressure connections. It will not read correctly if it is connected the wrong way round.
ABSOLUTE PRESSURE APPLIED TO THE CAPSULE
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Unit No. 3 - The pressure transmitter
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3.5
THE PNEUMATIC SIGNAL LOOP
Figure 3-5 The Pneumatic Signal Loop
Figure 3-5 shows a block diagram of the pneumatic signal loop : The process line to the pressure transmitter (usually 3/8" or 1/4" stainless steel tubing) has an isolation valve and a drain valve so it can be disconnected.
•
The pressure transmitter has an air supply set at 20 psi. This comes through the air pressure regulator from the main air supply (usually about 100 psi).
•
There is a signal line (usually 1/4" stainless steel tubing) which transmits the signal (3-15 psi) to the receiver in the controller.
Module No. 1: Instrumentation 1
•
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3.6
THE AIR PRESSURE REGULATOR The air pressure regulator is a simple device. It is used to lower the main instrument air supply of a plant to a pressure suitable for an air operated instrument; eg, a transmitter, control valve, etc. Normally, each air operated instrument has its own regulator. So an. air regulator is one of the most common devices in the plant. There are various manufacturers of air regulators, eg, Masoneilan and Fisher. However, they all work in much the same way. The example given is manufactured by Fisher (see Figure 3-6).
Figure 3-6 The Air Pressure Regulator
Module No. 1: Instrumentation 1
Operation: •
The main air supply is connected to the IN port. Air passes into the inlet chamber at the bottom of the regulator.
•
Air passes through the filter which removes dirt particles in the incoming air which may block nozzles etc. It then goes into the valve assembly.
•
The valve assembly is moved by the range spring pressing on the diaphragm.
•
The range spring will hold the valve assembly down until the output pressure is high enough to lift the diaphragm (via the air passage shown). At this point the small spring in the valve assembly closes the valve.
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•
Air is allowed to pass through a hole at the centre of the diaphragm and out of the vent. This maintains balanced pressure across the diaphragm.
•
If the outlet pressure is, above the pressure set by the range spring, the air will go out through the vent above the diaphragm. When the outlet pressure is correct, the valve assembly opens to set the correct pressure.
•
If the outlet pressure is below the pressure set by the range spring the valve assembly will stay open until the set pressure is reached.
Note : •
The drain valve should be opened regularly to drain any moisture in the inlet chamber.
•
Range springs come in various sizes. The most common is from 5-35 psi (0.34-2.4 Bar). This is set to give an output of 20 psi for transmitters, etc.
Module No. 1: Instrumentation 1
Unit No. 3 - The pressure transmitter
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Unit No. 3 - The pressure transmitter
3.7
THE ELECTRICAL PRESSURE TRANSMITTER Electrical transmitters have replaced pneumatic transmitters in most petrochemical plants. This is because they are cheaper to install and maintain. The transmission of the signal is also cheaper and easier to install. This is because an electrical transmitter has one pair of wires instead of expensive stainless steel signal tubing and air supply lines. There are three main types of transmitters. They use three kinds of capsules: capacitive (Rosemount), strain gauge (Honeywell) or vibrating wire (Foxboro). The output from the capsule is electronically converted to a STANDARD 4-20 mA SIGNAL for transmission to the control room. An electrical transmitter is calibrated with an instrument screwdriver or push buttons. There are two adjustments, zero and span (range). The calibration and servicing depends on the manufacturer. It must be carried out using the manual. Modern transmitters have become throw away items. If they cannot be calibrated they are not serviced. They are thrown away and replaced with a new transmitter.
Module No. 1: Instrumentation 1
Figure 3-7 below shows, as; an example, a Rosemount electrical transmitter.
Figure 3-7 The Rosemount Electrical Transmitter
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Any electrical transmitter uses an electrical series loop and it acts as a variable resistor. The basic diagram of the loop is shown in Figure 3-8.
PLANT
CONTROL ROOM
Figure 3-8 The Electrical Series Loop
The power supply provides the EMF around 24V D.C., to drive the series loop. This loop consists of: •
Two safety barriers(RB) which protect the plant from dangerous voltages in the case of a fault.
•
The transmitter, whose resistance (RT) changes with the measured pressure changes.
•
The controller with a resistance RC. The voltage across the resistance provides the signal for the controller electronics.
Module No. 1: Instrumentation 1
The current around the circuit (1) will be
RB and 'Rc are constant. So, the current changes as the resistance in the transmitter (RT) changes. The system is set so that with zero pressure, the current is 4 mA and 20 mA at the maximum value of the measured pressure. Note :
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Both pneumatic and electrical transmitter signals have a live zero. his means that a broken circuit can easily be detected
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UNITS IN THIS COURSE UNIT 1
INTRODUCTION TO INSTRUMENTATION
UNIT 2
PRESSURE MEASUREMENT
UNIT 3
THE PRESSURE TRANSMITTER
UNIT 4
FLOW MEASUREMENT
UNIT 5
MEASUREMENT OF LEVEL
UNIT 6
PRACTICAL TASKS
Module No. 1: Instrumentation 1
Unit No. 4 - Flow measurement
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Unit No. 4 - Flow measurement
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TABLE OF CONTENTS Para
Page
4.0
COURSE OBJECTIVE
3
4.1
INTRODUCTION
4
4.2
UNITS OF FLOW
4
4.3
QUANTITY METERS
7
4.3.1
Positive Displacement
7
4.3.2
Velocity Meters (semi-positive displacement)
9
4.4
RATE OF FLOW MEASUREMENT
10
4.4.1
Flow Basics
10
4.4.2
Flow Measurement by Differential Pressure
11
4.4.3
Differential Pressure Devices
15
VARIABLE AREA METERS
21
4.6
CALIBRATION OF FLOW MEASURING DEVICES
22
4.6.1
Quantity Meters
22
4.6.2
Calibration of Differential Pressure Devices
23
4.6.3
Flow Straighteners
23
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4.5
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Unit No. 4 - Flow measurement
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4.0
COURSE OBJECTIVE The student will be able to: •
Explain the difference between total flow and rate of flow measurements.
•
Explain the difference between mass and volume flow units.
•
Use volumetric and mass conversion tables.
•
Explain with the aid of a sketch how positive displacement flow meters work.
•
Explain with the aid of a sketch how velocity flow meters work.
•
Explain, with the aid of sketches, the use of a pipe restriction to produce a differential pressure for rate of flow measurement
•
Sketch the following rate of flow measuring devices and list their advantages and disadvantages. The orifice plate The Venturi The Venturi - nozzle. The nozzle Sketch a variable area meter.
•
State the calibration procedures that can be used to calibrate flow meters.
Module No. 1: Instrumentation 1
•
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Unit No. 4 - Flow measurement
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4.1
INTRODUCTION The aims of this unit are:
4.2
i)
to explain the measurement of liquid flow
ii)
describe devices used to measure liquid flow.
UNITS OF FLOW
Figure 4-1 Flow Measurements
Module No. 1: Instrumentation 1
Figure 4-1 shows a tanker being loaded from a storage tank. The amount of oil loaded must be accurately measured to know how much it costs (fiscal purposes). The total flow (quantity) of oil into the tanker can be measured in two ways: •
by volume, in barrels or cubic meters.
•
by mass, in metric or imperial tons (the international standard for oil/gas transfer).
For control purposes the rate of flow (how fast the ship is loaded) is also measured. Rate of flow units can also be given in either volumetric or mass units. For example •
Rate of flow by volume (volumetric) Barrels / Hour Cubic Feet / Min. Cubic Meters / Sec.
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•
Rate of flow by mass
Tons/ Hour Kilograms / Sec. Pounds / Min. The petrochemical industry uses many different units and-there is no common standard. The following list gives some of the units and their conversion.
Module No. 1: Instrumentation 1
Unit No. 4 - Flow measurement
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Module No. 1: Instrumentation 1
Unit No. 4 - Flow measurement
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Unit No. 4 - Flow measurement
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4.3
QUANTITY METERS There are two basic methods used to measure quantity (total flow)
4.3.1
(a)
Positive displacement.
(b)
Velocity meters (semi-positive displacement)
Positive Displacement The simplest form of positive displacement meter is the gasoline (petrol) pump. It will release an exact amount of gasoline in either imperial gallons or litres. A simple diagram to show its operation is shown in Figure 4-1
Figure 4-2 Reciprocating Piston Meter
Module No. 1: Instrumentation 1
Operation: When the piston is at the bottom of its stroke (see Figure (A)) the slide valve opens the inlet vent to the bottom of the cylinder. The liquid (petrol) flows into the cylinder below the piston and pushes the piston upwards. As the piston rises the liquid in the top half of the cylinder is pushed through the outlet vent into the outlet pipe. When the piston is at the top of its-stroke the slide valve closes the outlet at the top of the cylinder and opens the inlet vent. At the same time the slide valve opens the outlet vent at the bottom of the cylinder and closes the inlet (see Figure (B)). The pressure of the liquid coming into the top of the cylinder pushes the liquid at the bottom of the cylinder into the outlet pipe. The amount of liquid coming out of the cylinder during each stroke is measured. Each time the piston makes a stroke a meter connected to the top of the piston indicates how much liquid has been delivered.
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Unit No. 4 - Flow measurement
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The oil industry's positive displacement meter is the sliding vane meter. It can be used for measuring large quantities of liquid flow; eg, oil being loaded onto a tanker. A typical example is shown in Figure 4-3.
Figure 4-3 The Sliding Vane Meter
Operation:
Module No. 1: Instrumentation 1
This meter consists of a rotating drum with four sliding vanes (long blades) set inside it. The vanes move around a cam which is fixed in the centre of the drum. The liquid flowing through the meter pushes the vanes round with the drum. As the vanes rotate with the drum the cam pushes them in and out against the measuring wall. As the vanes are pushed against the measuring wall, they trap a measured volume of liquid between the drum and the measuring wall. Each revolution of the drum will measure 4 lots of the measured volume. The number of revolutions of the drum is counted and displayed. This gives the total flow passing through the meter. The calibration nut is used to adjust the side of the measurement chamber so that the volume of liquid passing through the meter can be measured exactly. Note: A static liquid chamber is added so that there is no differential pressure across the measurement wall.
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Unit No. 4 - Flow measurement
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4.3.2
Velocity Meters (semi-positive displacement) The sliding-vane type meter is not used much nowadays because it is slow. Most loading meters for shipping are now of the velocity type. The velocity meter measures the speed of the flow and works out the volume of flow using calibration figures.
Module No. 1: Instrumentation 1
Figure 4-4 The Turbine Meter Operation: The velocity (speed) at which the rotor turns depends on the flow rate. The pick-up coil gets a pulse induced for every rotation. The number of pulses is counted by an electronics unit. This unit displays the total quantity of flow. Note : In the oil/gas industry these two quantity meters are only for liquid measurements.
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Unit No. 4 - Flow measurement
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4.4
RATE OF FLOW MEASUREMENT The previous two quantity meters are used to calculate how much oil the customer pays for, so they must be extremely accurate. A modern turbine meter will measure to within ± 0.1 % of the true reading. Devices for measuring the rate of flow do not need to be so accurate. They are used mainly to give a flow signal to a controller.
4.4.1
Flow Basics
Figure 4-5 Flow in a Pipe
Figure 4-5 shows the flow of a fluid (gas or liquid) down a pipe. The flow is produced by the differential pressure across the ends of the pipe (P1-P2). The walls of a pipe are not perfectly smooth. The frictional force at the walls will cause the fluid to go slower at the edge than at the centre.
Module No. 1: Instrumentation 1
This leads to two different types of flow. •
LAMINAR FLOW The fluid flow rate is slow and the velocity of the wavefront down the pipe is much higher in the centre of the pipe than at the edges.
•
TURBULENT FLOW The fluid flow rate is high and the velocity of the wavefront is the same across the pipe. However, the flow is a little slower at the edges against the wall.
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The velocity lines on the diagram are called STREAMLINES. Engineers assume the streamlines are straight and parallel to each other.
4.4.2
Flow Measurement by Differential Pressure
Figure 4-6 Flow through a Restriction
Figure 4-6 shows the flow of a fluid through a closed pipe full of liquid or gas. It has a restriction in the pipe. Because there is a restriction, there is a difference between the pressure at the centre of the restriction (position Y) and the pressure in the normal pipeline (position X)
Module No. 1: Instrumentation 1
The difference in pressure between the centre of the restriction and the normal pipeline pressure (P1-P2) is proportional to the square root of the flow rate (Q) given as an equation.
or CD is called the "coefficient of discharge". We can get this from tables. It depends on what is used to make the restriction. Note :- Pl + P2 is often written as DP (Differential Pressure) or AP (Delta P).
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The above equation means that the measured DIP has to be square rooted before the flow rate can be calculated. Modern electronic transmitters do this automatically and a linear 4-20 mA signal for flow rate is produced. The older pneumatic systems produce a flow rate indication by using a square root scale or chart. A typical square root scale is shown below.
Module No. 1: Instrumentation 1
Unit No. 4 - Flow measurement
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Unit No. 4 - Flow measurement
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Theory
Bernoulli said "The sum of the kinetic energy (velocity energy) and pressure energy at any point in a closed pipe is a constant", i.e. the sum of these two types of energy is always the same. This is true if the pipe is horizontal and the temperature of the fluid does not change.
Module No. 1: Instrumentation 1
As the fluid flows through the restriction, it gets faster (the velocity increases). This is because FLOW = VELOCITY x AREA. If the area is smaller, the velocity is bigger. So, the pressure must fall and from the diagram;
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Unit No. 4 - Flow measurement
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Module No. 1: Instrumentation 1
CD is called the "coefficient of discharge". We can get this from tables. It depends on what is used to make the restriction.
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Unit No. 4 - Flow measurement
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4.4.3
Differential Pressure Devices There are many devices used to make a restriction in a pipeline so that rate of flow can be measured. The design of each device is fixed by either ISO or ISA (instrument Society of America) standards. There are standard tables which are used to calculate flow. In the petroleum industry engineers assume the flow is turbulent. However, you can get tables for laminar flow if you need them. A field technician will only need to calibrate the differential pressure transmitter. An engineer will give a technician the figures he needs to do this. Some of the more common devices are given below together with their uses. •
The Orifice Plate
Module No. 1: Instrumentation 1
Figure 4-7 The Orifice Plate and Tappings
Figure 4-7 shows an orifice-plate fitted into a pipeline to make a differential pressure. The orifice plate is a flat disc with a hole in it shaped as shown. The front edge is sharp and the back edge is chamfered. The fluid is squeezed as it passes through the hole and has a maximum velocity at a point called the VENA CONTRACTOR.
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Unit No. 4 - Flow measurement
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Taps (holes) are drilled into the pipeline and a differential pressure transmitter connected across the orifice plate. The differential pressure is measured. The square root of the differential is used to produce a flow signal which is proportional to the flow. Standard tables are produced for the tapping places as shown.
Module No. 1: Instrumentation 1
The position of the hole in the plate depends on the fluid being measured. The diagram below shows typical plates and their uses.
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Unit No. 4 - Flow measurement
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All orifice plates must be made to an exact standard to fit the reference tables. A typical example is given below for a D and D/2 tap fitting.
Figure 4-8 Dimensions for a D and D/2 orifice plate fitting
The advantages and disadvantages of an orifice plate. Advantages 1.
Simple in operation
2.
No moving parts
3.
Reliable for a long time
4.
Not expensive
Module No. 1: Instrumentation 1
Disadvantages 1.
Square root relationship
2.
2.Difficult to install
3.
3.Range of measurement small. Operator has to change plate (hole size) to change the range.
Note : The orifice plate is the only suitable device when measuring high gas flow rates.
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Unit No. 4 - Flow measurement
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•
THE VENTURI
This is a very expensive device. It is used when the energy of the flow is so low that the restriction could stop the flow (low pressure loss). The diagram below shows a typical Venturi with its pressure tappings (see Figure 4-9).
Figure 4-9 The Venturi
The advantages and disadvantages of the venturi.
Module No. 1: Instrumentation 1
Advantages 1.
Simple in operation
2
. Low pressure loss
3.
Can be used with liquids that contain solids
4.
Reliable for a long time
5.
No moving parts
Disadvantages 1.
Expensive
2.
Square root relationship
3.
Poor range. Designed for one job only
4.
Difficult to install
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•
NOZZLES These are a compromise between the orifice plate and the venturi. They are cheaper than a venturi but have a high pressure loss. They are more expensive than an orifice plate, but have lower pressure loss. A few examples are shown below.
Module No. 1: Instrumentation 1
Unit No. 4 - Flow measurement
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Unit No. 4 - Flow measurement
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•
PRESSURE LOSS GRAPH The graph below (see Figure 4-10) shows how much pressure is lost when these devices are used. The Y axis shows the hole size and the X axis shows the percentage of pressure lost. This shows the advantages of the venturi over the orifice plate. It also shows how the nozzle is between the two.
Module No. 1: Instrumentation 1
Figure 4-10 Pressure Loss Graph
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Unit No. 4 - Flow measurement
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4.5
VARIABLE AREA METERS These are simple devices used to indicate small rates of flow. They are used by an operator out in the field. Typical uses are: •
In seal oil and lubrication oil flow lines on large rotating machines; eg, diesel engines and gas compressors.
•
In cooling water lines for machines and processes.
Figure 4-11 shows a variable area meter (Rotameter).
Module No. 1: Instrumentation 1
Figure 4-11 The Rotameter
Operation The Rotameter is fitted vertically into the flow line. The flow of the fluid is from bottom to top through the cylinder. The cylinder increases in area from bottom to top. With no flow, the float is at the bottom (position A). When the flow increases, the pressure makes the float rise. It will rise to a position where the flow pressure on the float equals the weight of the float, (position B). If the flow gets faster there is more pressure on the float and it will rise higher (position C). The flow rate indicated depends on the size of the device. It is pre-calibrated by the manufacturer. The operator reads the flow rate from the transparent scale using the top of the float as a marker.
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Unit No. 4 - Flow measurement
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4.6
CALIBRATION OF FLOW MEASURING DEVICES
4.6.1
Quantity Meters The only way to calibrate a flow quantity meter accurately is to use a standard volumetric measure. This is easy when the volumes are small. A gasoline pump is checked by a standard measure; eg, a 10 litre can. When you fill the can the meter on the pump should read 10 litres. This method is impossible when you need to measure thousands of gallons per minute. The system used to check large liquid volumes is called a PROVER LOOP. A simple diagram of the system is shown in Figure 4-12.
Figure 4-12 The Prover Loop
The prover loop consists of a calibrated section of pipe. A tightly fitting rubber ball (sphere displacer) is pushed through the pipe by the flow of a liquid which must be measured. Two detector switches mark the travel of the ball. The computer works out the volume measured by the travelling ball and compares this measurement against the volume measured by the meter. The four way valve lets the operator send the ball in both directions through the calibrated section of pipe. This means that the ball can be passed a number of times to make sure the meter is checked accurately.
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Unit No. 4 - Flow measurement
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4.6.2
Calibration of Differential Pressure Devices In order to calibrate the differential pressure transmitter, the field technician uses figures given by the design engineer. For control purposes, the actual flow need not be exact. However, the movement of large amounts of gas can only be checked using an orifice plate. The true volume, at standard pressure, is worked out by a special programme on a computer. This work is done by a specialist called a metering engineer.
4.6.3
Flow Straighteners All flow measuring devices which use a restriction need a streamlined flow. Flow measuring devices must be placed away from things which disturb the flow; eg elbows, control valves, etc. If this is not possible then the flow is streamlined by flow straighteners. These are groups of small pipes placed in the pipe line as shown in Figure 413.
Module No. 1: Instrumentation 1
Figure 4-13 Flow Straighteners
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UNITS IN THIS COURSE
UNIT 1
INTRODUCTION TO INSTRUMENTATION
UNIT 2
PRESSURE MEASUREMENT
UNIT 3
THE PRESSURE TRANSMITTER
UNIT 4
FLOW MEASUREMENT
UNIT 5
MEASUREMENT OF LEVEL
UNIT 6
PRACTICAL TASKS
Module No. 1: Instrumentation 1
Unit No. 5 - Measurement of level
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Unit No. 5 - Measurement of level
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TABLE OF CONTENTS Para
Page
5.0
COURSE OBJECTIVE
3
5.1
INTRODUCTION
4
5.2
THE DIP STICK
4
5.3
THE DIP TAPE
5
5.4
THE SIGHT GLASS
5
5.5
FLOATS
6
5.6
5.5.1
The Simple Float
6
5.5.2
Industrial Float Systems
7
HYDROSTATIC TANK GAUGING (HTG)
9
5.6.1
Introduction
9
5.6.2
Offset Datum Lines
12
5.6.3
Wet Legs
13
DISPLACERS AND LOCAL LEVEL CONTROL
14
5.8
LEVEL SWITCHES
17
5.9
AIR BUBBLE METHOD
19
5.10
OTHER METHODS OF LEVEL MEASUREMENT
20
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5.7
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5.0
COURSE OBJECTIVE The student will be able to •
Explain the use of a dip stick and dip tape.
•
Sketch a typical sight glass installation.
•
Explain, with the aid of a diagram, typical level measurements using floats.
•
Explain, with the aid of diagrams, hydrostatic tank level measurement.
•
Explain, with the aid of a sketch, the operation of a typical buoyancy level transmitter.
•
Sketch typical float operated level switches.
•
Explain, with the aid of a diagram, the bubbler method of level measurement.
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Unit No. 5 - Measurement of level
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5.1
INTRODUCTION The aim of this unit is to introduce the measurement of level and the devices used in its indication, measurement and control
5.2
THE DIP STICK The dip stick shown in Figure 5-1 is the only true measurement of level. It is still used by operators and ships' captains to check that the instrumentation which measures the level of a liquid in a tank is correct. The dip stick is a long calibrated ruler. The depth of the liquid in the tank is indicated by a WET mark when the stick is removed. It's the same principle as checking the oil level of a car. Because there may be rubbish at the bottom of the tank the level may be taken from a bottom level datum line. A datum line is a base line from which things can be measured. There is also a top datum line which is used to measure the space above the liquid (the ullage).
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Unit No. 5 - Measurement of level
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5.3
THE DIP TAPE
Figure 5-2 Dip Tape
The dip tape shown in Figure 5-2 is a development of the dip stick for finding the level in large tanks. The tape is run out until the weight touches the bottom of the tank. It's then pulled up. The wet mark on the tape indicates the depth of the liquid. Note : The dip stick / tape is no good if the liquid does not leave a WET mark. An example of this type of liquid is mercury.
5.4
THE SIGHT GLASS
Module No. 1: Instrumentation 1
This is the level indicator used by operators in the plant. The device is connected to the side of a vessel and the level is seen by looking through the glass. A high pressure sight glass is shown in Figure 5
Figure 5-3 High Pressure Sight Glass
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There are many different types of sight glasses. A single glass tube is strong enough for low pressures. For high pressures you need a reinforced glass tube with a steel case, as shown in Figure 5-3. Most industrial sight glasses can be cleaned on site by closing the isolating valves, draining the tube via valve D and rodding through valve A. Good sight glasses also have an automatic shut-off valve. This operates if the glass breaks. It stops all the liquid draining out of the vessel. High pressure sight glasses have very specific instructions about how they are put together and taken apart. You must use the manufacturer's manual. A high pressure sight glass should never be used again because re-tensioning will make the glass break.
5.5
FLOATS
5.5.1
The Simple Float
Figure 5-4 Simple Float Indicator
Module No. 1: Instrumentation 1
Figure 5-4 shows a simple float level indicator. It is still used by water departments and on chemical tanks on older oil platforms. It is cheap to install and easy to operate. Operation The float and counter weight are connected together by a wire on pulleys. The system is in balance with the float on the surface of the liquid. If the level rises, the float rises and the counter weight falls to the new balance point. If the level falls the counter weight rises. The counter weight has a pointer which indicates the level on a scale on the outside of the tank. The scale shows "full" when the pointer is at the bottom and "empty" when it is at the top. The scale can be very large so that, for example, water tower levels can be seen from the ground.
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5.5.2
Industrial Float Systems The simple float is not very accurate and can be very difficult to read. If the surface of the liquid has waves then the float starts to swing. This problem is solved by fitting special devices inside the tank as shown in Figure 5-5.
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Unit No. 5 - Measurement of level
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Figure 5-5 Tank Constructions
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•
Guide wire system (Figure a) This is the cheapest system. The float; C, is held in place by wires; B. These are fixed to the bottom by a concrete block; A, and tightened by a spring; D. The float is connected by a wire (to the indicating unit K) via a pulley system (FGF) and pipe (1) supported on brackets (J). The indicating unit is the counter-weight and the level is indicated by a mechanical counter.
•
Still pipe system (Figure b and c) This is a more expensive method but it is more accurate. The float is contained inside a still pipe (a steel pipe with holes in it). The level inside the pipe doesn't move so it gives very accurate measurements of level. Figure b shows the older mechanical indication method. Figure c shows the modern method (Entis-Enraf). The system is electronically controlled and the level measurement is sent as an electronic signal to the control room.
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Unit No. 5 - Measurement of level
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5.6 HYDROSTATIC TANK GAUGING (HTG)
5.6.1
Introduction Many of the modern oil storage tank facilities (tank farms) use hydrostatic tank gauging to indicate the level in a tank. HTG is good because there is no equipment inside the tank. It is cheaper to install and maintain than float installations. BASIC PRINCIPLE
Module No. 1: Instrumentation 1
The higher the level of a liquid in a tank, the higher the pressure on the bottom of the tank. The nearer the outlet is to the bottom of the tank, the greater the pressure and the further the flow stream will reach. Figure (a) shows this effect. The pressure on the bottom of a tank only depends on the level of the liquid in the tank. Figure (b) shows this effect. No matter what the shape of the tank, the pressure (P) at the bottom of the tank is the same.
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Proof :
The force on the bottom of the tank is the weight of the liquid. WEIGHT OF LIQUID = VOLUME x DENSITY x GRAVITY but VOLUME = AREA (A) x HEIGHT (H)
Therefore WEIGHT OF LIQUID = AREA (A) x HEIGHT (H) x DENSITY (p) x GRAVITY (g)
Module No. 1: Instrumentation 1
but PRESSURE
=
FORCE(WEIGHT) AREA
AREA-x HEIGHT x DENSITY x GRAVITY AREA
PRESSURE (P) = HEIGHT(H) x DENSITY (p) x GRAVITY (g) or P = pgH
This equation shows that the pressure at the bottom of a column (level) of liquid does not depend on the shape of the container.
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Hydrostatic Tank Gauging (HTG) uses the pressure of a column of liquid to measure the level. The diagram below shows the basic layout of the system (see Figure 5-6)
Figure 5-6 Hydrostatic Tank Gauging Theory P1
=
Pressure above the liquid level
P2
=
Pressure at inlet to differential pressure transmitter
P2
=
P1 + Pressure of liquid above the datum line.
The pressure of the column of liquid above the datum line is given by the formula: P
=
Density x Gravity x Height
Gravity is a constant and providing the density of the liquid does not change then
Module No. 1: Instrumentation 1
and
P
=
KH
where K is a constant
P2
=
P1 + KH
The differential pressure (DP) across the transmitter is P2 - P1 =
P1+ KH - P1
DP
KH
=
This means that the DP transmitter signal gives a direct indication of level.
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Unit No. 5 - Measurement of level
5.6.2
Offset Datum Lines The above system works well if the transmitter can be placed at the same level as the datum line. This is often not possible and the offset (the difference between the levels of the datum line and the transmitter) must be allowed for. The diagram below illustrates the problem, (see Figure 5-7)
(a) Transmitter Below Datum Line
(b) Transmitter Above Datum Line
Figure 5-7 Transmitter Off-set
In figure (a) the transmitter is lower than the datum line. 'V' is the difference in height between the transmitter and the datum line. So, the transmitter will give the wrong reading, it will be 'V' units too high. Figure (b) shows the transmitter above the datum line. In this case the transmitter will give a level which is 'V' units too low, because the pressure of the liquid above the transmitter is less than the pressure of the liquid above the datum line.
Module No. 1: Instrumentation 1
Differential pressure transmitters have special units added to allow for the above problem. They are called elevation/depression units. These units move the zero to allow for the height difference between the transmitter and the datum line. Manufacturers use different methods for elevation/depression. The manual must be used when setting up a differential pressure transmitter on a tank.
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Wet Legs
Unit No. 5 - Measurement of level
5.6.3
Figure 5-8 Wet Legs
Module No. 1: Instrumentation 1
Some liquids produce heavy vapours. These vapours may condense to liquid in the pipe between the differential pressure cell and the top of the tank. This condensate can cause the transmitter to give the wrong reading. To stop this, the pipe is filled with a known liquid (eg glycol). This is called the "Wet Leg The differential pressure transmitter is adjusted using the elevation/depression units to offset the pressure caused by the height of the liquid in the wet leg (P3).
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DISPLACERS AND LOCAL LEVEL CONTROL The displacer is a locally mounted device which controls the level in a vessel. It is used on remote sites where it is too expensive to return signals to the control room. The most common types in use are manufactured by Fisher or Masoneilan. Figure 5-9 shows a Fisher device (The Level-Trol).
Module No. 1: Instrumentation 1
Unit No. 5 - Measurement of level
5.7
Figure 5-9 The Displacer
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The displacer unit is connected to both the vessel and the control valve. This makes a self contained local control loop as shown in Figure 5-10
Figure 5-10 Self-contained Local Control Loop
Operation The weight of the displacer changes as the level rises or falls in the displacer housing.
•
The displacer hangs on the torque tube via the connecting rod.
•
The changing weight of the displacer makes the torque tube twist or untwist.
•
The twisting motion of the torque tube moves a flapper against a nozzle. This sends a control signal to the pneumatic control valve.
•
The pneumatic control valve opens or closes to keep the level constant at the set point.
Module No. 1: Instrumentation 1
•
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Unit No. 5 - Measurement of level
Theory A displacer works on "Archimedes Principle" "The weight of a body immersed in a liquid depends on the weight of the volume of liquid displaced". In other words , if the displacer displaces a volume of liquid which weighs 1 kg, the displacer will seem to weigh 1 kg less than it weighs when it's not in the liquid.
Figure 5-11 Simple Example of Archimedes Principle
Module No. 1: Instrumentation 1
Figure 5-11 shows a simple example of Archimedes' principle. In 'A, the scale shows 3 Kg weight. The displacer weighs 3 kg. In 'B' the displacer has displaced a volume of water which weighs 1 kg. So, the scale shows a weight of 2 kg i.e. 3 kg minus 1 kg for the liquid displaced. The diameter of the container and displacer are kept constant. So, the weight loss on the displacer is directly proportional to the liquid level in the displacer housing. If the displacer weighs less then the torque tube is twisted less. The amount the torque tube twists depends on the level of liquid in the displacer housing. Note :- The weight of the liquid displaced is given by the formula Weight = Volume x Gravity x Density So, changing the density of the liquid in the vessel means the Level-Trol must be recalibrated.
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Unit No. 5 - Measurement of level
5.8
LEVEL SWITCHES A level switch is the last safety device when controlling level. If the level controller stops working the vessel can overfill. This can be dangerous. A level switch uses a float to operate a switch to shut down filling pumps in an emergency. The diagrams below show two typical examples.
Figure 5-12 Flexible-Shaft Float Switch
Module No. 1: Instrumentation 1
Figure 5-12 shows a pneumatic level switch. When the level of liquid is low the float hangs down. The operating screw on the end of the flexible shaft holds the flapper tight against the nozzle. The output signal is a maximum so the pumps continue to fill the vessel. If the level rises and lifts the float the screw on the end of the flexible shaft moves down. The flapper moves away from the nozzle and the output signal falls to zero. This shuts down the pumps so no more liquid comes into the vessel.
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Figure 5-13 Float Operated Mercury Switch
Module No. 1: Instrumentation 1
Figure 5-13 shows a typical electrically operated level switch. The mercury bottle has three connections, the mercury (a good conductor) acts as the switch to change over the contacts. The switch is operated magnetically, the two different positions being clearly shown. When the level is high the switch is in one position. When the level falls, the switch is in the other position.
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Unit No. 5 - Measurement of level
5.9
AIR BUBBLE METHOD The air bubble method is one of the oldest and simplest methods used to indicate and or transmit a signal. The diagram below shows a simplified layout of the method (see Figure 5-14).
Figure 5-14 Liquid Level Measurement by Air Bubbler Method
Module No. 1: Instrumentation 1
Operation •
An inert gas (air or nitrogen) is passed down the bubbler tube. There is just enough gas pressure for the bubbles to appear when the liquid is at the maximum level in the vessel.
•
When the vessel is full the pressure gauge or transmitter will read a maximum back pressure equal to the hydrostatic head (H), (the pressure of the liquid above the zero level).
•
At the zero level the back pressure will be zero and the gauge or transmitter will read zero.
•
The back pressure between zero and maximum levels is proportional to the liquid level in the vessel. The pressure gauge or transmitter can be calibrated to indicate the liquid level.
•
The gas pressure is adjusted by the regulator to give a steady flow of gas down the bubbler tube. The gas flow is indicated on the Rotameter.
•
This method can be very accurate. A modern differential pressure transmitter, open at one side, can easily be calibrated to give a span of 0-6" H20
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OTHER METHODS OF LEVEL MEASUREMENT In this unit we have introduced some common methods of measuring level used on most installations. There are many other methods using various types of high technology. These will be special for only one or two installations. You will have to learn them on the job. A few examples are: (a)
Radar, ultrasonic, gamma and infrared detectors.
(b)
Capacitive sensors.
(c)
Resistive sensors.
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Unit No. 5 - Measurement of level
5.10
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UNITS IN THIS COURSE UNIT 1
INTRODUCTION TO INSTRUMENTATION
UNIT 2
PRESSURE MEASUREMENT
UNIT 3
THE PRESSURE TRANSMITTER
UNIT 4
FLOW MEASUREMENT
UNIT 5
MEASUREMENT OF LEVEL
UNIT 6
PRACTICAL TASKS
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Unit No. 6 - Practical tasks
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TABLE OF CONTENTS Para
Page PRACTICAL TASK 1
3
PRACTICAL TASK 2
14
2.1
PNEUMATIC TRANSMITTER
14
2.2
ELECTRICAL TRANSMITTER
15
PRACTICAL TASK 3
25
3.1
PNEUMATIC DIFFERENTIAL PRESSURE TRANSMITTER CALIBRATION
25
3.2
ELECTRICAL DIFFERENTIAL PRESSURE TRANSMITTER CALIBRATION
26
PRACTICAL TASK 4
36
PRACTICAL TASK 5
39
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PRACTICAL TASK 1 CALIBRATION OF PRESSURE GAUGES INTRODUCTION The practical work you will do during this task will show you the common methods used to calibrate a pressure gauge. The test equipment used to calibrate the gauge may not be the newest type but it will work in the same way as newer equipment. New pressure sensors (e.g. piezoelectric, strain gauge etc.) are quickly changing the way test equipment is made from mechanical to electronic display. The particular test equipment used by an operating company must be learnt on site.
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Unit No. 6 - Practical tasks
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THE DEAD WEIGHT TESTER
Figure PT-1 Oil Dead Weight Tester
This is a very simple device. Figure PT-1 shows a typical dead weight tester. Oil is used as the testing fluid. When the handle is turned the increase in fluid pressure is applied to both the gauge and the weights. When the weights start to lift the gauge pressure should be the same as the pressure indicated by the weights. You can calibrate instruments very accurately if the weights. are correct and there is minimum friction between the weight piston and the cylinder. A good quality tester has a motor which keeps the weights spinning all the time. This reduces the friction. If the tester has no spinning motor, you should spin the weights by hand.
Module No. 1: Instrumentation 1
There are many kinds of dead weight testers. The operating fluid can be either oil, water or air, depending on the manufacturer. You might not be able to use an oil type dead weight tester on the job site. The oil left in the gauge can contaminate process fluids; particularly some gases. Modern dead weight testers do not use weights. The weights are replaced with a digital read out. Piezo-electric sensors are used to detect pressures. An example of a modern pressure calibration unit is given in Figure PT-2. Note:
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Liquid operated dead weight testers have a hand valve (HV) and a reservoir. This is to ensure that no air bubbles are trapped in the liquid before pressuring up the system.
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Figure PT-2 DRUCK Precision Pressure Calibrator
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THE PRECISION GAUGE AND TEST BENCH. Most instrument workshops calibrate site gauges using a test bench fitted with precision gauges. This is a much quicker method than using a dead weight test and accurate enough for field calibration (± 1 %). The precision gauges are calibrated regularly by a dead weight tester. This ensures that the gauges stay accurate. A typical example of gauge calibration using a precision gauge is shown in Figure PT 3.
Module No. 1: Instrumentation 1
Unit No. 6 - Practical tasks
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Figure PT 3 Gauge Calibration Using a Precision Gauge.
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•
GAUGE CALIBRATION There is no standard way to calibrate a pressure gauge. The way a gauge is calibrated depends on the way the gauge is used. The following notes show how to produce a test certificate for a gauge. However, this is not always needed on site. Producing a certificate will show the things to look for when doing a job site calibration. The CDC instrument workshop has different types of calibration equipment. The instructor will show you how to use each type. Use the manufacturer's instruction manual for each piece of test equipment when doing the following procedure.
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PRACTICAL TASK 1 CALIBRATION PROCEDURE 1.
Do not apply any pressure to the gauge. Set the pointer to read zero on the scale.
2.
Apply the full range pressure to the gauge. Adjust the linkage so that the pointer is at the maximum reading on the scale, (full scale deflection).
3.
Reduce the pressure to zero and check that the pointer reads zero on the scale. Adjust the pointer if necessary.
4
Repeat steps (2) and (3) until both readings are correct.
5.
If the gauge has a linearizing adjustment, set the applied pressure to 50% of the maximum scale reading. Adjust the linearizing adjustment so that the pointer reads at 50% of the maximum scale reading.
6.
Check the gauge reads correctly at 0, 50% and maximum reading. You may need to adjust the gauge many times before the gauge is correct. You must be patient and careful.
7.
When step (6) is completed, write down the reading on the gauge for the applied pressure readings. A calibration table is provided.
8.
Draw a graph of the gauge readings and the applied pressures (increasing and decreasing).
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Unit No. 6 - Practical tasks
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9.
The graph you get will look something like the graph shown above. The shaded area shows the hysterisis of the gauge (the hysterisis is the difference between rising and failing pressure readings). The biggest difference between the true reading and the gauge reading tells you how accurate the gauge is. Hysterisis is caused by friction and wear on the operating mechanism. If the gauge is not accurate enough, the mechanism cannot be replaced. In this case the gauge is thrown away.
Module No. 1: Instrumentation 1
You must decide if a gauge is accurate enough. If it is not accurate enough you must say so.
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10.
Well used gauges can have what is called a 'DEAD BAND'. This is a place on the scale where the pointer does not move when the pressure changes. You will see this when you find the calibration curve. An example of a large dead band is shown below. If the dead band is large you must get a new gauge.
CONCLUSION The above procedure shows you how to get a calibration curve for a gauge. This is very seldom done. It also shows the effects of hysterisis and a dead band.
Module No. 1: Instrumentation 1
A gauge is normally selected to operate around its mid range position. On the job, the gauge is often calibrated to be accurate only over its operational range. Large errors at each end of the scale (zero and maximum) are not important.
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Unit No. 6 - Practical tasks
CALIBRATION TABLE
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Unit No. 6 - Practical tasks
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Unit No. 6 - Practical tasks
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APPLIED PRESSURE
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Unit No. 6 - Practical tasks
PRACTICAL TASK 2 INTRODUCTION CALIBRATING A PRESSURE TRANSMITTER The basic procedure for calibrating a pressure transmitter is the same as for a pressure gauge. However, it is most important to check the linearity of the transmitter over its full range as it provides the input signal to the controller. This task has two parts. You will calibrate both a pneumatic and electrical transmitter.
2.1
PNEUMATIC TRANSMITTER
CALIBRATION PROCEDURES I)
Connect the inlet pressure connection to the calibrated pressure supply (e.g. dead weight tester, pressure bench etc.).
2)
Connect a 20 psiI (1.4 bar) air supply to the air supply connection.
3)
Use a pressure gauge to cover the range 0 - 20 psi (0 - 1.4 bar). Connect it to the output from the transmitter.
4)
Using the manufacturer's manual calibrate the transmitter. The setting for zero and the span of the operating range will be given by the instructor. Produce a calibration graph of output pressure against input pressure range. This will show if the transmitter is still linear. Check it is within the stated accuracy.
Module No. 1: Instrumentation 1
5)
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2.2
ELECTRICAL TRANSMITTER
CALIBRATION PROCEDURE 1.
Connect the inlet pressure connection to the calibrated pressure supply (e.g. dead weight tester, pressure bench etc.).
2)
Connect the 24V DC supply to the terminal board connections. Use the manual of the particular transmitter you have. A series load resistor may be required.
3)
You may need to use an ammeter in series to measure the 4 20 mA signal. However, most transmitters have a standard resistor (1 Ω) so 4 - 20 mV is measured instead. This is a more accurate measurement. The manual will tell you what to do. The setting zero and the span of the operating range will be given by the instructor. Produce a calibration graph of output range (4 - 20 mA) against input pressure range. This will show if the transmitter is still linear. Check it is within the stated accuracy.
Module No. 1: Instrumentation 1
4)
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Unit No. 6 - Practical tasks
CALIBRATION TABLE (psi)
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Unit No. 6 - Practical tasks
CALIBRATION TABLE (BAR)
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Unit No. 6 - Practical tasks
CALIBRATION TABLE (mA)
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CALIBRATION GRAPH INPUT / OUTPUT RELATIONSHIP OF THE PNEUMATIC PRESSURE TRANSMITTER
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Unit No. 6 - Practical tasks
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OUTPUT PRESSURE (psi)
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CALIBRATION GRAPH INPUT / OUTPUT RELATIONSHIP OF THE PNEUMATIC PRESSURE TRANSMITTER
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Unit No. 6 - Practical tasks
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OUTPUT PRESSURE (psi)
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CALIBRATION GRAPH INPUT / OUTPUT RELATIONSHIP OF THE PNEUMATIC PRESSURE TRANSMITTER
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Unit No. 6 - Practical tasks
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OUTPUT PRESSURE (bar)
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CALIBRATION GRAPH INPUT / OUTPUT RELATIONSHIP OF THE PNEUMATIC PRESSURE TRANSMITTER
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Unit No. 6 - Practical tasks
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OUTPUT PRESSURE (bar)
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CALIBRATION GRAPH INPUT / OUTPUT RELATIONSHIP OF AN ELECTRICAL PRESSURE TRANSMITTER
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Unit No. 6 - Practical tasks
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OUTPUT PRESSURE (mA)
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CALIBRATION GRAPH INPUT / OUTPUT RELATIONSHIP OF AN ELECTRICAL PRESSURE TRANSMITTER
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OUTPUT PRESSURE (mA)
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Unit No. 6 - Practical tasks
PRACTICAL TASK 3 CALIBRATING A DIFFERENTIAL PRESSURE TRANSMITTER INTRODUCTION Calibrating a differential pressure transmitter is much more difficult than calibrating a pressure transmitter. This is because the pressures are very small. A modern D.P. transmitter can have a calibration range of 0-5" H20 gauge. You can calibrate this with a manometer but normally a very low pressure test gauge is used. The equipment used in the lab is called a 'wally box'. This is a special pressure test kit made by Wallace and Tiernan. The 'wally box' is not used much now because of its size. Modern hand-held electronic devices (e.g. DRUCK) are becoming popular.
PNEUMATIC DIFFERENTIAL PRESSURE TRANSMITTER CALIBRATION 1.
Connect the calibrated pressure supply to the HIGH inlet pressure. Leave the LOW pressure inlet connection open.
2.
Connect the air supply port to the 20 psi (1.4 bar) supply.
3.
Use a pressure gauge to cover the range 0 - 20 psi (0 - 1.4bar). to the output port.
4.
Using the manufacturer's manual set up the differential pressure transmitter to values given by the instructor.
5.
Use the results to draw a calibration curve in the same way as for a pressure gauge. Ensure the device gives a linear output of 3-15 psi (0.2-1 bar). Check its accuracy is within specification. Remember: 3-15 psi is not equivalent to 0.2-1 bar. Calibrate using either 3-15 psi or 0.2-1 bar.
Connect it
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3.1
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3.2
ELECTRICAL DIFFERENTIAL PRESSURE TRANSMITTER CALIBRATION 1
Connect the "HIGH" inlet pressure connection to the calibrated pressure supply. Leave the "LOW" pressure connection open.
2.
Connect up the 24V D.C. supply to the transmitter output terminals using the manufacturer's calibration manual.
3.
The calibration method depends on the manufacturer. You may or may not need a series load resistor. The device may be current or voltage calibrated. The manual must be used.
4.
Set up the differential pressure transmitter to values given by the instructor.
5.
From results obtained in the same way as for a pressure gauge, draw a calibration curve. Ensure the device gives a linear output (4 - 20 mA) and check its accuracy is within specification.
Module No. 1: Instrumentation 1
Unit No. 6 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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Unit No. 6 - Practical tasks
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Module No. 1: Instrumentation 1
CALIBRATION TABLE (12sil
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Module No. 1: Instrumentation 1
Unit No. 6 - Practical tasks
CALIBRATION TABLE (bar)
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Unit No. 6 - Practical tasks
CALIBRATION TABLE (MA)
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CALIBRATION GRAPH INPUT / OUTPUT RELATIONSHIP OF THE PNEUMATIC DIFFERENTIAL PRESSURE TRANSMITTER
Module No. 1: Instrumentation 1
Unit No. 6 - Practical tasks
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OUTPUT PRESSURE (psi)
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CALIBRATION GRAPH INPUT / OUTPUT RELATIONSHIP OF THE PNEUMATIC DIFFERENTIAL PRESSURE TRANSMITTER
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Unit No. 6 - Practical tasks
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OUTPUT PRESSURE (psi)
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CALIBRATION GRAPH INPUT / OUTPUT RELATIONSHIP OF THE PNEUMATIC DIFFERENTIAL PRESSURE TRANSMITTER
Module No. 1: Instrumentation 1
Unit No. 6 - Practical tasks
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OUTPUT PRESSURE (bar)
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CALIBRATION GRAPH INPUT / OUTPUT RELATIONSHIP OF THE PNEUMATIC DIFFERENTIAL PRESSURE TRANSMITTER
Module No. 1: Instrumentation 1
Unit No. 6 - Practical tasks
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OUTPUT PRESSURE (bar)
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CALIBRATION GRAPH INPUT / OUTPUT RELATIONSHIP OF AN ELECTRICAL DIFFERENTIAL PRESSURE TRANSMITTER
Module No. 1: Instrumentation 1
Unit No. 6 - Practical tasks
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OUTPUT PRESSURE (mA)
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CALIBRATION GRAPH INPUT / OUTPUT RELATIONSHIP OF AN ELECTRICAL DIFFERENTIAL PRESSURE TRANSMITTER
Module No. 1: Instrumentation 1
Unit No. 6 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
OUTPUT PRESSURE (mA)
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Module No. 1: Instrumentation 1
Unit No. 6 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
PRACTICAL TASK 4 CALIBRATION OF A LEVEL TRANSMITTER (LEVEL-TROL) INTRODUCTION
Figure PT4 Fisher Level Trol
The level transmitter to be calibrated is a Fisher level-trol. This uses a displacer type level detector. A layout of the device is shown in Figure PT 4.
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Unit No. 6 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
The calibration adjustments are inside the transmitter housing. They are shown in Figure PT 5.
Module No. 1: Instrumentation 1
FIGURE PT 5 Transmitter With Cover Removed .
The instruction manual for calibrating this transmitter is difficult to understand. There is only one instruction manual for controller or transmitter operations for either level (gas/liquid), interface (liquid/liquid), or density. The following procedure will only help you calibrate a level transmitter (gas/liquid). The basic procedure used in this calibration is the same for any type of displacement level transmitter. It is possible to calibrate this unit using weights instead of the sensor (displacer unit). However, this is seldom done. You will calibrate the transmitter using water to set the required level. This is the normal workshop method. Remember the final calibration must be done on the job. This final calibration depends on where the transmitter is fitted (e.g. gas/oil, oil/water interface, etc.). You will have to learn this calibration on site.
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Unit No. 6 - Practical tasks
CALIBRATION PROCEDURE (WET TYPE)
Figure PT 6 Calibration Wet Type Pre-checks 1.
Set the air supply pressure to 20 psi on the supply gauge Adjust the air regulator if required.
2)
Set the S.G dial to 1.
3)
Put the zero adjustment dial to read zero.
Module No. 1: Instrumentation 1
Calibration 1.
When the water level is below the bottom of the displacer, set the zero adjustment. Check that the output gauge reads 3 psi.
2.
Fill the displacer housing with water until the level is above the displacer. Adjust the S.G. dial until the output reads 15 psi.
3)
Repeat steps (1 ) and (2) until both are correct.
4)
Raise the water level until it reaches the centre line (L) The output pressure should be 9 psi.
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Unit No. 6 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
PRACTICAL TASK 5 CALIBRATING A PRESSURE SWITCH
Module No. 1: Instrumentation 1
Figure PT 7 The Pressure Switch
Figure PT-7 shows a typical pressure switch. The inlet pressure is applied to the bottom of the operating piston. This piston is forced upwards by the inlet pressure against the range spring. The tension of the range spring can be adjusted so that it is compressed at a certain pressure. When this pressure is reached the operating pin will hit the trip button on the micro-switch and change it over. The normally open contacts (NO to C) will become closed and the normally closed contacts (NC to C) will open. The pressure at which the micro-switch changes over is set by adjusting the trip setting nut. This nut adjusts the tension of the range spring (e.g. if the nut is turned clockwise the trip pressure will be higher).
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Module No. 1: Instrumentation 1
Unit No. 6 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
1)
Connect the pressure switch to the workshop air supply via a hand pressure regulator and test gauge, as shown in the diagram.
2)
Use an Ohmmeter to check that the switch contacts are as indicated; NO and NC.
3)
Connect the Ohmmeter to the normally open contacts. The meter should read "open circuit". Adjust the hand pressure regulator to increases the pressure to the switch until the contacts change over. The meter should now read "short circuit". Note down the pressure reading on the sheet provided. This pressure is the switch setting for a "rising" pressure.
4)
Increase the pressure to the switch to it's maximum rating. Slowly reduce the pressure to the switch until the switch changes over from closed to normally open again. Note down this pressure reading on the sheet provided. This pressure is the switch setting for a "falling" pressure.
5)
From the readings you have taken work out the pressure difference between the rising and falling pressure settings. This is called the "dead band" of the switch.
6)
The maximum dead band is usually stated by the manufacturer. The switch is unserviceable if the maximum dead band is more than the manufacturer's recommendation.
7.
On job site the pressure switch will be set for either a falling or a rising pressure. This is stated on the maintenance sheet.
8)
After you have completed steps 1 to 7 and the instructor is sure that you have understood what you have done, try setting the switch to another position. You do this by adjusting the trip setting nut. The instructor will give a setting value for either rising or falling pressure inputs.
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Module No. 1: Instrumentation 1
Unit No. 6 - Practical tasks
CALIBRATION SHEET
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MODULE No. 2 INSTRUMENTATION 2
July 1999- Rev.0
UNITS IN THIS COURSE UNIT 1
MEASUREMENT OF TEMPERATURE
UNIT 2
TEMPERATURE TRANSMITTER
UNIT 3
THE CONTROLLER
UNIT 4
VALVES AND ACTUATOR
UNIT 5
PRACTICAL TASKS
Module No. 2: Instrumentation 2
Unit No. 1 - Measurement of temperature
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Unit No. 1 - Measurement of temperature
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para 1.0
COURSE OBJECTIVE
2
1.1
INTRODUCTION
4
1.2
TEMPERATURE SCALES
4
1.2.1
The Absolute Scale
4
1.2.2
Examples
5
1.3
1.4
Module No. 2: Instrumentation 2
Page
1.5
EXPANSION TYPE THERMOMETERS
6
1.3.1
Liquid In Glass Thermometers
6
1.3.2
Filled Systems
8
1.3.3
Solid Expansion Types.
10
1.3.4
Thermostats
10
1.3.5
Bi-metal Strip Thermometers
11
ELECTRICAL METHODS OF TEMPERATURE MEASUREMENT
12
1.4.1
The Resistance Temperature Detector (RTD)
12
1.4.2
The RTD Detector: The electrical circuit
15
1.4.3
The Thermocouple
16
1.4.4
The Thermistor
20
1.4.5
Radiation Temperature Detectors (Pyrometers)
21
THERMOWELLS
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Unit No. 1 - Measurement of temperature
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1.0
COURSE OBJECTIVE The student will be able to •
State and convert the common temperatures scales in use; Fahrenheit and Celsius.
•
Explain absolute temperature and carry out temperature conversions from Fahrenheit or Celsius to the absolute temperature scale.
•
Draw and explain the action of expansion type thermometers such as: Liquid, vapour and gas filled thermometers. Bi-metal strips.
•
Draw and explain the action of electrical temperature sensors. Resistance measuring devices. Thermocouple Thermistors Radiation measuring devices (Pyrometer) Explain the purpose of a Thermowell.
Module No. 2: Instrumentation 2
•
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Unit No. 1 - Measurement of temperature
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1.1
INTRODUCTION The aim of this unit is to introduce temperature measurement devices; both expansion types and modern electrical sensors.
1.2
TEMPERATURE SCALES
There are different scales for measuring temperatures. Figure 1-1 compares the two common temperature scales; Fahrenheit (Imperial) and Celsius (ISO). The fixed points for both scales are the temperature at which ice melts and water boils at standard pressure.
Module No. 2: Instrumentation 2
A temperature in Fahrenheit can easily be changed to Celsius and vice versa. The conversion equations depend on the number of divisions in each scale. Fahrenheit has 180 divisions between the freezing and boiling points of water but Celsius has only 100 divisions. Therefore, the ratio is 180/100 or 9:5. This gives:
1.2.1
The Absolute Scale In instrumentation, many temperature measurements have to be made from ABSOLUTE ZERO. Absolute zero is the temperature at which no heat energy (atomic movement) exists. This temperature is impossible to reach. On the Celsius scale, absolute zero is around - 273.15°C (- 459.67°F). This figure is used as the standard for absolute zero. So 0 Kelvin (K) is the same as -273.15°C Absolute temperature in ISO is called degrees KELVIN (°K) Absolute temperature in Imperial is called degrees RANKINE (°R) °C + 273.15
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Module No. 2: Instrumentation 2
Unit No. 1 - Measurement of temperature
TRAINING MANUAL INSTRUMENTATION
1.2.2 Examples
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Unit No. 1 - Measurement of temperature
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1.3
EXPANSION TYPE THERMOMETERS Most materials expand as they get hotter. An expansion type thermometer uses the expansion of a material to indicate temperature. There are several different types.
1.3.1
Liquid In Glass Thermometers
Module No. 2: Instrumentation 2
Figure 1-2 The Thermometer
The "liquid in glass" thermometer is the most common of all thermometers. It has industrial, domestic and medical uses. The instrument workshop uses these devices as a basic standard for calibration purposes. A good quality device is accurate to 0.1°C. The thermometer, (see Figure 1-2) consists of a glass tube (the stem) which has a very small but uniform bore (hole). At the bottom of this stem there is a thin, walled glass bulb. The bulb holds much more liquid than the stem. The bore in the stem is sealed under a vacuum so that there is no air in the system. The system works by differential expansion. The liquid expands over 20 times more than the glass when the bulb is heated.
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As the liquid expands it rises up the stem. The temperature is shown on a calibrated scale on the glass. A good workshop thermometer will have an immersion mark. The thermometer must be -placed in the liquid up to this mark for accurate temperature readings. Typical liquids in use are:
Module No. 2: Instrumentation 2
Unit No. 1 - Measurement of temperature
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Unit No. 1 - Measurement of temperature
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1.3.2
Filled Systems Liquid in glass thermometers are not strong enough for plant use so in industry the bulb and stem are made of steel. The bulb and stem are completely filled with the expansion liquid under pressure. The indicator is a spiral Bourdon tube, or a pressure cell (strain gauge) which gives an electronic signal. Figure 1-3 show the three basic systems.
Module No. 2: Instrumentation 2
Figure 1-3 Filled Systems
The steel bulb, stem and indicator (Bourdon tube) are completely filled under pressure with either; a liquid, eg. mercury, a gas, eg. freon or a vaporising liquid, eg. methyl chloride. Each system works in the same way. The system is totally filled to provide a constant volume. Expansion of the fluid in the tube is converted to a pressure. This pressure expands the Bourdon tube which moves the pointer on the scale. Note:
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The capillary (the stem) can be many meters long so that the indicator can be placed in a control room away from the fluid temperature being measured.
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Module No. 2: Instrumentation 2
Unit No. 1 - Measurement of temperature
TRAINING MANUAL INSTRUMENTATION
Applications: Filled systems are still used to indicate temperature particularly in places where there is no electrical supply. They are normally of the liquid filled type, to provide enough power to drive a "C" type Bourdon tube. The usual operating range is about 300°C. Gas and vapour filled systems are not used much as indicators because they have a short range and no driving power. However, they are used a lot in temperature control, eg. In air conditioning and refrigeration. A filled system is used to drive a pneumatic temperature transmitter. A typical example of this is the Foxboro type 12. This will be discussed in the next unit. Ambient Temperature Compensation: Filled system thermometers can be inaccurate if the capillary tube is long and the atmospheric (ambient) temperature around the capillary changes a lot from night to day. Accurate filled system thermometers use a dummy Bourdon tube and capillary to compensate for ambient temperature changes. This method is shown in Figure 14
Figure 1-4 Ambient Temperature Compensation for a filled System Thermometer
The dummy Bourdon tube and capillary are exactly the same as the measurement system. Any ambient temperature changes causes both the dummy and measurement system to move the pointer the same amount, but in opposite directions. So, the ambient temperature errors are cancelled.
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Unit No. 1 - Measurement of temperature
1.3.3
Solid Expansion Types. These devices use a solid instead of a fluid to measure or control temperature. The simplest form is the thermostat used to automatically control the temperature of a water heater. This device uses the expansion differential between brass and invar.
1.3.4
Thermostats
Figure 1-5 The Simple Rod Thermostat
Module No. 2: Instrumentation 2
Figure 1-5 shows a simple rod thermostat. The brass tube expands a lot as it gets hotter but invar expands very little . When the liquid is cool the brass does not expand so the switch is closed and the electric heater heats the water. When the water reaches the set temperature the brass tube has expanded enough to pull the invar rod away from the switch. This opens the switch and breaks the circuit. The electric heater will stay disconnected until the brass tube contracts enough to close the switch again. Normally the thermostat and heater are together in one unit. The temperature at which the switch is opened can be adjusted by changing the tension of the spring which closes the switch. Thermostats are not very accurate(± 3°C) but they are long lasting and cheap.
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Module No. 2: Instrumentation 2
Unit No. 1 - Measurement of temperature
1.3.5
Bi-metal Strip Thermometers
Figure 1-6 Action of a Bi-metal Strip
Another kind of solid expansion thermometer is the bi-metal strip. Figure 1-6 shows the action of a bi-metal strip. Two strips of metal, brass and invar, are tightly bonded together and fixed at one end. When the strip is heated the brass expands much more than the invar and the strip bends as shown. This action is used to make a dial thermometer as shown in Figure 1-7. The most common type is the Rototherm.
Figure 1-7 Dial Thermometer (Rotometer) Operation: The bi-metal strip is shaped into a helix. The helix is fixed at one end. The other end of the helix is free to rotate the shaft which is fixed to it. The heat applied to the bi-metal strip at the fixed end causes the helix to unwind and turn the pointer on the scale. Rototherm supplies these dial thermometers in ranges up to 560°C.
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TRAINING MANUAL INSTRUMENTATION
ELECTRICAL METHODS OF TEMPERATURE MEASUREMENT
1.4.1 The Resistance Temperature Detector (RTD) The Resistance Temperature Detector (RTD) is the international standard thermometer for measuring temperatures from -258°C to 727°C. It is accurate to 0.00001°C although in industry 0.1°C is usually good enough. Figure 1.8 shows the RTD together with a typical industrial unit (Kent Instruments) Industrial Unit
Module No. 2: Instrumentation 2
Unit No. 1 - Measurement of temperature
1.4
Figure 1-8 Basic Construction of the RTD
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Unit No. 1 - Measurement of temperature
Construction: This device indicates temperature by measuring the change in the electrical resistance of a metal. The sensing element has a platinum coil of about 100Ω at 0°C (the Pt 100) see Figure 1-9. The sensing element is connected to the terminal box by three wires. The ceramic spacers stop heat moving through the casing (the sheath). The terminal pins have glass to metal seals held in place with glass wool packing. The output cable connects the sensing head to the electronics unit. This converts the changes in resistance into temperature readings. Theory of Operation. When metals get hotter their resistance increases. This increase in resistance is almost linear. When it's measured it gives an accurate indication of temperature. The sensor is usually platinum because it is stable over a large temperature range and does not corrode. The normal platinum RTD is 100Ω at 0°C and rises to 1 38.5Ω at 100°C. The resistance of a Pt 100 at a particular temperature is given in a standard table. These tables must be used when calibrating this device. There is a Pt 100 table on the next page. Note:
Cheaper RTD metals (e.g. nickel and copper) are used where the temperature range is small.
Nickel is used in water heaters and air conditioners. Copper is used in oil product tank temperature sensing.
Module No. 2: Instrumentation 2
Manufacturers provide tables for the calibration of these RTD's.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 1 - Measurement of temperature
TEMPERATURE RESISTANCE VALUES FOR A Pt 100.
Examples of table use. =
175,84Ω
2) The resistance at - 150°C
=
39.73Ω
3) The resistance at 560°C
=
300.77Ω
Module No. 2: Instrumentation 2
1) The resistance at 200°C
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Unit No. 1 - Measurement of temperature
1.4.2
The RTD Detector:
The electrical circuit
Figure 1-9 Simple RTD Detector
The basic circuit (see Figure 1-9) for detecting the resistance of a 3 wire RTD is the unbalanced Wheatstone bridge. When the RTD is at 0°C (100Ω) the bridge is balanced so that the display unit reads 0°C As the temperature of the RTD changes the unbalanced current through the display unit is converted to give the temperature. The system is calibrated with a decade box (variable resistance unit) and the Pt 100 tables. For example; place a 335.92Ω resistance value across 1 and 2 when 2 and 3 are joined together and the display unit should read 670°C. The 3 wire system is used to cancel out unwanted changes in resistance. These can be caused by temperature changes in the air around the connecting leads. This is called ambient temperature compensation. The leads to 1 and 3 are on opposite sides- of the bridge so that any changes in resistance because of ambient temperature changes cancel each other out.
Module No. 2: Instrumentation 2
For greater accuracy, a 4 wire system of ambient temperature compensation is sometimes used. This is shown below (see Figure 1-10). Changes in the resistance of leads 1 and 2 are cancelled by changes in the resistance of leads 3 and 4.
Figure 1 -10 The 4 Wire System
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Unit No. 1 - Measurement of temperature
1.4.3
The Thermocouple
Figure 1-11 Seebeck Effect
The thermocouple uses thermoelectric EMF produced by the difference in temperature between two ends of a metal wire. This is an effect discovered by Seebeck, see Figure 1-11. Two different metals are joined to make two different junctions which are held at two different temperatures. The difference between the metals and the difference in temperature between the hot and cold junctions makes a current flow around the circuit. To use this effect you need standard tables which give the EMF produced by the temperature difference in various metal combinations. The EMF is picked up by an electronic amplifier. The indicator is pre-calibrated to show the temperature. The voltage can be checked against the tables, to make sure the indicator is correct.'
Module No. 2: Instrumentation 2
Figure 1-12 shows a typical thermocouple temperature measuring system, with a typical EMF/temperature curve
Figure 1-12 The Thermocouple Thermometer
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Unit No. 1 - Measurement of temperature
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There are many different thermocouples in use. They are classified by letter. Most modern thermocouple detecting instruments can use any standard thermocouple pair. However, they must be connected to the correct letter position. STANDARD THERMOCOUPLES IN USE
-200 to 850 °C
Module No. 2: Instrumentation 2
Low cost, standard for general use
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Unit No. 1 - Measurement of temperature
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The cold junction (inside the amplifier) must be kept at a constant temperature. This is done electronically using a cold junction sensor (a thermistor) Compensating Leads It is important for the thermocouple wires to be made of the same material all the way through even if there is a large distance between the hot and cold junctions.' However, this is not always possible for various reasons. Sometimes the high resistance of the wire reduces current flow. Sometimes the material is too expensive; (platinum rhodium). In these cases you can still get on accurate measurement by using compensating leads of similar thermo-electrical properties. The standard compensating leads are;
Module No. 2: Instrumentation 2
Figure 1-13 shows a typical arrangement of a thermocouple with compensating leads.
Figure 1-13 The Practical Thermocouple.
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Unit No. 1 - Measurement of temperature
TRAINING MANUAL INSTRUMENTATION
An industrial thermocouple sensing unit looks nearly the same as an industrial RTD sensing unit. Unfortunately some manufacturers (USA standard) mark the negative connection with a RED DOT or RED WIRE. The manual must be used when connecting or checking these devices. All thermocouple systems can be calibrated by checking the EMF generated. This requires standard tables such as the table for a type T thermocouple given below.
MILLIVOLTS (mV) VERSES TEMPERATURE TYPE 'T' THERMOCOUPLE
Cold junction at 0°C
Module No. 2: Instrumentation 2
Example: 1
A hot junction temperature of 350°C will give an output of 17.816 mV
2)
A voltage output of 7.718 mV will indicate a temperature of 170°C
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Module No. 2: Instrumentation 2
Unit No. 1 - Measurement of temperature
1.4.4
The Thermistor A thermistor is a semiconductor made of metal oxides. The thermistor's resistance increases or decreases with temperature. Thermistors are not very accurate but they are small. They are often used for ambient temperature compensation. Some of the cheaper hand-held devices use a thermistor as a sensor but indicate the temperature using the same system as an RTD The symbol for the thermistor is the same as an RTD.
Figure 1-14 Typical Industrial Thermistors.
Figure 1-14 shows two typical industrial thermistors. Both use negative temperature coefficient materials (n.t.c). This means that their resistance goes down as the temperature rises.
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Unit No. 1 - Measurement of temperature
1.4.5
Radiation Temperature Detectors (Pyrometers) Radiation temperature detectors (Pyrometers) are non contact devices. They are used to measure the temperature of something which is difficult to reach by other means, (eg. gas turbine combustion chambers). They are also the only way to measure very high temperatures (above about 1500°C) as all other devices melt. Figure 1-15 shows a typical radiation thermometer.
Figure 1-15 Typical Radiation Thermometer
Module No. 2: Instrumentation 2
The heat from the object is focused by lenses onto a sensor. The output from the sensor is electronically processed by the amplifier to give a reading in degrees. This device can also transmit a signal to the control room if required.
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Unit No. 1 - Measurement of temperature
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The sensor is usually a thermopile. A thermopile is a collection of thermocouples connected in series to produce a larger millivolt output. A typical thermopile is shown in Figure 1-16. The radiation is focused onto the black painted centre and the output connected to an electronic amplifier.
Module No. 2: Instrumentation 2
Figure 1-16 Thermopile for use in a Radiation Pyrometer
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Unit No. 1 - Measurement of temperature
TRAINING MANUAL INSTRUMENTATION
Using modern electronics these devices are now very common. One of the newest radiation Pyrometers is shown in Figure 1-17.
Figure 1-17 Pistol Grip Type Radiation Thermometer.
Module No. 2: Instrumentation 2
This device (Calex/Rayteil) is a pistol grip non-contact infrared thermometer. It has a laser sight for pin point accuracy and other advanced features. The Instrument has a measuring temperature range of -18°C to +540°C (0° to 1000°F). There is no need to focus or calibrate it and the operator does not need special training. A microprocessor makes the device accurate for repeated measurements on very small objects, (less than 25mm diameter). It gives current and maximum readings and it recalls the last reading. It's very good for measuring temperatures of hazardous materials or materials which are hard to reach or moving. The laser sight allows the operator to pinpoint small targets at a distance, even in the dark. It has an accuracy of ± 1 %
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THERMOWELLS
Unit No. 1 - Measurement of temperature
1.5
Figure 1-18 Thermowell Installation
Module No. 2: Instrumentation 2
The heat in a fluid takes longer to transfer through a thermowell, so changes in temperature take longer to show. Different methods are used to speed up heat transfer. Sometimes the space between the probe and the thermowell is filled with a liquid which conducts heat well. Sometimes the probe is placed in a corrugated aluminium cover to give a direct metal contact between the probe and the thermoweli. When you change the probe in a thermowell you must replace the new probe in the same way as the original. A thermowell is a device fitted in to a flow line so that the temperature of a fluid can be measured without shutting down the process. A Thermowell is placed in a flow line when the line is built. The thermometer is fitted into the thermowell. A typical thermowell installation is shown in Figure 1-18.
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UNITS IN THIS COURSE
UNIT 1
MEASUREMENT OF TEMPERATURE
UNIT 2
TEMPERATURE TRANSMITTER
UNIT 3
THE CONTROLLER
UNIT 4
VALVES AND ACTUATOR
UNIT 5
PRACTICAL TASKS
Module No. 2: Instrumentation 2
Unit No. 2 - Temperature transmitter
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 1/8
TABLE OF CONTENTS Para
Page
2.0
COURSE OBJECTIVE
3
2.1
INTRODUCTION
4
2.2 T HE PNEUMATIC TEMPERATURE TRANSMITTER 2.2.1 2.3
Foxboro Type 12 Construction
THE ELECTRICAL/ELECTRONIC TEMPERATURE TRANSMITTER
4 5 7
Module No. 2: Instrumentation 2
Unit No. 2 - Temperature transmitter
TRAINING MANUAL INSTRUMENTATION
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2.0
COURSE OBJECTIVE The student will be able to: •
Sketch and explain the basic construction and operation of a typical pneumatic temperature transmitter.
•
Sketch and explain the basic construction and operation of a typical electrical/electronic temperature transmitter.
Module No. 2: Instrumentation 2
Unit No. 2 - Temperature transmitter
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Unit No. 2 - Temperature transmitter
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2.1
INTRODUCTION The aim of this unit is to show the construction and operation of two types of temperature transmitters; the pneumatic temperature transmitter (Foxboro type 12) and the electronic temperature transmitter (Rosemount model 444).
2.2
THE PNEUMATIC TEMPERATURE TRANSMITTER
Figure 2-1 The Foxboro Pneumatic Temperature Transmitter
Module No. 2: Instrumentation 2
The most common pneumatic temperature transmitter used in the oil/gas industry is the Foxboro type 12 (see Figure 2-1). This is used as an example of a typical instrument.
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Unit No. 2 - Temperature transmitter
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2.2.1
Foxboro Type 12 Construction
Figure 2-2 Foxboro Type 12 P.T.T Construction Figure 2-2 shows a section of a Foxboro 12 Pneumatic Temperature Transmitter (P.T.T).
Module No. 2: Instrumentation 2
Operation: •
The gas filled thermal system (the bulb and capillary tube) is joined to a bellows called an element capsule. This forms a closed system. Heat on the sensor produces a change in gas pressure in the thermal system. This change in pressure makes the element capsule expand or contract.
•
The force bar is pivoted at the cross flexure. It is joined to the element capsule at one end and the flapper at the other end. It is moved by the force of the expanding bellows of the element capsule. As it moves it adjusts the position of the flapper at the other end.
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•
The output pressure of the nozzle varies with the change in pressure in the bellows caused by the change in temperature of the filled system.
•
The relay amplifies the nozzle output pressure to a standard 3-15 psi (0.2-1bar) signal.
•
The compensating bellows allows for ambient temperature and pressure changes.
•
The feedback bellows makes the system more stable in the same way as it does on a pressure transmitter.
Foxboro supply these instruments in temperature ranges between -75° to + 125°C and + 150° to 760°C. The range required must be specified when ordering. The span of any instrument can be changed within the specified range by the elevation spring. The stated accuracy is ± 0.5 % of span.
Module No. 2: Instrumentation 2
Unit No. 2 - Temperature transmitter
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Unit No. 2 - Temperature transmitter
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2.3
THE ELECTRICAL/ELECTRONIC TEMPERATURE TRANSMITTER One of the most common electrical/electronic temperature transmitters (E.T.T) is made by Rosemount (Model 444). This is used as an example of a typical instrument (see Figure 2-3).
Figure 2-3 Rosemount E.T.T Model 444
The Rosemount Model 444 consists of : Sensor The type of sensor depends on the range and application; eg: •
Thermocouple - Type E , J, K, R, S, T.
•
RTID - Copper (10Ω), Nickel (120Ω) and PT 100Ω with 2, 3 or 4 wire connections.
Module No. 2: Instrumentation 2
Electronics Unit This unit has two boards. A RANGE board, depending on the sensor in use and an AMPLIFIER / OUTPUT board which produces the standard 4 - 20mA signal for the control room.
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Unit No. 2 - Temperature transmitter
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Local Temperature Indicator This unit may or not be included. It provides either a moving coil type display or a digital display. It gives an on site temperature reading to the operator.
Figure 2-4 Typical E.T.T Installation
Module No. 2: Instrumentation 2
Figure 2-4 shows a typical installation for this transmitter. The accuracy of this transmitter is better than any pneumatic instrument. It is ± 0.2% of the calibrated span.
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UNITS IN THIS COURSE UNIT 1
MEASUREMENT OF TEMPERATURE
UNIT 2
TEMPERATURE TRANSMITTER
UNIT 3
THE CONTROLLER
UNIT 4
VALVES AND ACTUATOR
UNIT 5
PRACTICAL TASKS
Module No. 2: Instrumentation 2
Unit No. 3 - The controller
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Unit No. 3 - The controller
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para
Page
3.0
COURSE OBJECTIVE
3
3.1
INTRODUCTION
4
3.2
THE CONTROLLER
4
3.3
BASIC CONTROL THEORY
6
3.3.1
Proportional (P) Control
8
3.3.2
Proportional + Integral (PI) Control
10
3.3.3
Proportional + Integral + Derivative (PID) Control.
11
3.4
12
3.4.1
13
Pneumatic Controller Operation
THE ELECTRICAL / ELECTRONIC CONTROLLER
16
3.5.1
Introduction
16
3.5.2
The Standard Electrical Control Loop
16
3.5.3
The Smart Electrical Control Loop
17
3.5.4
The Digital Loop.
18
3.5.5
Electronic Controller
19
Module No. 2: Instrumentation 2
3.5
THE PNEUMATIC CONTROLLER
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3.0
COURSE OBJECTIVE •
The student will be able to:
•
Explain the following terms: measured value desired value (set point) error signal output signal
•
Describe the operation of a typical pneumatic controller.
•
Describe in general terms, the types of electronic controllers in use.
Module No. 2: Instrumentation 2
Unit No. 3 - The controller
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Unit No. 3 - The controller
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3.1
INTRODUCTION The aim of this unit is to introduce the function of the controller and describe typical pneumatic and electronic control systems.
3.2
THE CONTROLLER
Figure 3-1 The Controller Block Diagram
Figure 3-1 shows the main parts of a process loop controller: •
Receiver The receiver converts the signal from the process variable (flow, pressure, etc.) into a signal which is suitable for the controller operating system (e.g. pneumatic/mechanical, electronic or computer)
•
Error Detector
Module No. 2: Instrumentation 2
The error detector detects (finds) any difference between the measured process variable (measured value) and the set point (desired value). •
Control Amplifier This unit adjusts the output signal to the correcting unit (final control element e.g. control valve). The correcting unit corrects the error until the error signal is reduced to zero. When there is no error the control amplifier keeps the correcting unit at a fixed position. The operators can switch the system to manual and adjust the output signal by hand. The output signal is indicated in percentage from closed (0%) to maximum (100%).
•
Controller Functions There are up to three ways to adjust the controller. The older systems can be adjusted by a screwdriver. The new systems can be adjusted by
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TRAINING MANUAL INSTRUMENTATION changing the computer programme.
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The adjustments are: •
Proportional Band (gain). This controls how much the error signal is amplified.
•
Integral (reset). This is adjusted to cancel the final error which may be left after the proportional action has finished.
•
Derivative (rate). This is only used on slow moving loops (for example, temperature). It gives the system a quick start when an error occurs.
Module No. 2: Instrumentation 2
Unit No. 3 - The controller
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Unit No. 3 - The controller
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3.3
BASIC CONTROL THEORY
Figure 3-2 Simple Level Control Loop
Figure 3-2 shows a typical level control loop. This loop is for a two stage separator which separates oil from gas in a flow steam from a well head. This basic control system is used to explain the principles of control theory.
Module No. 2: Instrumentation 2
The object of good control is to automatically hold the level within safety limits. The safety limits are set by the system designer as shown in Figure 3-3. The indicator on the controller shows a percentage % of the minimum and maximum level. An instrument technician will set these levels from figures given by the engineer. Typical values are shown in the table (see Figure 3-3).
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Unit No. 3 - The controller
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Figure 3-3 Loop Operating Limits
The system is under control if the measured value stays within the high and low alarm levels. Normally there are two alarm levels. The first is a high/low alarm. This allows the operator to take action by going into manual control. In this way he can keep the process in operation. The second alarm is a high high/low low alarm. If this alarm is activated it means the system is unsafe and an automatic shutdown occurs.
Module No. 2: Instrumentation 2
The levels which are set for a control system depend on the process. In this example ± 5% may be fine. However, in some processes where the quality of the product is more important (eg. gasoline) the alarm levels may be less than ± 0.5%. Quality is becoming more and more important in modern industry. This means that control must be more and more accurate. For this reason, most modern instrument systems are changing from pneumatic to electrical or computer control. Except for the latest computer control, all systems use the same three control ideas. These are Proportional, Integral and Derivative (PID). These three ideas are explained below.
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3.3.1
Proportional (P) Control
Figure 3-4 Proportional Control
Figure 3-4 shows proportional control. The output changes by a proportion (K) of the error signal. The error signal is the difference between the measured value (MV) and the setpoint (SP). Therefore: OUTPUT
=
K x ERROR SIGNAL
Module No. 2: Instrumentation 2
The proportion (K) is also called "gain". It varies from 0.1 to 10 depending on:
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Unit No. 3 - The controller
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Example :
This means the higher the PB the lower the gain. If the gain is low then the correcting action will be slower. It's important to adjust the PB and the gain to the right level.
Module No. 2: Instrumentation 2
The graph (see Figure 3-4) shows the effect of changing the gain or PB. If the gain is too high the system will never recover from a set point change. It will go into oscillation. If the gain is too low the system will never reach the new set point. The best gain is, as shown, where the measured value settles down after a short time (T) but with an OFFSET. Proportional control will always produce an offset (a permanent difference between the MV and SP). This is because the output is a proportion of the error.
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3.3.2
Proportional + Integral (PI) Control
Figure 3-5 Proportional Integral Control
Module No. 2: Instrumentation 2
Figure 3-5 shows the effect of integral action added to proportional control. Integral action (reset) is added for one reason. It cancels the offset caused by proportional control in minimum time (T). However, if the integral action is too quick it will have the same effect as too much gain and the system will start to oscillate.
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3.3.3
Proportional + Integral + Derivative (PID) Control.
Figure 3-6 Proportional + Integral + Derivative Control
Module No. 2: Instrumentation 2
Figure 3-6 shows the effect of adding derivative action to a proportional and integral controller. Derivative action (rate) produces a signal which is a function of the rate of change of the error signal. This will give a quick start to the output change. Derivative action makes the system settle down at the new set point in a much quicker time (t). The problem with derivative action is that it is not stable. The quick changes in output can cause the control element to swing open and closed too fast. This can happen easily when the set point is changed. So, derivative action is only used if the control loop changes very slowly (e.g. temperature control of a boiler).
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Unit No. 3 - The controller
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3.4
THE PNEUMATIC CONTROLLER The pneumatic controller still has its uses. It is often used in remote places, because it needs no electrical supply. It can also be run on separated gas instead of an air supply. The most common pneumatic controller is the Foxboro 43 AP (air supply) or 43 APG (gas supply). This is an independent unit which will indicate and control pressure, vacuum, temperature or flow depending on the sensor fitted and the scale supplied. Figure 3-7 shows a 43 AP pressure controller.
Module No. 2: Instrumentation 2
Figure 3-7 The Foxboro Type 43 AP Pneumatic Controller
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3.4.1
Pneumatic Controller Operation The basic principles of a pneumatic controller will be shown using the Foxboro type 43 AP. Other manufacturers use similar methods but their manual must be used for exact operation. The PI and D controls can only be set up and adjusted on site, using the actual control loop. This will be practised using a loop simulator in the workshop.
Air Supply
Module No. 2: Instrumentation 2
Figure 3-8 Foxboro Type 43 AP Pneumatic Controller Figure 3-8 shows the layout of the Foxboro 43 AP, with reset and derivative units fitted. Operation: The error signal comes from the differential linkage. The difference between the measured value (in this case pressure) and the set point moves the differential linkage. The error signal linkage moves the proportioning lever which moves the flapper closer or further from the nozzle.
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•
The output pressure from the nozzle is amplified by the control relay. This amplified pressure produces the output signal through the auto/manual switch.
•
The balancing feedback for the system is fed, through the derivative unit, to the proportioning (feedback) bellows. The derivative restrictor sets the time before the feedback bellows can act. So, the output starts at a maximum before the system starts to balance. This activates the quick start for the controller.
•
The output signal also goes to the reset bellows via the reset (integral) unit. The reset restrictor slows down the filling of the reset bellows but eventually the pressure in the reset bellows will cancel the pressure in the feedback bellows. This will cancel the offset.
•
The setting of the proportional band (gain) is done by moving the flapper closer or further from the nozzle. The Foxboro system has a special fitting called a striker bar. This allows the operation of the controller to be reversed. An increasing error signal can produce either an increasing output or a decreasing output.
•
An auto-manual switch is included. The balance indicator should be in the middle before the system is changed from manual to automatic. If it is not in the middle the loop will be "bumped" changing from one output to the other.
•
This is a brief description of the operation of the Foxboro 43 AP. You will understand the system better after doing practical work in the workshop.
Module No. 2: Instrumentation 2
Unit No. 3 - The controller
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The combined signal of the output, for a set point change, will look as shown in Figure 3-9.
Module No. 2: Instrumentation 2
Figure 3-9 Controller Output Signal with P +I+ D Action
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3.5
THE ELECTRICAL / ELECTRONIC CONTROLLER'
3.5.1
Introduction The standard electrical control loop is a series connected system which uses a 4-20mA current signal for a 0-100% measurement range. It also uses a 4-20mA current signal to operate the final control element (control valve) which can move from fully open to fully closed. This method is being replaced by what are called SMART SYSTEMS and DIGITAL SYSTEMS. The following notes are given as a brief introduction to the systems in use.
Module No. 2: Instrumentation 2
3.5.2
The Standard Electrical Control Loop
Figure 3-10 The 4-20 mA Electrical Control Loop
Figure 3-10 shows a typical electrical single process control loop. The transmitter acts as a variable resistor. The process variable changes the current over the range 4-20 mA. Normally a 250 Ω resistor changes the signal for the controller to a 1.5 V signal. This resistor may be placed on a card called a CONDITIONING CARD which is separate from the controller, or it may be inside the controller itself.
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The controller produces proportional, integral and derivative actions electronically which have the same effects as a pneumatic controller. Reset, rate and proportional band (gain) controls are variable resistors, which can be adjusted with a screwdriver. The output from the controller is also a 4-20 mA series loop. The loop current is converted by a current to pneumatic converter (I/P). This provides the pneumatic signal for the control valve.
3.5.3
The Smart Electrical Control Loop
Module No. 2: Instrumentation 2
Figure 3-11 The Rosemount Smart Transmitter
Figure 3-11 shows a typical smart transmission loop. The transmission signal is 4-20 mA, as in a standard loop. However, pulsed signals are placed on top of this. The communicator (smart family interface 268) uses these pulses to look into the system. It checks the control loop and resets the 4mA (zero setting) and 20mA (span setting). Note :
The communicator can be placed anywhere in the loop but the loop resistance must be more than 250Ω. This resistance is normally provided by the safety barrier. Remember the safety barrier is included to prevent dangerous voltages and currents from happening if there is a fault.
The system shows a Rosemount transmitter but the Foxboro 800 series and the Honeywell 3000 series work in much the same way. However, each type has its own communicator.
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It is possible to have a smart output loop with a computer operated control valve. However, these are very specialised and must be learnt on the job.
3.5.4
The Digital Loop. he latest transmitters (Foxboro 860 series) use only pulsed (digital) signals to send the measured -value to the controller. There is no 4- mA loop and the system operates using a voltage supply from a special unit in the controller. A simplified diagram of the system is shown in Figure 3-12.
Figure 3-12 Digital Control Loop
Module No. 2: Instrumentation 2
The hand held communicator can be placed in any position in the loop to check the system and perform zero and span checks.
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3.5.5
Electronic Controller There are basically three types of electronic controller. •
The analog type (eg. Foxboro, Spec 200) These provide P and ID control of continuously changing (analog) signals. They uses a standard operational amplifier similar to the ones made in the electronics workshop.
•
Digital type (eg. Foxboro 760 Series) These provide single loop control using a small computer' (microprocessor). The settings are changed using a keyboard on the front of the unit.
•
Digital type - Workstation Operation The latest type of controller. Examples of these are listed below. Honeywell TDC 3000 Foxboro 1 A Bailey INFI 90
Module No. 2: Instrumentation 2
These controllers can operate a number of control loops at a time (around 20). The display for each loop is shown on a work station screen (similar to the office computer). The operator/technician changes the loop variables (eg. set point and PID settings) using a typewriter keyboard.
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UNITS IN COURSE UNIT 1
MEASUREMENT OF TEMPERATURE
UNIT 2
TEMPERATURE TRANSMITTER
UNIT 3
THE CONTROLLER
UNIT 4
VALVES AND ACTUATOR
UNIT 5
PRACTICAL TASKS
Module No. 2: Instrumentation 2
Unit No. 4 - Valves & Actuators
TRAINING MANUAL INSTRUMENTATION
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Module No. 2: Instrumentation 2
Unit No. 4 - Valves & Actuators
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para
Page
4.0
COURSE OBJECTIVE
3
4.1
INTRODUCTION
4
4.2
VALVE TYPES
4
4.3.
THE GATE VALVE
5
4.4
THE GLOBE VALVE
6
4.4.1
7
The Industrial Globe Valve.
4.5
THE BUTTERFLY VALVE
9
4.6
BALL AND PLUG VALVES
10
4.7
THE PINCH VALVE
13
4.8
THE NEEDLE VALVE
15
4.9
THE CHECK VALVE
16
4.10
PRESSURE RELIEF (SAFETY) VALVE
18
4.11
ACTUATORS
19
4.11.1
Introduction
19
4.11.2
The Pneumatic Actuator (Diaphragm Type)
19
4.11.3
The Pneumatic Actuator (Piston Type)
21
4.11.4
The Electrical Actuator
22
4.12
VALVE CHARACTERISTICS
24
4.12.1
Introduction
24
4.12.2
Flashing and Cavitation
26
4.13
EMERGENCY SHUTDOWN OPERATIONS
28
4.14
CONCLUSION
28
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Unit No. 4 - Valves & Actuators
4.0
COURSE OBJECTIVE The student will be able to: •
Describe, with the aid of a sketch, the following valve body types. Gate Globe Butterfly Pinch Needle Ball and Plug Check Valve Pressure Relief (Safety) Valve
•
Describe, with the aid of a sketch, the following types of actuator. Hand Diaphragm Piston Electrical Explain, with diagrams, the control characteristics of a complete control valve assembly.
•
Explain, with diagrams, the terms 'FAIL OPEN, 'FAIL CLOSE.
•
Explain, with diagrams, the causes and effects of flashing and cavitation.
Module No. 2: Instrumentation 2
•
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Unit No. 4 - Valves & Actuators
4.1
INTRODUCTION The aim of this unit is to describe and explain the types of valves and actuators used to control the flow of a fluid in a process.
4.2
VALVE TYPES The valves used on a plant can be grouped into two main types. Shut Off Valves:
Shut off valves can be operated by hand or operated automatically using an actuator. Their purpose is either to allow full flow through the valve or shut off the flow completely. They must not be used to control the amount of flow.
Control Valves:
Control valves can also be operated by hand. However, they are normally operated by an actuator automatically. Their purpose is to control the amount of fluid passing through the valve and to act as the correcting element for a control loop.
Module No. 2: Instrumentation 2
There are many different valve designs. They can be very simple (e.g. a water tap). They can be very complicated (e.g. low noise valves for the control of high pressure gas). The following notes are given as an introduction to the valve types available.
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Unit No. 4 - Valves & Actuators
4.3
THE GATE VALVE The gate valve is the common shut-off valve for a pipeline. It is designed to be either fully open or fully closed. Any position between the two can cause a lot of damage to the valve.
Figure 4-1 The Gate Valve
Module No. 2: Instrumentation 2
Figure 4-1 shows the two common forms of gate valves. These valves are hand operated. Rising Stem:
This is a simple device. As. the handle is turned the screw thread on the stem pulls up or pushes down the disc gate. The valve is designed for two positions only, fully open or fully closed.
Non Rising Stem:
The handle turns the stem. The stem fits into the sleeve which has an inside thread, As the stem turns, the inside thread causes the sleeve and gate to move up or down. This type of gate valve is used for higher pressures. The gate is split so that both sides are forced tight against the two seats. This gives a tight shut off.
Note:
July 1999- Rev.0
The rising stem valve is normally turned back a 1/4 of a turn after setting fully open or closed. This stops it from sticking if it is left for long periods in one position. This must never be done with a non rising stem valve.
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Unit No. 4 - Valves & Actuators
4.4
THE GLOBE VALVE This is the most common type of valve used to control the flow of fluid. The simplest type is the water tap. The flow can be controlled by hand. A simple diagram of how it works is in Figure 4-2.
Figure 4-2 The Simple Water Tap
Module No. 2: Instrumentation 2
The tap body is ball-shaped. The circular plug controls the flow of water by adjusting the gap between the plug and the seat ring. This action is called "throttling"; squeezing fluid through a smaller and smaller gap to reduce the flow.
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Unit No. 4 - Valves & Actuators
4.4.1
The Industrial Globe Valve. Industrial globe valves come in all shapes and sizes. However, they can be split into two main groups; the "shaped plug" and the "plug and cage". THE SHAPED PLUG GLOBE VALVE.
Module No. 2: Instrumentation 2
Figure 4-3 Typical Shaped Plug Globe Valves
Figure 4-3 shows the construction of a shaped plug globe valve. The single plug valve is used for flow control at low pressures. High pressure control is difficult as the line pressure pushes against the plug. Therefore, extra force must be applied to the stem to hold the plug in position. The double plug system (see Figure 4-3b) overcomes the problem of line pressure by providing two controlled flow streams. The pressure on the top plug forcing the stem up is balanced by the pressure on the bottom plug forcing the stem down. So, less force is needed to move the stem and you have good control at high pressures. The diagram also shows the two types of valve body. Figure (a), is direct. As the stem rises the flow increases. Figure (b) is reverse. As the stem rises the flow decreases.
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Unit No. 4 - Valves & Actuators
THE PLUG AND CAGE GLOBE VALVE
Figure 4-4 The Plug and Cage Globe Valve
Module No. 2: Instrumentation 2
Figure 4-4 shows a plug and cage globe valve. The flow through the valve is via holes cut in the cage. The amount of open hole, and thus the flow, depends on the position of the plug. The plug is held in position by a force on the stem. Most of the globe valves used in the oil/gas industry are of the plug and cage type. They are cheaper to manufacture and service, and provide a balanced action with a simple hole through the plug. The pressure at the top of the plug balances the pressure at the bottom of the plug. The diagram above shows a direct acting body. A reverse acting body is also available but it is very unusual.
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Unit No. 4 - Valves & Actuators
4.5
THE BUTTERFLY VALVE The butterfly valve is a thin disc which rotates across a pipe flow stream. A typical butterfly valve is shown in Figure 4-5.
Module No. 2: Instrumentation 2
Figure 4-5 The Butterfly Valve
The valve is rotated by an actuator. This rotates the disc a 1/4 turn (90°) from full closed to full open as shown in the simple diagram below.
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Unit No. 4 - Valves & Actuators
The discs in butterfly valves can have different shapes. Different shape discs can improve throttling characteristics and provide a tight shut-off. A popular model is the Fisher Fishtail. Figure 4-6 shows the shape of a Fishtail butterfly disc. The sectional drawing shows how a tight shut-off can be obtained.
Figure 4-6 The Fishtail Disc and Sealing Diagram
4.6
BALL AND PLUG VALVES
Module No. 2: Instrumentation 2
Ball and plug valves are 1 /4 turn valves which operate in the same way as a butterfly valve. The only difference between the two is the shape of the part being rotated in or out of the flow steam. Figure 4-7 shows a typical ball and plug valve.
Figure 4-7 The Ball and Plug Valve
The ball valve is a sphere with a hole drilled in it. The plug valve is a tapered cylinder with a hole drilled in it. All these valves are designed so that the actuator (handle) puts the hole in line for full flow as shown in the diagram below.
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These valves are popular as shut-off valves, particularly in air lines and process lines to instruments. They are also used as control valves. There are many different designs.. Two of the common ones used in the oil/gas industry are the Fisher "V" ball and the Masoneilan eccentric plug (eccentric means off-centre). The Masoneilan eccentric plug valve is shown in Figure 4-8.
Module No. 2: Instrumentation 2
ECCENTRIC PLUG
ACTUATOR ROD
Figure 4-8 The Masoneilan Eccentric Plug The actuator turns the plug a 1/4 turn, from fully closed to fully open. The plug is shaped to provide flow control similar to a globe valve.
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Ball/plug valves are often used to provide three/four way connections. This is done by drilling the hole in the ball in different ways. Some examples are given in Figure 4-9.
Module No. 2: Instrumentation 2
Figure 4-9 Multiway Valves
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Unit No. 4 - Valves & Actuators
4.7
THE PINCH VALVE The pinch valve is used to control the flow of very corrosive liquids, eg. acids. The flow passes through a flexible pipe or diaphragm. The pipe must be made of material which does not corrode easily. The pipe is squeezed or pinched to throttle the flow. There are two basic types which work in much the same way.
Figure 4-10 Saunders Half Pinch Valve
Module No. 2: Instrumentation 2
The SAUNDERS VALVE is a patented device. From the simplified diagram (see Figure 4-10) it can be seen that the actuator throttles the flow by pressing the diaphragm closer to the weir. The body itself is insulated from the corrosive liquid by a glass or plastic coating. The stem is connected to an actuator (either hand or automatic control)
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Figure 4-11 The Pinch Valve
Module No. 2: Instrumentation 2
The PINCH VALVE shown in Figure 4-11 uses a mechanical linkage to squeeze top and bottom together. The flexible sleeve is made of a synthetic rubber. When you turn the hand wheel it throttles the flow to the rate you want. An automatic actuator can be fitted to this device if required.
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THE NEEDLE VALVE The needle valve is used to control very low flows (e.g. chemical dosing of pipelines to stop corrosion). A typical example of a needle valve is shown in Figure 4-12. The flow is controlled by a needle plug which fits into a small hole in the seat. These valves can have manual or automatic actuators. When it's made from a solid block of stainless steel it can give very low flow control at very high pressures (e.g. 100,000 psi).
Module No. 2: Instrumentation 2
Unit No. 4 - Valves & Actuators
4.8
Figure 4-12 The Needle Valve
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Unit No. 4 - Valves & Actuators
4.9
THE CHECK VALVE The check valve is a non-return valve. It allows fluid to flow in one direction only in the same way as a diode allows an electrical current to flow in one direction only. There are two types of check valve; swing check and lift check. SWING CHECK This is the method used for large flow rates through big pipe lines. Figure 4-13 shows a typical swing check valve.
Figure 4-13 Swing Check Valve.
Module No. 2: Instrumentation 2
The swing check valve consists of a disc assembly which is free to rotate on the pivot pin. In the open position the fluid pressure on the disc swings it into the position shown. This allows full flow through the valve. In the reverse direction the pressure on the disc forces it hard against the valve seat and no flow is possible.
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Unit No. 4 - Valves & Actuators
LIFT CHECK. This is the method use for smaller flow rates and pipelines. Figure 4- shows a typical lift check valve.
Figure 4-14 Lift Check Valve
The lift check valve is similar to a globe valve. However, the plug is free to move. If there is no flow the weight of the plug closes the valve. Normal flow lifts the plug and allows the fluid to pass through the valve. If the pressure across the valve is reversed the plug is forced down against the seat and no reverse flow is possible. A lift check valve does not open to the full pipe diameter like a check valve so it is not suitable for high flow rates.
Module No. 2: Instrumentation 2
Note:
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Unit No. 4 - Valves & Actuators
4.10
PRESSURE RELIEF (SAFETY) VALVE Pressure Relief (Safety) Valves are mechanically set valves which open to relieve pressure when automatic control is lost. There are many different designs for this type of valve, divided into two basic groups. a)
Liquid pressure relief, normally called "Pressure Relief Valves (PRV)
b)
Gas pressure relief, normally called "Pressure Safety Valves" (PSV)
The basic difference is in the speed of operation. PSV's must relieve the pressure much faster than PRV's.
Figure 4-15 Pressure Relief Valve. Figure 4-15 shows a typical pressure relief (safety) valve.
Module No. 2: Instrumentation 2
Operation: 1.
Under normal conditions the valve is closed because of the force applied by the spring.
2.
The inlet pressure is applied to the under side of the disc.
3.
If the inlet pressure reaches the relief pressure the disc lifts against the spring. The valve opens and the excess pressure is released; in this example, to the flare.
4.
The device is calibrated using a dead weight tester. The relief pressure is set by using the adjusting bolt to control the force applied by the spring.
Note: The relief valve shown is only an example. Only certified technicians can calibrate safety relief valves. Each manufacturer holds special certification courses for its safety valves. You will do these special courses during training on the job.
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Module No. 2: Instrumentation 2
Unit No. 4 - Valves & Actuators
4.11
ACTUATORS
4.11.1 Introduction Actuators are the devices which drive the valve stems. There are many different actuators. They range from the simple handwheel to the latest microprocessor controlled electrical/hydraulic actuators. The following notes introduce the common types of actuator in general use by operating companies. You will learn about specialised actuators used on a particular site during advanced training.
4.11.2 The Pneumatic Actuator (Diaphragm Type)
Figure 4-16 Air to Close Pneumatic Actuator
Figure 4-16 shows a typical sectional view of an air to close, pneumatic actuator. With a minimum air signal (3 psi or 0.2 bar) applied to the loading pressure connection, the spring forces the stem to its maximum upwards position. With a maximum air signal (15 psi or 1 bar) the force on the diaphragm compresses the spring and the stem moves down to the closed position. Any signal between the two will hold the stem at an intermediate position to control the flow.
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Unit No. 4 - Valves & Actuators
TRAINING MANUAL INSTRUMENTATION
The actuator can be designed to work in the opposite direction, i.e. air to open. Figure 4-17 shows a typical air to open actuator. The air signal input is applied to the underside of the diaphragm (loading pressure connection).
Module No. 2: Instrumentation 2
Figure 4-17 Air to Open Pneumatic Actuator
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Unit No. 4 - Valves & Actuators
4.11.3 The Pneumatic Actuator (Piston Type)
Figure 4-18 A piston Pneumatic Actuator
Module No. 2: Instrumentation 2
Figure 4-18 shows a typical pneumatic piston actuator. It is used to operate a ball or butterfly valve. In the position shown the valve is about half way. An increase in the pressure (P1), forces the piston further down and rotates the ball or butterfly. This closes the valve more. The system is balanced by the feed back signal (P2). P2 is provided by the Actuator Control Unit (positioner). The operation of pneumatic positioners will be explained later in the course. If P1 fails the opposite occurs, P2 increases and the valve opens more. The actuator rotates the V ball through 90°. When the valve is closed the ball stops the fluid flow in the same way as an ordinary ball valve. As the valve opens the "V" cut in the ball allows fluid to pass at an increasing rate to the fully open position. So, the "V" ball is an efficient control valve similar to the globe type.
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Unit No. 4 - Valves & Actuators
4.11.4 The Electrical Actuator There are two common types of electrical actuator; the solenoid type used in emergency shut down systems and the motor operated type used when loading tankers from a marine terminal. THE SOLENOID OPERATED VALVE (SOV)
Module No. 2: Instrumentation 2
Figure 4-19 The Solenoid Valve
This on/off valve is used to remotely open or shut a flow line. These valves normally come in small sizes. (e.g. 2" diameter). It is often used for the control of air supply lines etc. in Emergency Shut-Down (ESD) systems.
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Unit No. 4 - Valves & Actuators
THE MOTOR OPERATED VALVE (MOV) Figure 4-20 shows one of the latest types of motor operated valve. The valve is either open or closed. An electrical signal goes to an electric pump. This drives the hydraulic fluid to open the valve. If the signal stops, the hydraulic pressure falls and a spring returns the valve to the closed position.
Figure 4-20 The Electro Hydraulic Actuator
Module No. 2: Instrumentation 2
These valves are used in large diameter pipelines (e.g. 20" diameter). They control the flow of petroleum products being loaded onto a tanker, etc.
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Module No. 2: Instrumentation 2
Unit No. 4 - Valves & Actuators
4.12
VALVE CHARACTERISTICS
4.12.1 Introduction The plug or cage of a valve can have different shapes. The different shapes can control- the flow in different ways. There are three main types of control characteristics; linear, equal percentage, and quick opening. The graph below shows how these control characteristics change the flow as the valve is opened.
A quick opening characteristic gives nearly maximum flow for a small % opening distance of the plug, (plug travel). A linear characteristic provides the same change of flow for the same change in plug travel (e.g. 50% open, 50% flow; 20% open, 20% flow; etc.) An equal percentage characteristic means the plug travel provides a constant percentage change in the flow rate. This is shown on the graph as a flow which gets faster as the valve opens. At a 20% flow rate a 10% increase means the valve opens to allow the flow rate to increase to 22%. However, a 10% increase in the flow rate at 80% means the valve opens to allow the flow rate to increase to 88%.
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Which characteristic is used depends on the property being controlled. A few examples are given below. Linear - Liquid level and flow control Equal Percentage - Pressure control Quick Opening - Pressure relief valves. The cages and plugs for the different characteristics are easy to tell apart as the following examples show:
Module No. 2: Instrumentation 2
Unit No. 4 - Valves & Actuators
TRAINING MANUAL INSTRUMENTATION
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Unit No. 4 - Valves & Actuators
4.12.2 Flashing and Cavitation
Figure 4-21 Pressure Curve through a Valve
Figure 4-21 shows the effect of a valve throttling a flow steam to control the flow. The valve acts like an orifice plate. The pressure will fall across the valve as the velocity increases through the restriction. If the pressure of the liquid falls below the bubble point the gases in the liquid under pressure will be released as bubbles. This is called the flashing point. As these bubbles keep hitting the valve plug and seat they can wear away the metal.
Module No. 2: Instrumentation 2
As the fluid leaves the valve the pressure increases again. This makes the bubbles implode. This is called the cavitation point. The sudden implosion of the bubbles can wear away the metal on the plug, seat and valve body. The two pictures below (see Figure 4-22) are examples of flashing and cavitation damage. Flashing and cavitation are two problems with valves which control liquid flow. When you service a control valve you must check the plug and cage for signs of damage caused by these problems. Report any damage as the valve may need to be redesigned to stop more damage which could cause a shutdown.
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Unit No. 4 - Valves & Actuators
TRAINING MANUAL INSTRUMENTATION
Figure 4-22 Flashing and Cavitation Damage
Module No. 2: Instrumentation 2
Most manufacturers make special cages to stop flashing and cavitation. These cages split the flow into small flow streams. This reduces the possibility of damage and noise as the flow rushes through the valve. Typical examples are shown in Figure 4-23.
Figure 4-23 Anti Cavitation/Noise Cages
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Unit No. 4 - Valves & Actuators
4.13
EMERGENCY SHUTDOWN OPERATIONS Various valve/ actuator assemblies on a plant are made to "fail open" or "fail closed" in an emergency shutdown. The valve and actuator can be made in the following ways to produce the "fail open" or "fail closed" conditions. FAIL OPEN: 1.
Air to close actuator with direct operating valve body
2.
Air to close actuator with reverse operating valve body.
FAIL CLOSED: 1
Air to open actuator with direct operating valve body
2.
Air to close actuator with reverse operating valve body.
Most plants use direct operating valve bodies. They get "fail open" or "fail closed" by using air to close or air to open actuators. However, the other combinations are sometimes used. A piston type actuator can be made either "fail open" or "fail closed" by the positioner. It is also possible for the positioner to hold the valve at it's last control position. That is usually called "fail intermediate".
4.14
CONCLUSION
Module No. 2: Instrumentation 2
The previous pages have provide a general introduction to valves and actuators. However, the subject can be very detailed and an engineer/technician can spend all his working life on valve operations only. A table is added at the end to summarise the facts you have learned. It gives a summary of the uses of valves.
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Module No. 2: Instrumentation 2
Unit No. 4 - Valves & Actuators
TRAINING MANUAL INSTRUMENTATION
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UNITS IN THIS COURSE
UNIT 1
MEASUREMENT OF TEMPERATURE
UNIT 2
TEMPERATURE TRANSMITTER
UNIT 3
THE CONTROLLER
UNIT 4
VALVES AND ACTUATOR
UNIT 5
PRACTICAL TASKS
Module No. 2: Instrumentation 2
Unit No. 5 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 1/8
TRAINING MANUAL INSTRUMENTATION
Para
Page
PRACTICAL TASK 1
3
Module No. 2: Instrumentation 2
Unit No. 5 - Practical tasks
TABLE OF CONTENTS
July 1999- Rev.0
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Unit No. 5 - Practical tasks
PRACTICAL TASK 1 CALIBRATING AN INDUSTRIAL THERMOMETER INTRODUCTION Most industrial thermometers cannot be calibrated. All you can do is check that they are reasonably correct. This is true for mercury in glass and bi-metal strip thermometers. However, some of the "filled system" types can be calibrated for zero and span. You can calibrate for zero by adjusting the pointer. You can calibrate for span by adjusting the Bourdon tube linkage, in the same way as with a pressure gauge. TEMPERATURE STANDARDS. The basic standards for all temperature calibration are: 1)
The ice point; ice melting in distilled water at a standard pressure of 101 325 Pa. This is 0° C or 32° F.
2.)
The boiling point of distilled water at a standard pressure of 101 325 Pa. This is 100°C or 212°F.
Other fixed points for temperatures outside this range are internationally agreed. This is called the International Practical Temperature Scale (IPTS). A few examples of these points are given below: Boiling point of oxygen
-
Module No. 2: Instrumentation 2
The freezing point of mercury
182.962 ° C -
38.862 ° C
The freezing point of zinc
-
419.58°C
The freezing point of silver
-
961.93 °C
The freezing point of gold
-
1064.43°C
You will probably never see any calibrations carried out using these standards. The calibration equipment in the instrument workshop is calibrated when it is manufactured. It is calibrated against the above standards and given a calibration certificate which shows its accuracy. When you get it from the manufacturer your equipment should be correct. A good instrument workshop sends its calibration equipment for re-calibration every year. Then a new calibration certificate is given.
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The most common industrial standard thermometer is the platinum PT 100 Ω. This should be accurate to 0.1° C over the range 15 to 1000° C. This thermometer is fitted into a temperature bath (usually sand filled or solid block). The electrical heater for the temperature bath is controlled by the PT 100 Ω. The temperature of the bath is given on a digital read out. An example of a workshop temperature calibration bath is shown in Figure PT-1. The AMETEK dry block calibrator has a range of -40 to 1 23°C and an accuracy of ± 0.5°C.
Module No. 2: Instrumentation 2
Unit No. 5 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
Figure PT-1 Arnetek Dry Block Calibrator
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Unit No. 5 - Practical tasks
CALIBRATION PROCEDURE The CDC does not have a modern temperature calibration bath. The following calibration is done using a "sand filled" temperature bath which does not keep such a steady temperature. The standard temperature is set using a semi-standard mercury in glass thermometer. Remember to use the immersion line for correct calibration. An ice/water mixture is used as the standard 0°C A layout of the calibration procedure is shown in Figure PT-2.
Figure PT-2 Basic.Thermometer Calibration
Module No. 2: Instrumentation 2
CALIBRATION STEPS 1)
Place the thermometer which you are testing in the ice/water mixture. It should read zero on the scale. If the thermometer is the filled system/Bourdon tube type, adjust the pointer to read zero.
2)
Place the thermometer which you are testing in the temperature bath and adjust the set temperature so that you get the maximum indication on the thermometer dial. If the thermometer is of the filled system/Bourdon tube type, adjust the linkage to the maximum indication point.
3)
Non adjustable thermometers are OK if they are within ± 4°F or ± 2°C
4)
The filled system types can, with care, be set to an accuracy of about ± 1° C.
Note:
July 1999- Rev.0
The above calibration procedure is the same using a modern temperature bath, but the ice is not needed. Also the quick response of the modern bath allows you to plot a graph to show linearity. The CDC bath needs an air supply. The air flows through the sand so that the temperature is the same in all parts of the sand bath.
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Unit No. 5 - Practical tasks
CALIBRATING A FILLED SYSTEM PNEUMATIC TRANSMITTER
Figure PT-4 Calibrating a Filled System Pneumatic Pressure Transmitter Figure PT-4 shows the layout required to calibrate a filled system, pneumatic, pressure transmitter.
Module No. 2: Instrumentation 2
CALIBRATION PROCEDURE 1)
The calibrated range of the transmitter will be given by the instructor.
2)
Adjust the temperature bath to the minimum value of the transmitter range.
3)
When the temperature of the bath is steady adjust the reference adjustment on the elevation spring so that you get 3 psi or 0.2 bar on the output test gauge.
4)
Adjust the temperature bath to the maximum value of the transmitter range.
5)
When the temperature of the bath is steady, adjust the feedback bellows so that you get 15 psi or 1 bar on the output test gauge.
6)
Repeat steps 2 through 5 until the outputs are correct at both temperatures.
7)
The output should be linear between 3 and 15 psi (0.2-1 bar) and no more calibration is needed.
The above procedure is a simple summary of the manufacturer's instruction manual. You will need this manual to find the components and to adjust the feedback bellows. July 1999- Rev.0
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Unit No. 5 - Practical tasks
CALIBRATING AN ELECTRICAL/ELECTRONIC TEMPERATURE TRANSMITTER INTRODUCTION Manufacturers of ectricaI/eIectronic temperature transmitters produce these transmitters for most types of T/C's and RTD's. The electronic board inside the housing is changed to suit a particular T/C (e.g. type J,K etc.) or RTD (e.g. PT 100Ω or copper 10Ω etc.). The transmitter has a special number to indicate what it does. This must be checked in the manual before calibration can be done. The CDC instrument workshop has different examples of the Rosemount model 444 Alphaline temperature transmitter. This transmitter is used a lot by the operating companies. The basic calibration procedures done on the model 444 are the same for any type of temperature transmitter. However, because of small differences in design the manufacturer's manual must be used in all cases. CALIBRATION PROCEDURE The calibration procedure must be done using the manufacturer's manual. The instructor will show you how to find out what type of transmitter is to be calibrated. This is complicated and will only apply to the Rosemount model 444. Other types, used on job site, will have similar complicated manuals. However, the basic calibration of a temperature transmitter is much the same and the same test equipment is used. There are two basic layouts.
Module No. 2: Instrumentation 2
•
When the transmitter is of the T/C type.
Figure PT-5 Calibration of a T/C Transmitter
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Unit No. 5 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
Figure PT-5 shows a block layout for the calibration of a T/C transmitter. The T/C calibrator simulates the T/C temperature by sending a mV signal which corresponds to the T/C type J, K, N etc.). The DVM is usually connected across the test position. The zero and span are adjusted to give a 4-20mA output signal corresponding to the T/C sensor range (eg. 100-400°C). Note :
•
Sometimes it is better to add your own standard resistor in series with the output. The DVM is then placed across this to measure the 4-20mA. A typical standard resistor is of the wire wound type (eg. 10Ω ± 0.1 %). This produces a 40mV to 200mV signal. When the transmitter is of the RTD type
Figure PT-6 Calibration of RTID Temperature Transmitter
Figure PT-6 shows the block layout for the calibration of an RTD temperature transmitter. The input RTD resistance value for the temperature comes from a decade box (standard variable resistor). The output 4-20mA is measured by using either the test position or a standard resistor.
Module No. 2: Instrumentation 2
The zero is adjusted to give 4mA for the minimum temperature set by the decade box. The span is -adjusted to give 20mA for the maximum temperature set by the decade box. Note :
July 1999- Rev.0
Modern calibrators produce a resistance output so a decade box is not required. The calibrator is connected to the transmitter. In this case a three wire system is used. The calibrator is then programmed to give the required RTD resistance values.
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MODULE No. 3 INSTRUMENTATION 3
July 1999- Rev.0
TRAINING MANUAL INSTRUMENTATION
UNIT 1
CONVERTERS AND POSITIONERS
UNIT 2
RECORDERS
UNIT 3
INDICATORS AND COMBINED UNITS
UNIT 4
HAZARDOUS AREAS AND INTRINSIC SAFETY
Module No. 3: Instrumentation 3
Unit No. 1 - Converters & Positioners
UNITS IN THIS COURSE
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Unit No. 1 - Converters & Positioners
TABLE OF CONTENTS Para
Page
1.0
COURSE OBJECTIVE
3
1.1
INTRODUCTION
4
1.2
THE ELECTRICAL TO PNEUMATIC SIGNAL CONVERTER (I/P)
4
1.2.1 Introduction
4
1.2.2 The Foxboro I/P Converter (E69F)
4
1.3
THE VALVE POSITIONER (DIAPHRAGM ACTUATORS)
6
1.4
THE VALVE POSITIONER (PISTON ACTUATORS)
8
1.5
THE COMBINED I/P AND POSITIONER
10
1.5.1
The Fisher Electro-Pneumatic Positioner
10
1.5.2.
Foxboro I/P Valve Positioner (E69P)
12
PRESSURE TO CURRENT CONVERTERS (P/1)
13
1.6.1
Introduction
13
1.6.2
Rosemount Type 1135F P/I Converter
13
1.6.3
The Foxboro 7010A P/I Converter
15
Module No. 3: Instrumentation 3
1.6
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1.0
COURSE OBJECTIVE
The student will be able to: •
Explain the need for I/P and P/l converters.
•
Explain the operation of a typical I/P converter e.g. Foxboro.
•
Explain the purpose of a positioner.
•
Explain the operation of a typical valve positioner e.g. Fisher.
•
Explain the operation of a modern combined converter/positioner, e.g. Fisher, Rosemount.
•
Explain the operation of typical pneumatic signal to electrical signal converters, e.g. Rosemount, Foxboro.
electrical/pneumatic
Module No. 3: Instrumentation 3
Unit No. 1 - Converters & Positioners
TRAINING MANUAL INSTRUMENTATION
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Unit No. 1 - Converters & Positioners
1.1 INTRODUCTION The aim of this unit is to explain: •
The operation and use of equipment which converts electrical signals to pneumatic signals (I/P)
•
The valve positioner and its uses
•
The combined valve positioner and I/P.
•
The pneumatic signal to electrical signal converter (P
1.2 THE ELECTRICAL TO PNEUMATIC SIGNAL CONVERTER (11/P)
1.2.1 Introduction The electrical to pneumatic signal converter (I/P) is an important piece of pneumatic equipment. This is because, even with modern digital transmission systems, most control is done with pneumatically operated valves. The 1/P is therefore an essential item in any electrical / lelectronic control system. The basic mode of operation is the "electric motor principle". A current passed through a conductor in a magnetic field will make the conductor move. The most common type of I/P which is used a lot in the field is the Foxboro (E69F).
Module No. 3: Instrumentation 3
1.2.2 The Foxboro I/P Converter (E69F)
Figure 1-1 The Foxboro I/P Converter July 1999- Rev.0
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Unit No. 1 - Converters & Positioners
TRAINING MANUAL INSTRUMENTATION
Figure 1-1 shows a schematic diagram of the Foxboro I/P converter. The coil, permanent magnet and coil flexure are mounted inside a can made of magnetic material, e.g. soft iron. The can is not shown in the diagram to make it easier to see. The can makes the magnetic field produced by the permanent magnet run at right angles to the coil. This makes a system similar to a moving coil meter, as shown in the sketch below.
Basic Operation: An increase in the input current through the coil makes the coil rotate. This moves the flapper towards the nozzle. The back pressure is increased. The change is amplified by the relay and applied to the feedback bellows. This makes the nozzle move away from the flapper until it reaches a new balance position. The system is arranged so that when the coil current reaches the upper range value (e.g. 20mA), the pneumatic output signal reaches its own upper range value (e.g. 1 bar). The signal can be adjusted a little by moving the nozzle radially at an angle to the axis of the coil.
Module No. 3: Instrumentation 3
Note:- The coil flexure acts as the restoring spring. With minimum current through the coil (e.g. 4mA), the flexure returns the coil to its - starting position and sets a minimum output pressure (e.g. 0.2 bar).
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THE VALVE POSITIONER (DIAPHRAGM ACTUATORS) A valve positioner is a device fitted onto a control valve to make the valve respond better. It's very good when long pneumatic control signal lines are used. It will position a valve accurately even if there are unbalanced forces in the valve body. It will also overcome stem friction. However, the positioner provides positive feedback to make the valve respond more quickly. Therefore, it can not be used on modern electronic controls systems as it will cause instability.
Module No. 3: Instrumentation 3
Unit No. 1 - Converters & Positioners
1.3
Figure 1-2 The Fisher Valve Positioner
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Unit No. 1 - Converters & Positioners
TRAINING MANUAL INSTRUMENTATION
Figure 1-2 shows a typical valve positioner (Fisher) fitted to a control valve. It has an "air to push down" actuator. The schematic diagram shows the main parts of the positioner. This schematic diagram is used to show the operation as follows. When the instrument pressure increases, the bellows expands and moves the beam. The flapper pivots on the beam and restricts the nozzle. The nozzle pressure increases and moves the relay diaphragm assembly. This opens the relay supply valve which moves the actuator stem downward. The stem movement is fed back to the beam by means of a cam. This causes the flapper to pivot slightly away from the nozzle. Nozzle pressure decreases and the relay supply valve closes to stop the output pressure from increasing more. The positioner is now in balance again but at a higher instrument pressure. It also has a slightly different flapper position and a new actuator stem position. When the instrument pressure decreases, the bellows contracts (helped by an internal range spring). This moves the beam to pivot the flapper slightly further from the nozzle. The nozzle pressure decreases and through relay operation, the exhaust valve in the relay opens. This releases the diaphragm actuator pressure to atmosphere, which allows the actuator stem to move upward. This stem movement is fed back to the beam by the cam to reposition the beam and flapper. When everything is in balance again, the exhaust valve closes to stop the pressure in the -diaphragm case from failing any more.
Module No. 3: Instrumentation 3
The speed of operation depends on how much feedback is applied by the feedback bellows. This can be adjusted by moving the position of the flapper on the quadrant. If the action of the actuator is "air to push up", the positioner must work in the reverse direction. This is done by placing the flapper in the reverse action quadrant of the beam.
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THE VALVE POSITIONER (PISTON ACTUATORS)
Unit No. 1 - Converters & Positioners
1.4
Module No. 3: Instrumentation 3
Figure 1-3 The Fisher Positioner for Piston Actuators
Figure 1-3 The Fisher positioner for Piston Actuators July 1999- Rev.0
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Unit No. 1 - Converters & Positioners
TRAINING MANUAL INSTRUMENTATION
Figure 1-3 shows a Fisher valve positioner for piston actuators. Figure 1-3(a) shows a diagram of the layout of the positioner. Figure 1-3(b) shows the positioner fitted to the piston actuator. OPERATION 1)
The input signal is applied to a bellows. This bellows applies a force to a double flapper assembly which moves using the flexure as a pivot.
2)
If the input signal increases, flapper "B", moves away from relay "B" and flapper 'W' moves nearer to relay "A". The signal pressure from relay 'W' increases and the signal pressure from relay W' decreases.
3)
The higher pressure on the top of the piston (relay A) moves the piston down. Therefore the stem moves down to close the valve.
4)
The range spring provides the feedback to the system so that the valve is repositioned at the new input controlled position.
5)
If t-he input signal decreases the opposite action occurs. The piston moves up to open she valve at the new input controlled position.
6)
The bias spring is adjusted to give the fully open position. This is the "Zero" adjustment where a 3 psi (0.2 bar) input sets the valve fully open.
7)
The "span" is set by the range spring. This is selected to give a fully closed valve with a 5 psi (1.0 Bar) input signal.
8)
The positioner is easily changed to provide reverse action (input signal 3 psi fully closed) by changing the position of the input signal bellows (as shown).
Note: The above operation assumes the valve body is direct acting (stem moving down closes the valve).
Module No. 3: Instrumentation 3
If the valve requires a reverse action, place the input bellows on the opposite side of the lever as shown. There are many different types of this positioner/actuator. The type used depends on the length of stroke of the valve and the operating pressures required. The manual must be consulted if the actuator/positioner is not of the standard type described.
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Unit No. 1 - Converters & Positioners
TRAINING MANUAL INSTRUMENTATION
1.5
THE COMBINED UP AND POSITIONER
1.5.1
The Fisher Electro-Pneumatic Positioner Schematic diagram of positioner
Module No. 3: Instrumentation 3
Figure 1-4 The "Fisher" Electro-Pneumatic Valve Positioner
Figure 1-4 The “Fisher” Electro-Pneumatic Valve Positioner
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TRAINING MANUAL INSTRUMENTATION
Unit No. 1 - Converters & Positioners
Figure 1-4 shows the Fisher combination UP and positioner. OPERATION •
The electric current is changed to pneumatic pressure using a 4 coil system and a magnetic armature.
•
The current passing through the coils produces a magnetic field as shown. The armature is pulled up by the positive connection coils and pushed down by the negative connection coils.
•
The armature acts as a flapper against the nozzle. A high signal current pushes the flapper nearer to the nozzle and produces a high output signal.
•
The relay amplifies the nozzle output and applies it to the diaphragm to move the valve stem.
•
Feedback is provided by the lever system.
The solid arrows show the movement as the diaphragm pressure increases. The feedback spring pulls the armature down. The supply current pulls it up. These two opposing forces stabilise the system as the signal current gets higher. If the signal current fails the action is reversed. This is shown by the dotted arrows. Note: The current to pneumatic conversion unit (torque motor) must never be taken to pieces. If you do this, you cannot set it up again, as the magnetic circuit is ruined.
2)
The setting up of this unit is complicated and will be done in the workshop using the manuals.
3)
The torsion rod acts like the controlling spring in a moving coil meter. When the signal current is removed the torsion rod puts the armature back to its horizontal position.
Module No. 3: Instrumentation 3
1)
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Unit No. 1 - Converters & Positioners
1.5.2. Foxboro UP Valve Positioner (E69P) The Foxboro I/P valve positioner (type E69P) is an adaptation of the standard I/P described in section 1. Figure 1-5 shows the layout of the system.
Figure 1-5 Foxboro UP Valve Positioner
OPERATION An increase in current through the coil rotates the flapper closer to the nozzle. The output pressure from the relay goes up.
•
The increase in output pressure moves the valve stem up.
•
The radius arm connected to the valve stem moves the lever which is pivoted about the feedback flexure. This makes the nozzle move away from the flapper. This provides the feedback required to reposition the valve at its new setting. The span adjustment sets the amount of feedback and thus the size of the stroke.
Module No. 3: Instrumentation 3
•
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Module No. 3: Instrumentation 3
Unit No. 1 - Converters & Positioners
1.6 PRESSURE TO CURRENT CONVERTERS (P/I
1.6.1
Introduction The pressure to current converter (P/1) is the opposite of the current to pressure converter (I/P). It changes an input pressure signal (e.g. 3-15 psi) to a standard electrical signal (e.g. 4-2OmA). The two examples given in the notes are the Rosemount type 11 35F and the Foxboro type 892.
1.6.2
Rosemount Type 1135F P/I Converter
Figure 1-6 Rosemount 1135F P/I
Figure 1-6 shows a Rosemount 1135F P/I fitted to a pipe for use in the field.
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Module No. 3: Instrumentation 3
Unit No. 1 - Converters & Positioners
OPERATION The 1135F P/I uses a capacitive cell (the same type as the Rosemount electrical pressure transmitter). The3-15 psi or 0.2-1 bar pneumatic signal is applied to one side of the cell. Atmospheric pressure is applied to the other side of the cell. The movement of the sensing diaphragm (maximum 0.10 mm) depends on the differential pressure across it. The difference in capacitance (C1-C2), as the sensing diaphragm moves, is measured by the electronics to provide the 4-20mA signal.
There is only one adjustment in the electronics. The zero adjustment. This sets the device to give 4mA output for a 3 psi or 0.2 bar input. The span of the device is set in the factory to meet customer requirements.
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TRAINING MANUAL INSTRUMENTATION
The Foxboro 7010A P/1 Converter
Unit No. 1 - Converters & Positioners
1.6.3
Figure 1-7 shows the basic principle of the device.
Module No. 3: Instrumentation 3
DUAL UNIT P/1 CARD 7010A
Figure 1-7 Foxboro P/1 Converter 7010A Foxboro produce a dual pneumatic to current converter for mounting in a rack instead of field mounting. The pneumatic signals are transmitted to the control room where they can be converted to an electrical signal in a safe area. The diagram above shows the Foxboro 7010A converter together with a rack mounted 7020A nest of these converters.
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Unit No. 1 - Converters & Positioners
TRAINING MANUAL INSTRUMENTATION
This is an analog system which uses operational amplifiers. A simplified diagram of the electronics is shown below.
T1
OPERATION
Module No. 3: Instrumentation 3
Component “A” is a strain gauge bridge. R1 and D2 provide a constant reference Voltage for the bridge. The supply to the unit is from an external 24V D.C supply. operational amplifier O1 detects the changes in resistance (voltage) across the bridge and the pressure changes. Operational amplifier O2 produces an output depending on the difference between O1 output and the reference voltage. O2 output controls the base voltage and thus the conduction of T1. T1 is a series transistor which varies the 4-20mA signal around the loop. D1 is added to stop any reverse current flow if the power supply is wrongly connected. The span adjustment adjusts the gain of O1 and the zero adjustment adjusts the reference level for O2.
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Unit No. 1 - Converters & Positioners
CALIBRATION The calibration of the equipment is simple. The following section is a copy of the procedure taken from the Foxboro manual. The power supply voltage required depends on the resistance of the output load, (e.g. controller input). This is the receiver on the diagram. This is 24V dc with a 500Ω resistor put in place of the receiver.
Equipment Setup
Module No. 3: Instrumentation 3
Procedure 1)
Set up the equipment as shown.
2)
Adjust the air supply so that the test gauge reads 3 psi or 0.2 bar.
3)
Adjust the zero screw (Channel 1) so that the digital voltmeter (DVM) output is 100 mV.
4)
Adjust the air supply so that the test gauge reads 1 5 psi or 1.0 bar
5)
Adjust the zero screw (Channel 1) so that the digital voltmeter (DVM) output is 500mV.
6)
Repeat steps 2 through 5 until you get the desired output.
7)
Repeat steps 1 through 6 for Channel 2.
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To determine the converter's output load resistance, add the input resistance of each component in the series loop which is connected to the converter output. The total loop resistance must not exceed 1000 Ω. As an example, if the resistance is 500 Ω, as shown in the following graph, the supply voltage must be 24 V minimum.
Module No. 3: Instrumentation 3
Unit No. 1 - Converters & Positioners
Output Load Resistance
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UNITS IN THIS COURSE UNIT 1
CONVERTERS AND POSITIONERS
UNIT 2
RECORDERS
UNIT 3
INDICATORS AND COMBINED UNITS
UNIT 4
HAZARDOUS AREAS AND INTRINSIC SAFETY
Module No. 3: Instrumentation 3
Unit No. 2 - Recorders
TRAINING MANUAL INSTRUMENTATION
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TABLE OF CONTENTS Para
Page
2.0
COURSE OBJECTIVE
3
2.1
INTRODUCTION
4
2.2
THE PNEUMATIC RECORDER
4
2.2.1
Introduction
4
2.2.2
The Foxboro Series 120 Consotrol Recorder
5
2.2.3
Circular Chart Recorder
8
2.3
ELECTRICAL/ELECTRONIC ANALOG RECORDERS
9
2.4
MODERN MICRO-PROCESSOR BASED RECORDER
11
2.4.1
Introduction
11
2.4.2
The Foxboro 740 Recorder
11
2.4.3
Conclusion
14
Module No. 3: Instrumentation 3
Unit No. 2 - Recorders
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COURSE OBJECTIVE The student will be able to •
Identify the parts of a pneumatic recorder.
•
Identify the parts of an electrical/electronic recorder.
•
Identify the parts of a microprocessor based recorder.
•
State the three common types of charts used in recorders: roll, flip and circular.
Module No. 3: Instrumentation 3
Unit No. 2 - Recorders
2.0
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Unit No. 2 - Recorders
2.1
INTRODUCTION The aim of this unit is to explain the principles of pneumatic, electrical/electronic and microprocessor based recorders. These are devices which produce a record of a process variable using an ink pen which draws a line on a chart. The chart is rotated by a motor to give a continuous record over a period of, for example, 24 hours.
2.2
THE PNEUMATIC RECORDER
2.2.1
Introduction
Module No. 3: Instrumentation 3
The pneumatically operated recorder is still used today, particularly in remote locations. It does not use electricity so it does not need any special intrinsic safety measures when it is used in hazardous areas. Examples of typical pneumatic recorders are shown below manufactured by Foxboro. They are the series 120 Consotrol recorder and the series 40 circular chart recorder; both made by Foxboro.
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Unit No. 2 - Recorders
2.2.2
The Foxboro Series 120 Consotrol Recorder Foxboro produces a range of pneumatic instruments called "Consotrol". The Foxboro series 120 Consotrol recorder is one of the most common pneumatic recorders. It is still manufactured and you can get spares for it. You may see other types of pneumatic recorders on older sites but they will work in much the same way as the Foxboro. If you come across a different recorder you should consult the manual before working on it. Figure 2-1 shows the front and top views of the Foxboro 120 pneumatic recorder.
TOP VIEW 4 PEN RECORDER (COVER REMOVED)
Module No. 3: Instrumentation 3
Figure 2-1 Foxboro 120 Pneumatic Consotrol Recorder
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The pens are operated by a bellows and pivot assembly. A sketch of the assembly is shown in Figure 2-2.
Module No. 3: Instrumentation 3
Unit No. 2 - Recorders
TRAINING MANUAL INSTRUMENTATION
Figure 2-2 Pen Driving Mechanism (Foxboro)
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Unit No. 2 - Recorders
Operation 1.
The incoming pneumatic signal is applied to a bellows unit which works by compression against the range spring.
2.
As the input signal increases the pen is rotated across the scale. The scale indicates from 0 to 100% for an input signal of 0.2 to 1 bar (3 to 15 psi).
3.
The rotating mechanism has span and zero adjustment screws (as shown)
4.
The driving rod from the bellows to the rotating mechanism has a linearity adjustment. This can adjust the linearity of the pen travel from 0-100%.
5.
The recorder can drive a maximum of 4 pens.
6.
The ink of the pens is supplied from bottles which are refillable. The colours available are red, green, blue and violet.
Module No. 3: Instrumentation 3
The chart is driven round by a motor (either electric or pneumatic) at a standard rate of 19 mm per hour. There is enough paper on a roll or flip chart to record around 30 days of continuous operation. The calibration and servicing of this recorder will be done in the workshop during practical tasks. The manual must be used.
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Unit No. 2 - Recorders
2.2.3
Circular Chart Recorder Foxboro also produces a circular chart recorder for field use. The pen drive system is the same as in the 120 Consotrol recorder. The chart is normally driven by clockwork. The instrument technician must wind it up from time to time. The circular chart completes one revolution every 24 hrs. The chart can be linear or square root. The square root chart can be calibrated to give a direct reading of flow rate. Figure 2-3 shows the instrument from the outside and a diagram of the pen operating mechanism behind the chart.
Module No. 3: Instrumentation 3
Foxboro Type 40 Circular Chart Recorder (outside view)
Figure 2-3 Circular Chart Recorder The measurement unit can be selected from various measuring elements, e.g. gauge pressure, absolute pressure, temperature, etc. The recorder can have a maximum of 4 elements to drive 4 pens. The newer recorders use disposable fibre tip pens but the older ones use a capillary tube with a refillable ink bottle
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Unit No. 2 - Recorders
2.3
ELECTRICAL/ELECTRONIC ANALOG RECORDERS One of the few electrical/electronic analog recorders still available is the Foxboro E 20S recorder. This recorder is a modification of the Spec 200 recorder which you can practice on in the workshop. This recorder can operate with roll or flip type charts. The inking system is the same as the pneumatic 120 series but you can get fibre tip disposable pens if required. Figure 2-4 shows the front view of a single pen recorder with a flip chart fitted.
Module No. 3: Instrumentation 3
Figure 2-4 Foxboro Type E 20S with Flip Chart
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Unit No. 2 - Recorders
TRAINING MANUAL INSTRUMENTATION
Figure 2-5 Foxboro E 20S Pen Assembly (top cover removed). Figure 2-5 shows ,the pen drive mechanism for a Foxboro E 20S recorder.
Module No. 3: Instrumentation 3
Operation: 1
The incoming 4-20mA signal is applied to the pen drive unit. This unit is fully sealed. You can not repair faulty units; you must change the whole unit.
2.
The pens are driven from the arbor assembly which is connected to the drive assembly by a link as shown.
3.
The arbor assembly contains the zero and span adjustments for setting the pen travel, as shown.
4.
The diagram shows a three pen unit. The pen drive units are underneath the plate holding the arbor and pen stop assembly. A screw type connection holds the lever assembly on to the shaft of the drive unit.
5.
A pen stop mechanism is added to prevent the pen from running off the chart at either end. (0%, 100%). These stops should not be used to set up zero and span.
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Unit No. 2 - Recorders
TRAINING MANUAL INSTRUMENTATION
2.4
MODERN MICRO-PROCESSOR BASED RECORDER
2.4.1
Introduction The latest recorders (either roll, flip or circular chart types) use a microprocessor to give multipurpose recording. The µP is programmed (configured) to set the alarm levels, engineering units, chart speeds etc. Normally the operator can use a keyboard to get exact indicated readings, totals etc. These are shown on a digital readout at the top of the chart. The Foxboro type 740R series is given as an example of this type of recorder. The following notes give an overview of how it works. Remember that you must use the manual when servicing this type of recorder. The manual will tell you exactly how the device should be configured and operated. The operator cannot change the programme. The instrument technician has a secret password, so that only a technician can change the configuration of the recorder (usually under instructions from an engineer).
2.4.2
The Foxboro 740 Recorder
Module No. 3: Instrumentation 3
Figure 2-6 shows the outside of the Foxboro 740 recorder. Note that the operator can use the keyboard to take process readings from the digital readout. He does not need to open the instrument or disturb the recordings on the charts.
Figure 2-6 Foxboro Type 740R µP Recorder
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Unit No. 2 - Recorders
TRAINING MANUAL INSTRUMENTATION
Figure 2-7 (a) Foxboro 740 Recorder (front door removed)
Module No. 3: Instrumentation 3
Figure 2-7 (a) shows the recorder with the door removed. Note the addition of a pen letter. This ensures that the pens make no mess on the chart when it Is changed. ' Also, if the pen is not in use it can be lifted out of the way so there are no useless lines on the charts.
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Unit No. 2 - Recorders
TRAINING MANUAL INSTRUMENTATION
Figure 2-7 (b) Foxboro 740 Electronics (platen open)
Figure 2-7 (b) shows the electronics of the recorder., These are under the platen (the name of the plate which holds the pens, chart and chart drive). Note that all connections and changes to the electronics can be done from the front of the recorder. There is no need to remove the instrument from its field position or control room panel mounting.
Module No. 3: Instrumentation 3
The electronics cards for the recorder can be selected to record and/or display the following. Temperature T/C
Types B, C, E, J, K, L, N, R, S, and T
RTD
100Ω Platinum, 10Ω Copper, 120Ω Nickel
Signals for pressure, flow rate, level etc. in the following forms: mA dc 4 to 20mA mV dc -80 to + 400 mV dc V dc
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0- 100V dc
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Unit No. 2 - Recorders
The electronics provides a)
Cold junction compensation
b)
RTD operating currents
c)
Mathematical operations on the signals: Square root, X 3/2, X 5/2 and Logx 10.
Instructions about how to fit these cards are given in the manual. The manual also tells you how to program the µP to provide the correct display and chart recording values (e.g. °C, bar, litres per minute etc.) The ink systems today do not use bottles and capillaries as seen in the workshop. They use disposable felt-tip pens which are easy to change. A Foxboro type, which is much the same as other manufacturers, is shown in Figure 2-8.
Figure 2-8 Modern Ink Pen Fitting
Module No. 3: Instrumentation 3
2.4.3
Conclusion The Foxboro 720 recorder is typical of any µP based recorder. Most manufacturers supply these devices in either roll, flip or circular chart form. Each has interchangeable electronic cards to measure, display, and record most of the process variables. A keyboard is provided to configure the system. The Foxboro manual is typical. It is large and contains all the information required to set up the recorder and programme it.
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UNITS IN THIS COURSE UNIT 1
CONVERTERS AND POSITIONERS
UNIT 2
RECORDERS
UNIT 3
INDICATORS AND COMBINED UNITS
UNIT 4
HAZARDOUS AREAS AND INTRINSIC SAFETY
Module No. 3: Instrumentation 3
Unit No. 3 - Indicators & Combined Units
TRAINING MANUAL INSTRUMENTATION
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Unit No. 3 - Indicators & Combined Units
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TABLE OF CONTENTS Para
Page
3.0
COURSE OBJECTIVE
3
3.1
INTRODUCTION
4
3.2
THE PNEUMATIC SIGNAL INDICATOR
4
3.2.1
Introduction
4
3.2.2
The Foxboro 110 Indicator
4
3.3
7
3.3.1
Introduction
7
3.3.2
The Foxboro 7601 Micro Indicator
7
3.3.3
The Bailey 531T5100 Indicator
9
3.3.4
Rosemount 580D Digital Process Indicator
10
COMBINATION UNITS
11
3.4.1
Introduction
11
3.4.2
The Foxboro 130 Pneumatic Controller
11
3.4.3
The Foxboro 760 Series Indicator/Controller
13
Module No. 3: Instrumentation 3
3.4
THE ELECTRONIC INDICATOR
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3.0
COURSE OBJECTIVE The student will be able to •
Use a given diagram to describe the operation of a pneumatic indicator.
•
Use a given diagram to describe the operation- of an electrical/electronic indicator.
•
Use a given diagram to describe the operation of a typical combined unit, (electrical/electronic and pneumatic) e.g. : recorder/indicator indicator/controller indicator/controller/recorder.
Module No. 3: Instrumentation 3
Unit No. 3 - Indicators & Combined Units
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Unit No. 3 - Indicators & Combined Units
TRAINING MANUAL INSTRUMENTATION
3.1
The aim of the unit is to explain the use and operation of a control room indicator. The indicator is not often used as a separate unit on its own. Therefore, some examples of recorder/indicators, indicator/controllers and indicator /controller/recorders are explained to show the more common combined control room unit.
3.2
THE PNEUMATIC SIGNAL INDICATOR
3.2.1
Introduction The pneumatic indicator is now obsolete. It is only seen on old installations or on production facilities at remote locations with no electrical supply. About the only manufacturer still supplying this device is Foxboro. They supply the type 110 Consotrol pneumatic indicator.
3.2.2
Module No. 3: Instrumentation 3
INTRODUCTION
The Foxboro 110 Indicator
Figure 3-1 Foxboro 110 Consotrol Pneumatic Indicator
Figure 3-1 shows the front view of a Foxboro 110 pneumatic indicator which displays two process variables. (There are indicators which display a maximum of three process variables). The scales are made to fit customers' requirements.
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Figure 3-2 below shows an enlarged view of the receiver and pointer drive assembly for a 3 variable indicator.
Module No. 3: Instrumentation 3
Unit No. 3 - Indicators & Combined Units
TRAINING MANUAL INSTRUMENTATION
Figure 3-2 Indicator Assembly
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OPERATION: The pneumatic signal from the transmitter (3-15 psi or 0.2-1 bar) is applied to the bellows unit (expansion type). The expanding bellows moves the moving plate. This moves the pointer across the scale by way of a link adjustment and the pointer drive unit. The amount of pointer movement depends on the input signal. The customer decides what variable must be measured, e.g. temperature, pressure, flow, etc. The receiver is calibrated using the span and zero adjustments as shown. A damping adjustment is added. This stops a pulsating signal from making the pointer difficult to read. The calibration of this type of indicator will be done in the workshop using the manual.
Module No. 3: Instrumentation 3
Unit No. 3 - Indicators & Combined Units
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3.3
THE ELECTRONIC INDICATOR
3.3.1
Introduction
The following notes are given to show three examples of the many kinds of electronic indicators available in the market.
3.3.2
The Foxboro 7601 Micro Indicator
Module No. 3: Instrumentation 3
Figure 3-3 Foxboro 7601 Micro Indicator
Figure 3-3 shows the front of a Foxboro 7601 micro indicator. This is typical of the modern µP based indicators. The keypad is used to programme (configure) the indicator to the measurements and alarm settings required. The two measurement bar graphs are calibrated from 0-100%. The actual value is indicated by the number of LEDs which light up on the bar graph. The alphanumeric display at the top shows the actual value of input measurement in words and numbers. The operator can use the keyboard to select which value is displayed.
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The operator can only use the keyboard if he is allowed to change the settings. If you work on this type of indicator you must use the manual both for operating and reconfiguring. The alarm indicator will flash if the alarm limits are reached. You can find the alarm readings and settings using the keyboard. However, you must use the manual to do this. The following input signals can be applied to the indicator. 1)
4-20mA
2)
1-5V
3)
RTD in various ranges
The actual units of measurement for the input signal (e.g. level, pressure, temperature) are configured using the input keyboard and the manual. The manual tells you how to calibrate the indicator. Remember this instrument is accurate to ± 0.2%. Therefore, the calibration equipment must be accurate to ± 0.1 %.
Module No. 3: Instrumentation 3
Unit No. 3 - Indicators & Combined Units
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3.3.3 The Bailey 531T5100 Indicator
Figure 3-4 Bailey 531T5100 Indicator
Module No. 3: Instrumentation 3
The Bailey 531T5100 will indicate 4 process variables at one time using what is called a "dot matrix" display. On this display the LEDs are points, not bars. Although points are more difficult to produce, they are clearer and easier to read. Like the Foxboro, the indicator will show a % indication on a bar and the actual value in the units required on the alphanumeric display (gallons in the diagram). The operator presses the buttons on the side to show the different process variables on the alphanumeric display. The indicator is configured using a special Bailey communicator. This can be plugged into the slot at the front. The inputs can be either 420mA or 1-5V. They can be transferred directly to an output connection so that they can be used by other units (e.g. recorder). Alarms can be set into the indicator. The outputs can be changed into serial data form using a RS 232C data port. Another advantage is that the indicator can provide a 24-26V dc output so it can drive a transmitter if required.
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3.3.4
Rosemount 580D Digital Process Indicator
Figure 3-5 Rosemount 580D Digital Process Indicator
Figure 3-5 shows a Rosemount 580D digital process indicator. This is a typical example of this type of indicator.
Module No. 3: Instrumentation 3
The input is the standard 4-20mA. The µLP inside the unit converts this signal to a 4 digit display. The range and units are programmed by the push buttons on the front. The example shows gallons per minute but most units can be displayed (e.g. pressure (psi, bar), level (meters, feet)). It has an accuracy of 0.02% of reading ± 1 count. Like the Bailey indicator, this unit can be provided with an internal 24dc power supply to drive a transmitter if required. Another model of this indicator, the 580T, can measure temperature. The 580T will indicate both 100Ω RTDs and most types of thermocouple.
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Module No. 3: Instrumentation 3
Unit No. 3 - Indicators & Combined Units
TRAINING MANUAL INSTRUMENTATION
3.4
COMBINATION UNITS
3.4.1
Introduction Most manufacturers provide both pneumatic and electronic panel mounted instruments. These instruments have a combination of functions; for example indicator/controller, indicator/recorder and indicator/controller/recorder. The two examples below are given to show the basics of these instruments. Both will be demonstrated in the workshop.
3.4.2
The Foxboro 130 Pneumatic Controller This unit is part of the consotrol pneumatic range. It combines a PID controller with an indicator. A circular chart recorder can also be added as an optional extra.
Figure 3-6 Foxboro 130 Pneumatic Indicator/Controller/Recorder
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Figure 3-6 shows the Foxboro combined pneumatic indicator/controller/recorder. It consists of the following parts. 1)
A process variable indicator which is operated by a receiver bellows and linkage in the same way as the 110 indicator.
2)
A set point indicator and manual adjustment knob.
3)
A set of derivative, proportional and reset units. These are adjusted with a screw driver, positioned as shown.
4)
A circular chart recorder (optional extra). The pen unit is driven from the indicator linkage.
5)
A manual control unit. This contains a transfer switch (automatic/manual). There is a wheel to adjust the output signal on manual. An output pointer and scale show the output signal as a percentage of the set range.
Module No. 3: Instrumentation 3
Unit No. 3 - Indicators & Combined Units
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3.4.3 The Foxboro 760 Series Indicator/Controller
Figure 3-7 Foxboro 760 Series Indicator/Controller
Module No. 3: Instrumentation 3
Figure 3-7 shows the overall view of a Foxboro 760 series indicator/controller.
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Unit No. 3 - Indicators & Combined Units
TRAINING MANUAL INSTRUMENTATION
The front uses the same type of display as the indicator 760. It is configured as shown in Figure 3-8.
Figure 3-8 Face Plate 760 Indicator/Controller
Module No. 3: Instrumentation 3
The following points should be noted. 1)
The set point is indicated by a single LED bar on the left hand side.
2)
The center scale indicates the measure value.
3)
The right hand scale indicates the output signal.
4)
The keypad is used to programme (configure) the controller and indicator. It can be programmed to give a measured value (scale and unit), PID settings, etc.
5)
The status light shows which column value is displayed on the alphanumeric display.
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Unit No. 3 - Indicators & Combined Units
TRAINING MANUAL INSTRUMENTATION
The function of each key on the keypad is as shown in Figure 3-9.
Figure 3-9 Foxboro 760 Keypad
W/P
LOCAL/REMOTE INDICATION
W
Indication at DCS Workstation VDU
P
Indication on front Panel
R/L
LOCAL / REMOTE SET POINT
L
Local setting by key pad
R
Set point from Remote position e.g. from a workstation.
A/M
Auto - Manual switch
or
In NORMAL OPERATION these arrows are used to change the set point or the output if in manual operation.
Module No. 3: Instrumentation 3
In READ or SET mode these arrows are used to programme the µP. TAG
This is used to change mode from NORMAL OPERATION to READ or SET mode and back again.
ACK
In NORMAL OPERATION this is used to ACKnowledge an alarm. In READ or SET mode this is used to enter changes in the measured value.
SEL
In NORMAL OPERATION this is used to SELect the bar graph status light. In READ or SET mode this changes the displaying steps to show what programme has been placed in the µP.
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760 Inputs 1)
4 analog 4-20mA signals.
2)
There are two frequency inputs from 1 to 9 999 Hz. These are used to accept different signals from, for example, a turbine flow meter.
3)
A single 3 wire RTD input.
4)
2 switched contact inputs.
760 Outputs There are 2 analog outputs; one for a 4 to 20 mA loop and one for a 1-5V dc loop. There are 2 digital switched outputs. Note 1.
The inputs to the 760, may require the addition of a 250Ω ± 0. 1 % resistor in series with the standard 4-20mA loop current input. This changes the 4-20mA signal to a 1-5 V signal for the controller electronics.
2.
The 760 can also provide an internal supply (25V dc) to power two transmitter loops
Module No. 3: Instrumentation 3
Unit No. 3 - Indicators & Combined Units
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UNITS IN THIS COURSE UNIT 1
CONVERTERS AND POSITIONERS
UNIT 2
RECORDERS
UNIT 3
INDICATORS AND COMBINED UNITS
UNIT 4
HAZARDOUS AREAS AND INTRINSIC SAFETY
Module No. 3: Instrumentation 3
Unit No. 4 - Hazardous areas & intrinsic safety
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Module No. 3: Instrumentation 3
Unit No. 4 - Hazardous areas & intrinsic safety
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para
Page
4.0
COURSE OBJECTIVE
3
4.1
INTRODUCTION
4
4.2
HAZARDOUS AREAS
4
4.3
EXAMPLES OF HAZARDOUS AREAS
5
4.4
HAZARDOUS AREA EQUIPMENT
7
4.4.1
Coding
7
4.4.2
Temperature Rating
7
4.4.3
Gas Grouping
8
4.5
EXAMPLES OF INTRINSIC SAFETY CODING
9
4.6
INTRINSIC SAFETY
12
4.7
THE SAFETY BARRIER
14
4.7.1
One Way Barrier
14
4.7.2
Two Way Barrier
15
4.7.3
References
17
4.8
MODERN TRENDS IN INTRINSIC SAFETY
18
4.9
PRACTICAL POINTS ON HAZARDOUS AREA EQUIPMENT
19
4.9.1
Testing a Barrier
19
4.9.2
Flameproof Equipment
20
4.9.3
Basic Rules
21
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4.0
COURSE OBJECTIVE The student will be able •
Define what is meant by Zone 0, Zone 1 and Zone 2 hazardous areas.
•
Explain the coding of equipment used in hazardous areas.
•
Sketch a one way barrier
•
Sketch a two way barrier
•
Explain how to check a barrier.
Module No. 3: Instrumentation 3
Unit No. 4 - Hazardous areas & intrinsic safety
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Unit No. 4 - Hazardous areas & intrinsic safety
TRAINING MANUAL INSTRUMENTATION
4.1
INTRODUCTION The aim of this unit is to explain the system of coding for hazardous areas. It also explains how safety barriers work.
4.2
HAZARDOUS AREAS A hazardous area is "an area where flammable gases, vapours and mists, and ignitable dust, fibres and flyings are present in a mixture with air and could be ignited by a spark of energy". Hazardous areas are found in many manufacturing processes (e.g. mines, grain silos, gas platforms, refineries etc.). Special electrical safety systems must be used to ensure that accidents are prevented. There are many regulations about electrical safety in hazardous areas. Unfortunately they are not the same from country to country. However, in general the regulations can be divided into two main groups. The European group uses the International Electrotechnical Commission (IEC) code of practice (IEC 79-14). The other group uses the USA code of practice (ANSI/ISA RP 12.6). The following diagrams and tables are designed to show the differences between the two groups. The table below gives the definitions of hazardous areas for both groups.
I.E.C
U.S.A
ZONE 0
CLASS 1 DIVISION 1 Hazardous concentrations of An explosive gas atmosphere is flammable gases or vapours or present continuously, or is present for combustible dusts in suspension long periods. continuously, intermittently or periodically present under normal operating conditions. ZONE 1
Module No. 3: Instrumentation 3
An explosive gas atmosphere is likely to occur in normal operation.
ZONE 2
CLASS 1 DIVISION 2
An explosive gas atmosphere is not likely to occur in normal operation and, if it does occur, it will exist for a short period only
Volatile flammable liquids or flammable gases present, but normally confined within closed containers or systems from which they can escape only under abnormal operating or fault conditions. Combustible dusts not normally in suspension nor likely to be thrown into suspension.
.
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Unit No. 4 - Hazardous areas & intrinsic safety
4.3
EXAMPLES OF HAZARDOUS AREAS Figure 4-1 shows, as an example, a product tank containing flammable liquid. The zone system has been applied to the product tank. The tank is surrounded by a wall (bund). The area between the tank and the bund must be large enough to contain all the liquid from a full tank in case the tank leaks.
Figure 4-1 Hazardous Areas Zones
Module No. 3: Instrumentation 3
The size of each zone depends on the actual construction of the site. Hazardous area maps of the plant are normally made so that the people concerned, (e.g. electrical and instrument staff) can make sure that the right kind of equipment is installed in the hazardous zone. A typical hazardous area plot for an installation is shown on the next page (see Figure 4-2).
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TRAINING MANUAL INSTRUMENTATION
ZONE 1 ZONE 2 Figure 4-2 Typical Hazardous Area Plot
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4.4
HAZARDOUS AREA EQUIPMENT
4.4.1
Coding Electrical and instrument equipment fitted in hazardous areas is tested and given a code to show the area where it can be used. A table of the equipment codes is given below:
4.4.2
Temperature Rating
Module No. 3: Instrumentation 3
The equipment in hazardous areas will also indicate the maximum surface temperature it may rise to, in an ambient temperature of 40 0 C. The code is as follows: Note:Equipment with a lower maximum surface temperature is safer than equipment with a higher temperature.
July 1999- Rev.0
TEMPERATURE CLASS
MAX.SURFACE TEMPERATURE 0°C
T1
450
T2
300
T3
200
T4
135
T5
100
T6
85
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Unit No. 4 - Hazardous areas & intrinsic safety
4.4.3 Gas Grouping Finally the equipment will indicate in what gas atmosphere it can safely be used. There are four main groups. An example gas is given.
In the IEC system, group H C is the safest equipment and in the USA system, group A is the safest group.
Module No. 3: Instrumentation 3
Note:
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EXAMPLES OF INTRINSIC SAFETY CODING
Figure 4-3 MTL Shunt Zener Diode Safety Barrier Figure 4-3 shows an MTL safety barrier as an example of equipment coding for a hazardous area. - Most international companies code for both IEC and USA standards as follows:
Module No. 3: Instrumentation 3
Unit No. 4 - Hazardous areas & intrinsic safety
4.5
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Figure 4-4 shows another example of a hazardous area coding; an electrical junction box. Note: BASEEFA
- British Approvals Service for Electrical Equipment in Flammable Atmosphere.
CENELEX
- Committee European De Normalisation Electrotechnique.
Figure 4-4 Electrical Junction Box
Field Device Labelling
Module No. 3: Instrumentation 3
Field device labelling (e.g. transmitter, I/P converters, switches, etc.) for hazardous areas depends on the manufacturer. Some will put on a label if requested. Some put on only the national code (e.g. Foxboro : factory mutual). Some put the code inside the serial number (e.g. Rosemount). Therefore, to find out if the device fits the area it is placed in, the manual must be consulted.
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Module No. 3: Instrumentation 3
Unit No. 4 - Hazardous areas & intrinsic safety
INGRESS PROTECTION SYSTEM
GLOSSARY: Ingress protection system
=
A system which prevents unwanted substances from getting in to equipment.
Immersion
=
Going under water for a short time
Submersion
=
Saying under water all the time.
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Unit No. 4 - Hazardous areas & intrinsic safety
4.6
INTRINSIC SAFETY Intrinsic safety means safety devices built in to the equipment. It is very unlikely that ordinary electrical equipment, e.g. Motors, fans, lights etc. will ever be placed in a zone 0 hazardous area. So, intrinsic safety systems are only used with instrumentation. There are two classes of intrinsically safe equipment under the European system; Ex ia and Ex ib. The definition of Ex ia is given below. It is almost the same as the USA definition of intrinsically safe equipment. Ex ia
Electrical apparatus (equipment) of category "ia" shall be incapable of causing ignition in normal operation, with a single fault, and with any combination of two faults applied, with the following safety factors: 1.5: in normal operation and with single fault. 1.0: with only combination of two faults
The following example is given to explain the definition
Module No. 3: Instrumentation 3
Figure 4-5
4-20mA Transmitter
Figure 4-5 shows a 4-20 mA transmitter located in the most dangerous gaseous atmosphere. The minimum energy required to ignite the atmosphere is 20 µJ. To be classed as Ex ia II C the barrier must; 1)
In normal operation and with one fault (e.g. too much current) break the circuit before 15 µJ can be released.
2)
With two faults (e.g. too much current and too much voltage) break the circuit before 20 µl can be released,
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If the supply voltage is 24Vd.c. with a current of 20mA then the power available is :
In order for the barrier (fuse) to be intrinsically safe Ex ia, the fuse must break in 4µs Ex ib has the same definition as Ex ia except that it only protects against one fault. Ex ib can only be used in Zone 1. A typical example of Ex ib equipment is the newer ENTIS-ENRAF tank level transmitters.
Module No. 3: Instrumentation 3
Unit No. 4 - Hazardous areas & intrinsic safety
A practical example of how this works is given below.
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4.7
THE SAFETY BARRIER
4.7.1
One Way Barrier The safety barrier makes sure that any fault in the safe area (control room) cannot provide enough energy to ignite the gaseous atmosphere in the hazardous area. However, the system will not be intrinsically safe unless all the equipment in the loop is also intrinsically safe (e.g. the transmitter is also Ex ia IIC UP is Ex ia IIC etc.) A typical one way safety barrier is shown in Figure 4-6. This is used mainly as a safety barrier for switch (digital) circuits.
Module No. 3: Instrumentation 3
Figure 4-6 Safety Barrier
The barrier consists of a series resistor and fuse plus a Zener diode to earth. The series resistor limits the current to about 100mA from a 28V supply when the hazardous terminals are short circuited. The Zener diode operates in the region of 30V and the fuse is rated at around 30 mA. The barrier thus ensures that either too much current or too much voltage will blow the fuse. This will keep any dangerous energy levels away from the hazardous area.
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Unit No. 4 - Hazardous areas & intrinsic safety
4.7.2
Two Way Barrier The one way barrier is not suitable for 4-20mA loops which are floating (neither side connected to earth). Figure 4-7 below shows a typical barrier used with a floating supply.
Figure 4-7 Two Way Barrier
This barrier has extra Zeners and resistors. The resistor R is actually 3 diodes in series. When they are working normally they act like a resistor and allow the return loop current to pass. If a fault occurs, it is possible for a dangerous reverse current to flow. R then acts as a diode to stop the reverse current.
Module No. 3: Instrumentation 3
Note: The reverse rating for the diodes must be high, around 600V.
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It is important that the earthing is good, i.e. less than IΩ to ground. All the earth wires must also be earthed to the same point. If the earths are not to the same point then currents may circulate in the earth line. If this happens then it's possible that a fault in other equipment will blow all the barriers in the system. he diagram below shows a typical Zener barrier earthing system (see Figure-4-8).
Figure 4-8 Zener Barrier Earthing System
Note:
July 1999- Rev.0
All electrical earthing systems are normally checked and tested by the electrical department.
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References The above notes show the two basic types of barrier in use. There are other types of barriers for special purposes. You must read the manual for the barriers on your installation, for special applications. The workshop has a wall chart produced by MTL (Measurement Technology Limited) which summarises the rules for intrinsic safety.
Module No. 3: Instrumentation 3
Unit No. 4 - Hazardous areas & intrinsic safety
4.7.3
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Unit No. 4 - Hazardous areas & intrinsic safety
4.8
MODERN TRENDS IN INTRINSIC SAFETY If a digital system of transmission is used it is possible to use what is called "galvanic isolation", (no electrical connection). Figures 4-9 & 4-10 shows two methods of doing this.
Figure 4-9 The Opto-Isolator
Figure 4-9 shows a typical opto-isolator. The digital input signals from the field produce pulses of light in the LED. These produce pulses of electrical current through the photo-transistor. The electronic amplifier converts the pulses of current into a digital signal output for control purposes. The photo diode and transistor come in a single package. They are fully insulated from each other. They are manufactured to meet Ex ia standards. Electro-Maqnetic Isolation
Module No. 3: Instrumentation 3
A transformer can provide galvanic isolation and produce the required output signals when the transmission system is a.c. However, it is difficult to design a transformer without interwinding capacitance. The relay is a more simple method which uses electro-magnetic isolation. Figure 4-10 shows a system which can be used.
Figure 4-10
A signal energises the coil in the safe area. The magnetic field closes a relay contact in the hazardous area. There is no electrical connection between the safe area and the hazardous area. This is sometimes called "voltless switching".
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4.9
PRACTICAL POINTS ON HAZARDOUS AREA EQUIPMENT
4.9.1
Testing a Barrier
Module No. 3: Instrumentation 3
Figure 4-11 Testing, a Barrier
To test the Zener safety barrier properly you need- speciatised test equipment. You also need to use pulse techniques to make sure that each Zener diode is working correctly. If you try to test the device fully without the correct test gear you may blow the fuse. You can assume that if the fuse in a barrier is not broken then the whole circuit is working correctly. Therefore, the most reasonable on-site test is to check that the end to end resistance is the same as the resistance value printed on the barrier (e.g. 300Ω). Figure 4-11 shows an ohmmeter used to check a dual barrier. Note that the driving voltage from the ohmmeter must be 9 volts or less. Remember that there are diodes in the barrier so only the correct polarity of ohmmeter supply will indicate the barrier resistance.
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Unit No. 4 - Hazardous areas & intrinsic safety
4.9.2
Flameproof Equipment The following is a basic explanation of "flameproofing". This technique is of more interest to the electrician but you should follow the basic rules written at the end of this section.
Figure 4-12 Flameproof Box
It is not practical to seal all equipment in hazardous areas (maintenance will be required from time to time). So, there is a system which allows for a controlled explosion. This system stops the resulting flame from igniting the hazardous area atmosphere around the equipment. Figure 4-12 shows a simple equipment box. The lid is fixed so that there is a small gap between the lid and the box. The combustion gases from the explosion can escape from the box, but the flame will be extinguished before it reaches the ends of the flanges. Experiments done using a 25mm flange have given a standard size for the working gap. The following table gives sample values from the I.E.C regulations.
GAS GROUP
GAP
Module No. 3: Instrumentation 3
I 0.5 mm
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Il A
0.4 mm
HB
0.2 mm
HC
0.025 mm
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Basic Rules 1)
When you replace the lid after servicing, make sure all the seals are fitted correctly and the lid is tightened down evenly. Check the gap is the correct size all round.
2)
Never make extra holes in Ex d fittings. Any damage to the box or lid makes the box unsafe.
3)
Any part which is changed in Ex d equipment must be replaced with a part which has the same safety factor as the original part.
4)
Follow the instructions on the label about cleaning, cable sizing etc.
Module No. 3: Instrumentation 3
Unit No. 4 - Hazardous areas & intrinsic safety
4.9.3
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MODULE No. 4 INDUSTRIAL ELECTRONICS 1
July 1999- Rev.0
UNITS IN THIS COURSE UNIT 1
THE ELECTRICAL CIRCUIT
UNIT 2
SERIES AND PARALLEL CIRCUITS
UNIT 3
ELECTROMAGNETIC PRINCIPLES
UNIT 4
BASIC ELECTROSTATICS AND THE CAPACITOR
UNIT 5
THE INDUCTOR, CAPACITOR AND D.C.
UNIT 6
A.C. PRINCIPLES
UNIT 7
COMMON ELECTRICAL SYMBOLS
UNIT 8
PRACTICAL TASKS
Module No. 4: Industrial electronics 1
Unit No. 1 - The electrical circuit
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Unit No. 1 - The electrical circuit
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para 1.0
COURSE OBJECTIVE
3
1.1
INTRODUCTION
4
1.2
THE ELECTRICAL CIRCUIT
4
1.2.1
The Electrical Supply
4
1.2.2
Connecting Wires
4
1.2.3
Electrical Loads
5
1.3
ELECTRICAL MEASUREMENT
5
1.4
THE LAWS OF ELECTRICITY
6
1.4.1
Electrical Power
7
1.4.2
Electrical Energy
8
1.5
1.6
Module No. 4: Industrial electronics 1
Page
1.7
1.8
RESISTANCE AND RESISTIVITY
9
1.5.1
Resistance
9
1.5.2
Resistivity
9
WORKED EXAMPLES
11
1.6.1
Example 1
12
1.6.2
Example 2
12
1.6.3
Example 3
13
1.6.4
Example 4
14
KIRCHHOFF'S LAWS
15
1.7.1
First Law
15
1.7.2
Second Law
15
SUMMARY OF IMPORTANT FORMULAS
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16
Page 2/16
1.0
COURSE OBJECTIVE The student will be able to: •
Draw a basic electrical circuit and place AMMETERS and VOLTMETERS correctly.
•
State OHM'S LAW and perform simple calculations using OHM'S LAW.
•
State the formulas for ELECTRICAL POWER and ENERGY. Carry out simple calculations using these formulas.
•
State the units in common use for electrical CURRENT, VOLTAGE, POWER, ENERGY and RESISTANCE.
•
Explain RESISTIVITY and its unit.
•
State KIRCHHOFF'S LAWS.
Module No. 4: Industrial electronics 1
Unit No. 1 - The electrical circuit
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Unit No. 1 - The electrical circuit
TRAINING MANUAL INSTRUMENTATION
1.1
INTRODUCTION The aim of this unit is to introduce the basic concepts of electricity to a trainee beginning a career in Instrumentation.
1.2
THE ELECTRICAL CIRCUIT
Figure 1-1 The Electrical Circuit
Figure 1-1 shows the basic electrical circuit. It consists of an electrical supply, a switch and a lamp (the electrical load). The parts are connected with insulated wires (conductors).
1.2.1
The Electrical Supply The electrical supply provides the electro-motive force (EMF) to drive the electrical current through the load. There are different forms of electrical supply.
Module No. 4: Industrial electronics 1
•
Batteries, provide a constant force, called Direct Current (D.C.). The current flows from the positive terminal (red) to the negative terminal (black).
Two examples of batteries are the dry cell (Walkman battery) which is not rechargeable and the lead-acid battery (car battery) which is rechargeable. •
Alternating supplies provide a changing positive and negative force called Alternating Current (A.C.).
The wall sockets around the workshop are examples of an A supply.
1.2.2
Connecting Wires The connecting wires, are made from material which allows electrical current to pass through it (conductors). Copper, aluminium and silver are good conductors.
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Unit No. 1 - The electrical circuit
The conductors must be insulated, to prevent accidents. An insulator stops the passage of an electrical current. Typical examples are the plastics (PVC, PTFE, etc.) and paper.
1.2.3
Electrical Loads The load in an electrical circuit is a device which uses electricity. The most common electrical load is a lamp. The electric current passes through the lamp and produces light. Other loads in the home are televisions, microwaves, and toasters. In industry, electric motors, industrial heaters and electrical/electronic instrumentation are common electrical loads.
1.3
ELECTRICAL MEASUREMENT •
The electrical circuit in Figure 1-1 shows the position of the two main instruments used for electrical measurement.
A
The Ammeter measures the electrical current flowing through the circuit. It is connected in series (as shown). It measures the current in a unit called THE AMPERE (Amp).
V
The Voltmeter measures the electro - motive force (EMF) of the supply. It is connected in parallel across the load (S supply (as shown). It measures the EMF in a unit called THE VOLT.
. WARNING:
Module No. 4: Industrial electronics 1
You must connect these meters as shown in the diagram. If you connect them wrongly you may damage the circuit
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Module No. 4: Industrial electronics 1
Unit No. 1 - The electrical circuit
TRAINING MANUAL INSTRUMENTATION
1.4
THE LAWS OF ELECTRICITY The first and most important law of electrical engineering is OHM's LAW. "The current in an electrical circuit is proportional to the electro motive - force applied" This can be written as
Figure 1-2 Ohm's Law Triangle
If you cover one of the quantities you can see how the other two will give you the answer.
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Unit No. 1 - The electrical circuit
Example:
1.4.1
Electrical Power The power (P) of the circuit is the multiplication of the voltage supplied and the current flowing.
P =IV = VOLTS x AMPS The unit of electrical power is called the
WATT
Module No. 4: Industrial electronics 1
Note:- Electrical power is related to mechanical power by the formula.
1 HORSE POWER = 746 WATTS.
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Electrical Energy The energy (work) provided by an electrical circuit is
WORK = POWER x TIME W = VOLTS x AMPS x SECONDS The unit is called the
JOULE Note:-
One JOULE is a very small amount of electrical energy. Normally, electrical energy is paid for in "electrical units". One electrical unit (The Kilowatt Hour) is 3,600,000 Joules.
Module No. 4: Industrial electronics 1
Unit No. 1 - The electrical circuit
1.4.2
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Unit No. 1 - The electrical circuit
1.5
RESISTANCE AND RESISTIVITY
1.5.1` Resistance
1.5.2
Resistivity An "OHMMETER" is a device for measuring the "RESISTANCE" in a circuit. You will practice using it in the workshop. The following points must be remembered. •
Conductors have very low resistance (less than 1Ω)
•
Insulators have very high resistance (more than 10,000,000 Ohm's)
•
Loads have a resistance fixed during manufacture. A fault will cause either an open circuit (very high resistance) or a short circuit (zero resistance)
Module No. 4: Industrial electronics 1
RESISTIVITY is a physical property of a material. The resistivity of a conductor is very low and the resistivity of an insulator is very high. The resistivity of a material is found by measuring the resistance of one cubic metre of a material between two faces. The unit of resistivity is then the OHM METRE.
Typical examples for resistivity are: Resistivity of copper
=
1.725 x 10 8 Ωm
Resistivity of PVC
=
1.2 x 10 10 Ωm
Resistivity is important because different types of conductor and insulator materials are used in instrumentation. The following points must be remembered when changing conductors/insulators and laying new cables.
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Unit No. 1 - The electrical circuit
TRAINING MANUAL INSTRUMENTATION
•
Changing the conductor material will change the conductor resistance (for example aluminium has about twice the resistivity of copper)
•
The longer the conductor (cable) the higher its resistance.
•
The thinner the conductor (cross sectional area) the higher the conductor resistance.
So, a long thin cable has more resistance than a short thick cable. Example : Question A copper cable has a resistance of 5Ω , a cross sectional area of 4mm2 and a length of 500 m. What would be the resistance of an 8 mm2 aluminium cable (resistivity two times copper) of the same length ? Solution As aluminium has twice the resistivity of copper, the aluminium cable's resistance will be 2 x 5Ω for the same size. However, the cross sectional area of the aluminium (8 mm2) is two times that of the copper(4 mm2) so its resistance will be half the copper cable
2x5 --------- Ω 2
Aluminium has more RESISTIVITY than copper but the RESISTANCE in the two cables is the same. This is because the aluminium cable is thicker than the copper one.
Module No. 4: Industrial electronics 1
So, the aluminium cable has the same resistance as the copper one.
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WORKED EXAMPLES The following worked examples are given to show how this theory is used, The examples use standard multiples and sub multiples. The following list is included as a reminder.
Module No. 4: Industrial electronics 1
Unit No. 1 - The electrical circuit
1.6
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Unit No. 1 - The electrical circuit
1.6.1
Example 1 Question: A load has a resistance of 500Ω. If the supply current is 2 Amps, find the supply voltage? Solution:
OHM's LAW EQUATION V V
=
IR
2A x 500 Ω
The supply Voltage
1.6.2
=
=
=
1,000 V
1,000 Volts.
Example 2 Question: An electric circuit has a supply current of 20mA and supply voltage of 3kV. Find: a)
The resistance
b)
The power used by the load.
Module No. 4: Industrial electronics 1
Solution:
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Unit No. 1 - The electrical circuit
b)
1.6.3
The power equation is :
Example 3 Question: An electric water heater has a rated power of 3.3 kW. If the supply voltage is 220V find: a)
The supply current
b)
The resistance of the heater.
Solution: The power equation is
b)
From the Ohm's Law triangle.
Module No. 4: Industrial electronics 1
a)
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Unit No. 1 - The electrical circuit
1.6.4
Example 4
Question: An electric light bulb has a resistance of 50Ω. If the supply voltage is 200V, find: a)
The supply current
b)
The power rating of the bulb.
c)
The energy used in one hour.
Solution: a)
From the OHM'S LAW TRIANGLE
b)
The power formula is P = IV = 4 x 200 = 800
The power rating of the bulb = 800 W
c)
The energy formula is P
=
CURRENT x VOLTAGE x SECONDS
Module No. 4: Industrial electronics 1
= 4 x 200 x 60 x 60 = 2,880,000 JOULES The energy used in one hour
July 1999- Rev.0
=
2.88MJ
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Unit No. 1 - The electrical circuit
1.7
KIRCHHOFF'S LAWS Kirchhoff's laws are extensions of Ohm's law. You must remember them. They will be used in the next unit on series and parallel circuits:
1.7.1
First Law
Figure 1-3 Kirchhoff's First Law
"The sum of electric currents flowing into a point (X) in an electrical circuit equals the sum of the electric currents flowing out of that point" Thus from Figure 1-3
1.7.2
Second Law
Module No. 4: Industrial electronics 1
"The sum of the voltages around a circuit must equal the supply EMF"
Figure 1-4 Kirchhoff's Second Law Thus from Figure 1-4
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TRAINING MANUAL INSTRUMENTATION
Unit No. 1 - The electrical circuit
1.8
SUMMARY OF IMPORTANT FORMULAS OHM'S LAW
V
=
IR
I
=
V/R
R
=
V/I
=
I2 R
V2 / R Watts
POWER
=
IV
ENERGY
=
AMPS x VOLTS x SECONDS
1 kWh =
=
Joules
3.6 MJ
KIRCHHOFF'S LAWS First Law: At a point in a circuit: Currents in = Currents out
Second Law:
Module No. 4: Industrial electronics 1
Supply EMF = Sum of voltages around the circuit.
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UNITS IN THIS COURSE UNIT 1
THE ELECTRICAL CIRCUIT
UNIT 2
SERIES AND PARALLEL CIRCUITS
UNIT 3
ELECTROMAGNETIC PRINCIPLES
UNIT 4
BASIC ELECTROSTATICS AND THE CAPACITOR
UNIT 5
THE INDUCTOR, CAPACITOR AND D.C.
UNIT 6
A.C. PRINCIPLES
UNIT 7
COMMON ELECTRICAL SYMBOLS
UNIT 8
PRACTICAL TASKS
Module No. 4: Industrial electronics 1
Unit No. 2 - Series & parallel circuits
TRAINING MANUAL INSTRUMENTATION
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Unit No. 2 - Series & parallel circuits
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para 2.0
COURSE OBJECTIVE
3
2.1
INTRODUCTION
4
2.2
THE SERIES CIRCUIT
4
2.3
SERIES CIRCUIT EXAMPLES
5
2.3.1
Example 1
5
2.3.2
Example 2
6
2.3.3
Example 3
7
2.4
THE PARALLEL CIRCUIT
8
2.5
PARALLEL CIRCUIT EXAMPLES
9
2.5.1
Example 1
9
2.5.2
Example 2
10
2.6
Module No. 4: Industrial electronics 1
Page
SERIES AND PARALLEL COMBINATION CIRCUITS
11
2.6.1
Example 1
11
2.6.2
Example 2
13
2.7
MEASUREMENT PROBLEMS
15
2.8
WHEATSTONE BRIDGE
17
2.8.1
17
2.9
The Bridge Circuit
SUMMARY OF IMPORTANT FORMULAS
July 1999- Rev.0
19
Page 2/19
2.0
COURSE OBJECTIVE The student will be able to: •
State the formula for resistors in series.
•
State the formula for resistors in parallel.
•
Carry out simple calculations on series, parallel and series-parallel combination circuits.
•
Sketch a typical Wheatstone bridge and calculate the value of the unknown resistor.
Module No. 4: Industrial electronics 1
Unit No. 2 - Series & parallel circuits
TRAINING MANUAL INSTRUMENTATION
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Unit No. 2 - Series & parallel circuits
TRAINING MANUAL INSTRUMENTATION
2.1
INTRODUCTION The aim of this unit is to explain the three common circuits used in instrument electrical work. The series circuit, the parallel circuit and the Wheatstone bridge.
2.2
THE SERIES CIRCUIT
Figure 2-1 The Series Circuit Figure 2-1 shows an example of the basic series circuit with four loads (resistors) connected to a supply. The rules for this circuit are: a) Kirchhoff's second law applies. The supply voltage equals the sum of the voltages around the circuit. b)
Module No. 4: Industrial electronics 1
b)
The supply current is the same in all parts
c) The TOTAL RESISTANCE of the circuit is the SUM OF THE INDIVIDUAL RESISTORS around the circuit. d)
NOTE:
The series circuit is not used much in industry because it has serious disadvantages. •
If extra loads are added to the circuit the voltage across each load will fall.
•
If one load is broken (open circuit) the supply is cut off to all the loads on the circuit.
However the series circuit is used for the basic instrument electrical signal circuit (loop). This circuit will be shown during the instrument course.
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Unit No. 2 - Series & parallel circuits
TRAINING MANUAL INSTRUMENTATION
2.3
SERIES CIRCUIT EXAMPLES The notes above give the rules for a series circuit. The example circuit has four loads. The following worked examples show the steps required to find unknown quantities in series circuits.
2.3.1
Example 1 Question Two 100Ω loads are connected in series to a 100V supply. Find a)
The total circuit resistance
b)
The supply current.
Solution
Module No. 4: Industrial electronics 1
Circuit Diagram
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2.3.2
Example 2 Solution: Using the circuit diagram given above. Find :a) The supply current (Is) b) The power of the 40Ω load. Total circuit resistance = 50EI + 1 M + 40Ω
Module No. 4: Industrial electronics 1
Unit No. 2 - Series & parallel circuits
TRAINING MANUAL INSTRUMENTATION
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2.3.3
Example 3 Question: 30 lights are connected in series across a 210V supply. Find the voltage across each bulb and the supply current if the resistance for each bulb = 2.93Ω Solution:
Module No. 4: Industrial electronics 1
Unit No. 2 - Series & parallel circuits
TRAINING MANUAL INSTRUMENTATION
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Unit No. 2 - Series & parallel circuits
TRAINING MANUAL INSTRUMENTATION
2.4
THE PARALLEL CIRCUIT
Figure 2-2 The Parallel Circuit
Figure 2-2 shows as an example the arrangement for three parallel electrical loads. The rules of this circuit are: a)
In this case Kirchhoff's first law applies, "Currents into a point on an electrical circuit equal the currents leaving that point " so that
Module No. 4: Industrial electronics 1
The supply current increases as more loads are added in parallel. b)
The supply voltage will be the same for each load.
c)
The total resistance of the circuit (RT) is given by the equation.
Note:
The parallel circuit is the standard industrial circuit because
•
The supply voltage is the same for each load.
•
If one load is disconnected (open circuited) the other loads will still work.
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Unit No. 2 - Series & parallel circuits
TRAINING MANUAL INSTRUMENTATION
2.5
PARALLEL CIRCUIT EXAMPLES The notes above give the rules for a parallel circuit. The example circuit has three loads. The following worked examples show the steps required to find unknown quantities in a parallel circuit.
2.5.1
Example 1 Question: Two 100Ω loads are connected in parallel to a 100V supply. Find (a)
The total circuit resistance.
(b)
The supply current.
Solution: Circuit Diagram.
The total circuit resistance is found using the formula given.
Module No. 4: Industrial electronics 1
(a)
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Unit No. 2 - Series & parallel circuits
TRAINING MANUAL INSTRUMENTATION
2.5.2
Example 2 Question
An electrical circuit consists of three loads connected in parallel across a supply as shown in the circuit diagram above. Find a)
The supply voltage (Vs)
b)
The supply current (Is)
Solution :
Module No. 4: Industrial electronics 1
In a parallel circuit the supply voltage is the same for each load so with 1A flowing through the 10Ω resistor.
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Unit No. 2 - Series & parallel circuits
TRAINING MANUAL INSTRUMENTATION
2.6
SERIES AND PARALLEL COMBINATION CIRCUITS It can be difficult to calculate unknown quantities in a series and parallel combination circuit. The following examples show ways of finding the answers.
2.6.1
Example 1 Question :
Using the circuit diagram above calculate a)
Total circuit resistance
b)
Supply current (Is)
c)
The voltage across each resistor.
Solution: Work out the total resistance of the parallel part first.
Module No. 4: Industrial electronics 1
a)
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Unit No. 2 - Series & parallel circuits
TRAINING MANUAL INSTRUMENTATION
b)
Redraw the circuit using the parallel result.
Total series resistance c)
=
10Ω + 12.Ω
=
22Ω
Supply current
Supply current = 4.55A d)
Voltage across the 10Ωresistor. V 10Ω
=
Is x 10
= 4.55A x 10Ω V10Ω
Module No. 4: Industrial electronics 1
e)
=
45.5 V
Voltage across 20Ω and 30Ω resistors will be the supply current times the parallel total resistance V20Ω or V 30 Ω
=
Is x 12
= 4.55A x 12Ω V20Ω or V 30 Ω =
54.6 V
Note: To check your answer, add V20Ω to V10Ω The result should be V the accuracy of the calculator.
July 1999- Rev.0
SUPPLY
within
Page 12/19
TRAINING MANUAL INSTRUMENTATION
Example 2
Using the above diagram calculate the supply voltage if Is = 2A Solution: a)
Work out the total resistance of each branch first.
Module No. 4: Industrial electronics 1
Unit No. 2 - Series & parallel circuits
2.6.2
July 1999- Rev.0
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TRAINING MANUAL INSTRUMENTATION
c)
Draw the new circuit
The supply voltage
Module No. 4: Industrial electronics 1
Unit No. 2 - Series & parallel circuits
b)
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Series & parallel circuits
2.7
MEASUREMENT PROBLEMS
Figure 2-3 Using an Ammeter and Voltmeter
Figure 2-3 shows a circuit with an ammeter (in series) and voltmeter (in parallel) measuring the circuit current and potential difference (PD) across the load. Another circuit can be drawn which includes the resistance of the meters (see Figure 2-4)
Figure 2-4 Equivalent Circuit with Ammeter and Voltmeter Added
For true readings the value of RA must be very small and Rv very big. Otherwise, the current and voltage readings are all wrong because the meters change circuit values.
Module No. 4: Industrial electronics 1
Normally the ammeter causes no problems as it has a very low resistance. However, the voltmeter can cause problems, especially the old type of moving coil meter. An example is given below.
The diagram shows a voltmeter (resistance 10kΩ) used to measure the PD across R2. Current with voltmeter not connected.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Series & parallel circuits
Voltage across R2 with voltmeter not connected.
Current with voltmeter connected
Voltage across R2 with voltmeter connected
Module No. 4: Industrial electronics 1
From the example it can be seen that using a voltmeter of low resistance changes the readings a lot. Most measurements in instrumentation must be carried out with a digital voltmeter (DVM), with a high input resistance (> 10MΩ). If the voltmeter does not have a high input resistance the measurements will not be correct.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Series & parallel circuits
2.8
WHEATSTONE BRIDGE The Wheatstone bridge is widely used in instrumentation to measure resistance accurately. It is also used to show changes in the resistance of sensors used to measure pressure, level, temperature, etc. The following notes explain the basic principle of the Wheatstone bridge. Practical applications will be shown later in the training .
2.8.1 The Bridge Circuit
Figure 2-5 The Wheatstone Bridge
The Wheatstone bridge circuit (see Figure 2-5) consists of two very accurate (standard) resistors(Rl & R2) called the ratio arms. There is an accurate variable resistor (Decade Box R3). A very sensitive ammeter which will detect very small currents (called a Galvanometer (G)) is connected across points D and B. A supply voltage is connected across points A and C. There are also two terminals to connect an unknown resistor Rx across the points A and B.
Module No. 4: Industrial electronics 1
The value of the unknown resistor is given by the equation.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Series & parallel circuits
Theory: The value of the unknown resistor is found by adjusting the value of the variable resistor until the galvanometer reads zero. This is called the balanced position so that at balance when I3 = 0
Module No. 4: Industrial electronics 1
This is the balance equation for a Wheatstone bridge and must be remembered.
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TRAINING MANUAL INSTRUMENTATION
SUMMARY OF IMPORTANT FORMULAS Resistors in Series RT
=
R1+ R2 + R3 etc.
Resistors in Parallel 1 / RT : =:
1 / R1 + 1 / R2 + 1 / R3
etc.
Wheatstone Bridge Rx
=
Ratio Arms x Decade Box Value.
Module No. 4: Industrial electronics 1
Unit No. 2 - Series & parallel circuits
2.9
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UNITS IN THIS COURSE UNIT 1
THE ELECTRICAL CIRCUIT
UNIT 2
SERIES AND PARALLEL CIRCUITS
UNIT 3
ELECTROMAGNETIC PRINCIPLES
UNIT 4
BASIC ELECTROSTATICS AND THE CAPACITOR
UNIT 5
THE INDUCTOR, CAPACITOR AND D.C.
UNIT 6
A.C. PRINCIPLES
UNIT 7
COMMON ELECTRICAL SYMBOLS
UNIT 8
PRACTICAL TASKS
Module No. 4: Industrial electronics 1
Unit No. 3 - Electromagnetic principles
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
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Unit No. 3 - Electromagnetic principles
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para
Page
3.0
COURSE OBJECTIVE
3
3.1
INTRODUCTION
4
3.2
BASIC MAGNETICS
4
3.3
THE ELECTRIC CURRENT AND MAGNETIC FIELDS
5
3.4
SOLENOID APPLICATIONS
6
3.4.1
The Relay
6
3.4.2
The Solenoid Valve
7
3.5
INTRODUCTION TO ALTERNATING CURRENT (A.C)
8
3.6
MOTOR PRINCIPLE
11
3.6.1 3.7
11
SELF AND MUTUAL INDUCTANCE.
12
3.7.1
Self Inductance
12
3.7.2
Mutual Inductance
12
3.7.3
The Transformer
13
3.7.4
Example
14
SUMMARY OF IMPORTANT FORMULAS
15
Module No. 4: Industrial electronics 1
3.8
The Moving Coil Meter
July 1999- Rev.0
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3.0
COURSE OBJECTIVE The student will be able to •
State the basic rules for magnets
•
Sketch typical magnetic fields around magnets.
•
Sketch the magnetic field produced by an electric current.
•
Sketch the magnetic field produced by a solenoid and give examples of its use.
•
Explain generator principle and the production of an alternating current waveform.
•
Explain the terms RMS, peak, peak to peak, cycle and frequency of an A.C. waveform.
•
State the standard workshop supply.
•
Explain, in simple terms, motor principle using the moving coil meter.
•
Explain self and mutual inductance; the Henry.
•
Explain the action of a transformer and state the formula used.
•
Carry out simple calculations using the transformer equation.
Module No. 4: Industrial electronics 1
Unit No. 3 - Electromagnetic principles
TRAINING MANUAL INSTRUMENTATION
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TRAINING MANUAL INSTRUMENTATION
INTRODUCTION The aim of this unit is to introduce the electric current and its magnetic field.
3.2
BASIC MAGNETICS Magnetic materials produce magnetic fields as shown below. Only three materials can be made to produce a large magnetic field. a)
Iron
b)
Cobalt
c
) Nickel
Ferro - magnetic elements
Rules 1)
Magnetic lines go from N to S.
2)
Magnetic lines never cross
3)
Like poles repel
4)
Unlike poles attract.
Module No. 4: Industrial electronics 1
Unit No. 3 - Electromagnetic principles
3.1
July 1999- Rev.0
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TRAINING MANUAL INSTRUMENTATION
Module No. 4: Industrial electronics 1
Unit No. 3 - Electromagnetic principles
3.3
THE ELECTRIC CURRENT AND MAGNETIC FIELDS When a current is passed through a conductor it will produce a magnetic field, as shown in the diagram.
Current (+) into the paper; the field is clockwise.
Current (0) out of the paper; the field is anti-clockwise.
The magnetic field around a conductor can be increased by increasing the current. However, a better method of increasing the magnetic field produced by electricity is to make a solenoid. A solenoid is made by coiling insulated wire around a cylinder. The greater the number of turns in the coil, the greater the magnetic field produced (see Figure 3-1).
Figure 3-1 The Simple Solenoid
When a current is passed through the coil, the magnetic field is concentrated. This field has a pattern similar to a bar magnet with N and S poles as shown.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 3 - Electromagnetic principles
Note: (a)
Reverse the current flow and the polarity reverses.
(b)
Reverse the direction of the turns in the coil and the polarity reverses.
(c)
Normally it is impossible to tell the direction of the turns in the coil. If you want to reverse the polarity, reverse the current flow.
3.4
SOLENOID APPLICATIONS
3.4.1
The Relay A typical relay is shown in Figure 3-2.
Module No. 4: Industrial electronics 1
Figure 3-2 The Relay
The relay consists of a solenoid and contacts. When the solenoid is energised it attracts a piece of iron (the armature) which changes over a set of contacts. The magnetic core of the solenoid is made of a material which is magnetic only when current flows through the coil (a temporary magnet). When the coil de-energises the return spring pulls the armature back and the contacts return to their normal positions. Relays operate using A.C or D.C supplies. The coils have many turns of small diameter insulated wire. This gives a strong magnetic field from a small energising current.
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TRAINING MANUAL INSTRUMENTATION
The Solenoid Valve The solenoid valve is an on-off device used to control the flow of liquids and gases through piping. When the supply voltage is applied to the coil, the solenoid is energised. It attracts the valve plunger and the valve opens. When the solenoid is de-energised the return spring closes the valve. A typical solenoid valve is shown in Figure 3-3.
Figure 3-3 The Solenoid Valve
Module No. 4: Industrial electronics 1
Unit No. 3 - Electromagnetic principles
3.4.2
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TRAINING MANUAL INSTRUMENTATION
INTRODUCTION TO ALTERNATING CURRENT (A.C)
Unit No. 3 - Electromagnetic principles
3.5
Figure 3-4 The Simple A.C Generator
Module No. 4: Industrial electronics 1
Figure 3-4 shows a simple alternating current generator. The single turn coil, rotated in the magnetic field, has an alternating voltage induced in it to produce an alternating current in the load resistor (R). The size of the voltage depends on the following: (a)
The strength of the magnetic field.
(b)
The speed of the rotating coil.
(c)
The length of the coil in the magnetic field.
(d)
The number of turns on the coil.
(e)
The voltage is maximum when the coil is at right angles to the magnetic field (moving across). The voltage is at zero when the coil is parallel to the magnetic field,- (moving in the same direction). See figure 3-5.
(f)
Industrial electric generators normally keep the coils steady and rotate the field. The output will still be the same.
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Module No. 4: Industrial electronics 1
Unit No. 3 - Electromagnetic principles
TRAINING MANUAL INSTRUMENTATION
Figure 3-5 The A.C Waveform
Figure 3-5 shows, in simple terms, a single rotation of a coil in a magnetic field:
July 1999- Rev.0
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Unit No. 3 - Electromagnetic principles
TRAINING MANUAL INSTRUMENTATION
Position A
Horizontal. The coil is moving parallel to the field. Zero volts induced
Position B
Vertical. The coil has rotated 90°. Maximum induced voltage.
Position C
Horizontal. The coil has rotated 180°. Zero induced voltage
Position D
Vertical. The coil has rotated 270°. Maximum induced voltage. The voltage is in the opposite direction as the coil is reversed in the field.
Position A
Back to position A. The coil has rotated 360° and completed one cycle.
The voltage between the given points changes to . produce what is called a SINEWAVE. A complete rotation through 360° is called a CYCLE. The number of cycles per second is called the FREQUENCY and is measured in HERTZ (Hz). Meters measuring A.C voltages and currents are set up to give the D.C equivalent. This is called the ROOT MEAN SQUARE (RMS) value. It is given by the formula:
Note:
Module No. 4: Industrial electronics 1
Electrical devices using an A.C supply have a fixed frequency. The standard frequency is 50Hz or 60 Hz , The voltage of a domestic supply is also fixed 120 or 140V. Make sure that electrical equipment used in the workshop is set to work at 240V@ 50Hz, otherwise you may damage the equipment.
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TRAINING MANUAL INSTRUMENTATION
Module No. 4: Industrial electronics 1
Unit No. 3 - Electromagnetic principles
3.6
MOTOR PRINCIPLE MOTOR PRINCIPLE is the reverse of GENERATOR PRINCIPLE described in the previous section, In an electric motor the current is applied to a coil placed in a magnetic field. This makes the coil rotate and rotates the mechanical load connected to it. The construction and operation of industrial electrical motors is complicated. They are studied by electrical technicians.
3.6.1
The Moving Coil Meter A simple example to show motor principle is the moving coil meter shown in Figure 3-6.
Figure 3-6 The Moving Coil Meter
When a D.C current is passed through the coil the coil moves, by motor principle, against the controlling spring. When the force produced by motor principle equals the force of the controlling spring, the position of the needle on the scale shows the size of the current.
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Unit No. 3 - Electromagnetic principles
TRAINING MANUAL INSTRUMENTATION
3.7
SELF AND MUTUAL INDUCTANCE.
3.7.1
Self Inductance
Figure 3-7 The Inductor
Self inductance occurs when a coil is supplied with an alternating current (see Figure 3-7). The changing magnetic field cutting the coil produces a back EMF which opposes the supplied current. The size of the back EMF depends on the construction of the coil and the rate of change of the supplied current. Written as an equation. Back EMF(e) =
A constant for the coil x rate of change of current.
The coil constant is called its SELF INDUCTANCE and is measured in a unit called the HENRY. The symbol for self inductance is L.
3.7.2
Mutual Inductance
Module No. 4: Industrial electronics 1
M
MUTUAL INDUCTANCE is the property of two coils placed side by side so that the magnetic field in one coil cuts the other one. When an A.C supply is connected to the primary coil the changing magnetic field cuts the secondary one. By generator principle this induces a voltage in the secondary coil. The amount of EMF produced in the secondary depends on the MUTUAL INDUCTANCE between the two coils, measured in HENRYS. The symbol for mutual inductance is M
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TRAINING MANUAL INSTRUMENTATION
Unit No. 3 - Electromagnetic principles
3.7.3
The Transformer
The transformer is a practical example of mutual inductance. The mutual inductance is made as high as possible by winding one coil on top of the other on a temporary magnetic core. All the magnetic field produced by the primary coil cuts the secondary coil. The secondary output voltage depends on the turns ratio of the two coils. Assuming no losses (100% efficient), the transformer equation is as follows:
Note: If VP or NP is greater than Vs or Ns it is a STEP DOWN transformer.
(2)
If Vs or Ns is greater than VP, or Np it is a STEP UP transformer.
(3)
transformer only works with an A.C supply. A D.C supply will produce no changing magnetic field or back EMF. The resistance to D.C. is very low so the transformer will burn out.
Module No. 4: Industrial electronics 1
(1)
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TRAINING MANUAL INSTRUMENTATION
Example Question: A transformer has 500 turns on the primary coil and 27 turns on the secondary coil. The supply voltage is 240V and the secondary load current is 10 A. Find the: (a)
Secondary voltage
(b)
Primary current.
Solution:
Module No. 4: Industrial electronics 1
Unit No. 3 - Electromagnetic principles
3.7.4
July 1999- Rev.0
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TRAINING MANUAL INSTRUMENTATION
SUMMARY OF IMPORTANT FORMULAS
Unit No. 3 - Electromagnetic principles
3.8
V RMS
=
0.707 V PEAK
I RMS
=
0.707 I PEAK
Module No. 4: Industrial electronics 1
Transformer Equation
July 1999- Rev.0
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UNITS IN THIS COURSE UNIT 1
THE ELECTRICAL CIRCUIT
UNIT 2
SERIES AND PARALLEL CIRCUITS
UNIT 3
ELECTROMAGNETIC PRINCIPLES
UNIT 4
BASIC ELECTROSTATICS AND THE CAPACITOR
UNIT 5
THE INDUCTOR, CAPACITOR AND D.C.
UNIT 6
A.C. PRINCIPLES
UNIT 7
COMMON ELECTRICAL SYMBOLS
UNIT 8
PRACTICAL TASKS
Module No. 4: Industrial electronics 1
Unit No. 4 - Basic electrostatics & the capacitor
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
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Unit No. 4 - Basic electrostatics & the capacitor
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para
Page
4.0
COURSE OBJECTIVE
3
4.1
INTRODUCTION
4
4.2
THE CAPACITOR
4
4.2.1
Capacitor Construction
5
4.2.2
Examples Of Capacitor Types
5
4.3
4.4
6
4.3.1
Series
6
4.3.2
Parallel
6
EXAMPLES ON CAPACITANCE
7
4.4.1
Example 1
7
4.4.2
Example 2
7
4.4.3
Examples 3
8
SUMMARY OF IMPORTANT FORMULAS
9
Module No. 4: Industrial electronics 1
4.5
CAPACITOR CONNECTIONS
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TRAINING MANUAL INSTRUMENTATION
COURSE OBJECTIVE The student will be able to: •
Sketch a simple capacitor
•
State the basic formulas for a capacitor Q = CV, C = εA/d. Understand the unit of capacitance; the Farad.
•
State the formula for capacitors in series.
•
State the formula for capacitors in parallel.
•
Carry out simple calculations on capacitors in series and parallel
•
State the formulas for energy stored in a capacitor.
Module No. 4: Industrial electronics 1
Unit No. 4 - Basic electrostatics & the capacitor
4.0
July 1999- Rev.0
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TRAINING MANUAL INSTRUMENTATION
Unit No. 4 - Basic electrostatics & the capacitor
4.1
INTRODUCTION The previous section (Unit 3) dealt with basic electro-magnetism. The ability of an electric current to produce energy in the form of a magnetic field. This unit will deal with the CAPACITOR; a device which stores electrical energy as an ELECTRIC FIELD.
4.2
THE CAPACITOR
Figure 4-1 The Capacitor Figure 4-1 shows the simplest form of a capacitor. It consists of two conducting plates separated by an insulator (dielectric). When a voltage (V) is applied across the plates the insulator will take in a charge and produce an electric field between the plates. The charge (Q) taken in is given by this equation
Module No. 4: Industrial electronics 1
Q
=CV
C
is called the CAPACITANCE of the device. The unit of measurement is the FARAD (F).
Q
The total charge stored has a unit called the COULOMB (C).
V
Is the voltage applied across the plates.
Total charge is also given by the equation Q
= AMPS. SUPPLIED x SECONDS
The electrostatic energy in a capacitor is given by the equation
Note : A battery stores electrical energy by chemical means. A car battery is measured by the charge stored in Ampere-Hours (Ahr). A 30 Ahr battery will store 30 x 3600 = 108,000 Coulombs. July 1999- Rev.0
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TRAINING MANUAL INSTRUMENTATION
Unit No. 4 - Basic electrostatics & the capacitor
4.2.1
Capacitor Construction The capacitance of a capacitor depends upon the following factors. •
The area of the plates (A).
•
The distance between the plates (d). The closer the plates, the higher the capacitance.
•
The dielectric strength of the insulator (ε).
This gives the equation
It is difficult to make large values of capacitance. Usually the plates and insulator are rolled up and placed in a can to increase the value of capacitance. The dielectric can also be made from chemicals (e.g. Tantalum and Aluminium oxides). This reduces the dielectric thickness to millionths of a metre (micron). This increases the capacitance value because the plates are closer together.
4.2.2
Examples Of Capacitor Types •
Electrolytic Dielectric : Aluminium and Tantalum. Approximate range of values from 0.1µF to 1 F. These are usually polarised and must be fitted correctly when used with D.C. The positive and negative sides are clearly marked.
Module No. 4: Industrial electronics 1
•
Plastic Dielectric : Polypropylene, Polycarbonates, Polyester. Approximate range of values from 100 µF to 1 µF. Non-polarised.
•
Ceramics, mica and air dielectric : Very small values. Approximate range of values 1 µF to 1000µF. Non-polarised.
July 1999- Rev.0
Page 6/9
Module No. 4: Industrial electronics 1
Unit No. 4 - Basic electrostatics & the capacitor
TRAINING MANUAL INSTRUMENTATION
4.3
CAPACITOR CONNECTIONS
4.3.1
Series Connecting capacitors in series has the same effect as pulling the two plates further apart.
The capacitance will go down. The formula is similar to resistors in parallel.
4.3.2
Parallel Connecting capacitors in parallel has the same effect as enlarging the area of the plates .
The capacitance will go up. The formula is similar to resistors in series.
July 1999- Rev.0
Page 7/9
Unit No. 4 - Basic electrostatics & the capacitor
TRAINING MANUAL INSTRUMENTATION
4.4
EXAMPLES ON CAPACITANCE
4.4.1
Example 1 Question Find the charge stored in a 1 00µ F capacitor connected to a 100 V D.C. supply. Solution : The charge in a capacitor is given by the equation Q Q
=
=C
100 X 10 -6 F x 100V
Q
10-2C
=
The charge stored
4.4.2
V
=
0.01 C
Example 2 Question Two 10,000 µF capacitors are connected in parallel across a 5,000 D.C. supply. Find (a)
The total capacitance of the circuit.
(b)
The charge stored.
Solution : (a)
Using the equation for capacitors in parallel
Module No. 4: Industrial electronics 1
CT
= C1 + C2
10,000 µF= + 10,000 µF
=
20,000 µF.
= Total capacitance (b)
20,000 µF
=
Stored charge equation is Q Q
=
CV
20,000 x 10-6 F x 5000V
= Q
=
100C
Stored charge = 100 Coulombs July 1999- Rev.0
Page 8/9
Module No. 4: Industrial electronics 1
Unit No. 4 - Basic electrostatics & the capacitor
TRAINING MANUAL INSTRUMENTATION
4.4.3 Examples 3
Question
July 1999- Rev.0
Page 9/9
TRAINING MANUAL INSTRUMENTATION
SUMMARY OF IMPORTANT FORMULAS For a Capacitor C
εA/d
=
Q
=
CV
Series connection: 1 / CT = 1 / Cl + 1 / C2 + 1 / C3 etc.
Parallel connection: CT
=
Cl + C2 + C3 etc.
Energy stored W
=
1/2 C V 2
Module No. 4: Industrial electronics 1
Unit No. 4 - Basic electrostatics & the capacitor
4.5
July 1999- Rev.0
Page 10/9
TRAINING MANUAL INSTRUMENTATION
UNIT 1
THE ELECTRICAL CIRCUIT
UNIT 2
SERIES AND PARALLEL CIRCUITS
UNIT 3
ELECTROMAGNETIC PRINCIPLES
UNIT 4
BASIC ELECTROSTATICS AND THE CAPACITOR
UNIT 5
THE INDUCTOR, CAPACITOR AND D.C.
UNIT 6
A.C. PRINCIPLES
UNIT 7
COMMON ELECTRICAL SYMBOLS
UNIT 8
PRACTICAL TASKS
Module No. 4: Industrial electronics 1
Unit No. 5 - The inductor, capacitor & DC
UNITS IN THIS COURSE
July 1999- Rev.0
Page 1/8
TRAINING MANUAL INSTRUMENTATION
Para
Page
5.0
COURSE OBJECTIVE
3
5.1
INTRODUCTION
4
5.2
D.C AND THE INDUCTOR
4
5.3
D.C. AND THE CAPACITOR
6
5.3.1
7
5.4
5.5
Circuit Operation
CAPACITOR TIMING CIRCUITS
7
5.4.1
8
Example
SUMMARY OF IMPORTANT FORMULAS
8
Module No. 4: Industrial electronics 1
Unit No. 5 - The inductor, capacitor & DC
TABLE OF CONTENTS
July 1999- Rev.0
Page 2/8
TRAINING MANUAL INSTRUMENTATION
COURSE OBJECTIVE The student will be able to: •
Explain with the aid of a diagram D.C. applied to an inductor.
•
Explain with the aid of a diagram D.C. applied to a capacitor.
•
Explain the time constant of a resistor - capacitor circuit and its use as an electronic timing circuit.
Module No. 4: Industrial electronics 1
Unit No. 5 - The inductor, capacitor & DC
5.0
July 1999- Rev.0
Page 3/8
TRAINING MANUAL INSTRUMENTATION
Unit No. 5 - The inductor, capacitor & DC
5.1
INTRODUCTION Units 2 and 3 explained the rules to be used when dealing with resistive circuits. The calculations can be carried out using either D.C. values or A.C RMS. values. The results will be the same. This is not true when inductors and capacitors are included in a circuit. This unit will explain what happens when D.C is applied to an inductor and capacitor.
5.2
D.C AND THE INDUCTOR
Figure 5-1 The Electrical Circuit
Module No. 4: Industrial electronics 1
Figure 5-1 shows D.C applied to an inductor via a switch. The graph below shows what happens when the switch is closed.
July 1999- Rev.0
Page 4/8
Unit No. 5 - The inductor, capacitor & DC
TRAINING MANUAL INSTRUMENTATION
At the moment the switch is closed the build up of the magnetic field in the coil produces a back EMF which opposes the applied EMF The starting current is zero. When the magnetic field is steady the back EMF is zero. The current is a maximum and is limited only by the winding resistance of the coil. The time it takes to reach maximum current flow depends on the ratio of the coil inductance to its winding resistance. Because of this effect relay coils and the solenoids of electrically operated valves have increased resistance. This reduces excessive hold on currents. If the switch is open the field in the coil collapses and a high voltage is produced. When the switch is open this high voltage can destroy the switch contacts by sparking. Therefore special circuits must be used to protect the switch and the connected supply voltages. These circuits will be explained in Industrial Electronics 11. Note: 1.
The above principle is used to ignite the fuel of a gasoline engine. The voltage produced when the circuit of an energised coil is broken is used to make a spark across the plug fitted in the cylinder.
2.
An energised coil stores energy in a magnetic form. The energy stored is given by the equation =
1/2 L I2
Module No. 4: Industrial electronics 1
W
July 1999- Rev.0
Page 5/8
TRAINING MANUAL INSTRUMENTATION
D.C. AND THE CAPACITOR
Figure 5-2 D.C Applied to a Capacitor
Figure, 5-2 shows D.C applied to a capacitor. Switch A closed and switch B open charges the capacitor. Switch A open and Switch B closed discharges the capacitor. The graph below shows what happens when the switches are operated.
Module No. 4: Industrial electronics 1
Unit No. 5 - The inductor, capacitor & DC
5.3
July 1999- Rev.0
Page 6/8
TRAINING MANUAL INSTRUMENTATION
Unit No. 5 - The inductor, capacitor & DC
5.3.1
Circuit Operation Charging: When switch A is first closed the current flow into the capacitor is only limited by the loss resistance of the insulator. As the capacitor charges the current flow will fall to zero and the voltage across the capacitor will equal the supply voltage. This means that a charged capacitor is an open circuit to D.C. Discharging: When switch B is closed the discharge current is in the opposite, direction to the charging current. It will start at maximum and then fall to zero as the capacitor voltage falls to zero.
5.4
CAPACITOR TIMING CIRCUITS The capacitor is often used in electronics as a timing circuit. An explanation of the basic principle is given in Figure 5-3. We will look at how this circuit can be used during more advanced work in later units.
Module No. 4: Industrial electronics 1
Figure 5-3 Basic RC Timing Circuit
Figure 5.3 shows a basic timing circuit using the voltage across the capacitor (C). The switch is closed and when the voltage (V) rises to a set value the timing unit operates. It is normal to use what is called the time constant for the circuit. The time constant is given by the equation.
TIME CONSTANT (T)
RESISTANCE (R)
(SECONDS
(OHMS)
x
CAPACITANCE (C) (FARADS)
The voltage across the capacitor will be 63.2% of the D.C supply voltage at the time constant (T = RC).
July 1999- Rev.0
Page 7/8
TRAINING MANUAL INSTRUMENTATION
Unit No. 5 - The inductor, capacitor & DC
5.4.1
Example Question: A 1 MΩ resistor is connected in series with a 100µF capacitor and supplied with D.C. Find the time taken for the capacitor to reach 63.2% of its maximum value after the supply is switched on. Solution: The time to reach 63.2% of the supply voltage is the time constant of the circuit RC. T.C
1 X 106Ω X 100 X 106 F
=
Time Constant
5.5
=
100 seconds.
SUMMARY OF IMPORTANT FORMULAS TIME CONSTANT
=
(SECONDS)
RESISTANCE (OHMS) =
CAPACITANCE (FARADS)
RC
Module No. 4: Industrial electronics 1
T
x
July 1999- Rev.0
Page 8/8
UNITS IN THIS COURSE
UNIT 1
THE ELECTRICAL CIRCUIT
UNIT 2
SERIES AND PARALLEL CIRCUITS
UNIT 3
ELECTROMAGNETIC PRINCIPLES
UNIT 4
BASIC ELECTROSTATICS AND THE CAPACITOR
UNIT 5
THE INDUCTOR, CAPACITOR AND D.C.
UNIT 6
A.C. PRINCIPLES
UNIT 7
COMMON ELECTRICAL SYMBOLS
UNIT 8
PRACTICAL TASKS
Module No. 4: Industrial electronics 1
Unit No. 6 - AC Principles
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 1/16
Unit No. 6 - AC Principles
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para 6.0
COURSE OBJECTIVE
3
6.1
INTRODUCTION
4
6.2
A.C. AND THE INDUCTOR
4
6.3
A.C. AND THE CAPACITOR
5
6.3.1
The Inductor-Resistor Combination
6
6.3.2
The Capacitor-Resistor Combination
7
6.3.3
RL and C in Combination
8
6.3.4
Resonance
9
WORKED EXAMPLES
9
6.4.1
Example 1
9
6.4.2
Example 2
10
6.4.3
Example 3
10
6.4.4
Example 4
12
6.4.5
Example 5
13
6.4.6
Example 6
14
6.4.7
Example 7
15
6.4
Module No. 4: Industrial electronics 1
Page
6.5
SUMMARY OF IMPORTANT FORMULAS
July 1999- Rev.0
16
Page 2/16
TRAINING MANUAL INSTRUMENTATION
COURSE OBJECTIVE The student will be able to: •
Explain with the aid of a diagram the effects of A.C. on an inductor.
•
Explain with the aid of a diagram the effects of A.C. on a capacitor.
•
State the formula for inductive reactance.
•
State the formula for capacitive reactance.
•
State the formulas for RL and C combination circuits supplied with A.C.
•
Explain the term impedance.
•
Carry out simple calculations using the formulas for RL and C combination circuits supplied with A.C.
•
Explain resonance.
Module No. 4: Industrial electronics 1
Unit No. 6 - AC Principles
6.0
July 1999- Rev.0
Page 3/16
TRAINING MANUAL INSTRUMENTATION
Unit No. 6 - AC Principles
6.1
INTRODUCTION The aim of this unit is to explain the operation of a capacitor and an inductor when supplied with A.C. both individually and in combination with resistors.
6.2
A.C. AND THE INDUCTOR
Figure 6-1 A.C. and the Inductor
Module No. 4: Industrial electronics 1
The previous unit showed the effect of applying D.C. to an inductor. The back EMF across the inductor falls from maximum to zero and the current rises from zero to maximum. A.C. is a form of continuously switching D.C. The diagram and the graph (see Figure 6-1) show the effect of supplying A.C. to an inductor. The voltage waveform leads the current waveform by 90'. When the voltage is maximum the current is zero. When the current is maximum the voltage is zero and so on. The faster the A.C. changes (the higher the frequency) the greater the back EMF which is produced. The back EMF reduces the current. The alternating current resistance provided by the coil is called INDUCTIVE REACTANCE (XL) and is given by the formula XL
=
2 ∏ f L Ohms,
XL
=
Inductive reactance (Ohms)
L
=
Coil inductance in Henrys (H)
f
=
Frequency in Hz
∏
=
Mathematical constant (3.142).
This formula must be remembered.
July 1999- Rev.0
Page 4/16
TRAINING MANUAL INSTRUMENTATION
A.C. AND THE CAPACITOR
Unit No. 6 - AC Principles
6.3
Figure 6-2 A.C. and the Capacitor
The previous unit showed the effect of applying D.C. to a capacitor. The charging current starts at a maximum and falls to zero and the voltage across the capacitor starts at zero and rises to a maximum. The effect is the exact opposite to an inductor. The diagram (see Figure 6-2) shows the effect of supplying A.C. to a capacitor. This time the graph shows the current leading the voltage by 90'. When the current is maximum the voltage is zero and so on.
Module No. 4: Industrial electronics 1
The faster the A.C. changes (the frequency) the less time the capacitor has to charge so the current through the device increases. The A.C. resistance (reactance) of a capacitor (Xc) goes down as the frequency goes up and is given by the formula Xc
=
I Ohms --------2∏fC
Xc
=
Capacitive reactance in Ohms
C
=
Capacitance in Farads (F)
f
=
Frequency in H2,
∏
=
Mathematical constant (3.142).
This formula must be remembered.
July 1999- Rev.0
Page 5/16
TRAINING MANUAL INSTRUMENTATION
The Inductor-Resistor Combination
Unit No. 6 - AC Principles
6.3.1
RL in series circuit
RL in parallel circuit
Figure 6-3 Inductor - Resistor Combination
Figure 6-3 shows the series and parallel circuits of an RL combination. Calculating the unknown values of Vs and Is is difficult because the current and voltage waveforms through the inductor are 90° apart.
The total A.C. resistance, IMPEDANCE (Z) for a series circuit is given by the formula:
Module No. 4: Industrial electronics 1
A simple line diagram is used to illustrate the problem. From the diagram we get the following formulas. These must be remembered.
July 1999- Rev.0
Page 6/16
TRAINING MANUAL INSTRUMENTATION
The Capacitor-Resistor Combination
Unit No. 6 - AC Principles
6.3.2
Figure 6-4 Inductor - Capacitor Combination
Module No. 4: Industrial electronics 1
Figure 6-4 shows the series and parallel circuits of an RC combination. The formulas for the total A. C. resistance (IMPEDANCE) of the above circuits follow the same principle as for the resistance and inductor circuits to give:
July 1999- Rev.0
Page 7/16
TRAINING MANUAL INSTRUMENTATION
Unit No. 6 - AC Principles
6.3.3
RL and C in Combination Figure 6-5 shows the series and parallel circuits for an RLC combination. In this case the reactance of the reactive parts oppose each other to give the formulas.
Module No. 4: Industrial electronics 1
Figure 6-5 Resistor - Capacitor - Inductor Combination
July 1999- Rev.0
Page 8/16
TRAINING MANUAL INSTRUMENTATION
Unit No. 6 - AC Principles
6.3.4
Resonance When the value of XL equals the value of XC then XL - XC= 0. So the formula for impedance changes to:
Therefore Z = R for both circuits This effect is called RESONANCE. At one frequency the circuit is only resistive. At resonance;
The frequency (f) is called the RESONANCE FREQUENCY of the circuit. It produces a circuit that is purely resistive. This circuit is useful because it is used to select one frequency from all others. A range of these circuits is used to select a television channel or radio station. Each channel transmits at a different frequency to stop interference
Module No. 4: Industrial electronics 1
6.4
WORKED EXAMPLES
6.4.1 Example 1 Question Find the inductive reactance of a 5 mH coil at 50 kHz. Solution Inductive Reactance
July 1999- Rev.0
(XL)
=
2∏fl
XL
=
2 ∏ x 103 (Hz) 5 x 10-3 (H)
XL
=
500 ∏ Ω
XL
=
1571 Ω
Page 9/16
TRAINING MANUAL INSTRUMENTATION Inductive Reactance
July 1999- Rev.0
=
1571Ω
Page 10/16
TRAINING MANUAL INSTRUMENTATION 6.4.2
Example 2
Unit No. 6 - AC Principles
Question Find the capacitive reactance of a 1∏ capacitor at 50 kHz. Solution Capacitive Reactance
Capacitive Reactance 6.4.3
(Xc)
=
1 -------2 ∏C
Xc
=
1 ------------------------------------2 ∏ 50 x103 (Hz ) x l x l 0-6 (F)
=
103 -------- = 100∏
=
= 3.183 Ω
3.183 Ω
Example 3 Question
Module No. 4: Industrial electronics 1
Find the impedance of a 50Ω resistor and 100 µF capacitor connected in series to a 50 Hz supply.
July 1999- Rev.0
Page 11/16
Module No. 4: Industrial electronics 1
Unit No. 6 - AC Principles
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 12/16
TRAINING MANUAL INSTRUMENTATION
Example 4 Question Find the impedance of a 50Ωresistor and inductive reactance of 50Ω connected in series. , Solution CIRCUIT DIAGRAM
R = 50Ω
Module No. 4: Industrial electronics 1
Unit No. 6 - AC Principles
6.4.4
July 1999- Rev.0
Page 13/16
TRAINING MANUAL INSTRUMENTATION
Example 5 Question Using the diagram above, find the circuit impedance.
Module No. 4: Industrial electronics 1
Unit No. 6 - AC Principles
6.4.5
July 1999- Rev.0
Page 14/16
TRAINING MANUAL INSTRUMENTATION
Example 6 Question Using the diagram above, find the circuit impedance. Solution
Module No. 4: Industrial electronics 1
Unit No. 6 - AC Principles
6.4.6
July 1999- Rev.0
Page 15/16
TRAINING MANUAL INSTRUMENTATION
Unit No. 6 - AC Principles
6.4.7 Example 7 Question
Using the circuit diagram above, find (a)
Resonant frequency
(b)
Circuit impedance at resonance
Module No. 4: Industrial electronics 1
Solution a)
At resonance
(b)
At resonance XL cancels X. and the circuit is resistive only. Circuit impedance
July 1999- Rev.0
=
.10 Ω
Page 16/16
TRAINING MANUAL INSTRUMENTATION
SUMMARY OF IMPORTANT FORMULAS Ohm's law and A.C. V
=
IR
V
=
I XC
V
=
1 XL
V
=
IZ
Module No. 4: Industrial electronics 1
Unit No. 6 - AC Principles
6.5
July 1999- Rev.0
Page 17/16
UNITS IN THIS COURSE UNIT 1
THE ELECTRICAL CIRCUIT
UNIT 2
SERIES AND PARALLEL CIRCUITS
UNIT 3
ELECTROMAGNETIC PRINCIPLES
UNIT 4
BASIC ELECTROSTATIC$ AND THE CAPACITOR
UNIT 5
THE INDUCTOR, CAPACITOR AND D.C.
UNIT 6
A.C. PRINCIPLES
UNIT 7
COMMON ELECTRICAL SYMBOLS
UNIT 8
PRACTICAL TASKS
Module No. 4: Industrial electronics 1
Unit No. 7 - Common electrical symbols
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 1/6
TABLE OF CONTENTS
Para 7
Page . COMMON ELECTRICAL SYMBOLS
3
Module No. 4: Industrial electronics 1
Unit No. 7 - Common electrical symbols
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 2/6
7.
COMMON ELECTRICAL SYMBOLS ELECTRICITY AND MAGNETISM
Module No. 4: Industrial electronics 1
Unit No. 7 - Common electrical symbols
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 3/6
Module No. 4: Industrial electronics 1
Unit No. 7 - Common electrical symbols
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 4/6
Module No. 4: Industrial electronics 1
Unit No. 7 - Common electrical symbols
TRAINING MANUAL INSTRUMENTATION
GRAPHIC SYMBOLS
July 1999- Rev.0
Page 5/6
Module No. 4: Industrial electronics 1
Unit No. 7 - Common electrical symbols
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 6/6
UNITS IN THIS COURSE UNIT 1
THE ELECTRICAL CIRCUIT
UNIT 2
SERIES AND PARALLEL CIRCUITS
UNIT 3
ELECTROMAGNETIC PRINCIPLES
UNIT 4
BASIC ELECTROSTATICS AND THE CAPACITOR
UNIT 5
THE INDUCTOR, CAPACITOR AND D.C.
UNIT 6
A.C. PRINCIPLES
UNIT 7
COMMON ELECTRICAL SYMBOLS
UNIT 8
PRACTICAL TASKS
Module No. 4: Industrial electronics 1
Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 1/41
TABLE OF CONTENTS Para
Page
PRACTICAL TASK 1
3
PRACTICAL TASK 2
6
PRACTICAL TASK 3
8
PRACTICAL TASK 4
10
PRACTICAL TASK 5
11
PRACTICAL TASK 6
12
PRACTICAL TASK 7
13
PRACTICAL TASK 8
16
PRACTICAL TASK 9
19
PRACTICAL TASK 10
22
PRACTICAL TASK 11
25
PRACTICAL TASK 12
28
PRACTICAL TASK 13
31
RESISTANCE COLOUR CODES
35
Module No. 4: Industrial electronics 1
Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 2/41
PRACTICAL TASK 1 PROOF OF OHM'S LAW
1.
Connect the circuit as shown using the components supplied.
2.
After the instructor has checked the circuit, switch on the D.C. supply and set the supply voltage to 1V on the voltmeter. Write down the current reading on the table provided.
3.
Repeat step (2) for a D.C. supply setting of 2V to 10V- Increase the supply in one volt steps. Note the current reading each time.
4.
Switch off the D.C. supply.
5.
Plot a graph of voltage against current from the readings obtained.
6.
The graph must be a straight line to show Ohm's law. V α 1.
7.
Find the slope of the graph.
8.
The slope of the graph will be the value of the resistor.
Module No. 4: Industrial electronics 1
Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 3/41
Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
Slope =
Distance X (Volts) ---------------------Distance Y (mA)
=
Resistance
RESULTS TABLE
Questions to be answered to show understanding. of the practical task.
Module No. 4: Industrial electronics 1
(1)
What is the current, if the supply voltage is 25 volts? -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
(2)
What is the circuit current with a supply voltage of 10 V, if the resistor is changed to 300Ω -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
July 1999- Rev.0
Page 4/41
Module No. 4: Industrial electronics 1
Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 5/41
TRAINING MANUAL INSTRUMENTATION
Module No. 4: Industrial electronics 1
Unit No. 8 - Practical tasks
PRACTICAL TASK 2
1.
Connect the circuit as shown using the components supplied. Do not connect the meters.
2.
After the instructor has checked the circuit, set the supply voltage to 10V with the voltmeter. Switch off.
3.
Connect an ammeter across A-B. Switch on and note the ammeter reading. Switch off.
4.
With the ammeter across A-B short out C-D. Switch on and note the reading. Switch off.
5.
Short out A-B and connect the ammeter across C-D. Switch on and note the reading. Switch off.
6.
above readings should show that in a broken series loop, the current is zero. Also the current is the same in all parts of a series circuit.
7.
Short out A-B and connect an ammeter across C-D. Switch on.
8.
Connect a voltmeter across P-Q, R-S and T-U in turn and note the reading of each. Switch off.
9.
The voltage readings obtained will prove KIRCHHOFFs second law.
10.
Divide the supply voltage (10V) by the total resistance (RT) for a series circuit.
RT = 10OΩ + 20Ω + 30Ω = 60Ω This will give the reading measured on the ammeter within the tolerance of the resistor values and the accuracy of the meters.
July 1999- Rev.0
Page 6/41
TRAINING MANUAL INSTRUMENTATION
Unit No. 8 - Practical tasks
RESULTS TABLE
Questions to be answered to show understanding of the practical task. (1)
What does the ammeter read, if the supply voltage is 5 volts? ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Module No. 4: Industrial electronics 1
-----------------------------------------------------------------------------------------------(2)
What happens to the voltage across the 20Ω resistor, if the short circuit across AB is replaced with a 40Ω resistor? ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
July 1999- Rev.0
Page 7/41
TRAINING MANUAL INSTRUMENTATION
Module No. 4: Industrial electronics 1
Unit No. 8 - Practical tasks
PRACTICAL TASK 3 THE PARALLEL CIRCUIT
1.
Connect the circuit as shown, without the meters connected.
2.
Set the D. C. supply to 10 V using the voltmeter. Switch off.
3.
Short out G - H and place the ammeter across A-B. Switch on and note the reading. Switch off.
4.
Repeat step (3) with the ammeter connected across C-D and E-F.
5.
Short out A-B, C-D and E-F. Reconnect the short circuit on G-H and replace with the ammeter across G-H. Switch on and note the reading. Switch off.
6.
KIRCHHOFFs first law is now proved as
7.
Short out G-H, A-B, C-D and E-F. Switch on. With a voltmeter measure VSUPPLY, VR1 VR2 and VR3
8.
July 1999- Rev.0
This will show that, in a parallel circuit, the voltage is the same across all loads.
Page 8/41
TRAINING MANUAL INSTRUMENTATION
Unit No. 8 - Practical tasks
RESULT TABLE
Questions to be answered to show understanding of the practical task. (1)
Calculate the resistance of the three resistors in parallel and find the supply current (is) for a supply voltage of 10V. This answer should be, within the accuracy of the equipment, the same as the measured Is. -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Module No. 4: Industrial electronics 1
---------------------------------------------------------------------------------------------
(2)
Are the measured values for 11, 12 and 1,3 the same as the following?
---------------------------------------------------------------------------------------------
July 1999- Rev.0
Page 9/41
TRAINING MANUAL INSTRUMENTATION
Unit No. 8 - Practical tasks
PRACTICAL TASK 4 THE SERIES - PARALLEL COMBINATION CIRCUIT
1
.Connect the circuit as shown, without the ammeter connected.
2.
After the instructor has checked the circuit, switch on the power supply. Set the supply and voltage to 11 V. Switch off.
3.
Connect the ammeter into the circuit. Switch on and note down the reading of the ammeter. Switch off.
RESULTS TABLE
VOLTMETER (VOLTS
AMMETER (mA
Module No. 4: Industrial electronics 1
Question to be answered to show understanding of the practical task. Calculate the total resistance of the circuit and the supply current (is) with supply voltage at 11 V. The calculated current will be, within the limits of accuracy, the value measured on the meter. -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
July 1999- Rev.0
Page 10/41
TRAINING MANUAL INSTRUMENTATION
Unit No. 8 - Practical tasks
PRACTICAL TASK 5 THE WHEATSTONE BRIDGE
1.
Connect the circuit as shown with the switch in the open position.
2.
After the instructor has checked the circuit, set the ammeter at its highest range. Close the switch.
3.
Adjust the decade box until the ammeter reads as near zero as possible on the lowest range. Note the value of the decade box. Switch off.
4. Calculate the value of Rx using the formula 5.
Check your answer by measuring Rx on an ohmmeter.
Module No. 4: Industrial electronics 1
5.
July 1999- Rev.0
Page 11/41
TRAINING MANUAL INSTRUMENTATION
Unit No. 8 - Practical tasks
PRACTICAL TASK 6 A.C. AND RMS VALUES
1.
Connect a 100Ω resistor across 6.3 V RMS supply, as shown.
2.
Connect an oscilloscope across the resistor and measure the peak to peak value.
3. 4.
Connect a dvm set to measure A.C. voltage across the resistor. This shows that, within limits, the device is calibrated in RMS.
5.
Connect an ammeter, which must be set to measure A.C. current, in the circuit. It should read V RMS / 100, to show it is also calibrated in RMS.
6.
Using the time basis scale, show the supply frequency is 50Hz. Remember the frequency = 1/2 period.
Question to be answered to show understanding of the practical task.
Module No. 4: Industrial electronics 1
Why is it not possible to display the workshop socket A.C. waveform on the oscilloscope? -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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D.C. AND THE CAPACITOR
1.
Connect the circuit as shown in the diagram.
2.
After the instructor has checked the circuit, switch on the supply. Note the reading on the DVM every 10 seconds as the capacitor charges. When the DVM is steady, switch-off the supply. Discharge the capacitor by switching the DVM to the D.C. ampere range.
3.
Plot the results obtained on a graph and estimate the RC time of the circuit (time to reach 6.32V).
Module No. 4: Industrial electronics 1
Unit No. 8 - Practical tasks
PRACTICAL TASK 7
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Unit No. 8 - Practical tasks
RESULTS TABLE
TIME (S)
DVM (V)
0 10 20 30 40 50 60 70 80 90 100 110 120
Module No. 4: Industrial electronics 1
CALCULATIONS Estimated RC time
=
_______________ secs
Calculated RC time
=
_______________ secs
QUESTION Why are the estimated and calculated RC times different? -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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THE INDUCTOR AND A.C.
1.
Connect the circuit, as shown, with the function generator set at 100 Hz with a 1 V output. Write down the reading indicated on the mA meter.
.
Repeat step (1) with a function generator output of 1V for frequencies of 200, 400, 800, 1000, 1200 and 1400 Hz. Note the reading of the mA meter each time.
3.
Work out the reactance of the coil at each frequency (divide 1V by mA reading).
4.
Plot a graph of reactance against frequency.
5.
The graph will show that inductive reactance increases linearly (in a straight line) with frequency.
Module No. 4: Industrial electronics 1
Unit No. 8 - Practical tasks
PRACTICAL TASK 8
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RESULTS TABLE
Supply voltage constant at 1 V.
Questions to be answered to show understanding of the practical task. (1)
Calculate the reactance of the inductor at 1000 Hz. -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Module No. 4: Industrial electronics 1
-----------------------------------------------------------------------------------------------(2)
Why is it different from the measured value? ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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Unit No. 8 - Practical tasks
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THE CAPACITOR AND A.C.
1.
Connect the circuit as shown, with the function generator set at 100 Hz with a 1 V output. Write down the reading indicated on the mA meter.
2.
Repeat step (1) with a function generator output of 1V for frequencies of 200, 400, 800, 1000, 1200 and 1400 Hz. Note the reading of the mA m meter each time.
3.
Work out the reactance of the capacitor at each frequency (divide 1 V by mA reading).
4.
Plot a graph of reactance against frequency.
5.
The graph will show that capacitive reactance falls exponentially with frequency.
Module No. 4: Industrial electronics 1
Unit No. 8 - Practical tasks
PRACTICAL TASK 9
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Supply voltage constant at 1 V.
Questions to be answered to show understanding of the practical task. (1)
Calculate the reactance of the capacitor at 1000 Hz. --------------------------------------------------------------------------------------------------
Module No. 4: Industrial electronics 1
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
(2)
Why is it different from the measured value? -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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Unit No. 8 - Practical tasks
PRACTICAL TASK 10 A.C. CURRENT AND VOLTAGE IN AN INDUCTANCE
1.
Connect the circuit as shown. Connect the earth leads of the oscilloscope (OSC) to the LOW side of the function generator.
2.
Adjust the waveforms to approximately the same size and sketch the waveforms shown on the oscillator.
3.
The waveforms should, show approximately a 90° phase shift between the voltage across the inductor and the current (voltage across the 20Ωresistor) through the inductor.
Question to be answered to show understanding of the practical task. Does the voltage lead the current or the current lead the voltage?
Module No. 4: Industrial electronics 1
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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Unit No. 8 - Practical tasks
PRACTICAL TASK 11 A.C. CURRENT AND VOLTAGE IN A CAPACITANCE
1.
Connect the circuit as shown. Connect the earth leads of the oscilloscope (OSC), to the low side of the function generator.
2.
Adjust the waveforms to approximately the same size. Sketch the waveforms displayed.
3.
The waveforms should show approximately a 90° phase shift between the voltage across the capacitor and the current (voltage across the 20Ω resistor) through the capacitor.
Question to be answered to show understanding of the practical task. 1
Does the voltage lead the current or the current lead the voltage? ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Module No. 4: Industrial electronics 1
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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Unit No. 8 - Practical tasks
PRACTICAL TASK 12 A.C. AND THE INDUCTOR / CAPACITOR RESISTIVE CIRCUIT INTRODUCTION The previous two tasks (10 and 11) showed the phase shift between the current and voltage waveform when a capacitor or inductor is supplied with A.C. This task will show how the phase shift between the current and the voltage can be adjusted. This is done using an inductor / capacitor with a resistor in a series circuit.
1.
Connect the circuit as shown. Sketch the two waveforms shown on the oscilloscope.
2.
Change the frequency to 500 Hz. Sketch the waveforms.
3.
Change the frequency to 2000 Hz. Sketch the waveforms.
4.
Replace the 100 mH inductor with a 0.1µF capacitor.
5.
Repeat steps (1) through (3) and sketch the waveforms.
Questions to be answered to show understanding Of the practical task. Module No. 4: Industrial electronics 1
(1)
Work out the phase shift at the various frequencies from the sketches you have made. -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
(2)
Do these sketches show that the inductive reactance goes up with frequency and the capacitive reactance goes down with frequency?
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Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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Unit No. 8 - Practical tasks
PRACTICAL TASK 13 RESONANCE SERIES
1.
Connect the circuit as shown.
2.
Set the function generator to 200 Hz and note the reading on the DVM
3.
Repeat step (2) for frequencies of 400, 600, 800, 1000, 1200, 1400. 1800, 2000 and 2200 Hz.
4.
Adjust the frequency on the function generator to obtain the largest reading on the DMV. Note the frequency.
5.
Draw a graph of DVM reading against frequency.
6.
The graph should show the circuit resonates. It. has a maximum voltage (current) at one frequency.
Module No. 4: Industrial electronics 1
Questions to be answered to show understanding of the practical task.
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
(2)
Does your calculation agree with the result obtained from step (4) ? ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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Unit No. 8 - Practical tasks
PARALLEL
1.
Connect the circuit as shown.
2.
Set the function generator to 200 Hz. Note this reading on the DVMI.
3.
Repeat step (2) for frequencies of 400, 600, 800, 1000, 1200, 1400. 1800, 2000 and 2200 Hz.
4.
Adjust the frequency on the function generator to obtain the smallest reading on the DMV. Note the frequency.
5.
Draw a graph of the DVM reading against the frequency.
6.
The graph should show the circuit resonates. It has a minimum voltage (current) at one frequency.
Module No. 4: Industrial electronics 1
Questions to be answered to show understanding of the practical task.
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
(2)
Does your calculation agree with the result obtained from step (3) ? ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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Module No. 4: Industrial electronics 1
Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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Unit No. 8 - Practical tasks
RESISTANCE COLOUR CODES Method 1
Module No. 4: Industrial electronics 1
The diagram shows a colour coded resistor. The colour code is there to show you the resistance of the resistor. Reading from left to right, the first and seconds bands indicate a number, (eg. if the first colour band is 4 and the second colour band is 7 then the number is 47). The third band is the 3 multiplier in power form, (eg. 10 ) . The fourth band indicates the tolerance of the resistor (eg. ± 5%). The numbers to match the colours are internationally fixed and are given below.
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Unit No. 8 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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Unit No. 8 - Practical tasks
Example 1
GREEN
The example has the bands red, violet, green and gold. This means 27 x 100 000 (105)
=
2.7 MΩ
Tolerance ± 5%
Example 2
The example has the bands black, green, gold and red. This means 05 x 0. 1
=
0. 5Ω
Tolerance ± 2%
Module No. 4: Industrial electronics 1
Example 3
The example has the bands red, black, black and brown. This means 20 x 1 =
20.Ω
Tolerance ± 1 %
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Unit No. 8 - Practical tasks
Method 2 The newer types of resistor have the value and tolerance printed on them. These use letters to show the powers. R
=
K M
1
=
1000
=
1 000 000
Examples 100 R
=
1R1
=
1 K5 4M7
= =
100 Ω 1.1Ω 1 500Ω
4 700 000Ω
This method of numbering resistors is now used on circuit diagrams and in catalogues when ordering resistors.
Module No. 4: Industrial electronics 1
Note: Various colour codes for capacitors have been devised. However none of these are widely accepted so they are not worth learning.
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Unit No. 8 - Practical tasks
Tolerance: This is the range over which a component is allowed to vary from its stated value. The closer the tolerance- the more accurate the component. However, the closer the tolerance the higher the cost. Electrical/electronic circuits are designed so that close tolerance components are only used when it is necessary for correct operation. Example Find the acceptable range of values for a 1 K resistor with a tolerance of ± 5%. 5% of 1 000 = 50Ω
Acceptable range will be
1 000 ± 50Ω
Module No. 4: Industrial electronics 1
or 950 Ω to 1 050Ω
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Unit No. 8 - Practical tasks
Power Rating. When an electric current is passed through a resistor, energy is dissipated (used) in the form of heat. So, resistors are made according to how much heat (power) they can take without burning out. This is called the power rating. The power -rating of a resistor is not usually printed on the resistor. The manufacturer's catalogue will tell you the maximum power it can handle. The construction of resistors depends on their power rating. There are two basic kinds. (1)
Film Resistors
Figure PT-1 Film Resistor
Module No. 4: Industrial electronics 1
Figure PT-1 shows a film resistor used in electronics. It is made by putting a thin coating (a film) of a carbon or metal compound onto a ceramic cylinder. The resistivity of the coating compound is varied to give the necessary resistance value. The resistance can be made more accurate by cutting grooves in the film. The grooves change. the area and thus the resistance. The connections to the resistor are made by brass or nickel caps and copper connecting leads. The device is coated with a plastic insulator and painted with the colour code. These resistors are made in various values from about I Ω- to 10MΩ with a power rating to about 2W.
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Unit No. 8 - Practical tasks
(2)
Wire Wound
These resistors are made for high power applications. An instrument technician will only see these types in power amplifiers driving field devices (e.g. relays, solenoid valves, etc.) however, they are often used in electrical work.
Figure PT-2 Wire Wound Resistor
Figure PT-2 shows a wire wound resistor. It consists of a ceramic cylinder with a resistance wire wound around it. The resistance value depends on the resistivity of the wire and the number of turns. These resistors can be very accurate. They are used as standard resistors. The insulation on the device depends on what the device will be used for. Very high power resistors which need to dissipate kilowatts usually have no insulation. They lose the heat by radiating it outwards like the sun. Electronic power resistors are usually insulated with what is called 'vitreous enamel". This provides good insulation with good heat radiation properties. The wire wound resistor usually has its resistance value and its tolerance written on the device (no colour code). Wire wound resistors are only produced in the lower resistance ranges with a maximum value of about 100kΩ. The power ratings produced for electronics vary from about 2.5W to 50W. However, some electrical systems may use resistors with power ratings of many kilowatts.
Module No. 4: Industrial electronics 1
Note:
The latest type of resistors come in what are- called "chips". These are film resistors. They are constructed on a ceramic chip. There are connecting pads on the bottom so it can be surface mounted on a printed circuit board. The "chips" come from the factory stuck on a tape. They are removed one at a time when they are needed. Figure PT-3 shows a typical chip resistor.
Figure PT-3 Chip Resistor
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MODULE No. 5 INDUSTRIAL ELECTRONICS 2
July 1999- Rev.0
UNITS IN THIS COURSE
UNIT 1
BASIC SEMICONDUCTOR THEORY
UNIT 2
DIODE APPLICATION
UNIT 3
THE CONTROLLED DIODE
UNIT 4
TRANSISTOR S
UNIT 5
PRACTICAL TASKS
Module No. 5: Industrial electronics 2
Unit No. 1 - Basic semiconductor theory
TRAINING MANUAL INSTRUMENTATION
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Page 1/13
TRAINING MANUAL INSTRUMENTATION
Para
Page
1.0
COURSE OBJECTIVE
3
1.1
INTRODUCTION
4
1.2
BASIC ATOMIC THEORY
4
1.2.1
6
1.3
Electrical Conduction
THE INTRINSIC SEMICONDUCTOR
7
1.3.1
Doping an Intrinsic Semiconductor
8
1.3.2
The PN Junction
10
1.3.3
The PN Junction Diode
11
1.3.4
Diode Symbols
12
Module No. 5: Industrial electronics 2
Unit No. 1 - Basic semiconductor theory
TABLE OF CONTENTS
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COURSE OBJECTIVE The student will be able to •
Explain an electric current as a movement of electrons or holes.
•
Explain the difference between a conductor, insulator and semiconductor by electron shell theory.
•
Explain the terms 'P' type and 'N' type material.
•
Describe the action of a PN junction.
•
With the aid of a sketch explain the action of a PN junction diode.
Module No. 5: Industrial electronics 2
Unit No. 1 - Basic semiconductor theory
1.0
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Unit No. 1 - Basic semiconductor theory
1.1
INTRODUCTION The aim of this unit is to explain, in basic terms, the physics of the conductor, insulator, semiconductor and the PN junction.
1.2
BASIC ATOMIC THEORY The basic unit of all matter is the atom. Figure 1-1 shows a very simplified diagram of an atom.
Figure 1-1 The Simple Atom
Module No. 5: Industrial electronics 2
The atom contains a central group of particles called the NUCLEUS. This nucleus is made up of PROTONS with a POSITIVE CHARGE and NEUTRONS with ZERO CHARGE. The ELECTRON clouds, with a NEGATIVE CHARGE, move around the nucleus in a spherical space at a fixed distance from the nucleus called a SHELL. The number of electrons and protons in an atom are equal, so an atom has no charge. The number of electrons or protons in an atom depends on the substance (element). For example:
There are about 100 elements. Every element has an atomic number. The atomic number tells you how many protons/electrons are in the element. So, the atomic number of Hydrogen is 1, Oxygen is 8 and so on. Everything is made, of atoms. If all the atoms in a substance are of one type that substance is an element. If the atoms are of two or more types the substance is a compound. A typical compound is water, which is made of Hydrogen and Oxygen. It's basic unit (molecule) is shown July 1999- Rev.0
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Unit No. 1 - Basic semiconductor theory
TRAINING MANUAL INSTRUMENTATION
The electrical properties of an element depend on how many electrons are in the outside shell. Good conductors like copper only have 1 electron in the outside shell. Good insulators like neon have a full outside shell (8 electrons). Semiconductors are half-full; eg, silicon. Elements are classified according to the number of electrons in the outside shell from group 1 (good conductors) to group 8 (good insulators). Group 3, 4 and 5 materials are used to make semiconductor devices in use today, e.g. diode, transistor, integrated circuit etc.(see Figure 1-2) •
Group 1 example Lithium
One electron in the outside shell
Module No. 5: Industrial electronics 2
•
Group 4 example Silicon
Four electrons in the outside Shell
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Unit No. 1 - Basic semiconductor theory
•
Group 8 example Neon
Eight electrons in the outside shell Figure 1-2 Atomic Shell Structure examples
Module No. 5: Industrial electronics 2
1.2.1
Electrical Conduction
Figure 1-3 Simple Electrical Conduction
Figure 1-3 shows a few atoms in a conductor. Each atom has a single (free) electron in the outside shell. When an EMF is applied, the free electron (which has a negative charge) is pulled towards the positive charge. The 'hole' left by the electron is filled by the next moving electron. So, the hole (the positive charge) appears to move to the negative. The electron flow of an electric current is from negative to positive. Hole flow is from positive to negative. It is hole flow that is used to show the direction of an electric current, (conventional flow). Note :
July 1999- Rev.0
The charge of an electron is very small. A current of 1 Amp means that 1.6 x 1019 electrons are moving per second.
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Unit No. 1 - Basic semiconductor theory
1.3
THE INTRINSIC SEMICONDUCTOR The intrinsic semiconductor is a group 4 material with 4 electrons in the outside shell. This means there are four (4) empty places in the outside shell into which an electron can move. The most common intrinsic semiconductors are silicon and germanium. They are produced as crystals. Crystals are an extremely pure form in which the atoms have a regular pattern (see Figure 1-4).
Module No. 5: Industrial electronics 2
Figure 1-4 Arrangement of Atoms in a Crystal
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Unit No. 1 - Basic semiconductor theory
The atoms hold together as a solid, by sharing the electrons between them to keep the outside shell full some of the time. arrangement in which atoms share electrons is called a covalent bond. A simple diagram of this is shown in Figure 1-5.
Figure 1-5 Covalent Bonding
Note : A good quality crystal has less than 1 part in 10,000,000,000 impurities.
Module No. 5: Industrial electronics 2
1.3.1
Doping an Intrinsic Semiconductor Doping an intrinsic semiconductor is the trick which makes all semiconductor devices work. There are only two types of doping that can be done. N Type A small amount of a group 5 material; eg, arsenic, which has 5 electrons in its outer shell, is added to an intrinsic semiconductor. This causes the shared area around a covalent bond to have a 'spare' electron which is free to move; 4 from silicon 5 from arsenic = 9 electrons. There is one spare electron because 8 is a full shell. Figure 1-6 shows the action of an N type material which has free electrons to give away: doNate.
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Unit No. 1 - Basic semiconductor theory
TRAINING MANUAL INSTRUMENTATION
Figure 1-6 ‘N’ Type Material P Type
Module No. 5: Industrial electronics 2
A small quantity of a group 3 material, eg, aluminium, which has 3 electrons in the outer shell, is added to an intrinsic semiconductor. This causes the shared area around a covalent bond to be one electron short. There are 4 electrons from silicon and 3 from aluminium which makes 7 electrons in total. This is one short of a full shell so a 'hole' is produced where an electron can go. The P type material will take in electrons, accePtor. Figure 1-7 shows the action of a 'P' type material.
Figure 1-7 'P' Type Material
Note:
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Unit No. 1 - Basic semiconductor theory
1.3.2
The PN Junction
Figure 1-8 The PN Junction
Figure 1-8 shows what happens when a piece of 'P' type material and a piece of 'N' type material are joined together.
Module No. 5: Industrial electronics 2
The excess holes from the P side and excess electrons from the side cross the barrier and cancel each other out. The 'P' area gain electrons and goes NEGATIVE, the 'N' area gains holes and goes POSITIVE. A CONTACT POTENTIAL is formed. The electric field which it produces stops any more hole-electron connections. The area where these holes and electrons cancel each other is called the DEPLETION LAYER.
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Module No. 5: Industrial electronics 2
Unit No. 1 - Basic semiconductor theory
TRAINING MANUAL INSTRUMENTATION
1.3.3
The PN Junction Diode
Figure 1-9 The PN Junction Diode
Figure 1-9 shows what happens when an external voltage is applied to a PN Junction. If the 'N' region is made positive to the 'P' region the depletion layer gets stronger and no current flows. If the 'P' region is made positive to the 'N' region, the contact potential and depletion layer are broken down and current will flow. The PN junction acts like a non-return valve. It allows current to flow in one direction only. Therefore, it is a diode. The current depends on the size of the voltage applied July 1999- Rev.0
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Unit No. 1 - Basic semiconductor theory
Note :
The current will only flow in the forward direction after the contact potential (barrier voltages) has been overcome. This is about 0.7v with silicon diodes and about 0.2v with germanium diodes. If enough voltage is applied in the reverse direction, the diode will break down and high current will flow. The amount of voltage needed to do this is called the ZENER breakdown voltage.
Module No. 5: Industrial electronics 2
Figure 1-10 Diode Symbols
Figure 1 -10 shows the common symbols for a diode. Figures (a) and (b) show the symbol used on an electronic diagram. The 'P' region is called the 'ANODE' and the 'N' region the "CATHODE'. Forward current flow is from anode to cathode. Figures (c) and (d) show the markings on the diode itself. Two examples of industrial semiconductor diodes are shown below.
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WIRE CONNECTIONS
Module No. 5: Industrial electronics 2
Unit No. 1 - Basic semiconductor theory
STUD MOUNTING
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UNITS IN THIS COURSE
UNIT 1
BASIC SEMICONDUCTOR THEORY
UNIT 2
DIODE APPLICATION
UNIT 3
THE CONTROLLED DIODE
UNIT 4
TRANSISTOR S
UNIT 5
PRACTICAL TASKS
Module No. 5 : Industrial electronics 2
Unit No. 2- Diode application
TRAINING MANUAL INSTRUMENTATION
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TABLE OF CONTENTS Para
Page
3.0
COURSE OBJECTIVE
3
3.1
INTRODUCTION
4
3.2
SYMBOLS
4
3.3
CHARACTERISTICS
5
3.3.1
Forward Characteristics
6
3.3.2
Reverse Characteristics
7
3.4
PARAMETERS
8
3.5
IDENTIFICATION
9
3.6
TESTING DIODES
11
3.7
APPLICATIONS
13
3.8
LIGHT-EMITTING DIODES
15
3.9
REVIEW
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17
Page 2/17
Unit No. 2- Diode application
TRAINING MANUAL INSTRUMENTATION
3.0
COURSE OBJECTIVE The trainee will be able to: •
Recognise the common diode symbols
•
Explain the meaning of ∗
Forward bias
∗
Reverse bias
∗
Anode
∗
Cathode
∗
Reverse (breakdown) voltage
∗
Forward voltage/current
∗
LED
∗
Photodiode
• Explain how to test a diode • Briefly explain how diodes can be identified
Module No. 5 : Industrial electronics 2
• Describe one application for a diode (or an LED)
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Unit No. 2- Diode application
TRAINING MANUAL INSTRUMENTATION
3.1
INTRODUCTION A diode is a semiconductor device. It is usually made of Silicon (Si) or Germanium (Ge). It has two terminal connections and, within voltage limits, conducts current only in one direction. It does the same job as a non-return valve. It is this characteristic which is important in electronics. Diodes are used to convert Alternating Current (AC) to Direct Current (DC). Direct Current is used in all electronic equipment. The connections of a diode are called the ANODE and the CATHODE (see figure 3.1). If the diode is FORWARD BIASED, the anode is connected to the positive voltage and the cathode to the negative voltage. When the diode is forward biased, current flows through it. When the diode is REVERSED BIASED, only a very small amount of current (leakage current) flows through it.
Figure 3-1 Symbol of Diode
Diodes allow current to flow only in one direction. It is this characteristic which is very useful in electronics. Diodes are used to convert Alternating Current (AC) to Direct Current (DC). Direct Current is used for all electronic equipment. You will learn about this when you study Rectifiers in Unit 4.
Module No. 5 : Industrial electronics 2
3.2
SYMBOLS The general symbols for diodes are:
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Unit No. 2- Diode application
TRAINING MANUAL INSTRUMENTATION
Figure 3-2 Symbols for Diodes
Some symbols also show the polarity of the diode (see figures 3.3 and 3.4).
Module No. 5 : Industrial electronics 2
Figure 3-3 Polarity of Diodes - Polarity shown by symbol
Figure 3-4 Polarity of Diodes - Polarity shown by line
3.3
CHARACTERISTICS When a diode is connected to a supply with a positive anode and negative cathode polarity, the diode is FORWARD BIASED.
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Unit No. 2- Diode application
TRAINING MANUAL INSTRUMENTATION
When a diode is connected to a supply with a negative anode and positive cathode polarity, the diode is REVERSE BIASED. When the diode is Forward Biased, it conducts current. When the diode is Reverse Biased, it does not conduct much current. Each of these two conditions has a different characteristic (see figure 3.5).
Module No. 5 : Industrial electronics 2
Figure 3-5 Characteristics of Forward and Reverse Biased Diodes In figure 3.5, note that the current is shown in mA and µA and that the forward voltage is positive and reverse voltage is negative. Note also that the forward voltage is shown in tenths but the reverse voltage is shown in tens. In figure 3.5 in FORWARD bias, the current through a Silicon diode begins to flow when the voltage reaches about 0.6V. In REVERSE bias, the current through the Silicon diode begins to flow when the voltage reaches about -50V. What are the two limits for a Germanium diode?
3.3.1
Forward Characteristics Figure 3.5 shows that Silicon diodes and Germanium Diodes have different characteristics. The Silicon diode has a higher "turn-on" voltage and a higher "breakdown" voltage. The "turn-on" voltage is the voltage at which the diode begins to work. The "breakdown" voltage is the voltage at which the diode cannot control
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Unit No. 2- Diode application
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The Silicon diode has a higher FORWARD VOLTAGE DROP than the Germanium diode (see figure 3.6). In figure 3.6 the Silicon diode is shown with a FORWARD VOLTAGE DROP from about 0.6V. What is the FORWARD VOLTAGE DROP of the Germanium diode shown in figure 3.6?
Figure 3.6 Forward Characteristics
There are many different types of diode. Some diodes break when only a few milliamperes (mA) of current flow through them. Other diodes break when over 100 amperes of current flows through them.
3.3.2
Reverse Characteristics
Module No. 5 : Industrial electronics 2
In theory, no current flows when the diode is reverse biased. In practice, however, some LEAKAGE current does flow. The Germanium diode has a larger leakage current at lower voltages than the Silicon diode. At some values of reverse voltage the current suddenly increases very quickly. This is called the REVERSE BREAKDOWN VOLTAGE. Voltage at this level will destroy the diode. So it must be controlled by a resistor connected in series (see figure 3.7).
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Figure 3-7 Reverse Characteristics
The leakage current of diodes increases when the temperature rises. The leakage current of Germanium diodes increases faster than Silicon diodes.
3.4
PARAMETERS
Module No. 5 : Industrial electronics 2
Diode manufactures give the following ratings, or information, about their diodes: V-f
Forward Voltage for a particular forward current (eg up to 1.5 @ 5A for a silicon diode but the value will usually be less than 1 V for most diodes) - or the voltage in a forward bias direction - at which the diode begins to work.
I-f
the Forward Current at which a diode begins to work.
V-rrm
the Repetitive Reverse Maximum Voltage (sometimes referred to as PRV - Peak Reverse Voltage - or PIV - Peak Inverse Voltage). The amount of voltage a diode can withstand in a reverse bias direction without breaking down.
When choosing a diode, you should consider: • how much FORWARD CURRENT it will carry.(or at what voltage it will begin to work). • how much REVERSE VOLTAGE it will tolerate (or at what voltage it will break down). • whether it is made of Silicon or Germanium (remember that they have different characteristics).
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Unit No. 2- Diode application
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3.5
IDENTIFICATION There are many different types, sizes, styles and ratings for diodes (see figure 3.8, 3.9 and 3.10). The diodes therefore have to be identified so that we know what they can do.
Module No. 5 : Industrial electronics 2
Figure 3-8 Diode Case Styles
Figure 3-9 Diode Case Styles
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There are two numbering systems for diodes. The AMERICAN system starts with a 1 N. This means that the device has one PN junction. For example, a 1 N4004 is a Silicon diode with a voltage rating of 400V. A 1 N4020 is a Zener diode made of Silicon with a voltage rating of 12V and which can handle 5W. A 1 N2326 diode is made of Germanium with a rating of 1 V and 2mA. The numbers after 1 N are reference numbers. The EUROPEAN Pro-Electron system starts with two letters. The first letters are A or B. The second letters are Y or Z. A
=
Germanium (a detection or switching diode)
B
=
Silicon (a variance capacitance diode)
Y
=
a rectifier diode
Z
=
a voltage reference diode (or a transient suppresser diode)
For example, a BY200 is a type of Silicon Rectifier diode. The numbers after the two letters are reference numbers.
Module No. 5 : Industrial electronics 2
Some diodes have ratings which say that they can handle only a few milliamperes. Other diodes can handle several hundred amperes (see figure 3.10).
Figure 3-10 Diode Case Styles
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Unit No. 2- Diode application
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3.6
TESTING DIODES Diodes can be tested with an ohmmeter: •
connect the ohmmeter across the diode
•
look at the reading
•
reverse the ohmmeter leads
•
look at the reading
If the readings are exactly the same, the diode is probably not working. Can you explain why? Remember that a diode works like a one-way valve. The ohmmeter test is about 98% accurate. It is not 100% accurate because the diode could be overheating when the circuit is operating. Diodes can also be tested with analogue and digital voltmeters. Using a DIGITAL voltmeter (see figure 3.11), set it to the DIODE CHECK position. Note that the RED lead is the POSITIVE lead and the BLACK lead is the NEGATIVE lead.
Module No. 5 : Industrial electronics 2
The digital voltmeter uses a constant current to check resistance. On the resistance range, the voltage would not be enough to "turn on" the diode since it is less than 0.6V. On the DIODE CHECK range, the current is still constant but there is enough voltage for the diode to conduct. The reading is the forward voltage drop of the diode. This reading is useful to find diodes with similar V-fs.
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Figure 3-11 Using a Digital Voltmeter (DVM) In figure 3.11 the lamp lights up when the supply is connected. When the current is reduced, the lamp becomes dimmer and the voltage falls. In figure 3.11 B when the diode is reversed, the lamp does not light up. Can you explain why?
Module No. 5 : Industrial electronics 2
Using an ANALOGUE voltmeter (or Multimeter), set it to the Ohms x 1 range. In this range, the BLACK lead is POSITIVE and the RED lead is NEGATIVE (see figure 3.12).
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Unit No. 2- Diode application
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Figure 3.12 Using an Analogue Multimeter
All the meters will show an OPEN CIRCUIT diode as an open circuit and a SHORT CIRCUIT diode as a short circuit. Some diodes will show a resistance other than infinity when in REVERSE BIAS. For Germanium diodes, this could be correct because Germanium diodes have more leakage current than Silicon diodes. For Silicon diodes, a resistance other than infinity when in REVERSE BIAS probably means the diode is not working.
Module No. 5 : Industrial electronics 2
3.7
APPLICATIONS The diode has many uses but two uses will be described here. Look at figure 3.13. It is a circuit for a dimmer control for a lamp. The lamp is OFF when the switch is in the centre position. When the movable contact of the switch meets the upper contact, the lamp is ON at FULL brightness because FULL voltage is supplied to it. 'When the movable contact meets the lower contact, the current flows through the diode. The diode allows current to flow only one way. HALF of the AC Waveform is blocked during each cycle. So HALF of the voltage is supplied to the lamp. So the lamp is ON - but at HALF brightness.
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Figure 3.13 Lamp Dimmer Control
This is a dimmer control. The advantage of this type of control is that ordinary lamps can be used.
Module No. 5 : Industrial electronics 2
Look at figure 3.14. It is a circuit is for a dynamic brake for a small AC induction motor. AC induction motors can be braked by applying DIRECT CURRENT to their stator windings. Remember that diodes allow current to flow only in one direction. The current becomes DIRECT CURRENT because it does not reverse direction.
Figure 3-14 Dynamic Braking Circuit for AC Induction Motor
The AC induction motor is OFF when the switch is in the centre position. When the switch is in the MAINTAINED CONTACT position, the AC motor is ON. When the switch is in the MOMENTARY CONTACT position, the current flows through the diode and the limiting resistor. This applies direct current to the stator winding and brakes the motor. The resistor limits the amount of current to a safe value for the diode and the winding of the motor.
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3.8
LIGHT-EMITTING DIODES Light-emitting diodes (LEDs) give out light when current passes through them (see figure 3.15).
Figure 3-15 - Light Emitting Diode
Photodiodes are turned on by light (see figure 3.16). They will not work without light.
Module No. 5 : Industrial electronics 2
Figure 3-16 - Photodiode
Note
the different direction of the arrows on the symbols.
LEDs and ordinary diodes are similar. They both allow current to flow in only one direction but an LED needs a higher voltage to turn it on. An ordinary Silicon junction diode needs about 0.7V to turn it on. An LED needs about 1.7V to turn it on. An LED has a higher voltage drop than an ordinary junction diode. The higher voltage drop makes an LED difficult to test with some ohmmeters. The best way of testing an LED is put it in a circuit and see if it works. When it is in a circuit, an LED works with about 20mA (0.020A) or less of current. So when an LED is put in a 12V DC circuit, the current must be limited by resistor which is connected in series (see figure 3.17). Can you calculate the value for the resistor? Use R=V ÷ I
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Unit No. 2- Diode application
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Figure 3.17 Current Flow limited by Resistor
When an LED is connected in a circuit, it is important to know which lead is the anode and which lead is the cathode. Hold the LED with the leads towards you. The plastic case has a flat side. The flat side should match the line on the diode symbol (see figure 3.18).
Module No. 5 : Industrial electronics 2
Figure 3-18 LED Polarity
LEDs are used for pilot lights and numerical and figure displays on electronic equipment. They are inexpensive. Unlike light bulbs, they do not have any filaments. LEDs are used for seven segment displays. The seven segments can be lit in different combinations to make a variety of numbers and figures.
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3.9
REVIEW •
symbols for diodes, including LEDs.
•
Forward Bias is when the diode is connected to supply so that, within limits, it conducts current.
•
Reverse Bias is when the diode is connected to a supply so that, within limits, it does not conduct current.
•
The anode and cathode are the diode connections.
•
When the diode is Forward Biased the anode is connected to the positive and the cathode to the negative voltage.
•
Reverse Breakdown Voltage is the voltage at which the diode cannot control the current.
•
Forward Voltage is the voltage at which the diode begins to work.
•
LEDs are Light Emitting Diodes which give out light when current passes through them.
•
Photodiodes are diodes which need light to work.
•
Diodes can be tested to find out if they working by using ohmmeters or multimeters connected across them.
Module No. 5 : Industrial electronics 2
Unit No. 2- Diode application
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UNITS IN THIS COURSE UNIT 1
BASIC SEMICONDUCTOR THEORY
UNIT 2
DIODE APPLICATION
UNIT 3
THE CONTROLLED DIODE
UNIT 4
TRANSISTORS
UNIT 5
PRACTICAL TASKS
Module No. 5: Industrial electronics 2
Unit No. 3 - The controlled diode
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 1/8
TABLE OF CONTENTS Para
Page
3.0
COURSE OBJECTIVE
3
3.1
INTRODUCTION
4
3.2
THE THYRISTOR (SCR)
4
3.2.1
A Typical SCR Control Circuit
5
3.2.2
The SCR in Industry
6
3.3
THE TRIAC
7
Module No. 5: Industrial electronics 2
Unit No. 3 - The controlled diode
TRAINING MANUAL INSTRUMENTATION
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TRAINING MANUAL INSTRUMENTATION
COURSE OBJECTIVE The student will be able to • Describe the operation of a Thyristor (SCR). • Draw and explain the operation of a typical variable D.C. supply using an SCR. • Describe the operation of a TRIAC. • Draw and explain the operation of a typical variable A.C. supply using a TRIAC.
Module No. 5: Industrial electronics 2
Unit No. 3 - The controlled diode
3.0
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Unit No. 3 - The controlled diode
3.1
INTRODUCTION Units 1 and 2 described the diode and its applications. This unit will explain the operation of diode devices which can produce a variable D.C. output from an A.C. supply, or a variable A.C. output from an A.C. supply.
3.2
THE THYRISTOR (SCR) The thyristor or Silicon Controlled Rectifier (SCR) is a 3 PN Junction device. A very enlarged view of the construction of an SCR is shown in Figure 3-1.
Figure 3-1 The Three Junction SCR
Operation
Module No. 5: Industrial electronics 2
An AC supply is connected anode to cathode. This device will only conduct when a voltage is applied to the gate. With no voltage on the gate, the device will not conduct in either direction. If a positive voltage of above 3V is applied to the gate a current will flow. The forward bias of the bottom PN junction produces enough current carriers for a positive voltage on the anode to produce a current. This current will flow from anode to cathode. However, a negative voltage on the anode produces no current flow even if the gate has a positive voltage. This means the device acts as a diode but its point of conduction is controlled by the voltage on the gate. The SCR cannot be switched off once it is conducting. It will continue to conduct until the anode voltage falls to zero.
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A Typical SCR Control Circuit
Unit No. 3 - The controlled diode
3.2.1
Figure 3-2 Simple SCR Light Control Circuit
Figure 3-2 shows a simple SCR control circuit. It is used to control the brightness of a lamp (the load).
Module No. 5: Industrial electronics 2
Operation. •
The timing of the pulse onto the SCR gate is controlled by an RC timing circuit and a UJT. The UJT (Uni-Junction Transistor) has only one junction so it only conducts when the emitter is positive enough.
•
The capacitor C charges through R on the positive half cycle to a point where the M conducts. The high current passing through R, passes a pulse of voltage, via the diode, onto the SCR gate. When a voltage is applied to the gate of the SCR it conducts and the bulb lights.
•
The low resistance of the EMITTER-B1 junction shorts out the capacitor so it is ready for charging on the next positive halfcycle.
•
The time at which the SCR conducts on the positive half-cycle depends on the RC time. The RC time can be adjusted by R.
•
If the RC time is long, the SCR will switch on later. If the SCR takes longer to switch on, the positive A.C. pulse across the lamp will be shorter. If the AC pulse is shorter, the lamp will get dimmer (less bright).
•
The SCR is a diode so it only operates on positive half-cycles.
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TRAINING MANUAL INSTRUMENTATION
The graph below shows the action of the circuit.
Unit No. 3 - The controlled diode
•
3.2.2
The SCR in Industry
Module No. 5: Industrial electronics 2
The simple SCR control circuit has no practical use. It is only used to show SCR action in a training workshop. An industrial SCR controller uses a complicated switching circuit. This is a separate electronic unit (card). Industrial SCRs are used in different ways. You must use the manufacturer's manual to set them up. A typical example of SCR control is the large industrial battery charger. A typical circuit for this is shown in Figure 3-3. The supply is three-phase with fullwave rectification. This gives six D.C. pulses per cycle which is more efficient (see Figure 3-3b).
Figure 3-3 The Basic Industrial Battery Charger
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Unit No. 3 - The controlled diode
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Figure 3-3b
3.3
THE TRIAC The triac is basically two SCRs back-to-back in one unit. It is used to control an A.C. wave. The symbol for a triac is
Module No. 5: Industrial electronics 2
With a triac, the gate can be triggered by both negative and positive pulses (usually positive). Current will flow in both directions when either MT2 is positive to MT1 or MT1 is positive to MT2. The simple light control circuit (see Figure 3-4) can be adapted to use a triac which gives better control of light dimming as shown below.
Figure 3-4 Simple Light Control Circuit
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Unit No. 3 - The controlled diode
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The dimming circuit works in the same way as before but the output voltage across the lamp will be controlled A.C. The graph below shows the basic action.
Module No. 5: Industrial electronics 2
Triacs are seldom used in industry as the distorted A.C. waveform causes unwanted electrical noise. This is difficult to filter out. However, they are used in instrumentation as A.C. power switches. They are very good for switching devices such as valves, relays, turbine control valves, etc which are actuated by electrical solenoids. Full AC voltage is applied when the triac is switched on, but there is no voltage when the triac is switched off.
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UNITS IN THIS COURSE UNIT 1
BASIC SEMICONDUCTOR THEORY
UNIT 2
DIODE APPLICATION
UNIT 3
THE CONTROLLED DIODE
UNIT 4
TRANSISTORS
UNIT 5
PRACTICAL TASKS
Module No. 5: Industrial electronics 2
Unit No. 4 - Transistor
TRAINING MANUAL INSTRUMENTATION
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Unit No. 4 - Transistor
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TABLE OF CONTENTS Para
Page
4.0
COURSE OBJECTIVE
3
4.1
INTRODUCTION
4
4.2
THE BIPOLAR JUNCTION TRANSISTOR (BJT)
4
4.2.1
The BJT NPN Amplifier Circuit
6
4.2.2
The PNP BJT Transistor
7
4.2.3
The BJT PNP Transistor Amplifier
7
4.3
THE COMMON COLLECTOR (EMITTER FOLLOWER) AMPLIFIER
8
4.4
THE FIELD EFFECT TRANSISTOR (FET)
9
4.5
THE FET AMPLIFIER
10
4.6
THE METAL OXIDE SEMICONDUCTOR FIELD EFFECT 11
THE TRANSISTOR AS AN ELECTRONIC SWITCH
13
Module No. 5: Industrial electronics 2
4.7
TRANSISTOR (MOSFET)
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Unit No. 4 - Transistor
4.0
COURSE OBJECTIVE The student will be able to • Explain with the aid of a diagram, the operation of a Bipolar Junction Transistor (BJT) amplifier. • Explain with the aid of a diagram, the operation of a Field Effect Transistor (FET) amplifier. • Explain with the aid of a diagram, the operation of a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) amplifier.
Module No. 5: Industrial electronics 2
• Explain with the aid of a diagram, the operation of typical transistor switching circuits.
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Unit No. 4 - Transistor
4.1
INTRODUCTION The aim of this unit is to introduce the basic construction of three common transistors in use: the Bipolar Junction Transistor (BJT), the Field Effect Transistor (FET) and the Metal Oxide Semiconductor Field Effect Transistor (MOSFET). These devices use a D.C. power supply as an energy source to increase the level of an alternating current signal to a usable level. No modern electronic devices (radio, stereo, television etc.) will work without transistors. This is because the signals received from an aerial, tape or C.D. player are too small to see or hear. They must be amplified.
4.2
THE BIPOLAR JUNCTION TRANSISTOR (BJT)
Figure 4-1 (BJT) NPN Construction
Module No. 5: Industrial electronics 2
Figure 4-1 shows an enlarged diagram of the construction of a BJT. It has two PN Junctions to make an NPN BJT. The three areas are called EMITTER, BASE and COLLECTOR. Operation •
The 'N' regions are more heavily doped than the 'P' region.
•
The base is much thinner than the emitter and collector regions.
•
The emitter-base junction is FORWARD BIASED. The- current which flows is mainly ELECTRONS. These are the MAJORITY CURRENT CARRIERS.
•
The base-collector junction is REVERSE BIASED. This means that the electric field across the junction will pull the majority current carriers (electrons) into the collector region.
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Unit No. 4 - Transistor
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The base current is very small because it is not heavily doped. So, there are few holes for the electrons to combine with. Most of the electrons are pulled into the collector. The current passing into the collector, for a fixed collector voltage, is proportional to the emitter-base bias. The device amplifies because small changes in the base current produce large changes in the collector current. The gain of a BJT is called HFE. HFE reflects the change in the collector current compared to the change in the base current. =
CHANGE IN COLLECTOR CURRENT CHANGE IN BASE CURRENT
Module No. 5: Industrial electronics 2
HFE (GAIN)
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The BJT NPN Amplifier Circuit
Unit No. 4 - Transistor
4.2.1
Figure 4-2 The NPN BJT Amplifier Figure 4-2 shows a typical NPN BJT small signal amplifier. The output signal is an amplified version of the input signal. The signal is amplified but it's not distorted.
Module No. 5: Industrial electronics 2
Operation •
R1 and R2 set the bias voltage of the base-emitter junction so that the BJT is conducting at it's midpoint and the voltage at the collector (Vc) is half of +Vcc.
•
The load resistor (RL) is added to develop a voltage output from the changes in the collector current.
•
C1 and C2 are added to block any D.C. levels which may affect the bias set by R1 and R2.
•
If the input signal goes positive the forward bias of the base-emitter junction is increased. This causes the collector current to go up so Vc will fall.
•
If the input signal goes negative the forward bias of the baseemitter junction will fall. This causes the collector current to fall which means Vc will rise.
•
The output signal is 180° out of phase with the input signal (inverted) but amplified.
•
This transistor is called a 'common emitter' amplifier because the emitter is connected to both the input and the output.
•
The symbol on an electrical diagram for an NPN BJT is
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he PNP BJT Transistor
Unit No. 4 - Transistor
4.2.2
Figure 4-3 PNP BJT Construction
Figure 4-3 shows the basic construction of a PNP BJT. It works in exactly the same way as an NPN BJT. However, the power supply are reversed and the majority current carriers are the holes (the spaces which attracts free electrons).
Module No. 5: Industrial electronics 2
4.2.3
The BJT PNP Transistor Amplifier
Figure 4-4 PNP BJT Amplifier
Figure 4-4 shows a typical PNP BJT amplifier. The use of each component is exactly the same as for an NPN BJT but the DC supply is negative (-Vcc). he symbol for a PNP Transistor is:
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Unit No. 4 - Transistor
4.3
THE COMMON COLLECTOR (EMITTER FOLLOWER) AMPLIFIER The normal small signal voltage amplifier is the one shown previously for both the PNP and NPN. This circuit is not suitable for giving power to small resistance loads. The circuit used to provide power gain is called the common collector (emitter follower) and shown in Figure 4-5.
Figure 4-5 Common Collector Amplifier
Module No. 5: Industrial electronics 2
The load (RL) is connected in the emitter circuit of the BJT. 1 The voltage gain is less than one but the current gain and power gain. ~, high. This circuit will thus provide the power to drive loads requiring high power, eg. stereo speakers, solenoid valves, relays, etc. The output waveform is in phase with the input waveform.
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THE FIELD EFFECT TRANSISTOR (FET)
Unit No. 4 - Transistor
4.4
Figure 4-6 FET Construction
Figure 4-6 shows the construction of a field effect transistor (FET). It consists of a piece of 'N' material (the N channel) with P type material inset around the middle as shown. When there is no voltage on the 'P' region (gate), current flows from the drain (D) to the source (S). If a negative voltage is applied to the gate a depletion layer is created as shown. This makes the N channel smaller so less current flows through it. The resistance of the 'N' channel goes up and the drain-source current (W goes down. If a big enough negative voltage is applied to the gate the depletion layers close off the 'N' channel so that no current can pass through it. Then ID fails to zero. The gate voltage to do this is called the 'pinch off' voltage. It is possible to change this device to a 'P channel` with inset 'N' type material. A 'P' channel FET works the same way as an 'N' channel except the `pinch off' voltages are positive.
Module No. 5: Industrial electronics 2
This gives a unit of conductance (1/Resistance). This unit of conductance is called the 'SIEMEN'. FET Symbols
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THE FET AMPLIFIER
Unit No. 4 - Transistor
4.5
Figure 4-7 The "N" Channel FET Amplifier
Figure 4-7 shows a typical N channel FET amplifier. The bias is set by R,. The voltage at Vs being ID R, positive. The voltage on the gate must be negative with respect to the source. The input signal developed across RG will move the gate voltage up and down. This produces an inverted output signal similar to a BJT amplifier. The gain of an FET amplifier is small (around 10) compared with a BJT amplifier (around 100). But the input resistance is very high unlike the BJT. However, the input resistance on an FET amplifier is much higher than on a BJT. A P channel FET amplifier looks exactly the same as an N channel amplifier. The big difference is that VDD is negative.
Module No. 5: Industrial electronics 2
Note : The capacitor Cl is used to short out any A.C. signals that would affect the bias developed across R,.
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Unit No. 4 - Transistor
4.6
THE METAL (MOSFET)
OXIDE
SEMICONDUCTOR
FIELD
EFFECT
TRANSISTOR
There are two types of MOSFET; depletion (see Figure 4-8a) and enhanced (see Figure 4-8b)
Figure 4-8 The MOSFET
The N-channel in the P substrate is insulated from the gate by silicon dioxide (Si02). When a voltage is applied across source and drain, a current will flow. The gate, insulator and N-channel act as a capacitor. When a voltage is applied to the gate a charge comes out of or goes into the N-channel. This increases or decreases the current which flows from the drain to the source.
Module No. 5: Industrial electronics 2
Any change in the gate voltage will produce a greater change in the drain circuit. This produces a gain similar to an ordinary FET. This device is usually made to be half conducting when there is no gate voltage so an amplifier circuit does not need biasing (see Figure 4-9).
Figure 4-9 Depletion MOSFET Amplifier
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Unit No. 4 - Transistor
Depletion MOSFET symbol
Enhanced MOSFET The enhanced type MOSFET has no inlaid N-channel. If there is no voltage on the gate there is no current flow between the drain and the source. A positive voltage on the gate makes a current flow in the channel between the N+ regions. The size of this current depends on the size of the gate voltage. These devices are not normally used as signal amplifiers because the biasing is complicated. However, they are very useful as electronic switches. Enhanced MOSFET symbol
Module No. 5: Industrial electronics 2
Note : P channel devices work in the same way as N channel devices except the gate voltage must be applied in the opposite direction. A negative gate voltage increases the current flow from drain to source. This means a P channel MOSFET amplifier does not reverse the input signal. Input and output signals are in phase.
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Unit No. 4 - Transistor
4.7
THE TRANSISTOR AS AN ELECTRONIC SWITCH The previous sections explained the use of a transistor to amplify a small alternative signal. This section deals with the use of a transistor as an electronic switch. The only transistors which are useful as switches are ones that are OFF normally and will switch ON when a signal is applied. The transistors discussed so far are : BJT
Normally off without bias
FET
Normally on without bias
MOSFET (Depletion)
Normally neither on or off.
MOSFET (Enhanced)
Normally off.
This means the transistors used as switches are usually either the BJT or the MOSFET (enhanced). The circuit below (see Figure 4-10) shows the construction of a typical BJT and MOSFET (enhanced) electronic switch.
Module No. 5: Industrial electronics 2
Figure 4-10 Transistor Switches Neither the BJT nor the MOSFET have any bias. A positive voltage applied to the base or gate is sufficient to drive both devices into full conduction and to operate the relay coil or solenoid valve. Remove the positive voltage and the device switches OFF. A capacitor, C, is sometimes added, particularly to a BJT, to quicken the switching action. The power MOSFET used as a switch is very popular today as it can be made to switch at least 200A as required.
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UNIT 1
BASIC SEMICONDUCTOR THEORY
UNIT 2
DIODE APPLICATION
UNIT 3
THE CONTROLLED DIODE
UNIT 4
TRANSISTORS
UNIT 5
PRACTICAL TASKS
Module No. 5: Industrial electronics 2
Unit No. 5 - Practical tasks
UNITS IN THIS COURSE
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Para
Page
INTRODUCTION
3
PRACTICAL TASK 1
4
PRACTICAL TASK 2
7
PRACTICAL TASK 3
10
PRACTICAL TASK 4
18
PRACTICAL TASK 5
21
PRACTICAL TASK 6
24
PRACTICAL TASK 7
30
PRACTICAL TASK 8
34
PRACTICAL TASK 9
39
PRACTICAL TASK 10
40
Module No. 5: Industrial electronics 2
Unit No. 5 - Practical tasks
TABLE OF CONTENTS
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Unit No. 5 - Practical tasks
INTRODUCTION The previous units (theory) have described the discrete (separate) components which are used in electronics. These discrete components are only used in instrumentation for simple tasks. Diodes for rectification. Transistors as output devices to drive 4-20 mA loops, relays, solenoids etc. Most electronic tasks are done using Integrated Circuits (IC). These IC's provide the amplification, computing, switching, etc. of the electronic signals. The IC has thousands of diodes, transistors etc. in one package using either BJT or MOSFET TECHNIQUES. The practical tasks given for this unit will enable you to learn the use of discrete components and identify the circuits in an overall instrument circuit diagram. The next unit will show you how to use IC's.
Module No. 5: Industrial electronics 2
The connections to a component depend on the way it is made. The correct connections to a device are given in data books which are held in the workshop. You must learn how to use these data books, with the assistance of the instructor.
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THE PN JUNCTION DIODE
(1)
Connect-up the circuit as shown in the diagram above.
(2)
After the instructor has checked the circuit, set the variable DC supply to give 0.25V and note the mA reading.
(3)
Repeat step (2) for 0.5V, 0.75V, 1 V, 1.5V and 2V.
(4)
Reverse the positive and negative connections to the DC supply. Switch on the power supply and note the mA reading for a 2V and 3V supply. Switch off.
(5)
Plot a graph of diode current (mA) against supply voltage.
Module No. 5: Industrial electronics 2
Unit No. 5 - Practical tasks
PRACTICAL TASK 1
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RESULTS TABLE
SUPPLY VOLTAGE (V)
DIODE CURRENT (mA)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 2 -1 (Reversed connections) -2 (Reversed connections)
Module No. 5: Industrial electronics 2
Questions to be answered to show understanding of the task. (1)
IN 4001 is a silicon diode. From the graph estimate the forward bias required to cancel the barrier (contact) potential. -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
(2)
Does your graph show that a diode is an electric check valve? -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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Unit No. 5 - Practical tasks
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THE ZENER DIODE
(1)
Connect up the circuit as shown using the components supplied.
(2)
After the instructor has checked the circuit, set the DC supply to 1 V and note the mA reading.
(3)
Repeat step (2) for a DC supply of 2V, 3V, 4V, 5V, 6V, 7V, 8V, 9V, 10V noting the mA reading each time. Switch off.
(4)
Reverse the connections to the DC supply and the DMM. Switch on and note the mA reading for a supply voltage of 0.5V, 1V, 1.5V and 2V.
(5)
Draw a graph of zener diode current against supply voltage.
Module No. 5: Industrial electronics 2
Unit No. 5 - Practical tasks
PRACTICAL TASK 2
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Unit No. 5 - Practical tasks
RESULTS TABLE
REVERSE VOLTAGE
REVERSE CURRENT
0 1 2 3 4 5 6 7 8 9 FORWARD VOLTAGE
FORWARD CURRENT
0.5V 0.6V 0.7V
Module No. 5: Industrial electronics 2
1V 2V
Questions to be answered to show understanding of the practical task. (1)
Is the reverse zener breakdown voltage written on the zener correct? -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
(2)
Does a zener diode act like an ordinary diode in the forward direction? ---------------------------------------------------------------------------------------------------
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Module No. 5: Industrial electronics 2
Unit No. 5 - Practical tasks
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Unit No. 5 - Practical tasks
PRACTICAL TASK 3 THE DIODE AND RECTIFICATION •
HALF-WAVE RECTIFICATION
(1)
Connect the circuit as shown using the components supplied.
(2)
After the instructor has checked the circuit, switch on the AC supply and measure the peak voltage and period of the waveform shown on the oscilloscope (OSC).
(3)
Sketch the waveform 'accurately on the graph paper supplied.
Questions to be answered to show understanding of the practical task. (1)
What happens if the diode is reversed?
Module No. 5: Industrial electronics 2
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
(2)
What is the RMS value of the AC supply? ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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FULL WAVE RECTIFIER
(1)
Connect the circuit as shown using the components supplied.
(2)
After the instructor has checked the circuit, place the plug connected to the primary side into the wall socket.
(3)
Switch on. Measure the peak voltage of waveform seen on the oscilloscope (OSC).
(4)
Sketch the oscilloscope waveform accurately on the graph paper provided.
Unit No. 5 - Practical tasks
•
Questions to be answered to show understanding of the practical task. (1)
Does your sketch show full wave rectification? -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Module No. 5: Industrial electronics 2
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------(2)
What happens if the diodes are reversed? -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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Unit No. 5 - Practical tasks
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Unit No. 5 - Practical tasks
BRIDGE RECTIFIER
(1)
Connect the circuit as shown using the components supplied.
(2)
After the instructor has checked the circuit, put the plug connected to the primary side into the wall socket.
(3)
Switch on the 240V - supply and measure the peak voltage shown on the oscilloscope.
(4)
Accurately sketch the waveform shown on the oscilloscope on the graph paper provided.
Questions to be answered to show understanding of the practical task. (1)
Does the bridge rectifier circuit provide full wave rectification? -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Module No. 5: Industrial electronics 2
----------------------------------------------------------------------------------------------------
(2)
What happens if the diodes are reversed? ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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Unit No. 5 - Practical tasks
PRACTICAL TASK 4 SMOOTHING CIRCUITS THE RESERVOIR CAPACITOR
(1)
Connect the circuit as shown using the components supplied.
(2)
After the instructor has checked the circuit, put the plug connected to the primary side into the 240V wall socket.
(3)
Switch on the 240V supply and carefully sketch the waveform seen on the oscilloscope. Switch off.
(4)
Replace the 10 µF capacitor with a 100 µF capacitor.
(5)
Switch on and carefully sketch the waveform seen on the oscilloscope on the same sketch drawn during step (3). Switch off.
QUESTION to be answered to show understanding of the practical task. What is the effect of changing the size of the reservoir capacitor?
Module No. 5: Industrial electronics 2
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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Unit No. 5 - Practical tasks
THE RESERVOIR CAPACITOR WITH FILTER CIRCUIT
(1)
Connect the circuit as shown using the components supplied.
(2)
After the instructor has checked the circuit, put the plug connected to the primary side into the 240V wall socket.
(3)
Switch on the 240V supply and carefully sketch the waveform seen on the oscilloscope.
(4)
Measure the size of the peak to peak ripple and the DC level using the oscilloscope.
(5)
Find the frequency of the ripple using the oscilloscope. Switch off.
Questions to be answered to show understanding of the practical task. (1)
Why is a 200Ω resistor used instead of an inductor? ---------------------------------------------------------------------------------------------------
Module No. 5: Industrial electronics 2
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------2)
Does the RC filter reduce the ripple? -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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Unit No. 5 - Practical tasks
PRACTICAL TASK 5 THE 3 LEG REGULATOR
(1)
Connect the circuit as shown using the components supplied.
(2)
After the instructor has checked the circuit, put the plug from the primary side into the wall socket.
(3)
Switch on and adjust the variable resistor so that the ammeter reads 0. 1 A. Note down the reading on the voltmeter.
(4)
Repeat step (3) for ammeters reading 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0A. Note the voltmeter reading each time. Do not exceed 1 A. Switch off Draw a graph of load current against output voltage.
Module No. 5: Industrial electronics 2
(5)
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Unit No. 5 - Practical tasks
RESULTS TABLE
LOAD CURRENT (A)
LOAD VOLTAGE (V)
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Questions to be answered to show understanding of the practical task. (1)
How good is the regulation of the 3 leg regulator? -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Module No. 5: Industrial electronics 2
------------------------------------------------------------------------------------------------(2) Find the size of the ripple across the load, if load current is 0.5A. -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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THE THYRISTOR (SCR)
(1)
Connect the circuit as shown using the components supplied.
(2)
After the instructor has checked the circuit, switch on the A supply.
(3)
Sketch the waveforms shown on the oscilloscope, for different settings of the variable resistor. Switch off.
(4)
Place a 10µF capacitor (with the correct polarity) across the 5kΩ resistor. Switch on and prove to yourself that a variable DC level can be obtained when the variable resistor is adjusted.
Module No. 5: Industrial electronics 2
Unit No. 5 - Practical tasks
PRACTICAL TASK 6
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Do your findings agree with the theory given? --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Module No. 5: Industrial electronics 2
Unit No. 5 - Practical tasks
Question to be answered to show understanding of the practical task.
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THE TRIAC
(1)
Connect the circuit as shown using the components supplied.
(2)
After the instructor has checked the circuit, switch on the A and DC supplies.
(3)
Sketch the waveforms seen on the oscilloscope for various settings of the variable resistor.
(4)
Prove to yourself that the output is a variable AC waveform controlled by the timing circuit.
Module No. 5: Industrial electronics 2
Unit No. 5 - Practical tasks
PRACTICAL TASK 7
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Unit No. 5 - Practical tasks
PRACTICAL TASK 8 THE BJT TRANSISTOR AMPLIFIER
Introduction
Module No. 5: Industrial electronics 2
The BC 109 NPN BJT is not an accurate device. The gain (HFE) can vary over a range from about 100 to 400. The circuit shown is thus a compromise. Your friend's results may be different from yours. However, if your circuit works then the results you get are good. (1)
Connect the circuit as shown using the components supplied.
(2)
After the instructor has checked the circuit, switch on the DC supply (9v).
(3)
With a voltmeter check that the collector / earth voltage is about +4.5V. Check the base / emitter voltage is about 0.7V.
(4)
Using an oscilloscope display the output waveform on the screen. Adjust the input signal to get the biggest undistorted signal (good sinewave). Measure the peak to peak value of this output signal. Measure the peak to peak value of the input signal.
(5)
Switch off DC supply and AC sinewave generator. Change the BC 109 for a BC179 (the P type equipment of the BC109).
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Unit No. 5 - Practical tasks
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TRAINING MANUAL INSTRUMENTATION
(6)
Connect the DC supply with reversed connections. Switch on the DC supply and AC generator.
(7)
Repeat step (4). The results should be similar to the BC 109, the only difference is the reversed supply.
RESULTS TABLE
INPUT SIGNAL
OUTPUT SIGNAL
GAIN
BC 109 BC 179
QUESTION to be answered to show understanding of the practical task. 1)
Is the output signal the same as an inverted amplified input signal. -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
2)
What happens if the 1 OK resistor is shorted out? -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Module No. 5: Industrial electronics 2
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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Unit No. 5 - Practical tasks
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THE COMMON COLLECTOR AMPLIFIER
(1)
Connect the circuit as shown using the components supplied.
(2)
After the instructor has checked the circuit, switch on the DC supply
(3)
With an input signal of 1 V p to p, find the p to p value of the output signal on an oscilloscope. Prove to yourself that this circuit has a voltage gain of less than one.
(4)
Using both traces on the oscilloscope, show that the input and output waveforms are in phase.
Module No. 5: Industrial electronics 2
Unit No. 5 - Practical tasks
PRACTICAL TASK 9
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Unit No. 5 - Practical tasks
PRACTICAL TASK 10 THE FET AMPLIFIER
(1)
Connect the circuit as shown using the components supplied.
(2)
After the instructor has checked the circuit, switch on the D supply and the signal generator.
(3)
With an oscilloscope across the output, adjust the input A signal to obtain the largest undistorted signal on the screen.
(4)
Measure the input and output peak to peak signals. Find the gain of the amplifier.
RESULTS TABLE
OUTPUT SIGNAL
GAIN
Module No. 5: Industrial electronics 2
INPUT SIGNAL
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Unit No. 5 - Practical tasks
Questions to be answered to show understanding of the practical task. (1)
Is the output signal an inverted version of the input? --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
(2)
Which device gives the best gain; an FET or a BJT?
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Module No. 5: Industrial electronics 2
------------------------------------------------------------------------------------------------
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TRAINING MANUAL INSTRUMENTATION
MODULE No. 6 INDUSTRIAL ELECTRONICS 3
July 1999- Rev.0
TRAINING MANUAL INSTRUMENTATION
UNIT 1
DIGITAL MATHEMATICS
UNIT 2
INTRODUCTION TO DIGITAL SYSTEMS
UNIT 3
LOGIC GATES, FLIP-FLOPS, COUNTERS AND REGISTERS
UNIT 4
MEMORIES AND CLOCKS
UNIT 5
MULTIPLEXERS, DECODERS AND DISPLAYS
UNIT 6
DIGITAL / ANALOG & ANALOG / DIGITAL CONVERTERS
UNIT 7
THE COMPUTER
UNIT 8
INTRODUCTION TO DIGITAL TRANSMISSION
UNIT 9
PRACTICAL TASKS
Module No. 6: Industrial electronics 3
Unit No. 1 - Digital mathematics
UNITS IN THIS COURSE
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Unit No. 1 - Digital mathematics
TABLE OF CONTENTS Para 1.0
COURSE OBJECTIVE
3
1.1
INTRODUCTION
4
1.2
BINARY NUMBERS
4
1.2.1
The Decimal System
4
1.2.2
The Binary System
5
1.2.3
Binary and Numbers Less Than One
7
1.3
OCTAL NUMBERS
10
1.4
HEXADECIMAL NUMBERS
11
1.5
BINARY CODED DECIMAL, OCTAL AND HEXADECIMAL
13
1.5.1
Binary Coded Decimal
13
1.5.2
Binary coded octal
14
1.5.3
Binary Coded Hexadecimal
14
1.6
Module No. 6: Industrial electronics 3
Page
BASIC BINARY ARITHMETIC
15
1.6.1
Addition
15
1.6.2
Binary Subtraction
16
1.7
BINARY MULTIPLICATION AND DIVISION
19
1.8
ALPHANUMERICAL CODES
20
1.9
CONCLUSION
22
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COURSE OBJECTIVE The student will be able to • Explain the binary system of numbers and convert from binary to decimal and decimal to binary. • Explain octal and hexadecimal number systems and convert these numbers to decimal. Convert decimals to octal and hexadecimal. • Explain binary coded decimals (BCD), binary coded octal and binary coded hexadecimal. • Carry out simple addition and subtraction of binary, octal and hexadecimal numbers using one's and two's directed complements.
Module No. 6: Industrial electronics 3
Unit No. 1 - Digital mathematics
1.0
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Unit No. 1 - Digital mathematics
1.1
INTRODUCTION The aim of this unit is to introduce the mathematics required to understand digital number systems which are now in common use throughout Companies.
1.2 BINARY NUMBERS The binary system is a counting system used by computers. It's different from our normal system of counting because it has a base of two. To understand the binary system it helps to start with our normal counting system and then compare it to the binary system.
1.2.1 The Decimal System Our normal counting system is the decimal system. In the decimal system there are ten digits: 0,1,2,3,4,5,6,7,8,9. In this system, we write numbers as multiples of ten. So, the number 694 means 6 hundreds, 9 tens and 4 ones. We can show ones, tens, hundreds, thousands etc. as powers of ten:
Note: 1.
Any number to the power of zero is 1. So 10°= 1, 5° = 1 694° = 1 and 20° =
Module No. 6: Industrial electronics 3
In the decimal system you can use the ten digits 0 to 9 in any column. The highest number in any column is 9. To write the next number you must use the next column to the left. So, 9 means 9 ones. 10 means 1 ten and 0 ones. 19 means 1 ten and 9 ones, 99 means 9 tens and 9 ones. 100 means 1 hundred, 0 tens and 0 ones. 999 means 9 hundreds, 9 tens and 9 ones, and so on.
Figure 1-1 Try and write one thousand on the table (Figure 1 -1 ). July 1999- Rev.0
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Unit No. 1 - Digital mathematics
1.2.2
The Binary System in _the binary system there are only two digits: 0 and 1. In this system we write numbers as multiples of two. We can show ones, twos, fours, eights etc. as values of two.
Figure 1-2
Module No. 6: Industrial electronics 3
Note: Any number to the power of zero is 1. So, 2° = 1. In the binary system you can only use the two digits 0 or 1 in any column. The highest number in any column is 1. So, 0 in the 2° column means 0 ones which is zero. 1 in the 2° means 1 one which is 1.1 in the 21 column and 0 in the 2° column means 1 two and 0 ones which is 2. 1 and 1 means 1 two and 1 one which is 3. To write 4 you must move to the next column (2 2). 1, 0 and 0 means 1 four, 0 twos and 0 ones which is 4. 5 would be written as 1 four, 0 twos and 1 one; 6 is 1 four, 1 two and 0 ones, 7 is 1 four, 1 two and 1 one and so on. Try and write 8, 9, and 10 as binary numbers on the table.
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Modern electronic gates only have 2 states, on and off, (1 and 0). So, the binary system of numbers is the only one a modern computer understands. The previous examples showed how a binary number can be translated to a decimal number. The next example shows a quick way of changing a-decimal number to a binary number. The trick is to keep dividing the number by two. The binary number is the final quotient, which is one (called the Most Significant Bit (MSB)). It is followed by all the remainders in reverse order.
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Unit No. 1 - Digital mathematics
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Module No. 6: Industrial electronics 3
Unit No. 1 - Digital mathematics
1.2.3
Binary and Numbers Less Than One The previous paragraph explained how binary and decimal numbers are made for whole numbers (e.g. 57, or 10110). This paragraph shows how to write numbers less than one.
The binary conversion to decimal is:
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Module No. 6: Industrial electronics 3
Unit No. 1 - Digital mathematics
A further example is given to show the conversion of a binary number to decimal:
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Unit No. 1 - Digital mathematics
Converting a Decimal Number to Binary You can convert a decimal number to a binary number by continually multiplying by two as follows:
Module No. 6: Industrial electronics 3
The binary number is given by the carry column (from top to bottom).
The check is not exactly the same as the original number because the binary number was rounded down to 6 bits. The conversion is more exact if you use more bits. Your calculator will usually calculate and display to an accuracy of about 10 bits. A good multimeter will calculate and display to 4 bits.
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Unit No. 1 - Digital mathematics
1.3
OCTAL NUMBERS The decimal system has a base of ten which means it uses ten digits. The binary system has base two. The octal system has base eight which means it uses eight digits. The highest in any one column is 7. The octal number 172 can be converted to the decimal system as follows:
Module No. 6: Industrial electronics 3
To change a decimal number to an octal number you keep dividing the number by 8. The octal number is the final quotient (MSB) followed by all the remainders. Converting the decimal number 247 into octal.
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Unit No. 1 - Digital mathematics
1.4 HEXADECIMAL NUMBERS The hexadecimal number system has base sixteen which means it uses sixteen digits. The highest number in any one column is 15. To avoid using two digits eg. 1 and 5) in any one column, the hexadecimal system uses letters to represent the numbers 10 to 15 as follows:
DECIMAL 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
HEXADECIMAL 0 1 2 3 4 5 6 7 8 9 A B C D E F 10 11
A typical hexadecimal number is written below :
Module No. 6: Industrial electronics 3
The hexadecimal number 3AC
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Unit No. 1 - Digital mathematics
TRAINING MANUAL INSTRUMENTATION
To change a decimal number to a hexadecimal number you keep dividing the number by 16. The hexadecimal number is the final quotient (MSB) followed by all the remainders. Converting the decimal number 247 into hexadecimal.
Module No. 6: Industrial electronics 3
Note :
Computers count in twos. However, large binary numbers can be very long. For this reason large numbers are converted into octal and hexadecimal systems. These numbers systems are easy for computers because they are powers of two.
So, octal and hexadecimal numbers can be easily expressed in the binary system using 4 and 8 bit units (bytes). A good scientific calculator will change binary, octal and hexadecimal numbers into decimal and vice-versa. An instrument technician will need a good calculator when programming Programmable Logic Control (PLC) systems at work.
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Unit No. 1 - Digital mathematics
1.5
BINARY CODED DECIMAL, OCTAL AND HEXADECIMAL This is a way of presenting decimal, octal and hexadecimal numbers for binary processing in a computer. The table below shows the decimal (0-17), octal and hexadecimal numbers as they are changed to binary.
DECIMAL 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
1.5.1
OCTAL 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17 20 21
HEXADECIMAL 0 1 2 3 4 5 6 7 8 9 A B C D E F 10 11
BINARY 0 1 10 11 100 101 110 111 1000 1001 1010 1011 1100 1101 1110 1111 10000 10001
Binary Coded Decimal
Module No. 6: Industrial electronics 3
This is used to change the digits in decimal numbers to binary numbers for computer calculations '(eg. putting decimal numbers into a calculator). The decimal number is binary coded as follows :
This code must be reversed by a BCD decoder before it can be shown as a digital read-out (eg. on a multimeter). The binary code for the decimal number is now processed in blocks of FOUR binary BITS called a BYTE.
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Unit No. 1 - Digital mathematics
1.5.2
Binary coded octal This is used to change the digits in octal numbers to binary numbers for computer calculations. The octal number is binary coded as follows.
The binary code for the octal number is now processed in units of 3 binary BITS called a BYTE. This code was popular when processing chips could only manage 4 bits at a time. Modern computer chips can use 32 bits at a time (one byte has 32 bits). This means that hexadecimal systems are mostly used today.
1.5.3
Binary Coded Hexadecimal
Module No. 6: Industrial electronics 3
This is used to change a hexadecimal number (hex) to a binary number for computer calculations. The hexadecimal number is binary coded as follows :
The binary code for the hex number is now processed in blocks of 4 bits. These can be grouped by a modern processor or chip into a BYTE of 32 bits.
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Unit No. 1 - Digital mathematics
TRAINING MANUAL INSTRUMENTATION
1.6
BASIC BINARY ARITHMETIC
1.6.1
Addition In the decimal counting system, the highest number in any column is 9. When you add numbers in the decimal system you "carry one". Carry one means adding one to the next column on the left if the total is more than 9. So:
Module No. 6: Industrial electronics 3
In the binary counting system, the highest number in a column is 1. When you add numbers in the binary system you still "carry one". However, carry one in the binary system means adding one to the next column on the left if the total is more than 1 .
A further example is given below. In the left column there are 3 ones. The sum of the 3 ones is 1 carry 1. In the next column there are now four ones. The sum of four ones is "zero" carry two (1 and 1 in the next column 1011 1001 1011 0010 100001 1111
For every two ones a carry. each carry added to next column
11
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Unit No. 1 - Digital mathematics
1.6.2
Binary Subtraction Binary Subtraction can be done in exactly the same way as with decimals using the method of borrowing one from the next highest power (the next column to the left). However, the number which is borrowed from the next highest power is written as 11 in the next column to the right:
As can be seen this method of subtraction is difficult in binary. it is not used in a computer. The following two methods are used in computers. ONE'S COMPLEMENT One's complement is a method of subtraction which can be done easily in a computer. The binary number to be subtracted is inverted (all the ones become zero and all the zeros become ones). The two numbers are then added together. The digit on the left shows whether the final number is positive (1) or negative (0).
Module No. 6: Industrial electronics 3
Invert the number to be subtracted (the bottom number) and add the two numbers as shown below.
+ 00100
The "1 " in the extra column on the left indicates a POSITIVE number and must be added to obtain the answer:
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Unit No. 1 - Digital mathematics
TRAINING MANUAL INSTRUMENTATION
Example 2:
Subtract 11011 from 10111
Remember, invert the number to be subtracted and add the two numbers together:
The '0' in the first column means it is a NEGATIVE number. If the result is a negative number then the one's complement of the result (invert the result) provides the answer. 11011 inverted = -00100 So, that 10111 - 11011 = -00100
Module No. 6: Industrial electronics 3
Proof :
Note : ONE'S COMPLEMENT IS ONLY APPLIED TO THE NEGATIVE NUMBER.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 1 - Digital mathematics
In the following example (example 2) the result is a negative number (it begins with 1). In this case reverse two's complement is applied to the result.
The reverse two's complement of -1110 = - (0001 + 1) = -0010 Proof :
Module No. 6: Industrial electronics 3
In the following example, both the numbers are negative. So, both numbers are inverted using two's compliment.
2's reverse of - 01100 =-(1 0011 + 1) = -10100 Proof :
Note :- TWO'S COMPLEMENT IS ONLY APPLIED TO THE NEGATIVE NUMBER.
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Unit No. 1 - Digital mathematics
1.7
BINARY MULTIPLICATION AND DIVISION Both multiplication and division are done in the same way as in decimals. The two examples below show how this done. The computer does this process by either continuous addition or subtraction and this need not be remembered. Multiplication:
Module No. 6: Industrial electronics 3
Division:
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ALPHANUMERICAL CODES The binary code for the computer is called the "Machine Code". This is the code the computer (machine) understands. The previous section showed how to get the binary code for numbers. However, a binary code must also be made to represent letters. The standard computer keyboard is arranged in the same way as a typewriter; it has a "qwerty" keyboard. There is an international standard code which "assembles" both numbers and letters from the keyboard into the "Machine Code". This code is called "ASCII" (American standard code for information interchange). It is an 8 bit code which provides 256 characters for upper and lower case alphabets, numbers, punctuation marks, symbols etc. A few examples of this code are given in the table below. The code is given in hexadecimal for convenience. The machine code for the computer is in binary coded Hex. The following examples are given as a reminder.
Module No. 6: Industrial electronics 3
Unit No. 1 - Digital mathematics
1.8
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Module No. 6: Industrial electronics 3
Unit No. 1 - Digital mathematics
Some examples of ASCII (8 Bit code)
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Unit No. 1 - Digital mathematics
There are many other alphanumberic codes used by manufacturers. These codes will have to be learnt during specialised training, e.g. using the TDC 3000 or Foxboro ]A keyboards. You have already learnt one of these codes when using the Autodynamic simulator keyboard. The best known example of a modern alphanumeric code is the "Bar Code" as used on supermarket products. This code allows the price to be taken automatically and is used to keep control of stock. Note:
1.9
Setting the rules for a computer using a "machine code" or "alphanumeric code" would take a very long time. To make things go faster a high level code (language) is used which does whole sentences at one time. These high level languages are learnt by the systems engineers. Then they programme the system to do what is wanted (e.g. PID control, Logic control etc.). Examples of these languages are Fortran, Pascal, C, Unix, etc.
CONCLUSION
Module No. 6: Industrial electronics 3
This unit has shown you the basics of computer mathematics. You have learned the binary, octal and hexadecimal number systems and how to add up and subtract in binary. A computer uses these methods and others to do basic arithmetic. How a computer makes up a number for processing and transmission will be shown on a more advanced training course.
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UNITS IN THIS COURSE UNIT 1
DIGITAL MATHEMATICS
UNIT 2
INTRODUCTION TO DIGITAL SYSTEMS
UNIT 3
LOGIC GATES, FLIP-FLOPS, COUNTERS AND REGISTERS
UNIT 4
MEMORIES AND CLOCKS
UNIT 5
MULTIPLEXERS, DECODERS AND DISPLAYS
UNIT 6
DIGITAL/ANALOG & ANALOG/DIGITAL CONVERTERS
UNIT 7
THE COMPUTER
UNIT 8
INTRODUCTION TO DIGITAL TRANSMISSION
UNIT 9
PRACTICAL TASKS
Module No. 6: Industrial electronics 3
Unit No. 2 - Introduction to digital system
TRAINING MANUAL INSTRUMENTATION
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TRAINING MANUAL INSTRUMENTATION
Para
Page
2.0
COURSE OBJECTIVE
3
2.1
INTRODUCTION
4
2.2
THE ANALOG SIGNAL
4
2.2.1
4
2.3
2.4
Analog Systems
THE DIGITAL SIGNAL
6
2.3.1
8
Digital Systems
ANALOG/DIGITAL COMPARISON
10
2.4.1
Analog
10
2.4.2
Digital
10
2.5
THE 4-20 mA ANALOG CONTROL LOOP
11
2.6
THE DIGITAL CONTROL LOOP
12
2.7
DIGITAL TRANSMISSION
13
2.8
EXAMPLES OF A DIGITAL TRANSMISSION SYSTEM
13
2.8.1
E.S.D Systems
13
2.8.2
Tank Farm Systems
14
Module No. 6: Industrial electronics 3
Unit No. 2 - Introduction to digital system
TABLE OF CONTENTS
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TRAINING MANUAL INSTRUMENTATION
COURSE OBJECTIVE The student will be able to •
Explain with the aid of a diagram an analog signal.
•
Explain with the aid of a diagram a digital signal.
•
Explain with the aid of a diagram an analog instrument loop.
•
Explain with the aid of a diagram a digital instrument loop.
•
Describe, in general terms, how a digital transmission system works.
•
Use block diagrams to explain the use of digital transmission systems in the petroleum industry, e.g. ESD systems and tank farm operations.
Module No. 6: Industrial electronics 3
Unit No. 2 - Introduction to digital system
2.0
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Unit No. 2 - Introduction to digital system
2.1
INTRODUCTION The aim of this unit is to describe the differences between analog and digital systems. It explains the basics of digital transmission, and gives general examples of how digital transmission is used in the petroleum industry.
2.2
THE ANALOG SIGNAL
Module No. 6: Industrial electronics 3
Figure 2-1 Analog Signal (as shown on the oscilloscope) Figure 2-1 shows a simple circuit which produces an analog signal. The variable resistor is placed across a supply. The slider (moveable arm) of the variable resistor is moved by the process variable. The output signal shown on the oscilloscope, changes continuously with time. The output signal depends on the position of the variable resistor slider.
2.2.1
Analog Systems Analog systems are still used because they are easier to make. The sensor itself produces an analog signal. The control valve needs an analog signal. So, it is simplest to use a (4-20 mA) analog signal for the loop. Also people must have analog systems in order to see and hear etc. That's why televisions, radios, and cassette players etc. must produce analog signals.
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Module No. 6: Industrial electronics 3
Unit No. 2 - Introduction to digital system
TRAINING MANUAL INSTRUMENTATION
However, the analog system needs a large bandwidth of frequencies when sending a signal. This reduces the number of signals (television channels, etc.) that can be sent along one cable. Analog systems are also very sensitive to noise and cannot produce a good quality television picture or good quality sound over a radio or cassette player. . Figure 2-2 shows how amplifiers are connected to make an Analog Process Controller.
Figure 2-2 Analog Process Controller
The error detector senses any difference between the set point (SP) and the measured value (MV). It sends a signal to three amplifiers in parallel. The proportional amplifier sets the gain of the system. The Integral reduces any offset to zero. The derivative speeds up the initial change in the system (particularly used for slow response loops). The responses of these three amplifiers are combined and converted to a current signal. This signal adjusts the I/P converter which adjusts the position of the control valve. The action of the three amplifiers is adjusted so that the control valve responds as quickly as possible to changes in the loop, with minimum overshoot. The curve shows this effect using a set point change.
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Unit No. 2 - Introduction to digital system
TRAINING MANUAL INSTRUMENTATION
The response time will be slow (minutes) on most control loops and can be hours on some temperatures loops. Each individuals loop has its own controller. On a large refinery the control room is very large. There are hundreds of individual control loops for the various physical properties being measured (e.g. pressure, temperature, pH, 02 content, etc.)
2.3
THE DIGITAL SIGNAL
Figure 2-3 Digital Signal
Figure 2-3 shows a simple circuit which produces a digital signal. The process variable operates a switch which opens and closes the circuit. The output signal is always the same level but comes as a series of pulses (digital) depending on whether the switch is open or closed. The system has a fixed timing sequence which produces a series of "ONES" and ZEROS" to represent the process variable.
Module No. 6: Industrial electronics 3
A typical example of a digital signal is shown in Figure 2-4.
Figure 2-4 A Digital Signal The process variable timing sequence takes 1 6us There are 8 timing intervals of 2us Each digit ("1") is 1.2~is long. The digital presentation of the process variable is 10111001 Each following 16~is will produce a similar 8 digit sequence which reflects the value of the process variable.
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From previous units it has already been shown that the transistor (FET) is ideal as a high speed switch to provide either a maximum (on) or minimum (off) signal. By manufacturing thousands of these FET switches on an integrated circuit the basics of a digital computer is obtained. However, there are only two states for any one switch. The system is binary so it can only count in twos giving either a one or zero output. Each ONE or ZERO is called a "BIT" of information, a "BYTE" is the word length of the processing system, e.g. 8 bits or 16 bits taken at one time. Note :
Bit rates are an approximation for powers of 2, thus:
Module No. 6: Industrial electronics 3
Unit No. 2 - Introduction to digital system
TRAINING MANUAL INSTRUMENTATION
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Digital Systems Digital systems produce better quality signals than analog systems (e.g. a compact disc player has better sound than a cassette player). The bandwidth is also reduced. There are at least 5 times as many channels on a television when the new digital television signals are used. The picture is also much better. Unfortunately, new television sets are needed to process digital signals. Digital systems are more complicated than analog systems. This is because the analog signals from the sensor must be converted to digital signals using an analog to digital converter (A/D). Digital signals must be converted to analog for the control valve using a digital to analog converter (D/A). Both these devices will be explained later in the course. Digital systems are slowly coming into the petroleum industry as they use less cable and cost less to install. The reduced bandwidth needed for each loop means that many signal loops can be sent from one area to another using one cable or radio link. The digital system of control is based on the micro-computer which operates in micro-seconds. So, using digital techniques it is possible to control all the loops from one place. This means that the operator can use a VDU to control the plant from one position. Therefore, there is no need for a control room with an individual indicator controller recorder for each loop.
Module No. 6: Industrial electronics 3
Unit No. 2 - Introduction to digital system
2.3.1
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Unit No. 2 - Introduction to digital system
TRAINING MANUAL INSTRUMENTATION
Figure 2-5 The Digital Controller Figure 2-5 shows a typical layout of a control system using a microcomputer. It has the following components. a)
Signal Conditioning Cards (SCC): These take the analog signals from the input transmitters etc. and put them into a form acceptable to the data acquisition system (usually 1-5 volts). Typical signals are 4-20 mA, mV (thermo-couples), pulse trains (turbine meters) and resistance (RTD).
b)
Data Acquisition System (DAS): This unit changes all input signals into a digital format which is acceptable to the microcomputer. It also switches the loops for processing.
Module No. 6: Industrial electronics 3
Note:
Modern digital systems combine (a) and (b) in one unit.
c)
Input Ports: These connect the digital input signals from the field to the computer.
d)
Output Ports: These connect the computer to the control devices, e.g. relays, digital/analog converters for 4-20 mA signals, solenoid valves, etc. They also provide the signals for the VDU display.
e)
The Keyboard: This lets the operator communicate with the computer. Using a set program the operator can call up the required loop, display it and change the PI and D configuration as required. This is usually done under engineering supervision.
(f)
The Micro-Computer: This controls the system. Using a set program it will check each loop and adjust the control as required. It has a memory to store each loop's operating data so that it has a reference for each check. Each check only takes micro-seconds so each loop can be checked every hundred milli-seconds or so. The type of micro-computer used depends on the system. The Foxboro !A is based on the Intel 80286 16 bit micro-processor.
(g)
Timer: This is an electronic clock which times the operation of the system. Each operation is in time with a clock pulse. In this way the micro-computer is synchronised with the information coming in and going out.
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Unit No. 2 - Introduction to digital system
TRAINING MANUAL INSTRUMENTATION
2.4
ANALOG/DIGITAL COMPARISON
2.4.1
Analog
2.4.2
Digital Analog technology is well known and has been in use for years. Most sensors use an analog system. So, for single loop and small system control the analog system is cheap and easy to service using standard equipment. However, for larger systems, e.g. chemical plants, oil/gas production platforms etc., The analog system is difficult and expensive to install. It needs a lot of wiring to complete all the loops. It also needs a large control room with many discrete controllers/indicators/recorders which must be supervised by the operator on duty. Analog signals are also very sensitive to noise. Noise degrades the signal and cannot be removed, Digital control systems are only possible because of the development of the integrated circuit. It is much more complicated to process information using a digital system than an analog system. However, once they are installed, digital systems are more reliable than analog systems. They are noise free so there is not much interference from atmospherics, etc. A Distributed Control System (DCS) with modems and fibre optic cables added needs much less wiring. Also, it is cheaper to supervise the control room, if smart sensors are used which control between themselves without using the main computer.
Module No. 6: Industrial electronics 3
Unfortunately, the introduction of digital control means that the instrument technician has to improve his knowledge of electronics and learn a completely new method of control. That is the object of this training module.
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THE 4-20 mA ANALOG CONTROL LOOP
Unit No. 2 - Introduction to digital system
2.5
Figure 2-6 The 4-20 mA Analog Control Loop
Figure 2-6 shows a 4-20 mA analog loop using a microprocessor (µP) controller and Video Display Unit (VDU).
Module No. 6: Industrial electronics 3
The resistance of the transmitter changes as the process variable changes. The variable, analog current signal which is produced is passed through the input of the conditioning unit. This unit converts the analog signal to a digital signal so that the µP controller can process the signal. The digital signal output from the j-LP controller is returned through the output unit. This converts the digital signal back to an analog current signal. The analog signal is converted to a pneumatic signal by the I/P to position the valve.
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THE DIGITAL CONTROL LOOP
Unit No. 2 - Introduction to digital system
2.6
Figure 2-7 The Digital Control Loop
Figure 2-7 is a simplified diagram of the latest type of digital control. The transmitter itself converts the analog sensor signal to a digital signal. The input loop sends only digital signals. These do not need to go through a conditioner so they go directly to the control processor. The output loop sends only digital signals. The valve positioner uses a µP to set the correct position of the valve. The valve actuator is usually either electro/hydraulic or pneumatic in operation.
Module No. 6: Industrial electronics 3
Note:
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There is a half-way system in use called "Smart". You have already worked on these in the workshop. The. "Smart" system is basically analog in operation. The digital signals placed on top of the analog signal are used for calibration.
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Unit No. 2 - Introduction to digital system
2.7
DIGITAL TRANSMISSION There are many methods of organising the digital pulses so that the microprocessor controller can understand the signal that is being sent. The method used is called a "protocol". Most manufacturers now use a protocol called "Hart", developed by Rosemount. The "Smart" transmitters you have worked on use "Hart". Understanding "protocols" and fault finding digital transmission systems will be learnt on advanced courses later in your career.
2.8
EXAMPLES OF A DIGITAL TRANSMISSION SYSTEM The following examples show in simple terms the use of digital transmission systems in the petroleum industry.
2.8. 1 E .S.D Systems
Module No. 6: Industrial electronics 3
Figure 2-8 Digital ESD System
Figure 2-8 shows a typical digital control system used for ESD processing. The local control unit has many field alarms connected to it. The controller in the local control unit puts all the separate input signals into a timed sequence (multiplexed) digital stream. This means that all the input signals can be sent down one cable to the control room. The control room µP de-multiplexes (separates) the digital stream and shows the alarms separately on the alarm panel. This process of sending all the alarm signals digitally down one cable saves kilometres of cable and reduces the time it takes to install the system.
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Tank Farm Systems
Module No. 6: Industrial electronics 3
Unit No. 2 - Introduction to digital system
2.8.2
Figure 2-9 Tank Farm Systems
Figure 2-9 shows a popular tank farm management system for indicating tank levels in a control room. The level transmitters (e.g. ENTIS-ENRAF) process the level of each tank into digital signals. The field communication unit combines these signals and sends a multiplexed signal down one cable to the control room, The µP in the control room de-multiplexes the signal and displays each level separately on the VDU.
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UNITS IN THIS COURSE UNIT 1
DIGITAL MATHEMATICS
UNIT 2
INTRODUCTION TO DIGITAL SYSTEMS
UNIT 3
LOGIC GATES, FLIP-FLOPS, COUNTERS AND REGISTERS
UNIT 4
MEMORIES AND CLOCKS
UNIT 5
MULTIPLEXERS, DECODERS AND DISPLAYS
UNIT 6
DIGITAL/ANALOG & ANALOG/DIGITAL CONVERTERS
UNIT 7
THE COMPUTER
UNIT 8
INTRODUCTION TO DIGITAL TRANSMISSION
UNIT 9
PRACTICAL TASKS
Module No. 6: Industrial electronics 3
Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
TRAINING MANUAL INSTRUMENTATION
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Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para
Page
3.0
COURSE OBJECTIVE
3
3.1
INTRODUCTION
4
3.2
LOGIC GATES
4
3.3
THE SR FLIP-FLOP
7
3.4
THE JK FLIP-FLOP
8
3.5
THE "D" FLIP-FLOP.
10
3.6
THE PRACTICAL JK FLIP-FLOP
11
3.7
COUNTERS
12
3.7.1
The Non-Synchronous Counter; (Ripple Counter)
12
3.7.2
The Synchronous Counter
14
3.7.3
The Decade Counter
15
3.7.4
Counter Chips
16
THE REGISTER (ACCUMULATOR)
17
Module No. 6: Industrial electronics 3
3.8
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COURSE OBJECTIVE The student will be able to: 1
Draw the basic symbols for logic gates to include multiple units.
2)
Draw a typical SR flip-flop.
3)
Make a truth table for an SR flip-flop.
4)
Draw a JK flip-flop and make a truth table to show its operation.
5)
Draw the following counters made from flip-flop's and explain how they count. •
Ripple counter
•
The synchronous counter
•
Decade counter
7)
Draw and explain the register.
8)
Explain the meaning of SISO, SIPO, PISO and PIPO registers.
Module No. 6: Industrial electronics 3
Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
3.0
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INTRODUCTION The aim of. this unit is to review logic gates and the use of these gates to make the building blocks for digital systems.
3.2
LOGIC GATES The following diagrams are given as a review of the logic gates learnt in Unit 2 Industrial Electronics 3. Extra symbols have been added to show the latest symbols which are an American (IEEE) and International (IEC) joint standard.
Module No. 6: Industrial electronics 3
Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
3.1
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Module No. 6: Industrial electronics 3
Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
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The old symbol for a "NOT" function was a circle. The new IEEE/IEC symbols use the triangle to show a "NOT" function. It is possible for symbols to have inverters shown on the input so that:
Most gates today come as multiple units and the new standard symbol for a 2 input quad NAND gate (e.g. 7400) is drawn as follows (the old symbol is shown on the right so that you can compare the two).
Module No. 6: Industrial electronics 3
Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
Note:
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Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
3.3
THE SIR FLIP-FLOP The SR (set-reset) flip-flop is the basic building block for most digital devices, (e.g. memories, registers, converters, etc.). It will keep either a "V' (high) or "0" (low) as a continuous output. The output will not change unless the input is changed.
Figure 3-1 SR Flip - Flop Block Diagram
Figure 3-1 shows an SR flip-flop in block diagram form. The SR flip-flop has two input connections; S and R. It also has two output connections; Q and Q1 (Q1 = not Q or opposite of Q). Normally there is a " 1 " on S and “0" on R. The outputs are " 1 on Q and “0" on Q1. If the " 1 " on S is removed nothing happens. If a " 1 " is now applied to "R" the flip-flop changes over. This means that the Q output is "0" and Q1 output is " 1 ". If there is a " 1 " on both R and S the flip-flop may change its position or it may not. This is called the "undefined" position and is not allowed.
Module No. 6: Industrial electronics 3
A simple truth table below is given to shows its operation.
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TRAINING MANUAL INSTRUMENTATION
Module No. 6: Industrial electronics 3
Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
3.4
THE JK FLIP-FLOP The SR flip-flop as a chip is now obsolete and is little used today. The JK flip-flop has taken its place. The JK flip-flop has the advantage of having no "undefined" position. It can be connected as an SR flip-flop if required. The JK flip-flop can be operated in two ways; synchronous or asynchronous. The two words are often used in digital systems and you must learn what they mean. Synchronous means the system runs like a clock. There are a fixed number of pulses per second (e.g. 4M bit). The pulses from the clock and the changes in the input both change the outputs. Asynchronous means the system runs without timing like the SR flip-flop. You can only change the output by changing the input.
Figure 3-2 JK Flip-Flop Block Diagram
Figure 3-2 shows the JK flip-flop. The pulses from the clock can switch the flip-f lop. With a " 1 on the J terminal and a '0' on the K terminal, the outputs are Q = “1 " and Q1 = 0. If there is a “0” on the J terminal and a "'I " on the K terminal then the outputs reverse. If a " 1 " is applied to both J and K then the outputs follow the clock with the Q1 output out of phase with the Q output. This position is called a "toggle".
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The JK flip-flop has the advantage of having no "undefined" position. It can be used as an SR flip-flip simply by not connecting the clock. Note:
The above flip-flops can be made using either "NAND" or "NOR" gates. The actual circuit need not be remembered as these devices are manufactured as IC chips. You are not given information about how it is made. An example is the CMOS 4027 dual JK flip-flop.
Module No. 6: Industrial electronics 3
Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
The truth table for a JK flip-flop is given together with the pulse diagram
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Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
3.5
THE "D" FLIP-FLOP. This is an adaptation of the JK flip-flop. It allows for the switching of one input only. It delays the passing of the input signal by one clock pulse. That's why it's called the "Delay" or M" flip-flop.
Figure 3-3 M" Flip-Flop Block Diagram
Module No. 6: Industrial electronics 3
Figure 3-3 shows the block diagram for a "D" flip-flop. There is only one input. An "on chip" inverter reverses the D input signal to the K input. The output at Q will be a "1' if D is "1' and "0" if "D" is zero. The output changes at the end of the clock pulse so that a change in D is delayed by one clock pulse. "D" flip-flops are like JK flips-flops as they are manufactured in DIL packages, e.g. CMOS 4013 dual D flip-flop.
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Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
3.6
THE PRACTICAL JK FLIP-FLOP
The manufactured JK flip-flop comes with extra connections; "preset" and "clear". Set :
Pre-set
= 0: Clear
=
1:
therefore
Q
=
1
Reset: Pre-set
= 1: Clear
=
0:
therefore
Q
=
0
Toggle: Pre-set
= 1; Clear
1:
therefore
=
?
Note
:
Q
Pre-set 0 and Clear 0 is not allowed.
The output Q depends on the data at J and K and the clock. It is the same as in a JK flip-flop.
Module No. 6: Industrial electronics 3
JK flip-flops come in two types. Type 1 switches on the positive going edge of the clock pulse. Type 2 switches on the negative going edge of the pulse. The JK flip-flop described was of the negative edge type.
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Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
3.7
COUNTERS These are integrated circuits. When they are connected to a data stream of bits, they count them.
3.7.1
The Non-Synchronous Counter; (Ripple Counter) The simplest type of counter is called a RIPPLE COUNTER. The data passes through the device like waves hitting the sea shore. Figure 3- shows a typical ripple counter using 4 JK flip-flops.
Figure 3-4 Typical Ripple Counter
Module No. 6: Industrial electronics 3
Remember all the J and K inputs are at "1'. Each JK flip-flop gives an output signal as it receives a pulse from the clock terminal. The table below gives the count and the waveform for each particular JK output.
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A ripple counter is only used if speed is not important. This is because it may take too much time for the clock pulse to ripple through several stages. The time it takes for the ripple to pass can be shortened by using a synchronous counter. In this system all the flip-flops are switched at the same time.
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Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
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Module No. 6: Industrial electronics 3
Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
3.7.2
The Synchronous Counter Figure 3-5 shows a typical synchronous counter. Note that the inputs J and K on flip-flop A are ONE and all other flip-flop X inputs are joined together and controlled by the AND gates X and Y. The waveforms of the 4 outputs are given below.
Figure 3-5 Synchronous Counter
Note:
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The output at QD comes after 16 input bits. Thus the circuit can be used to divide things by 16. In the same way Qc will divide by 8, QB by 4 and QA by 2.
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The Decade Counter The most popular type of decade counter (a device which counts in tens) uses flip-flops with a reset connection. If a ONE is removed from this connection the output goes to zero. An example of the use of this type of flip-flop is given in Figure 3-6.
Figure 3-6 The Decade Counter
The counter works as follows: All the J and K inputs are connected to the high " 1 " position (e.g. 5V). The data to be counted is applied to the clock connection of flip-flop A.
Module No. 6: Industrial electronics 3
Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
3.7.3
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Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
3.7.4
Counter Chips The previous pages have shown how individual JK flip-flops are connected to make counters. There is no need to remember how this is done as they are manufactured as IC chips. A typical example of a counter chip is the 7493 14 pin TTL 4 bit binary counter. The pin connections are the same for every manufacturer. They are given in standard electronic data books. Normally the data book shows the basic block diagram of how the chips are connected. It also shows how to connect them. An example is shown in Figure 3-7.
Module No. 6: Industrial electronics 3
Figure 3-7 Pin Connections for a 7493 4 Bit Binary Counter
If pin 12 is connected to pin 1 it is a 4-stage ripple counter. R1 and R2 are re-set connections. A " 1 " applied to both positions clears the counter. This can be turned into a decade counter by connecting pin 9 to pin 2, pin 11 to pin 3, and pin 12 to pin 1.
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Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
3.8
THE REGISTER (ACCUMULATOR) A register (accumulator) is a temporary storage device for data coming into or out of the processor. It also stores data which is not completely processed. When it is connected in the right way it can take this stored data and switch it to either a parallel or a serial output. It is the basic block for storing a digital number and moving it about. Figure 3-8 shows a typical register using JK flip-flops.
Figure 3-8 Register
This unit will store a 4 bit binary number as follows. At the start, the reset has cleared the register. QA
= 0,
QB
= 0,
QC
= 01,
QD = 0
Module No. 6: Industrial electronics 3
Let us suppose that the number to be stored is 1101. This is fed into the data input in the order that it reads from left to right. The register will then store the number as follows.
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If the clock is now stopped the accumulated number will stay in the register as long as the supply stays switched on. So, this unit can be a 4 bit memory circuit. If the clock is started again then the data will leave the output (QD) in the order it was entered.
This kind of register can be used to hold a number so it can be processed later. It is then called an ACCUMULATOR. If the data is moved in and out it is called a SHIFT REGISTER. The shift register shown moves the data in and out in serial form (one after the other). However, shift registers can be used to provide a parallel output (all the data comes out at the same time). Thus the shift register can come in 4 different forms, depending on requirements, as follows.
Module No. 6: Industrial electronics 3
Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
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Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
1)
SISO (Serial in-Serial out) This is the method shown above. It acts as a short term storage device or a delay circuit. The data can only be accessed in the order in which it is stored. The first bit of data IN must be the first bit of data OUT.
2)
SIPO (Serial in-Parallel out) The information is stored as described above. However, when the information is accessed by the next clock pulse, all 4 bits of data come out of the outputs at the same time. It is used to change information from serial to parallel form.
SIPO Shift Register
3)
PISO (Parallel in-Serial out)
Module No. 6: Industrial electronics 3
This is the opposite of SIPO. After a clock pulse is applied, the next clock pulse will apply the 4 bits (byte) to all flip-flops simultaneously (at the same time). The following 4 clock pulses will then read out the number in serial form.
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Module No. 6: Industrial electronics 3
Unit No. 3 - Logic gates, Flip-flops, Counters and Registers
4)
PIPO (Parallel in-Parallel out)
The PIPO acts as a storage device but keeps and processes the byte in parallel form.
PIPO Shift Register
Special Devices 1.
There are counters which will count in both directions. They are called "Up/Down" counters.
2.
Some shift registers will move out in both directions. These are called "Shift Right/ Shift Left" registers.
3.
Very high speed systems use special flip-flops called "Master/Slave Flip-Flops". The external connections are the same as an ordinary JK flip-flop.
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UNITS IN THIS COURSE UNIT 1
DIGITAL MATHEMATICS
UNIT 2
INTRODUCTION TO DIGITAL SYSTEMS
UNIT 3
LOGIC GATES, FLIP-FLOPS, COUNTERS AND REGISTERS
UNIT 4
MEMORIES AND CLOCKS
UNIT 5
MULTIPLEXERS, DECODERS AND DISPLAYS
UNIT 6
DIGITAL/ANALOG & ANALOG/DIGITAL CONVERTERS
UNIT 7
THE COMPUTER
UNIT 8
INTRODUCTION TO DIGITAL TRANSMISSION
UNIT 9
PRACTICAL TASKS
Module No. 6: Industrial electronics 3
Unit No. 4 - Memories and clocks
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TABLE OF CONTENTS Para 4.0
COURSE OBJECTIVE
3
4.1
INTRODUCTION
4
4.2
MEMORIES
4
4.2.1
Introduction
4
READ ONLY MEMORIES (ROMS)
4
4.3.1
7
4.3
4.4
4.5
4.6
Module No. 6: Industrial electronics 3
Page
EPROM's and EEPROM's
RANDOM ACCESS MEMORIES (RAM)
8
4.4.1
Static RAMs (SRAM)
8
4.4.2
Dynamic RAMs (DRAM)
8
HARD DISKS AND FLOPPY DISKS
10
4.5.1
Hard disk
10
4.5.2
Floppy Disk
12
4.5.3
Magnetic Data Readers
13
CLOCKS
14
4.6.1
The Multivibrator
14
4.6.2
The Mains Supply and a Schmitt Switch
15
4.6.3
The 555 Timer
17
4.6.4
The Crystal Oscillator
19
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4.0
COURSE OBJECTIVE The student will be able to •
Explain the terms ROM and CD ROM.
•
Explain the terms PROM, EPROM and EEPROM.
•
Explain the terms RAM, SRAM and DRAM.
•
Explain the terms Hard disk and Floppy.
•
Explain the use of a Streaming Tape,
•
Explain the terms Hardware, Firmware and Software.
•
Sketch a typical Multi-Vibrator and show how the timing can be changed.
•
Sketch the circuit of a 555 timer, and show how the timing can be changed.
•
Sketch a typical Crystal Clock and explain its frequency of operation.
Module No. 6: Industrial electronics 3
Unit No. 4 - Memories and clocks
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4.1
INTRODUCTION The aim of this unit is to explain in basic terms the memories and clocks used in a computer.
4.2
MEMORIES
4.2.1
Introduction Digital memories are units which store information. The information is for use when checking control loops, storing operating programmes (PI and D settings), or remembering numbers during calculations etc. This unit will introduce the basic concepts of how digital memories are made.
4.3
READ ONLY MEMORIES (ROMS)
Module No. 6: Industrial electronics 3
A ROM is a permanent memory. It cannot be changed by the operator. It has data written into it which can be read from the memory but it cannot be removed. The data cannot be destroyed even if the supply is switched off.
Figure 4-1 Simple Diode Operated ROM
Figure 4-1 shows a simple diode operated ROM. The programmer positions the diodes to link the rows and columns into a matrix. In this program there are three address lines and 4 output data lines. With the diodes in the positions shown the program operates as follows.
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If there is no diode linking the row to the column, the output is high (1). If there is a diode linking the row to the column when the row is earthed, the output is low (0). The programmer can change this program by moving the diodes to new Positions. The resistors are added to limit the current through the diodes. The data output for a ROM with diodes as shown in Figure 4-1 is given below:
This type of memory is usually called HARD WIRED. The program can only be changed by manually changing the position of the diodes. For example, if diode A is moved to position B (see Figure 4-1) then the data output for address 101 will change from 1110 to 0111.
Module No. 6: Industrial electronics 3
This type of memory is still used in ESD systems (e.g. Thorn-EMI fire and gas detectors). This gives maximum safety as only an instrument engineer can move the diodes to change the programme. The keyboard operator cannot change the ROM program by mistake. The diode matrix shown in the diagram is also manufactured as an integrated circuit. However bipolar or field effect transistors are usually used to link the rows and columns (not diodes). It works in the same way but the switch is more accurate and faster. This type of fixed memory is used when a manufacturer intends to mass produce a device, eg. a calculator. You use the keyboard to address the set programme of the memory. The answer is displayed on a screen.
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Another kind of ROM is the Programmable Read Only Memory (PROM). These come with all the rows and columns linked by transistors so that all the outputs are set at "0" as shown below.
The user can then program the device by selecting different line/column junctions. When you pass enough current through the junction it will blow the fuse link, and change the output from 'TO" to “1” There are several TTL PROM's available e.g. 74186 (64 x 8) and 74470 (256 x 8). There are also devices to program them. Note: 64 x 8 means 64 output words of 8 bits. 64 = 26 so you need 6 address lines to get the correct output words.
Module No. 6: Industrial electronics 3
PROM's are popular with manufacturers of washing machines, ovens, etc. They allow flexibility in design.
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4.3.1
EPROM's and EEPROM's Both these devices store a charge. The MOSFET insulation gate is used like a capacitor. It either holds a charge: " 1 " or it doesn't hold a charge; "0". The EPROM (Erasable Programmable ROM) has a window in the top. By shining ultraviolet light through the window the program in the chip can be erased. Then the chip can be reprogrammed. The basic problem with this type of chip is that all the program must be destroyed before any changes can be made. The EEPROM (Electrically Erasable Programmable ROM) is a device which is programmed using an electrical signal. A voltage of about 21 volts applied for about 10ms will erase a single bit of the programme. This device is must easier to use than the EPROM, as no ultraviolet light is required. You do not need to remove it to reprogram it. Examples of the above chips are: EPROM
Intel 2716
EEPROM
Intel 2817
EEPROM's provide the operating program in many of the PLC systems used in the field. You will see these and program them when you practice basic configuring of the Allen Bradley PLC in the workshop. Note The latest type of EEPROM is called a FLASH memory. This is an EEPROM which can be erased and programmed while it's in place. However, all the memory is removed when changes are required. The memory is removed in a "flash". An example is the INTEL N 28FO10-200 chip. This will remember 131072 words (word length 8 bit) sometimes written as a 132 k byte memory.
2.
PROMS, EPROM's and EEPROMS are programmed using special devices supplied by the manufacturer. Normally the engineer programmes these chips in the office. Then you change the whole chip in the field.
Module No. 6: Industrial electronics 3
1.
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4.4
RANDOM ACCESS MEMORIES (RAM) As discussed in the previous section the ROM will store data permanently, (it is non-volatile). However changing the programme is difficult, if not impossible. EPROM's and EEPROM's can be erased and rewritten but they cannot be changed at normal operating speeds. Therefore, another type of memory is required which can be re-programmed as required. These memories are called read/write or Random Access Memories (RAMs).
4.4.1
Static RAMs (SRAM) The simplest RAM is the flip-flop which will store 1 bit of data. FlipFlops connected to make a resister can store a word. Connecting resisters together will produce a very simple memory. The TTL 74170 4x4 register contains 16 flip-flop's. It can store 4 words of 4 bits each. The TTL type RAM cannot be made very large because it loses a lot of power (heat). Normally MOSFET technology is used to make what are called "STATIC RAMs". These static RAMs contain thousands of flip-flop's on one IC chip. This produces a large random access memory. A typical static RAM is the Toshiba CMOS TC 55257 BPI-102. This will remember 1 32k byte and pass them out 8 bits at a time. This chip comes in a 28 pin DIL package so you can connect it to a PCB.
Module No. 6: Industrial electronics 3
4.4.2 Dynamic RAMs (DRAM)
Single Transistor Dynamic RAM Storage Cell
Dynamic RAMs are devices which can store a charge. A single cell is shown above. A charge in the capacitor indicates a "ONE". These devices can only store the information for about a milli-second, so that the data must be refreshed (rewritten after a short period of time). These devices are popular because each cell area is small so a large memory can be created in a single device. An example is the Motorola 65536 (64k) x 1 high speed dynamic RAM. This will store 64k bits but only send them out one at time.
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Module No. 6: Industrial electronics 3
Unit No. 4 - Memories and clocks
Note 1)
Dynamic RAMs need a lot of external hardware. For example, they need a counter, a clock for the counter and a mulitplexer to send row addresses, column addresses and a refresh count. They are still popular because dynamic RAMs are much cheaper per bit of information stored,
2.
Chip manufacturers are developing larger and faster static and dynamic RAMs all the time. Any electronic magazine will give you the latest developments.
Static and dynamic RAMs are "volatile" memories. If you switch off the power, the data is lost. Therefore, most systems have standby batteries or an Uninteruptable Power Supply (UPS) system. This provides power if mains power is lost. The Motorola 64 K DRAM comes in a 16 PIN DIL package with pin connections as shown.
This type of memory is used in most personal computers. It is also the type of memory used when changing the operating conditions for a loop.
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HARD DISKS AND FLOPPY DISKS Static RAMs and Dynamic RAMs are used inside computers. They remember changes that the typist makes on the P.C. or the operator makes on a control loop. However, these devices soon become full. Therefore, other types of RAM are used to store large' quantities of information which must be kept for a long time. These are called hard disks and floppy disks (floppies).
4.5.1
Hard disk
Module No. 6: Industrial electronics 3
Unit No. 4 - Memories and clocks
4.5
Figure 4-2 A Typical Hard Disk Drive Unit
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Figure 4-2 shows the layout of a hard disk drive unit. It consists of a number of magnetic disks. These store the information in much the same way as a music cassette tape. Information can be written on or read off the disk electronically. The electronics drive motors. The motors set the position of the disk and the position of the reading head so it will process the required data. The hard disk drive used in the office P.C. will store about 0.5 G bytes of information. The hard disk will hold all the necessary information. This information can be changed and rewritten as required. The hard disk unit on a DCS in a plant is called the HISTORIAN. The historian can be very large (many Giga bytes). It will hold and update information about the plant over many weeks. The hard disk will also hold the software programmes. These are programs which are designed to do particular jobs such as typing, making diagrams, doing calculations, etc. Windows 95 is a good example of a modern software system.
Module No. 6: Industrial electronics 3
A DCS system has software specially made to show the PFD's for the plant on the VDU. These software programmes are supplied by the manufacturer to fit your plant and are stored on the system hard disk.
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Floppy Disk
Unit No. 4 - Memories and clocks
4.5.2
Figure 4-3 Floppy Disks (Diskettes)
Figure 4-3 shows 2 typical floppy disks. These disks come in various sizes (e.g. 8 in., 51/4 in., 3.5 in., etc.). They are used to store information that is no longer needed on the hard disk. When this course was written it was kept on a hard disk until it was finished. Then it was put onto a floppy and kept until it was needed. The floppy also acts as a back-up for the hard disk in case of hard disk failure. A DCS system keeps the plant programmes on floppy disks as a back-up for the hard disk drive. Also, the engineer may have his own floppies which he has programmed himself. These may be used to run a maintenance schedule to check the system micro-processors etc.
Module No. 6: Industrial electronics 3
A floppy disk drive works in much the same way as a hard disk drive. The difference is that you must insert the disk (the floppy) into the disk drive yourself.
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Unit No. 4 - Memories and clocks
4.5.3 Magnetic Data Readers
The diagram shows the basic principle of a magnetic data reader. The tape/disk has a thin coating of magnetic iron oxide. When a current is passed through the coil a flux is produced across the gap in the iron core. This magnetises the tape/disk so that a ONE or ZERO is recorded at that particular position. The digit depends on the direction of the current (i.e. the polarity of the induced magnet). The system is reversible. The magnetised tape/disk is passed over the head. As magnetised areas on the tape pass under the tape head gap, a magnetic field is formed in the soft iron core. The variations in the field induce voltages across the coil. These voltages are amplified to produce ONE's and ZERO's according to the data stored on the tape.
Module No. 6: Industrial electronics 3
Note: 1.
The latest type of programme storage method is the Compact Disc Read Only Memory (CD ROM). This is the same type of disk as is used in a CD in a home music system. These are read using a light source (laser). They are better than the old magnetic disks because the data cannot be damaged by stray magnetic fields.
2.
In some systems the RAM is a cassette type tape unit. These are common in large computer facilities as they can store Tera bytes of data. Foxboro uses them as back-up units for the main hard drives. Foxboro calls them "Streaming Tape" drives.
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Unit No. 4 - Memories and clocks
4.6
CLOCKS
4.6.1 The Multivibrator The basic clock or timer for digital work is the free running flip-flop (multi-vibrator). This is easily made using two BJTs as shown in Figure 4-4.
Figure 4-4 The Multi-Vibrator
Module No. 6: Industrial electronics 3
OPERATION: When the circuit is turned on let's assume that T1 conducts more than T2. The collector of T1 will fall and the resulting negative charge is transferred through C1. This will tend to cut off T2 and produce a rise in T2 collector. This rise will drive T1 further on so that T1 will go "FULL ON" and T2 "OFF" almost immediately. This state will not last. The charge of C1 will immediately drain through R2 and RA until the voltage on T2 base reaches the "CUT ON" point. At this point T2 Will "CUT ON". When T2 Cuts on its collector voltage will fall. This fall will "CUT OFF" T1. This state is also unstable as C2 drains through R, and RB, so that T1 will again "CUT ON" as T2 "CUTS OFF". This operation continues as long as the supply is switched on. The outputs from the two load resistors are opposing square waves as shown below. The frequency of operation is approximately 1/2 C (R + RA) Hz (if C1 = C2, RA = RB & R1= R2). If the transistors are the same the output will have an even mark to space ratio
This circuit is, not very stable and operates only at low frequencies. However, it is a very simple and cheap clock.
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The Mains Supply and a Schmitt Switch In some countries the frequency of the mains supply is very stable all day. So, it can be used as a timer for electronic clocks, etc. The diagram below shows a circuit used to obtain 50 Hz T.T.L level pulses from the mains supply.
Module No. 6: Industrial electronics 3
Unit No. 4 - Memories and clocks
4.6.2
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The Schmitt switch is a very useful device. You can buy it as T.T.L (7414) or CMOS (4554). It has two trigger levels. One level is for positive going signals and one is for negative going signals. The Schmitt switch is usually shown with a "NOT" output. This means the output is high when the input is low. Another important use for the Schmitt switch is for telephone and radio signals etc. The digital signals are distorted as they pass from the transmitter to the receiver. The Schmitt switch can be used to restore the distorted signal. The diagram below shows this using a 7414.
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Unit No. 4 - Memories and clocks
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Unit No. 4 - Memories and clocks
4.6.3
The 555 Timer The 555 timer is an IC which is good for low frequency non-precision timing work. It comes in either T.T.L (LM 555) or CMOS (LM7555). The basic circuit is given in Figure 4-5.
Module No. 6: Industrial electronics 3
Figure 4-5 Basic Circuit of 555 Timer
The chip consists of an RS flip-flop with reset, upper and lower comparators and a constant current discharge transistor.
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Unit No. 4 - Memories and clocks
OPERATION: 1.
The three 5 kΩ resistors inside the chip set the control voltages for the upper and lower comparators. When the unit is switched on (+Vcc applied), the RS flip-flop is reset so that Q is low.
.
External capacitor C charges through RA and RB. When the voltage across C (trigger) reaches the control voltage of the lower comparator, the comparator output changes over. The change applied to the terminal of the RS flip-flop switches the output Q to High. This change switches on the discharge transistor.
3.
The capacitor C will now discharge through this transistor. At a certain low voltage the threshold of the upper comparator is reached. At this point the output to the R terminal of the R/S flipflop output changes back to its starting position. Then C starts to charge again.
4.
The operation is continuous as long as the VCC is connected.
The output is a series of timed pulses depending on the charge time of the capacitor.
Module No. 6: Industrial electronics 3
The frequency of the pulse depends on the added components RA, RB and C where
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Unit No. 4 - Memories and clocks
4.6.4
The Crystal Oscillator Most computers must have a more accurate timing system than the timers described in this unit. This is achieved using a "quartz crystal" such as the ones used in top quality watches. A "quartz crystal" is a specially cut crystal that has a "pieso-electric" effect. At a particular frequency it will vibrate to produce an electrical signal and vice-versa. In effect it is a very accurate tuned circuit. It's frequency of resonance depends mainly on the size of the crystal and the way it is cut. A typical 1 MHz TTL crystal oscillator is given in Figure 4-6. The crystal provides positive feedback at its resonant frequency. This means the circuit will oscillate at one frequency only. The crystal sets the frequency.
Module No. 6: Industrial electronics 3
Figure 4-6 1 MHz Crystal
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UNITS IN THIS COURSE UNIT 1
DIGITAL MATHEMATICS
UNIT 2
INTRODUCTION TO DIGITAL SYSTEMS
UNIT 3
LOGIC GATES, FLIP-FLOPS, COUNTERS AND REGISTERS
UNIT 4
MEMORIES AND CLOCKS
UNIT 5
MULTIPLEXERS, DECODERS AND DISPLAYS
UNIT 6
DIGITAL/ANALOG & ANALOG/DIGITAL CONVERTERS
UNIT 7
THE COMPUTER
UNIT 8
INTRODUCTION TO DIGITAL TRANSMISSION
UNIT 9
PRACTICAL TASKS
Module No. 6: Industrial electronics 3
Unit No. 5 - Multiplexers, Decoders and Displays
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TABLE OF CONTENTS Para
Page
5.0
COURSE OBJECTIVE
3
5.1
INTRODUCTION
4
5.2
MULTIPLEXERS
4
5.3
DECODERS (DE-MULTIPLEXERS)
5
5.4
THE BCD DECODER
5
5.5
THE SEVEN SEGMENT DISPLAY
6
5.6
THE LIQUID CRYSTAL DISPLAY.
8
5.7
MATHEMATICAL NOTE
9
Module No. 6: Industrial electronics 3
Unit No. 5 - Multiplexers, Decoders and Displays
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COURSE OBJECTIVE The student will be able to •
Explain the purpose of a multiplexer
•
Explain the purpose of a decoder
•
Describe the LED and LCD 7 segment display
•
Explain the use of the 7 segment code.
Module No. 6: Industrial electronics 3
Unit No. 5 - Multiplexers, Decoders and Displays
5.0
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INTRODUCTION The aim of this unit is to explain the purpose of multiplexers, decoders and displays.
5.2
MULTIPLEXERS Multiplexers are devices which combine different analog or digital data signals into groups. This data can then be transmitted over a single cable. Multiplexers are very complicated devices. They are normally serviced and operated by the telecommunication department. However, simple single chip multiplexers are found in some instruments. A typical example is the MAX. 378 which will multiplex 8 analog signals for onward transmission. One type of multiplexer is a device called a "UART". The UART (Universal Asynchronous Receiver Transmitter) takes signals from the µP system in parallel form and changes them to a serial form for transmission. It will also, receive serial input signals and convert them to parallel 'for µP processing. Figure 5-1 shows in simple terms the use of a UART.
Figure 5-1 UART
Module No. 6: Industrial electronics 3
Unit No. 5 - Multiplexers, Decoders and Displays
5.1
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Unit No. 5 - Multiplexers, Decoders and Displays
5.3
DECODERS (DE-MULTIPLEXERS) Decoders (de-multiplexers) take in a stream of data. They convert the data so that it can be displayed or processed by a µP. There are various types of decoder (de-multiplexer) depending on their use.
5.4
74138 :
This decoder/de-multiplexer takes in 3 input data lines and produces 8 output lines.
74154:
This decoder/de-multiplexer takes in 4 input data lines to provide 16 output lines. This chip is used in your workshop project.
THE BCD DECODER An important decoder is the BCD decoder. This converts BCD into a special code to drive a digital display (seven segment display). A block diagram of how a digital display is done is shown below.
Module No. 6: Industrial electronics 3
The decade counter counts the input signal pulses and provides a 4 line address in BCD to the "BCD to 7 segment.7 decoder. The output from the decoder is amplified by the driver to operate the display.
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Unit No. 5 - Multiplexers, Decoders and Displays
5.5
THE SEVEN SEGMENT DISPLAY This is the standard method for digital displays on all electronic indicators. The two common types of seven segment display use either LEDs or an LCD as follows:
Figure 5-2 The LED 7 Segment Display
Figure 5-2 shows a typical 7 segment LED display. Seven LED's are shaped and placed in an insulating substrate (usually a ceramic). The position and-connection of each lettered LED is standard. Therefore, all LEDs will have "a" at the top and "d" at the bottom and so on. The device shown has the anodes of the LEDs connected together (common anode). The LED is lit by the BCD decoder-driver which earth different cathodes as required.
Module No. 6: Industrial electronics 3
The LED display can also be obtained with all the cathodes connected together (common cathode). In this case a positive voltage Is provided by the BCD decoder-driver which lights the correct LED.
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Figure 5-3 Digital Display of Numbers using Seven Segment Code
Figure 5-3 shows the illumination required to make a seven segment digital display. For example, to display the number 3 - a,b,c,d and g must light, but f and e must not light.
Module No. 6: Industrial electronics 3
Unit No. 5 - Multiplexers, Decoders and Displays
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Unit No. 5 - Multiplexers, Decoders and Displays
5.6
THE LIQUID CRYSTAL DISPLAY. This is the display usually used by a calculator. The seven segment code is used for the display in the same way as an LED. However, the method of illumination is different, as follows:
Figure 5-4 The Basic LCD Display
Module No. 6: Industrial electronics 3
Figure 5-4 shows a simplified diagram of an LCID display. The liquid crystal is sealed between two plates of glass. The segments are set into the liquid. When a voltage is applied to the segment it produces an electrical field across the liquid. This makes it absorb light and so the segment looks black. When the voltage is removed it will reflect light and so it looks white again. In the diagram a b and c have a voltage applied (black),f, e, g and d have no voltage applied (white). The number 7 is displayed as a black figure on a white background.
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Module No. 6: Industrial electronics 3
Unit No. 5 - Multiplexers, Decoders and Displays
5.7
MATHEMATICAL NOTE ADDING BCD There is a problem when adding BCD as a 4 bit binary code. It will produce numbers from 0000(0) to 1111 (15). However, decimal will only allow 0000(0) to 1001 (9) in any one column. After this the number must revert to 0000 and carry. Therefore, adding two BCD numbers is complicated. The calculator actually uses the following method.
Note:
July 1999- Rev.0
Subtraction is done using ONE's complement and adding.
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UNITS IN THIS COURSE
UNIT 1
DIGITAL MATHEMATICS
UNIT 2
INTRODUCTION TO DIGITAL SYSTEMS
UNIT 3
LOGIC GATES, FLIP-FLOPS, COUNTERS AND REGISTERS
UNIT 4
MEMORIES AND CLOCKS
UNIT 5
MULTIPLEXERS, DECODERS AND DISPLAYS
UNIT 6
DIGITAL/ANALOG & ANALOG/DIGITAL CONVERTERS
UNIT 7
THE COMPUTER
UNIT 8
INTRODUCTION TO DIGITAL TRANSMISSION
UNIT 9
PRACTICAL TASKS
Module No. 6: Industrial electronics 3
Unit No. 6 - Digital / Analog & Analog / Digital Converters
TRAINING MANUAL INSTRUMENTATION
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TABLE OF CONTENTS Para
Page
6.0
COURSE OBJECTIVE
3
6.1
INTRODUCTION
4
6.2
D/A AND A/D CONVERTERS
4
6.3
BINARY WEIGHTED RESISTOR DAC
4
6.3.1
5
6.4
R/2R Ladder DAC
ANALOG TO DIGITAL CONVERTER (ADC)
6
6.4.1
Ramp Type ADC
6
6.4.2
Successive-Approximation ADC
7
Module No. 6: Industrial electronics 3
Unit No. 6 - Digital / Analog & Analog / Digital Converters
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COURSE OBJECTIVE The student will be able to: •
Explain the use of a DAC
•
Explain with a simple diagram the binary weighted resistor DAC.
•
State the advantages of an R/2R DAC.
•
Explain using a block diagram a modern DAC chip.
•
Explain the use of an ADC.
•
State the advantages of a successive approximation ADC over a ramp type ADC.
Module No. 6: Industrial electronics 3
Unit No. 6 - Digital / Analog & Analog / Digital Converters
6.0
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Unit No. 6 - Digital / Analog & Analog / Digital Converters
6.1
INTRODUCTION The aim of this unit is to explain the Digital to Analog Converter (DAC) and Analog to Digital Converter (ADC).
6.2
D/A AND A/D CONVERTERS Digital to Analog Converters (DAC) and Analog to Digital Converters (AD C) are an essential part of digital control. The DAC turns digital control signals into analog signals for 4-20mA output loops, etc. The ADC turns analog signals from the input loops (mA, mV, resistance, etc.) into digital signals for computer processing. The following notes describe the techniques used. remember these converters are obtained as an I.C (not as discrete components
6.3
BINARY WEIGHTED RESISTOR DAC The basic principle of a Binary Weighted Resistor DAC is simple. It uses a summing operational amplifier. This is the circuit which was made during practical tasks in Industrial Electronics Ill. Figure 6-1 shows the basic circuit for 'a 4 bit binary to analog converter using binarv weighted resistors.
Figure 6-1 Basic Circuit for a 4 Bit DAC
Module No. 6: Industrial electronics 3
The switches represent the input binary code and it works as follows. 1.
With all switches open the output is 0 Volts.
2.
If D0 is closed the output is -10 / 100 x 5V = - 0. 5V.
3.
If D0 is opened and D1 is closed the output is -10/50 x 5V = -1 V.
4.
If all four switches are closed the output is -5 x (10/12.5 +10/25+10/50+10/100) =-5 x(0.8+0.4+0.2+0.1) = -5 x (1 .5) = -7. 5 V.
5.
So if the switches are operated by a binary code the output will be from 0 volts to 7.5 in 15 steps of -05V.
The range of the output can be adjusted by RF but the maximum will be about -14V due to the saturation of the OP-AMO. The above system is not much good if the input is larger than 4 bits as the range of the resistors becomes to large. July 1999- Rev.0
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Module No. 6: Industrial electronics 3
Unit No. 6 - Digital / Analog & Analog / Digital Converters
6.3.1
R/2R Ladder DAC
Figure 6-2 Basic R/2R Ladder DAC
A basic R/2R ladder DAC is shown in Figure 6-2. This is the normal method used by the I.C. manufacturer. The circuit is shown at the zero position with all inputs at zero. The binary input changes the switches (D0 to D3) to provide an output with the same voltage steps as the previous DAC. However, the analysis of the circuit is complicated and not worth remembering. The R/2R DAC has the advantage of only requiring 2 values of resistor. Therefore, it is much easier to manufacture on an IC. chip. Modern D/A converter chips have input latches, decode logic (electronic switch), an R/2R DAC and an output amplifier all on one chip. They will accept either serial or parallel digital data inputs. A typical example of an instrumentation DAC is the MAX. 502. This accepts a 12 bit parallel input and produces an output voltage suitable for driving a 4-20mA output loop.
Note:
The latches collect the 12 bit input data. They pass the input data to the decoder logic and DAC when ordered by the µP using the write line.
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Module No. 6: Industrial electronics 3
Unit No. 6 - Digital / Analog & Analog / Digital Converters
TRAINING MANUAL INSTRUMENTATION
6.4
ANALOG TO DIGITAL CONVERTER (ADC)
6.4.1
Ramp Type ADC
Figure 6-3 Ramp Type ADC
Figure 6-3 shows a ramp type ADC. The principle of operation of this type of ADC is simple. The ANALOG and RAMP signals are applied to a single comparator OP-AMP. The comparator will have an output if the analog signal is greater than the ramp voltage. The comparator output and a clock signal are both fed to an AND gate together. The pulsing output from the AND gate is fed to a binary counter. The counter will count until the RAMP voltage reaches the level of the analog signal. Then it will stop. The system then resets and counts again. The binary counter will change every count cycle so that it follows the changes in the analog signal. The timing and control unit opens the latches at the end of each count. This provides a 16 bit parallel data output. As an example, suppose the RAMP is set to rise at 1V every 1 ms. If the analog signal is 2 volts then the counter will count for 2ms before stopping and resetting. The number of counts will be 2/1000 x 1000000/1 = 2000 if the analog signal then falls to 0.5 volts then the count reduces to 500. The problem with this type of ADC is that there are variations in the ramp generator slope. This is caused by temperature and voltage variations. Also it is slow. A count of 20 000 will take 100ms or more per conversion cycle.
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Unit No. 6 - Digital / Analog & Analog / Digital Converters
TRAINING MANUAL INSTRUMENTATION
6.4.2
Successive-Approximation ADC
Figure 6-4 Basic layout of Successive Approximation ADC Chip
Figure 6-4 shows the basic layout of a successive approximation ADC. Remember all of this is made on one chip (e.g. MAX 166 8bit ADC). The great advantage of this type of converter is its speed. It will produce ONE bit of resolution for only ONE clock pulse. The problem is that it is more complicated and it needs a DAC to make it work. Therefore, it is expensive.
Module No. 6: Industrial electronics 3
A brief explanation of the successive approximation method is given below: 1.
The circuit consists of four units. A successive-approximation register (SAR), a DAC and two operational amplifiers.
2.
At the start the output is zero. On the first clock pulse the full output from OP-AMP A will be applied to the SAR. This produces a ONE at the MS13 position. This ONE is then D/A converted and returned to OP-AMP A via OP-AMP B. The SAR waits for the result of this comparison.
3.
If the DAC signal is greater than the input then the SAR MSB reverts to zero and the next clock pulse operates the next MSB.
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4.
If the D/A signal is less than the input then the SAR keeps a ONE in MSB position.
5.
The SAR will then continue trying the next MSB until it is full.
6.
The output will be a series of ones and zeros according to the analog input successively approximated to the number of digits the SAR will accommodate.
7.
At the end of the count the latches open to provide a parallel output. The SAR also provides a serial output if required.
8.
If the output "End of Conversion" is connected to the "Start of Conversion" (SC) then the converter will continuously recycle.
Module No. 6: Industrial electronics 3
Unit No. 6 - Digital / Analog & Analog / Digital Converters
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UNITS IN THIS COURSE UNIT 1
DIGITAL MATHEMATICS
UNIT 2
INTRODUCTION TO DIGITAL SYSTEMS
UNIT 3
LOGIC GATES, FLIP-FLOPS, COUNTERS AND REGISTERS
UNIT 4
MEMORIES AND CLOCKS
UNIT 5
MULTIPLEXERS, DECODERS AND DISPLAYS
UNIT 6
DIGITAL/ANALOG & ANALOG/DIGITAL CONVERTERS
UNIT 7
THE COMPUTER
UNIT 8
INTRODUCTION TO DIGITAL TRANSMISSION
UNIT 9
PRACTICAL TASKS
Module No. 6: Industrial electronics 3
Unit No. 7 - The Computer
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TABLE OF CONTENTS Para
Page
7.0
COURSE OBJECTIVE
3
7.1
INTRODUCTION
4
7.2
THE COMPUTER BLOCK DIAGRAM
4
7.3
THE MICROPROCESSOR
6
7.4
CONCLUSION
7
Module No. 6: Industrial electronics 3
Unit No. 7 - The Computer
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COURSE OBJECTIVE The student will be able to •
List the main parts of a computer and explain their use.
•
Explain the terms hardware, firmware and software.
•
Explain the term bus.
Module No. 6: Industrial electronics 3
Unit No. 7 - The Computer
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Unit No. 7 - The Computer
7.1
INTRODUCTION The previous chapters have discussed the basic digital building blocks. The purpose of this chapter is to show how these blocks go together.
7.2
THE COMPUTER BLOCK DIAGRAM
Figure 7-1 Computer Block Diagram
Module No. 6: Industrial electronics 3
Figure 7-1 shows the components of a simple single-board computer. The functions of each components are as follows: a)
Keyboard: This may be the same as a typewriter keyboard (e.g. secretary's PC). It may be specially designed for a particular purpose (e.g. refinery control). You will learn how to use a particular keyboard during specialist equipment training (e.g. Foxboro IA, Honeywell TDC 3000 etc.).
b)
Keyboard Input Unit: This changes the signals from the keyboard into an ASCII code so that the signals can be understood by the microprocessor. These signals are changed from serial to parallel form using standard shift registers. The keyboard will also provide signals so that the computer can be programmed.
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Unit No. 7 - The Computer
c)
Memory Unit: This is the working memory for the computer. RAM's depends on the size of the computer. The number of RAM’s depends on the size of the computer.
d)
Timing Unit (Clock): This consists of a crystal oscillator and ROM. The ROM programmes the timing pulses from the clock. Different microprocessors operate timers in different ways.
e)
1/O Units (input/output Data Chips) : These connect the computer to the outside world. They may include parallel/serial converters, DACs and ADCs etc. The type of chip which is used depends on the data being sent or received.
f)
Power Supply Unit: This is a standard 230V/5V DC power supply. It's similar in design to the ones made during Industrial Electronics 2.
g)
VDU Output:
This unit produces the characters which will be displayed on the VDU screen. The RAM stores the data to be displayed. When the data is called up a character generator displays the information on the screen. h)
Interface Unit to Disk Drives: This unit collects/sends data to/from the disk drives fitted into the computer. There are usually two drives. One drive takes a floppy disk at the front of the machine. The other drive is the hard disk drive inside the machine.
Module No. 6: Industrial electronics 3
Note:
July 1999- Rev.0
The connections between the chips are extremely complicated. It is only possible to do these using a multilayered printed circuit board. They are designed by the manufacturer and made as a particular item for the PC in use. You can not make your own computer without a special machine for making PC boards
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THE MICROPROCESSOR This is the most important unit in the computer. It processes the incoming data, carries out calculations as required, and returns the results to the outside world. Figure 7-2 shows a block diagram of the ZILOG Z 80 CPU (Central Processing Unit) microprocessor. The diagram shows the pin configuration i.e. the way the pins are arranged. This operates on an 8 bit data bus (8 wires) and a 16 bit address bus (16 wires). 8 BIT DATA
8 BIT DATA
Module No. 6: Industrial electronics 3
Unit No. 7 - The Computer
7.3
Figure 7-2 ZILOG Z 80 CPU
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Unit No. 7 - The Computer
7.4
CONCLUSION The previous units have introduced the chips required to make a digital computer control system. The actual instrument control system used in the field varies from company to company. This must be learnt on the job. The instrument course on the "Introduction to Process Control" gave a short introduction to the Foxboro IA and Honeywell TDC 3000 as examples of DCS control. However, you may work on the Bailey INFI 90 DCS or the Rosemount RS3 DCS. This course, "Industrial Electronics 4", has introduced what is called the HARDWARE (the chips and components that make up digital control). The operating instructions for the microprocessors and the computer are set by the manufacturer in the operating ROM's. These instructions are called FIRMWARE. The systems engineer programmes the DCS to fit the plant he is working on using what is called SOFTWARE. This software is provided by the manufacturer (e.g. Foxboro) to operate his system only. Usually this comes in the form of a floppy disc or streaming tape. This data is loaded into the hard disk on start-up. The Z80 is used as an example of a microprocessor chip. There are many more types of chips in use. The newest chips have a 64 bit data address. However, they all work in the same way.
Module No. 6: Industrial electronics 3
The blocks provide the following functions: 1.
The 16 bit address bus is used by the CPU to address the RAM's, ROM's etc. to gather or send data required for processing.
2.
Data comes in and out through an 8-bit bus.
3.
The ALU (Arithmetic Logic Unit) performs the required data calculations.
4.
The Inst. Reg. (instruction register) holds the instructions as required. This allows the "Instruction Decode and CPU control" unit to perform the required system and CPU control.
5
The CPU registers do many jobs;
July 1999- Rev.0
•
They act as short time stores (accumulators) for the ALU.
•
They acts as memory refresh for dynamic RAM's.
•
They act as a program counter
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UNITS IN THIS COURSE UNIT 1
DIGITAL MATHEMATICS
UNIT 2
INTRODUCTION TO DIGITAL SYSTEMS
UNIT 3
LOGIC GATES, FLIP-FLOPS, COUNTERS AND REGISTERS
UNIT 4
MEMORIES AND CLOCKS
UNIT 5
MULTIPLEXERS, DECODERS AND DISPLAYS
UNIT 6
DIGITAL/ANALOG & ANALOG/DIGITAL CONVERTERS
UNIT 7
THE COMPUTER
UNIT 8
INTRODUCTION TO DIGITAL TRANSMISSION
UNIT 9
PRACTICAL TASKS
Module No. 6: Industrial electronics 3
Unit No. 8 - Introduction to data transmission
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Para
Page
8.0
COURSE OBJECTIVE
3
8.1
INTRODUCTION
4
8.2
SERIAL TRANSMISSION
4
8.3
PARITY
5
8.4
A TYPICAL TRANSMISSION SYSTEM
6
8.5
THE MODEM
7
8.6
RS 232C
9
8.6.1 The RS 232C Connector
10
8.7
HANDSHAKING
11
8.8
GLOSSARY
12
Module No. 6: Industrial electronics 3
Unit No. 8 - Introduction to data transmission
TABLE OF CONTENTS
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COURSE OBJECTIVE The student will be able to: •
Sketch a typical serial digital transmission signal.
•
Explain parity
•
Explain the use of a modem
•
State the main types of modulation
•
Explain the terms DTE and DCE.
•
Explain why RS 232C is used.
•
Explain handshaking
Module No. 6: Industrial electronics 3
Unit No. 8 - Introduction to data transmission
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Unit No. 8 - Introduction to data transmission
8.1
INTRODUCTION In the oil industry a Distributed Control System must be able to send operational data from the outlying platforms to the main control room. This final chapter tells you something about how this data is sent using digital transmission.
8.2
SERIAL TRANSMISSION Serial transmission reduces the number of communication links (e.g. telephone wire pairs or micro-wave links etc.) which a system needs. Serial transmission allows the-required data to be sent as a series of bits on a single pair of wires or a channel.
Module No. 6: Industrial electronics 3
Figure 8-1 A Serial Digital Transmission Signal
Figure 8-1 shows a typical byte (character) sent in a serial form. It consists of 11 bits of data. The start and stop bits are always the same so that the receiver of the serial signal knows which part of the byte to decode. Parity is explained in the next section. The actual message is a seven bit word. A group of these bytes makes up the full data. The data is sent one word at a time. The real meaning of the message depends on the programme used. It will have to be decoded correctly at the receiving end before it can be displayed. The speed of the transmission is called the "Baud Rate". This is 1 divided by the time for one bit. For example if the bit time is 1.66 ms then the baud rate is
A high "Baud Rate" means faster transmission.
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Unit No. 8 - Introduction to data transmission
8.3
PARITY As can been seen from the previous work if a "ONE" bit changes to a "ZERO" during the transmission of digital information the whole meaning of the message changes. To reduce errors a PARITY system is used. Figure 8-2 shows a typical parity transmission system. The "parity generator and checker" on the sender finds out if the message has an ODD or EVEN number of ONEs (e.g. 01101101 has an odd number of ones). If the number is ODD a high signal is sent to the receiving end, if EVEN a low signal. The "parity generator and checker" on the receiver also works out if the number is ODD or EVEN. If it agrees with the sender then the data is passed. If it does not agree it sends a return signal so that the sender transmits the data again.
Module No. 6: Industrial electronics 3
Figure 8-2 Parity Checking of Parallel Digital Transmission
The example given shows parity checking for a parallel data transmission system. Normally this is only used over short distances, such as within the computer itself. Parity checking is also used on serial transmission. The parity bit is placed at the end of the message (see Figure 8-1). This is a 1 if the message bits are odd and 0 if the message bits are even. The receiving equipment adds up the number of ones in each message. It checks this number against the parity bit to see if a bit has been lost in transmission. If the parity check shows a fault the receiver sends a serial message back to tell the transmitter to send again. The "parity" method does not work if two errors occur. If two bits are lost the message will still appear odd or even and the system will think the information is correct. There are error correction systems which use more than one "parity" link. These systems use what are called "Hamming codes". These codes are not covered in this unit. However, students working on data transmission "unit to unit" will cover these codes later on.
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Unit No. 8 - Introduction to data transmission
8.4
A TYPICAL TRANSMISSION SYSTEM
Figure 8-3 Basic Data Transmission System
Figure 8-3 shows the basic steps used to send data by radio link. The information from the transmitters is digitised and processed through the microcomputer. The processed information is put into serial form by a PISO register and fed into a modulator (modem). The modulator drives the radio transmitter (Tx). The receiver (Rx) picks up the radio signal. The signal is then changed back to serial digital form by the receiving demodulator. Next, it is put into parallel form by a SIPO register. Finally, it is processed by the microcomputer for display on the receiving station's VDUs. The block diagram shows a signal being sent from a remote station to the main control center. Most systems are capable of working in both directions; each end can work as a receiver or transmitter. The modulator and demodulator are combined into what is called a MODEM.
Module No. 6: Industrial electronics 3
Note:
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Unit No. 8 - Introduction to data transmission
8.5
THE MODEM A MODEM is a unit which changes the serial data of the input so that it can modulate the transmission system (e.g. telephone wire pairs, radio link, satellite channel, etc.). Modulation is the process which puts the data onto a carrier wave so that it can be sent long distances through the atmosphere. It can also do the opposite. It converts the incoming signal to a serial data form (demodulates). There are many systems for doing this. These systems will not be dealt with in this unit. They are the work of telecoms technicians. However, some trainees may cover these systems at a later date. The four main methods of modulation are: 1)
Amplitude Modulation (AM) There is a carrier frequency for a "ONE" but not for a "ZERO"
2)
Phase Shift Modulation (PSM)
Module No. 6: Industrial electronics 3
The sine wave carrier reverses phase (inverts) from a "ONE" to a "ZERO".
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Unit No. 8 - Introduction to data transmission
3)
Frequency Shift Keying (FSK) The frequency jumps from one frequency to another depending on the data.
Note: a)
A similar system is used by the new digital telephones. A different frequency is used for each number.
b)
This method is popular as modems can pass information in both directions at once (duplex mode) using four different frequencies. Two frequencies are used to send data and two frequencies are used to receive data.
4)
Time Division Multiplex (TDM)
Module No. 6: Industrial electronics 3
In this system, various groups of data are sent on the same transmission frequency. However, they are separated in time. This system is also used to multiplex data within the microprocessor. The diagram show three messages being sent at one time.
The messages A B and C are split into small parts and sent together in one second intervals. The receiver then separates the small parts and puts all the As, all the Bs, and all the Cs together to make the real message. This method is used for satellite telephone calls. About 20 calls can be made at the same time on one channel. The time for each part of the call is in milliseconds. It is so fast that the listener cannot detect that he is listening to a message in bits.
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Unit No. 8 - Introduction to data transmission
8.6
RS 232C Modems and the transmitting and receiving equipment that go with them are often called Data Communication Equipment (DCE). The terminals and computers which send or receive the serial data are called Data Terminal Equipment (DTE). In order to have standard connections between DCE's and DTE's the USA "Electronics Industries Association" (EIA) published the following standard; RS232C. This standard is used all over the world. This standard defines the voltage levels of the signal, the handshake signals, and a standard 25 pin connector.
DTE DCE
-
DATA TERMINAL EQUIPMENT DATA COMMUNICATIONS EQUIPMENT
Module No. 6: Industrial electronics 3
Figure 8-4 Block Diagram of a Communication System
Figure 8-4 shows the basic block diagram of a communication system using modems.
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The IRS 232C Connector
Module No. 6: Industrial electronics 3
Unit No. 8 - Introduction to data transmission
8.6.1
Figure 8-5 RS 232C Pin Connections
Figure 8-5 shows the RS 232C signal names and pin numbers A basic explanation of the operation of the system is as follows: 1
The "Power on" terminal runs self - checking routines.
2.
The Terminal sends a DTR signal to the MODEM
3.
The MODEM replies DSR to indicate it is ready.
4.
The MODEM at the receiver end is dialled.
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8.7
5.
The receiver MODEM sends a ready signal. This is on a frequency of 2225 HZ on telephone lines.
6.
When the transmitter MODEM gets this ready signal it sends a- CD signal to the terminal.
7.
The terminal sends an RTS signal back to its MODEM.
8.
After a set time the transmitter MODEM sends a CTS signal to the terminal.
9.
The terminal sends the data in serial form through the TXD output.
10
.The same steps are followed at the receiving end.
HANDSHAKING Handshaking is used to ensure that the two ends of a communication link are ready to receive transmitted information. This is particularly important when a duplex system is used. For example, using telephone wires, the transmitting modem will not send until it receives a high signal (2225 Hz) from the receiving modem (answer mode modem).
Module No. 6: Industrial electronics 3
Unit No. 8 - Introduction to data transmission
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GLOSSARY The following is a list of definitions of the terms used in industrial electronics and can be used as a review of the course:
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Unit No. 8 - Introduction to data transmission
8.9
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Unit No. 8 - Introduction to data transmission
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Module No. 6: Industrial electronics 3
Unit No. 8 - Introduction to data transmission
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
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Module No. 6: Industrial electronics 3
Unit No. 8 - Introduction to data transmission
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
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UNITS IN THIS COURSE
UNIT 1
DIGITAL MATHEMATICS
UNIT 2
INTRODUCTION TO DIGITAL SYSTEMS
UNIT 3
LOGIC GATES, FLIP-FLOPS, COUNTERS AND REGISTERS
UNIT 4
MEMORIES AND CLOCKS
UNIT 5
MULTIPLEXERS, DECODERS AND DISPLAYS
UNIT 6
DIGITAL/ANALOG & ANALOG/DIGITAL CONVERTERS
UNIT 7
THE COMPUTER
UNIT 8
INTRODUCTION TO DIGITAL TRANSMISSION
UNIT 9
PRACTICAL TASKS
Module No. 6: Industrial electronics 3
Unit No. 9 - Practical tasks
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July 1999- Rev.0
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TRAINING MANUAL INSTRUMENTATION
Para
Page
INSTRUCTOR'S NOTE
3
PRACTICAL TASK 1
4
PRACTICAL TASK 2
7
PRACTICAL TASK 3
10
Module No. 6: Industrial electronics 3
Unit No. 9 - Practical tasks
TABLE OF CONTENTS
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TRAINING MANUAL INSTRUMENTATION
An instrument technician is not expected to service electronic cards. The digital electronic course is concerned with what a chip does, not how it works. Therefore it is suggested that the practical tasks for this course be done in the form of projects. Three simple projects are enclosed which have proven successful in the past. However, other similar projects can be carried out to fit the chips readily available in the Company. A book which may help is "301 Circuits" by Micro-Tech Publishers.
Module No. 6: Industrial electronics 3
Unit No. 9 - Practical tasks
INSTRUCTOR'S NOTE
July 1999- Rev.0
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TRAINING MANUAL INSTRUMENTATION
Unit No. 9 - Practical tasks
PRACTICAL TASK 1 THE MULTIVIBRATOR AND BINARY COUNTER Components required.
QUANTITY
DESCRIPTION
2
BC 109
2
10k Resistors
2
2.2k Resistors
2
10µF Capacitors
1
4024
7
250Ω Resistors
7
Red LED’s
1
PC Board
Module No. 6: Industrial electronics 3
Procedure: 1)
.Construct the given circuit on a PCB board. Remember to make sure that the copper strips are broken in the correct places.
2)
Connect up the 9V supply (power supply switched OFF).
3)
Switch ON the power supply. If the circuit is counting correctly the LED's will light in a binary sequence until they are all lit (the binary number 1111111).
4)
If the circuit does not work. Start fault finding by checking that the multi-vibrator produces pulses first.
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Unit No. 9 - Practical tasks
Questions to show understanding of project.
Q1)
What is the maximum decimal count of the binary counter?
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Q2)
How could you re-set the counter manually?
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Module No. 6: Industrial electronics 3
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Unit No. 9 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
Module No. 6: Industrial electronics 3
CIRCUIT DIAGRAM BINARY COUNTER
4024 PIN CONNECTIONS
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Unit No. 9 - Practical tasks
PRACTICAL TASK 2 BACK AND FORTH FLASHER The object of this project is to produce a back and forth flasher. This is often used as a brake warning light on a car. It will demonstrate the use of a 555 timer, up and down counter, 7400 NAND logic gate and a 44ME to 16 4ME decoder (demultiplexer). Components required.
Module No. 6: Industrial electronics 3
Procedure: 1)
Construct the circuit as given in the diagram. Remember to break the copper strips as required.
2)
When you have made the circuit check it for mistakes.
3)
Connect the circuit Vcc to the positive terminal of a 5V dc supply. Earth the negative terminal together with the circuit earth (make sure the power supply is switched OFF).
4)
If you have been successful in your construction, the LEDs will flash back and forth when you switch the supply ON.
If it does not work switch OFF. Check the circuit again. With patience you will find the fault and get it to work.
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Unit No. 9 - Practical tasks
Questions to show understanding of project.
1)
How could you change the fishing rate ? ----------------------------------------------------------------------------------------------------
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2)
Why is the UP/DOWN counter added?
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Module No. 6: Industrial electronics 3
Unit No. 9 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
Note: Pin connections on diagram. Vcc = + 5V Circuit Diagram Back and Forth Flasher
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Unit No. 9 - Practical tasks
PRACTICAL TASK 3 A 0-99 COUNTER The aim of this project is to use a decade counter, BCD decoder/driver and a 7 segment display to make a 0-99 counter. This counter is made using a common cathode LED display. If a common anode display is used the BCD decoder/driver will be 7447.
Components required. Procedure: Construct the circuit as shown in the given diagram.
2)
Set the function generator to give a 1 Hz square wave output. Apply the TTL output to pin 14 of the first counter (IN position). See if you can get it to work.
Module No. 6: Industrial electronics 3
1)
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Module No. 6: Industrial electronics 3
Unit No. 9 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
Note: Pin connections on diagram. Vcc = + 5V Circuit Diagram 0-99 Counter
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MODULE No. 7 INSTRUMENT WORKSHOP
July 1999- Rev.0
UNITS IN THIS COURSE UNIT 1
PRACTICAL TASKS
Module No. 7: Instrument workshop
Unit No. 1 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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TABLE OF CONTENTS Para
Page
PRACTICAL TASK 1-
4
1.1
OBJECTIVES
4
1.2
PRACTICAL NOTES
5
1.2.1
5
1.3
Types of Packing
PACKING REPLACEMENT
8
1.3.1
Removing Old Packing
8
1.3.2
Installing New Packing
8
PRACTICAL TASK 2
12
2.1
OBJECTIVES
12
PRACTICAL TASK 3
18
3.1
SOLDERING
18
3.2
INTRODUCTION
18
3.3
GOOD P.C. BOARD SOLDERING
18
Module No. 7: Instrument workshop
Unit No. 1 - Practical tasks
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TRAINING MANUAL INSTRUMENTATION
Module No. 7: Instrument workshop
Unit No. 1 - Practical tasks
WORKSHOP SAFETY The following safety procedures must be followed when doing practical work on valves in the workshop or out in the field. •
The shop should be kept clean and free from hazards which might cause you to slip or fall.
•
Inspect your tools regularly; if your tools are worn and unsafe you should throw them away.
•
Neutralise any valve body which has been in contact with flammable, corrosive or toxic fluids.
•
The employees should wear suitable clothing. If the clothing is contaminated with any harmful or flammable substances it must be cleaned before it is used again.
•
Wear approved eye protection when you use compressed air to remove rubbish from the equipment.
•
To avoid a fire hazard or explosion, do not apply grease to valves which are used in oxygen systems unless the grease can be removed before the valve is put back in service.
•
The disc in a butterfly valve will fall out of the valve body if the shaft is removed. To avoid injury to personnel and damage to the disc, you must support the disc to prevent it from failing when you remove the shaft.
•
Before you begin maintenance work or removal of any equipment in the field, you must tell the process supervisor and get a work permit.
•
If you are working above ground level you will need a platform of some kind. The platform-must be strong enough to support you, and your equipment and tools. Platforms should have a raised edge to prevent objects from falling off.
•
To avoid personal injury and damage to the process equipment, isolate the control valve from the system and release all the pressure from the body and actuator before you take it to pieces. You must also relieve all compressive actuator load on the shaft connection before performing any maintenance operations. Do not clear out gas lines with air. Use steam, nitrogen or other approved chemicals.
• •
Be very careful of compressed air or gases. Unfortunately, many employees have died while working on compressed air lines that they thought were depressurised.
•
Finally, read the manufacturer's instruction manual before starting work on any piece of equipment.
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TRAINING MANUAL INSTRUMENTATION
REPACKING A STUFFING BOX AND DOING A LEAK TEST 1.1
OBJECTIVES The student will : 1.
Repack a gland following the procedure in the attached instructions.
2.
Carry out a pressure test on the valve using the attached instructions.
Module No. 7: Instrument workshop
Unit No. 1 - Practical tasks
PRACTICAL TASK 1.
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Unit No. 1 - Practical tasks
1.2
PRACTICAL NOTES VALVE STEM PACKING The purpose of valve packing is to prevent leakage. In a typical valve, the gland follower is tightened until there is no leakage. The valve is then test-operated to make sure that the packing is not compressed so tightly that the valve does not turn easily. Note:
Constant friction is not a problem in valve packing applications. This is because the opening or closing of a valve is a low speed operation.
Figure PT-1 Valve Stem Packing
Module No. 7: Instrument workshop
1.2.1
Types of Packing Packing comes in two types; in pre-shaped rings or in rope form which is cut to size. The common pre-shaped ring packing material is Teflon. This a plastic material which is self lubricating. Normally the Teflon rings are held in place with a spring below the gland follower. Another form of this type of packing is called "Chevron" packing. Teflon or rubber packing rings are shaped like a "V" (Chevron) and placed in the stuffing box as shown in Figure PT-2. The process pressure forces the edges of the chevrons outwards against the shaft and stuffing box wall to produce a good seal. Chevron packing is very common in hydraulic and pneumatic systems.
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Module No. 7: Instrument workshop
Unit No. 1 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
Figure PT-2 Types of Packing
Rope type packing Rope type packing comes in various styles and a few examples are shown in Figure PT-2. The most common materials used to make these ropes are Teflon/Asbestos or graphited asbestos. Both are cheaper than Teflon and can be used over a larger range of temperatures. Also, they will seal better if the stem is roughened with wear. Rope type packing is also easier to change during maintenance. However, it usually needs a lubricator to work well.
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Unit No. 1 - Practical tasks
LANTERN RINGS AND LUBRICATORS
Module No. 7: Instrument workshop
Figure PT-3 Valve Stem Packing with Lantern Ring and Lubricator
Figure PT-3 shows valve stem packing with a lantern ring and lubricator. The lantern ring is made of metal with holes in it. it allows a lubricating compound to be forced into the space between the ring and the stem. The lubricating compound is usually silicon grease. It is used to ensure there is no sticking as the valve stem moves up and down. To fill a lubricator, remove the lubricator nut and fill the lubricator with grease. Open the isolating valve and turn the replaced lubricator nut to force the grease into the system. You will know that there is enough grease in the lantern ring when the lubricator nut starts to tighten. Remember to close the isolation valve when you recharge the lubricator. Leave the valve open when you recharge the lantern ring.
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TRAINING MANUAL INSTRUMENTATION
1.3
PACKING REPLACEMENT
1.3.1
Removing Old Packing The most common sign of packing damage is too much leakage from the gland. If you can't control this leakage by adjusting the gland follower, then the packing should be replaced. Before the packing in a valve can be replaced, the valve must be locked off using the platform's usual procedures. Next you must check manufacturer's specifications to make sure that the old packing is replaced with packing of the right size and type. Then you must carry out the following procedures. Loosen the gland follower nuts.
Module No. 7: Instrument workshop
1.3.2
b)
Swing open the gland follower dog bolts.
c)
Open (or remove for a split type) the gland follower.
d)
Remove the first few packing rings. The easy way is to use a packing tool. This works exactly like a corkscrew.
e)
Take care not to scratch the shaft with the packing tool.
f)
If there is a lantern ring, remove this with a piece of wire bent into a hook.
g)
Remove the remaining packing rings.
h)
Make sure all packing scraps have been removed.
i)
Check the old packing and the shaft to see if it is only worn, or if there is a more serious reason for the leakage.
Installing New Packing a)
Make sure that the exposed portion of the shaft is completely clean. It is important to get rid of all the grit particles so that they do not get pushed into the stuffing box with the new packing.
b)
Clean the shaft and stuffing box with a non-flammable, non-toxic solvent.
c)
Brush down and then wipe the area with a clean rag.
d) If no manufacturer's information is available, measure the gap between the shaft and the stuffing box. e)
Similarly, measure the depth of the stuffing box.
f)
Measure the thickness of the lantern ring.
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Unit No. 1 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
g)
Calculate how many rings will be required. h) Find a suitable mandrel (the same diameter as the shaft).
h)
Wind the packing material round the mandrel as many times as the number of rings required.
i)
Cut the rings with a sharp knife.
k)
Check how many rings you need to put below the lantern ring. The correct number must be replaced, otherwise the lantern ring will not be in line with the lubricator.
l)
Lubricate the rings with an anti-seize compound, so they will go in more easily.
m) Insert the packing rings and lantern ring one by one, pushing them into the stuffing box as far as they can go. The joints of the rings should be staggered and cut on a slant as shown below.
Module No. 7: Instrument workshop
n)
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A pressure test is carried out to find out if the packing has made a good seal around the stem. If the valve is a "gate valve" the seal must be perfect at the rated pressure. Most valves in the industry are rated using the American National Standards Institute (ANSI) code in psi. This rating is usually stamped on the valve; for example, 600. This means the valve has a maximum rated pressure of 600 psi. Control valves however, may be allowed to pass a small amount of process fluid as they move up and down. This amount depends on the valve and its position in the plant. To make it easier to complete this task in the workshop, a gate valve is used to show how to re-pack a stuffing box and how to do a pressure test. Both procedures are also done on a control valve, with the actuator removed.
Module No. 7: Instrument workshop
Unit No. 1 - Practical tasks
PERFORMING A PRESSURE TEST
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Unit No. 1 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
Figure PT-4 Pressure Testing a Gate Valve
Figure PT-4 shows the layout for pressure testing a "gate valve". The outlet is sealed off with a 'blanking plate. The inlet is connected to a pressure test rig. Water is used as the pressurising liquid. The hand pump is operated until the test gauge shows the maximum working pressure of the valve. If the valve leaks through the packing, open the depressurising valve and tighten the packing. Repeat the pressure test. If the valve still leaks through the packing, depressurise the valve and replace the packing. If the valve does not leak when fully open you can be sure it will not leak fully closed. This is because there is less force on the packing when the valve is closed.
Module No. 7: Instrument workshop
Note:
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2.1
OBJECTIVES OVERHAUL CONTROL VALVE. The student will : 1)
Remove the actuator from the valve body.
2)
Completely dismantle (take apart) the valve.
3)
Inspect all parts for wear and damage.
4)
Reassemble (put back together) the valve.
5)
Re-pack the gland and carry out a pressure test.
6)
Completely dismantle the control valve actuator.
7)
Inspect all parts for wear and damage.
8)
Reassemble the control valve actuator.
9)
Refit the control valve actuator to the valve body.
10)
Set the control valve stroke using the attached instructions.
11)
Carry out a leak rate test on the complete control valve assembly using the attached instructions.
Module No. 7: Instrument workshop
Unit No. 1 - Practical tasks
PRACTICAL TASK 2
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There is a class 6 which defines leakage for very tight shut off. This test requires air or nitrogen to be used as the test fluid. The leakage rate is given in bubbles of gas through water per minute. Checking this class is not practical in a normal instrument workshop.
A P & ID may indicate a control valve must be "Tight Shut Off" (TSO). How tight this shut off is depends on the design engineer, but class 5 and class 6 are usually considered tight shut off.
Module No. 7: Instrument workshop
Unit No. 1 - Practical tasks
Note:
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Unit No. 1 - Practical tasks
LEAK TEST PROCEDURE
Figure PT-5 Leak Test
Module No. 7: Instrument workshop
Use the workshop equipment supplied and connect the pressure test rig to the valve inlet. Connect the outlet to a measuring vessel. Connect an air supply to the actuator signal input. Figure PT-5 shows a block diagram of the leak test arrangement. Test as follows. 1.
If the control valve assembly is air to close, apply 15 psi to the actuator to fully close the valve
2.
If the control valve assembly is air to open, apply 3 psi to the actuator, to fully close the valve.
3.
Operate the pressure test rig to apply 50 psi to the valve inlet . Time the rate at which water is collected in the measuring vessel (litres per min).
4.
Use figures given -by the instructor to work out if the valve is serviceable.
5.
TSO valves may be checked for leakage using air. In this case the number of bubbles passing through the water in the measuring vessel gives the leakage rate, e.g. 10 bubbles per minute.
Note:
July 1999- Rev.0
Most of the CDC valves are made by "Fisher" and are standard class 2. The rated capacity depends on the size of the valve and the way it is made. It is calculated using manufacturers tables.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 1 - Practical tasks
LAPPING
How well a control valve is seated when it is fully closed, depends on the metal to metal seal between the plug and the seat.
The above diagram shows a simplified, metal to metal seal. The plug is forced against the seal by the actuator. The metal to metal seal can be worn away during use because of corrosion, cavitation, sand, etc. After a time the valve leaks badly when fully closed. The seal can be restored in the workshop using a technique called "lapping". A grinding paste is applied between the plug and the seat. Then, the plug is rotated against the seat. This action smoothes the surface of the plug and seat so that the metal to metal seal is restored. The process can take a long time, particularly with big valves. Some instrument workshops use electric lapping machines to rotate the plug against the seat. The actual process of lapping is difficult to describe so the instructor will demonstrate "lapping" a valve. If a valve fails its leak test then "lapping" is done and then the leak test is carried out again.
Module No. 7: Instrument workshop
It may be impossible to improve the valve seal by lapping. If this happens the seat and plug must be replaced.
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Unit No. 1 - Practical tasks
SETTING UP A CONTROL VALVE ASSEMBLY There is a plate on the yoke of the actuator. This tells you the operating range of the valve and the diaphragm pressure needed to make the stem do its full travel, (called the valve stroke). Most modern valves will give these figures for both workshop and field operation as the valves are balanced. Non-balanced valves however, will give two stroke ranges. One is an operating range when in the field, under full operating pressure, (e.g. 3-15 psi). The other is the range for a "bench test". This range is used when you set up a valve on a bench in the workshop. The range allows for the unbalanced forces which occur in the field. A typical example of a bench test range would be 5-15 psi. This would give a field operating range of 3-15 psi
Module No. 7: Instrument workshop
The "bench" setting of a control valve assembly can also be done for "split range" operation. Normally a special range spring is fitted. The actuator is then adjusted so that the stroke is completed over a diaphragm pressure range of for example, 3-9 psi or 9-15 psi.
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TRAINING MANUAL INSTRUMENTATION
1)
Use a calibration standard to see how much pressure is applied to the actuator.
2)
If the valve opens on air failure, the travel indicator should show the valve to be closed when full loading pressure is applied. The valve should be open when no pressure is applied to the diaphragm case. If the travel indicator does not show this, then you should loosen the travel indicator screws and shaft until it does.
3)
Vary the pressure on the diaphragm case over its full range and observe the valve travel. Make sure that the valve plug is seated on the seat rings. If the travel is not correct, it can be changed by screwing the valve plug stem into or out of the stem connector. Use a wrench on the stem locknuts to turn the stem. Do not turn the stem when the valve plug is on the seat. When the travel is set properly, lock the stem locknuts against the stem connector and tighten the cap screws in the stem connector.
4)
If travel starts at a pressure lower or higher than the required pressure, then you can adjust it by turning the spring adjuster. Turning the spring adjuster counterclockwise will decrease the spring compression. This causes the valve travel to start at a lower loading pressure. Turning the spring adjuster clockwise will increase the compression on the spring. This causes travel to start at a higher loading pressure. For a 0.2 to 1 bar diaphragm pressure range, the valve should start travelling at 0.2 bar pressure.
5)
When the control valve is installed and connected to the controller, it should be checked again. You must check for correct travel, freedom from friction and correct action, air to open, to match the controlling instrument. In order to work properly the actuator stem and valve -plug stem must move freely when the loading pressure on the diaphragm is changed.
Module No. 7: Instrument workshop
Unit No. 1 - Practical tasks
CALIBRATION PROCEDURES
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Unit No. 1 - Practical tasks
PRACTICAL TASK 3 3.1
SOLDERING
3.2
INTRODUCTION SOLDERING AND WIRE WRAPPING The older methods of soldering connectors onto cables are not used any more and are normally banned on oil/gas installations. All cables are joined to connectors by crimping, the method practised during instrument craft practice. However, you may need to do basic soldering of components onto a printed circuit (P.C.) board. This is the type of soldering that will be practised in this task.
3.3
GOOD P.C. BOARD SOLDERING
Module No. 7: Instrument workshop
The P.C. board consists of copper conducting strips stuck onto an insulation board. The component is fitted onto the board using holes already drilled by the board manufacturer. The only connection between the component and the copper strip is the solder which is applied. A well soldered joint (low resistance connection) and a badly soldered joint (high resistance connections) are shown in the diagram below.
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TRAINING MANUAL INSTRUMENTATION
MODULE No. 8 P & ID’s
July 1999- Rev.0
UNITS IN THIS COURSE
UNIT 1
GENERAL SYMBOLS
UNIT 2
READING A P & ID
UNIT 3
PRACTICAL TASKS
Module No. 8 : P & ID’s
Unit No. 1 - General symbols
TRAINING MANUAL INSTRUMENTATION
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Unit No. 1 - General symbols
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para
Page
1.0
COURSE OBJECTIVE
3
1.1
INTRODUCTION
4
1.2
GENERAL SYMBOLS FOR VESSELS, PUMPS, COMPRESSORS AND TANKS
5
1.3
GENERAL PIPING AND INSTRUMENT LINES
7
1.4
GENERAL SYMBOLS FOR FIELD DEVICES
8
1.5
INSTRUMENT 1.5.1
Instrument Symbol Examples
12 14
COMPUTER SYMBOLS
16
1.7
PIPELINE DESIGNATION
18
Module No. 8 : P & ID’s
1.6
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COURSE OBJECTIVE The student will be able to •
Recognise the general symbols used for valves and actuators.
•
Recognise the general symbols used for vessels, pumps, compressors and tanks.
•
Recognise the general symbols used for field devices, e.g. orifice plates, filters, Venturis etc.
•
Recognise the general symbols used for piping and instrument signal lines.
•
Describe the letter symbols which indicate a particular instrument.
Module No. 8 : P & ID’s
Unit No. 1 - General symbols
1.0
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INTRODUCTION Piping and instrumentation diagrams show the instrumentation which is fitted to a particular plant. These diagrams may contain only two or three sheets of paper or many hundreds. They must be used when any plant maintenance work is carried out. Unfortunately there is no standard method for P & ID's and only the basics of the system can be shown. This unit shows the general symbols.
Module No. 8 : P & ID’s
Unit No. 1 - General symbols
1.1
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GENERAL SYMBOLS FOR VESSELS, PUMPS, COMPRESSORS AND TANKS
Module No. 8 : P & ID’s
Unit No. 1 - General symbols
1.2
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Module No. 8 : P & ID’s
Unit No. 1 - General symbols
TRAINING MANUAL INSTRUMENTATION
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GENERAL PIPING AND INSTRUMENT LINES
Module No. 8 : P & ID’s
Unit No. 1 - General symbols
1.3
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GENERAL SYMBOLS FOR FIELD DEVICES
Module No. 8 : P & ID’s
Unit No. 1 - General symbols
1.4
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Module No. 8 : P & ID’s
Unit No. 1 - General symbols
VALVES
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Module No. 8 : P & ID’s
The P & ID indicates the position of a control valve when the ESD is operated as follows.
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Unit No. 1 - General symbols
Note : The symbol for manual operation is:
Module No. 8 : P & ID’s
Most P & ID's only indicate the manual operation of important process valves. Other manual valves e.g. drain valves, bypass valves, instrument block valves etc. do not have the manual symbol.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 1 - General symbols
1.5
INSTRUMENT SYMBOLS The instruments fitted into a control loop on a P & ID are shown by a circle. Letters and numbers are written inside the circle to show the function of the instrument and its identification (tag) number. The first letter indicates the process variable being measured and the following letters indicate what it does (function). Normally the maximum number of letters is 4. All instruments in the same loop i.e. transmitter, controller and control valve, have the same tag number. The letters used on a P & ID diagram to show the operation of an instrument are not always the same in every diagram.
Module No. 8 : P & ID’s
Table 1 shows a list of common letters and their meanings. Lines are drawn in the circle to show the instrument's position in the plant as follows :
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Module No. 8 : P & ID’s
Unit No. 1 - General symbols
TRAINING MANUAL INSTRUMENTATION
TABLE 1 COMMON INSTRUMENT IDENTIFICATION LETTERS
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Instrument Symbol Examples
Unit No. 1 - General symbols
1.5.1
All these instruments are on control loop 101 -
2. Alarms are drawn with the level high or low either inside the circle or outside. The example shows a temperature alarm high.
Module No. 8 : P & ID’s
Shut down alarms are indicated using two letters for high or low, e.g.
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Unit No. 1 - General symbols
TRAINING MANUAL INSTRUMENTATION
Some alarms indicate both high and low eg.
Module No. 8 : P & ID’s
3.
July 1999- Rev.0
Motor operated valves have their position shown by a position switch. The control room indicator shows open or closed. This is drawn as follows :
Z
for
Position
S
for
Switch
I
for
Indicator
O
for
Open
C
for
Closed
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Unit No. 1 - General symbols
1.6
COMPUTER SYMBOLS Modern control systems are controlled by computers (microprocessors). The recorders, indicators, alarms etc. are displays on a video screen. The controller function is part of the microprocessor. To indicate this on a P & ID, it is usual to put the circles in a square box. e.g.
Module No. 8 : P & ID’s
Some computer operations are done separately from the computer control system. This is shown by using the circle symbol with the computer function written outside, for example:
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Module No. 8 : P & ID’s
Unit No. 1 - General symbols
The following computer symbols are reasonably standard.
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Unit No. 1 - General symbols
1.7
PIPELINE DESIGNATION Piping on a P & ID is indicated by : 1 ) Usage: For example, process, drain, nitrogen, blowdown, etc. 2)
Line Number: The identification number of the line on the plant.
3)
Size: Usually in inches.
4)
Piping Class: The piping specification, both material and pressure rating.
The specification is usually given using American standards e.g. American Society of Mechanical Engineers (ASME). or American Petroleum Institute (API).
Module No. 8 : P & ID’s
Each installation uses slightly different methods to do this but the end result is the same. A typical example is given below.
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UNITS IN THIS COURSE
UNIT 1
GENERAL SYMBOLS
UNIT 2
READING A P & ID
UNIT 3
PRACTICAL TASKS
Module No. 8 : P & ID’s
Unit No. 2 - Reading P &ID’s
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Unit No. 2 - Reading P &ID’s
TABLE OF CONTENTS Para
Page
2.0
COURSE OBJECTIVE
3
2.1
INTRODUCTION
4
2.2
WHAT'S ON A P & ID
4
2.3
EXAMPLE 1
5
2.3.1
Process Description
5
2.3.2
Instrumentation
6
2.3.3
Safety System
8
2.3.4
Operator's Aids
8
EXAMPLE 2
11
2.4.1
Process Description
11
2.4.2
Instrumentation
11
2.4.3
Safety Instrumentation
12
2.4.4
Operator's Aids
13
Module No. 8 : P & ID’s
2.4
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COURSE OBJECTIVE The student will be able to •
Understand P & ID for an operating company which he has not seen before.
•
Get a 70% pass in a test on the understanding of a sight unseen operating company P & ID.
Module No. 8 : P & ID’s
Unit No. 2 - Reading P &ID’s
2.0
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Unit No. 2 - Reading P &ID’s
2.1
INTRODUCTION Reading a P & ID is difficult and requires practice. Companies use different methods for making their diagrams. This course is an introduction to reading a P & ID. You will have to learn the P & lDs for any particular plant on the job. The P & lDs on the job site have a 'legend'. The legend is a table which tells you exactly what all the symbols mean. You must study this table before you try to read the P & ID.
2.2
WHAT'S ON A P & ID The following gives a general outline of what you will see on a P & 1 D. 1.
A layout of the process equipment, e.g. vessels, pumps, heat exchangers etc. A description of the pressure, flow rate, size etc. of the main parts.
2.
All the pipelines fitted to the process equipment. Each line has a code on it to show its specification, size etc.
3.
All the instrumentation fitted to the vessels, e.g. transmitters, controllers, control valves, relief valves etc.
4.
Signal lines to and from the instrumentation.
5.
All control loops should be complete. The diagram should show where the instrument is, e.g. control room or field.
6.
A P & ID is only a drawing of the process. The actual position of the instrumentation may not be correct. Sometimes there is a note on the drawing to tell you where the instrument is actually placed.
Module No. 8 : P & ID’s
The example P & ID only shows one part of the plant. There are many P & ID's to show all of a plant. Each P & ID tells you what other drawing you must look at to learn about the next part of the process
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Unit No. 2 - Reading P &ID’s
2.3
EXAMPLE 1 Figure 2-1 shows a typical P & ID from a plant. The legend for this P & ID is also given (page 10). Remember this legend only applies to this P & ID. Other P & ID's use different symbols. With reference to the P & ID. 1.
The code number for the drawing is 062-Dl 12. It shows the piping and instrumentation for vessel 5C-1 64 and pump 5G-1 55.
2.
Vessel 5C-164 is a Propane/ADIP settler. It's size is 3 500 mm in diameter (0) and 10 500 mm T/T in length.
3.
Pump 5G-155 is a lean ADIP recycle pump. It has a capacity of 122 m3 /h.
Note : T/T means tangent to tangent
2.3.1
Process Description The vessel is used to separate propane from ADIP (AmineDiIsoPropanol). The Propane/ADIP mixture is used to remove sulphur compounds from gases produced in the oil field. The input to the vessel is from vessel 5C-1630 (Propane/ADIP mixer) on pipe line P-55211 -10"-EA 2DX, shown on P & ID drawing 05-AD-1054 The outlets are: Propane from the top of the tank to 5C-180 on drawing 05-AD-1054. ADIP drained from the bottom of the tank to 5C-156 on drawing 062-D110.
Module No. 8 : P & ID’s
The main process flow is shown by the thick black lines. All other lines are secondary to the process.
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Unit No. 2 - Reading P &ID’s
2.3.2
Instrumentation •
LOOP 803 Loop 803 is the only control loop on the diagram (area A). It is a pneumatic control loop. It keeps the interface level between the propane and the ADIP constant (the interface level is the level at which the propane and the ADIP meet). The loop consists of: Level transmitter (LT 803). This is of the differential pressure type with a pneumatic signal output. Level switch low (LSL 803). This is mounted behind the control panel. Level alarm low (LAL 803). This is an indicating light mounted on the front of the control panel. Level switch high (LSH 803) behind the control panel. Level alarm high (LAH 803) indicating light on the control panel. Level recorder (LR 803) on the control panel. The recorder is a three pen unit which combines LR 803 and FIR 789 (2 readings). The level indicator controller (LIC 803) is of the pneumatic type. It is mounted on the control panel. The control valve is a globe valve with a pneumatic diaphragm (LV 803).
•
LOOP 804
Module No. 8 : P & ID’s
Loop 804 is a shut down loop for both the pump and the control valve (LV 803). It operates when the interface level reaches the low low point. The level is measured using a displacer type pneumatic transmitter (LT 804). The level controller low low is an on/off controller (LCLL 804 and LCLL 804A). LCLL 804 switches off the pump and closes LV 803 when the interface level falls below 500 mm. LCLL 804A switches the system on (reset) when the interface level rises to 650 mm.
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Unit No. 2 - Reading P &ID’s
TRAINING MANUAL INSTRUMENTATION
A time delay is fitted in the pump motor control line to allow the valve to move before the pump starts or stops (note 1). The pneumatic signals operate pressure switches. The switches send electrical control signals to the pump motor and LCV 803 solenoid operated valve (SOV 803). When the low low level is reached SOV 803 de-energises. This breaks the air signal to the actuator diaphragm and the valve closes. ESD HS-042/1 (Drawing 05-AD-1063) is also connected into the circuit. If the ESD is pressed, the pump stops and LV 803 closes. (The valve is fail closed (FC)). •
LOOP 1812 This loop indicates to the operator in the control room the differential pressure across HV 1816. HV 1816 is a gate valve on the propane outlet line. The loop consists of : Electrical pressure differential transmitter (PDT 1812) with a locally mounted indicator (PDI 1812/2). Pressure differential indicator in the control room (PDI 1812/1). This is combined with temperature indicator (TI 666) into one unit. Pressure differential switch high (PDSH 1812) behind the control panel.
Module No. 8 : P & ID’s
Pressure differential alarm high (PDAH 1812) on the control panel.
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Unit No. 2 - Reading P &ID’s
2.3.3
Safety System The inlet to the vessel (5C-164) and the propane outlet from the vessel are controlled by pneumatically operated ball valves, set by hand. HV 1841 inlet. HV 1842 outlet. Both these valves fail closed (FC) with tight shut off (TSO) if the ESD system is operated. The ESD operates on SOV which breaks the air supply to the valve for shutdown. ESD reset is done by hand in the field (HS 1841 and HS 1842). The pressure safety valve (PSV 948) prevents the pressure in the vessel reaching a dangerous level. It vents the vessel to the HP (high pressure) flare header (line VF71556-10"-EA28) and to the flare, where it is burnt. The PSV is set to operate at 20 barg.
2.3.4
Operator's Aids A sight glass is fitted to the vessel (LG 947). A pressure gauge (PG 949) is fitted to the discharge of the Lean ADIP Recycling Pump.
Module No. 8 : P & ID’s
An orifice plate and pneumatic flow transmitter are fitted in the ADIP outlet line. The flow transmitter's signals are used to record the ADIP flow rate in the control room.
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Module No. 8 : P & ID’s
Unit No. 2 - Reading P &ID’s
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Unit No. 2 - Reading P &ID’s
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Unit No. 2 - Reading P &ID’s
2.4
EXAMPLE 2 Figure 2-2 shows a simplified P & ]D from a more modern plant. In this plant the control is done using a distributed control system with microprocessors. The drawing uses the same symbols as introduced in Unit 1. You should look at this unit if you don't remember the general symbols.
2.4.1
Process Description The Sweet Gas K.O. (knock out) Drum is used to separate gas/condensate/water. The input fluid stream comes from vessel 32V-101. The vessel is fitted with a mist extractor pad and a vortex breaker. It produces three outputs. 1.
Treated gas comes out of the top of the vessel. It goes to dehydration (water removal) vessel 42E-309 This is shown on drawing 42.00.30.022
2.
Water comes out of the bottom of the vessel. It goes to vessel 32V-184. This is shown on drawing 32.00.30.030
3.
Condensate (hydrocarbons) comes of the middle of the vessel. It goes to vessel H.P. separator 32V-22 This is shown on drawing 32.00.30.002
2.4.2
Instrumentation
The basic control of the vessel is done using two level indicating controllers; LIC 2819 and LIC 2823. •
LOOP 2823
Module No. 8 : P & ID’s
This is the main level control loop for the vessel. It maintains the gas/liquid level in the drum. The level transmitter (LT 2823) is of the D/P type (4-20mA) and provides an electrical signal to LIC 2823. This is a computer function in the control microprocessor. The high alarm (LAH set at 4450 mm) and low alarm (LAL set at 588 mm) are indicated on the VDU display in the control room. The control microprocessor provides an electrical signal (4-20mA) to set the position of the control valve (LV 2823). The control valve is pneumatically operated. The conversion from an electrical to a pneumatic signal is done using an I/P converter LY 2823. LIC 2823 sends an internal data signal to a level compute unit inside the microprocessor. This unit closes the interface loop 2819 if the liquid level in the tank falls below 1800 mm. If this operates, a low alarm (LAL 2823) is indicated on the control room VDU.
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•
LOOP 2819
Unit No. 2 - Reading P &ID’s
This loop controls the interface level between the hydrocarbons and the water. The system uses on/off control. If the level fails below 589 mm LIC 2819 closes the valve. LIC 2819 reopens the valve when the level rises above 988 mm. There is a connection inside the microprocessor which closes the valve if the liquid level falls below 1 800 mm. A switch (LZSC 2819) on LV 2819 is used to indicate the position of the valve. It indicates closed on the control room VDU (LZAC 2819). •
FLOW MEASUREMENT
The collection of instruments in area A shows the modern computer measurement of flow rate and total flow. It consists of a)
Temperature Transmitter TT 5886
b)
Pressure Transmitter PT 3996
c)
Two Flow Transmitters FT 1988A and FT 1988B
The signals from these transmitters are connected to the microprocessor. The microprocessor internally works out the gas flow rate and total flow in standard units (Nm3 hr and N m3). Temperature (TI 5886), pressure (PI 3996) flow (FI 1998) and total flow (FQI 1998) are all indicated on the control room VDU. Note : Two flow transmitters are used to make the system more reliable. If one FT fails the other still provides a signal. The microprocessor will indicate transmitter failure.
2.4.3
Safety Instrumentation
Module No. 8 : P & ID’s
The ESD system is applied via a computer which ensures XV 9829 and XV 9887 close when the ESD is operated. provides a signal to solenoid operated three way valves. If the ESD is operated the signal to the solenoid valves is removed. The three way valve vents the air supply to the piston actuator and the valve closes. A mechanical type pressure safety valve is fitted (PSV 8005). This releases the gas to the flare if the pressure in the vessel exceeds 75 bar g.
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Unit No. 2 - Reading P &ID’s
2.4.4
Operator's Aids A sight glass (LG 2828) indicates the hydrocarbon/water interface. Level sight glass (LG 2822) indicates the liquid level in the vessel. This is made from three separate sight glasses to cover the distance required. A typical layout is given in Figure 2-3.
Figure 2-3 Combination Sight glass
Module No. 8 : P & ID’s
Note : The flow through the vessel is controlled from the previous vessel on diagram 32.00.30.001. The control valve (FV 1821) has pneumatic diaphragm type actuator. It is operated by an I/P converter (FY 1821). The valve is designed to fail locked (intermediate). This means , that the loss of the control signal will not stop operations immediately.
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Module No. 8 : P & ID’s
Unit No. 2 - Reading P &ID’s
TRAINING MANUAL INSTRUMENTATION
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UNITS IN THIS COURSE UNIT 1
GENERAL SYMBOLS
UNIT 2
READING A P & ID
UNIT 3
PRACTICAL TASKS
Module No. 8 : P & ID’s
Unit No. 3 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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Para
Page
3.0
COURSE OBJECTIVE
3
3.1
INTRODUCTION
5
3.3
PRACTICAL TASK 1
7
3.4
PRACTICAL TASK 2
15
Module No. 8 : P & ID’s
Unit No. 3 - Practical tasks
TABLE OF CONTENTS
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COURSE OBJECTIVE The student will complete the following practical tasks on reading a P & I D.
Module No. 8 : P & ID’s
Unit No. 3 - Practical tasks
3.0
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Module No. 8 : P & ID’s
Unit No. 3 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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INTRODUCTION The following two practical tasks have been given for you to practice reading a P & ID.
Module No. 8 : P & ID’s
Unit No. 3 - Practical tasks
3.1
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Module No. 8 : P & ID’s
Unit No. 3 - Practical tasks
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Unit No. 3 - Practical tasks
3.2
PRACTICAL TASK 1 With reference to drawing 062-D110 (page 4), complete the following tasks. 1)
Using a coloured pencil, outline the main process flow lines.
2)
The size of the input feed drum
3)
The size of the ADIP absorber
The purpose of the ADIP absorber is to remove the sulphur compound hydrogen sulphide from the untreated DEO (De-Ethaniser Overheads), mostly methane gas. The gas moves up the absorber, as the lean (clean) ADIP moves down. The clean ADIP removes the sulphur compounds from the gas and leaves the bottom as rich (dirty) ADIP. 4)
The treated gas goes to
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5)
The dirty ADIP goes to
Module No. 8 : P & ID’s
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Unit No. 3 - Practical tasks
6)
Name each instrument in flow loop 785 ----------------------------------------------------------------------------------------------------
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7)
When would you expect FV 785 to open.
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8)
Name each instrument in loop 1844. ----------------------------------------------------------------------------------------------------
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9)
What does loop 1844 do? ----------------------------------------------------------------------------------------------------
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10)
What are PSV 925A and PSV 925B? ----------------------------------------------------------------------------------------------------
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Module No. 8 : P & ID’s
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Unit No. 3 - Practical tasks
11)
What does the interlock between PSV 925A and PSV 925B do. ----------------------------------------------------------------------------------------------------
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12)
How many interconnected sight glasses are there for LG 923? ----------------------------------------------------------------------------------------------------
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13)
Name each instrument in loop 781 and say where they are mounted. ----------------------------------------------------------------------------------------------------
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14)
What does LCLL 782 do? ----------------------------------------------------------------------------------------------------
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15) What is PDT 783 for? ----------------------------------------------------------------------------------------------------
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16) What does the symbol below mean?
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Unit No. 3 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
17)
What does the symbol below mean in a vessel?
18)
What does the symbol below mean in a vessel?
19)
What are the following?
P - 55200 - 10" - EA2D ----------------------------------------------------------------------------------------------------
Module No. 8 : P & ID’s
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Unit No. 3 - Practical tasks
20)
Where do the semi - lean and lean ADIP's come from? ----------------------------------------------------------------------------------------------------
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Unit No. 3 - Practical tasks
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Unit No. 3 - Practical tasks
3.3
PRACTICAL TASK 2 INTRODUCTION The previous task used a P & ID produced by Kellogg. This task uses a P & ID produced by Bechtel-Technip. This is done so that you can see that although construction companies follow the same basic rules for P & ID, each P & ID is a little different. BASIC PROCESS DESCRIPTION The P & ID 24-00-30016 (see page 13) shows the 2nd stage of a two stage centrifugal gas compressor. At this stage, the process is controlled by an anti-surge controller. Using a microprocessor, the anti-surge controller controls flow through and the pressure increase across the stage. The product of flow and pressure increases (AP) must stay constant. If it does not stay constant then surge occurs. (Surge means the rotor of the compressor moves backwards and forwards). Surge can badly damage the compressor. The output of the anti-surge controller adjusts the position of the recycle valve. This valve allows the compressed gas to flow back to the input of the stage. (This gas returns to the input via a cooler and suction scrubber (KO. drum)). This recycled gas increases the flow through the stage and reduces the differential pressure across it. This stops the surge. 1)
What does UIC mean for the anti-surge controller?
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2)
Write down the inputs to UIC 1632
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Module No. 8 : P & ID’s
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Unit No. 3 - Practical tasks
3)
The number of the re-cycle valve is:
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4)
Describe loop 1638.
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5)
What is TG 6611 and where is it?
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6)
Where is TI 5641 ? When will the alarm light?
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7)
What happens if TAHH 5842 lights?
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8)
If PAL 3842 is light, what can you do?
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9)
What is PDG 4838 for?
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Module No. 8 : P & ID’s
10)
What pressure are the mechanical pressure safely valves set at?
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11)
Where does the gas go if the ESD opens the compressor depressuring line?
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12)
What do the letters "HVSO" and "HZLO" mean?
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13)
How are the two anti surge controllers inter-connected?
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14)
Explain Note 1:
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Where does the discharged gas go?
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What is the average increase in pressure across this stage?
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What is the size of the suction line?
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What is the size of the discharge line?
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Why is the discharge line smaller?
Module No. 8 : P & ID’s
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What is meant by Nm3 /h?
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MODULE No. 9 CONTROL SYSTEM 1
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UNITS IN THIS COURSE
UNIT 1
COURSE INTRODUCTION
UNIT 2
PRACTICAL TASKS
Module No. 9 : Control systems 1
Unit No. 1 - Introduction
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Para
Page
1.0
COURSE OBJECTIVE
3
1.1
INTRODUCTION
4
1.2
BASIC THEORY
4
1.2.1
Introduction
4
1.2.2
On/Off control
4
1.2.3
Manual Control
7
1.2.4
Automatic Flow Control
8
1.2.5
Temperature Control
9
1.2.6
Pressure Control
12
1.2.7
Centrifugal Pump Control
14
Module No. 9 : Control systems 1
Unit No. 1 - Introduction
TABLE OF CONTENTS
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Unit No. 1 - Introduction
1.0
COURSE OBJECTIVE The student will be able to •
Use the Autodynamics simulator keyboard to adjust values on the video display.
•
Perform the following tasks on the simulator to demonstrate the use of the following. On/Off control Hand control Flow control Temperature control Pressure control Pump control
Module No. 9 : Control systems 1
Centrifugal pump
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Unit No. 1 - Introduction
1.1
INTRODUCTION The aim of this unit is to introduce the student to instrument control systems using the Autodynamics simulator.
1.2
BASIC THEORY
1.2.1
Introduction The following theory is a back-up to the practical exercises carried out on the Autodynamics simulator.
1.2.2
On/Off control
Module No. 9 : Control systems 1
On/Off control is the simplest form of control. The controller works between two levels. It switches 'ON' at one level and it switches 'OFF' at the other. A simple diagram of the system is shown in Figure 1-1.
Figure 1-1 Simple On/Off Control On/Off control is shown on the simulator by a block valve. An example of the block valve is the gate valve. It is either open (on) or closed (off).
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TRAINING MANUAL INSTRUMENTATION
Figure 1-2a shows the graphic display seen on the video display unit (VDU).This, is the operator's Process Flow Diagram (PFD). The P & ID to match the PFD is also given.
Figure 1-2b P & ID
Figure 1-2 ON/OFF Process Control
Module No. 9 : Control systems 1
The instrumentation provided for ON/OFF flow control is: 1.
Inlet (upstream) pressure by loop 111. Indication on the workstation VDU.
2.
Flow rate by loop 101. Indication on the workstation VDU.
3.
Outlet (downstream) pressure by loop 113. Indication on the workstation VDU.
4.
Pressure loop 112 is added to show pressure loss across the block valve. The pressure loss is the indication on loop 111 minus the indication on loop 112.
5.
The position of block valve (HV-131) is indicated as open or closed on the workstation VDU.
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Unit No. 1 - Introduction
CORRECT LOOP OPERATION OFF(closed):
FIR reads ZI 131 indicates PT 112 and PT 113 read
zero. closed. 129 kg /cm2
ON (Open):
FR 101 PI 111 ZI 131 PI 112 PT 113
125 TPH 146.4 4 kg/ cm2 open 144.4 kg / cm2 129 kg / cm2
reads reads indicates reads reads
The correct 'OPEN' 2 position shows the downstream (outlet) pressure fixed at 129 kg/cm . At this pressure, the flow rate for a differential pressure of 146.4-129 (17.4 kg cm2) is 125 TPH.
Module No. 9 : Control systems 1
The correct pressure loss across the valve is 2 kg / cm2 (146.4-144.4). If the pressure loss across the valve is higher than this it means there is a fault. The valve is not fully open or it's in bad condition. It needs maintenance work.
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1.2.3
Manual Control
Unit No. 1 - Introduction
Figure 1-3a shows the process flow diagram seen on the VDU for manual control. The matching P & ID is shown in Figure 1-3b. The operator can use the keyboard to open and close the control valve manually. In this way the operator can set the required flow rate.
Figure 1-3 Manual Control
The instrumentation provided for the manual flow control is :
Module No. 9 : Control systems 1
1.
5.
Inlet (upstream) pressure by loop 211. Indication on the workstation VDU set at 2 146.4 kg/ cm . 2.
Flow rate by loop 201. Indication on the workstation VDU.
3.
Piston operated control valve FCV 231. The position of FCV 231 is set in the control room by HC 231.
4.
Loop 212 indicates the outlet pressure from the control valve.
Loop 213 indicates the downstream pressure (set at 129 kg/ cm2). CORRECT LOOP OPERATION_ The upstream pressure is fixed at 146.4 kg/ cm2. The downstream pressure is fixed at 129 kg/ cm2. The operator controls the flow by adjusting HC 231. The throttling effect of the control valve is shown by the difference in pressure between loops 211 and 212 on either side of the control valve. Remember that when the control valve is closed PI 212 will show the downstream pressure at 129.0 kg / cm2.
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Unit No. 1 - Introduction
1.2.4
Automatic Flow Control INTRODUCTION The flow of liquid is automatically controlled using a Flow Indicating Recorder Controller (FIRC). The control actions (PID) are set to give good control. High and low alarms are set at 150 TPH and 50 TPH. Figure 1-4 gives the PFD and the P & ID for the system.
Figure 1-4b P & ID
Figure 1-4 Automatic Control using an FIRC
Module No. 9 : Control systems 1
The FIRC will control at a designed flow rate of 129 TPH. The upstream and downstream pressures are fixed at 146.4 kg/ cm2 and 129.0 kg/ cm2 . The control valve is 60% open.
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Unit No. 1 - Introduction
1.2.5
Temperature Control The simulator is designed to show temperature control using an industrial shell/tube heat exchanger. You will see many of these in use throughout operating companies. THE SHELL/TUBE HEAT EXCHANGER
Figure 1-5 Shell/Tube Heat Exchanger
Module No. 9 : Control systems 1
Figure 1-5 shows an example of a shell/tube heat exchanger. The cooling fluid flows round inside the tubes and the process fluid flows outside the tubes inside the shell. The cooling fluid takes heat from the process fluid as it passes through the tube. So, the process fluid is cooled. The amount of cooling depends on four factors: 1.
The rate of flow of the cooling fluid.
2.
The rate of flow of the process fluid.
3.
The initial temperature of the cooling fluid.
4.
The initial temperature of the process fluid.
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SIMULATOR PFD AND P&ID
Unit No. 1 - Introduction
70.6
Module No. 9 : Control systems 1
Figure 1-6b Heat Exchanger P & ID
Figure 1-6a Heat Exchanger PFD Figure 1-6 Temperature Control (Shell-Tube Exchanger)
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Unit No. 1 - Introduction
CONTROL PRINCIPLES The operating conditions for the heat exchanger are as follows : Coolant flow rate 104.2 TPH with an inlet temperature of 25°'C and an outlet temperature of 70.6°C. Process fluid flow rate 150 TPH with an inlet temperature of 180°C and an outlet temperature of 116.7°C (temperature reduction of 63.3°C) The controlled variable is temperature of the process fluid at the outlet. This is maintained by a TIRC (Temperature Indicator Recorder Controller). The TIRC operates the 3-way valve (TCV-624). This allows some of the hot process fluid to bypass the system. The mixture of cooled process fluid and bypass hot process fluid keeps the output process fluid at a constant 116.7°C. Two operator alarms are added. Low coolant flow rate. The alarm operates if the coolant flow falls below 10 TPH
Module No. 9 : Control systems 1
There is a high/low temperature alarm at the process fluid outlet. The high alarm operates at 150°C, and the low alarm at 90°C.
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Unit No. 1 - Introduction
1.2.6
Pressure Control INTRODUCTION The simulation of pressure control is done using an expansion vessel. The principle of operation is 'BOYLE's LAW' (at a constant temperature Pressure x Volume = Constant). This means that when the volume increases the pressure must fall. Figure 1-7 shows the PFD and P & ID for simulator pressure control.
Module No. 9 : Control systems 1
Figure 1-7a Pressure Control PFD
Figure 1-7 Pressure Control
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Unit No. 1 - Introduction
CONTROL DESIGN For correct operation the inlet pressure of the process gas falls as it expands in the vessel from 15.5 kg / cm2 to an outlet pressure of 4 kg / cm2 . The flow rate is fixed at 10.2 TPH. The system is controlled by PIRC-712. This positions the outlet pressure control valve in order to maintain the set-point pressure in the vessel. OPERATOR ALARMS These are set as follows : Inlet flow High alarm above 13.2 TPH Low alarm below 2 TPH Outlet pressure High alarm above 14 kg,/ cm2
Module No. 9 : Control systems 1
Pressure safety valve (PSV-710) is set at 15 k g/ cm2
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Centrifugal Pump Control INTRODUCTION The centrifugal pump is a common oil field device used to move liquids. It can send oil from an offshore platform, via a pipeline, to a refinery. It can also pump oil onto a tanker etc. •
Module No. 9 : Control systems 1
Unit No. 1 - Introduction
1.2.7
Principle of operation
Figure 1-8 The Simple Centrifugal Pump
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Operation 1.
Liquid enters through the hole at the centre (the eye).
2.
The impeller rotates very fast.
3.
The vanes on the impeller drive the liquid outwards (centrifugal force). The velocity of the liquid increases.
4.
Drops of liquid fly off the ends of the vanes and hit the pump casing.
5.
When the drops of liquid hit the pipe casing their kinetic energy is changed to pressure energy. The higher pressure of the drops forces the liquid out of the outlet at a higher pressure.
6.
The size of the pump depends on the mass flow rate and the gain in head pressure of the liquid as it passes through the pump.
7.
Figure 1-9 shows a multi-stage impeller. Most large pumps use a multi-stage impeller where the output of one impeller is the input to the eye of the next.
Module No. 9 : Control systems 1
Unit No. 1 - Introduction
Figure 1-8 shows a simple centrifugal pump.
Figure 1-9 Multi-stage Impeller
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Unit No. 1 - Introduction
SIMULATOR PFD AND P & ID Figure 1-10 shows the simulator PFD of the centrifugal pump action as seen by the operator on the workstation screen. It also shows the P & ID.
Module No. 9 : Control systems 1
Figure 1-10a Centrifugal Pump PFD
Figure 1-10 Centrifugal Pump Control
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Unit No. 1 - Introduction
CONTROL OPERATION 1.
Under normal operation, the level in the vessel (V-500) is constant. it provides a suction pressure of 4.5 kg / cm2.
2.
The normal flow rate is 125 TPH set by FIRC-502. The outlet (discharge) pressure is 18 kg / cm2.
3.
HCV-544 allows the liquid to return to the vessel (recycle line) so that the pump can be kept running even if FIRC-502 closes the outlet valve.
4.
The minimum flow line reduces pump vibration which may occur when the flow rate is very low.
PROCESS ALARMS Suction alarm low (PAL 511) which operates at below 0.8 kg/cm2
2.
Discharge -pressure alarm low (PAL-512) which operates at below 1.0 Kg/cm2
3.
Electric motor overcurrent alarm (IAH-551) which operates when the current is above 325 A.
4.
Discharge flow alarm low (FAL-501) which operates when the flow fails below 50 TPH.
Module No. 9 : Control systems 1
1.
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UNITS IN THIS COURSE UNIT 1
COURSE INTRODUCTION
UNIT 2
PRACTICAL TASKS
Module No. 9 : Control systems 1
Unit No. 2 - Practical Tasks
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TABLE OF CONTENTS Para
Page PRACTICAL TASK 1
3
PRACTICAL TASK 2
5
PRACTICAL TASK 3
13
PRACTICAL TASK 4
16
PRACTICAL TASK 5
18
PRACTICAL TASK 6
19
NOTES FOR INSTRUCTOR
20
Module No. 9 : Control systems 1
Unit No. 2 - Practical Tasks
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Unit No. 2 - Practical Tasks
PRACTICAL TASK 1 ON/OFF CONTROL 1)
Make sure the Autodynamics simulator is operating as designed with the valve open. The screen VDU shows : PI-111 FR-101 ZI-131 PI-112 PI-113
reads reads reads reads reads
146.4 kg/cm2 125 TPH Valve open 144.4 kg/ cm2 129 kg/ cm2
2)
Close the valve manually using the keyboard.
3)
Fill in the blanks. ZI-131 PI-111 PI-112 PI-113 FR-101
4)
reads reads reads reads reads
.............................. .............................. .............................. .............................. ..............................
Open the valve. Does the VDU return to the normal operating position with the valve open.
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Unit No. 2 - Practical Tasks
5)
Look at the screen. Describe what is happening.
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Unit No. 2 - Practical Tasks
PRACTICAL TASK 2 MANUAL CONTROL PART 1 1)
Make sure the autodynamics simulator is operating as designed with HCV231 60% open. The VDU screen shows : reads reads
146.4 kg/ cm2 96 TPH
HCV-231
reads
60%
PI-212 PI-213
reads reads
130.4 kg/ cm2 129 kg/ cm2
Close the valve. The VDU screen shows : PI-211
reads ..............................
FR-201
reads ..............................
HCV-231
reads ..............................
PI-212
reads ..............................
PI-213
reads ..............................
Module No. 9 : Control systems 1
2)
PI-211 FR-201
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Unit No. 2 - Practical Tasks
3)
Open the valve in 10% steps to fully open (100%). Fill in the table for each step. % Open
Flow Rate
PI 212
PI 211-PI 212
10
0
129
17.4
20 30 40 50 60 70 80 90 100
4)
Plot a graph of flow rate against value % open.
5)
Plot a graph of pressure drop across the valve (PI 211 -PI 212) against valve % open.
6)
Does the flow rate increase linearly with valve % open.
Module No. 9 : Control systems 1
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Unit No. 2 - Practical Tasks
7)
Does the pressure drop across the valve decrease linearly as the valve opens.
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8)
Do your results show that a control valve when fully open causes a much larger pressure loss than the block valve used during practical task 1.
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Module No. 9 : Control systems 1
Unit No. 2 - Practical Tasks
VALUE % OPEN
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Module No. 9 : Control systems 1
Unit No. 2 - Practical Tasks
VALUE % OPEN
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Unit No. 2 - Practical Tasks
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Unit No. 2 - Practical Tasks
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1)
Return the manual control to its design position. HCV-231 is 60% open.
2) .
Look at the screen. The upstream pressure has risen to 155 kg / cm2 Adjust the HCV-231 so that the MV returns to a SP of 96 TPH. The new setting of the valve is ……………………….% open.
Module No. 9 : Control systems 1
Unit No. 2 - Practical Tasks
PART 2
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Unit No. 2 - Practical Tasks
PRACTICAL TASK 3 AUTOMATIC FLOW CONTROL 1)
Make sure the Autodynamics simulator is operating as designed.
The screen VDU shows :
2)
PI-311
reads
146.4 kg/ cm2 g
FIRC-301
reads
129.0 TPH
PR-312
reads
133.5 kg/cm2 g
PI-313
reads
129 kg/cm2 g
Put the flow controller into manual and open the valve. The high alarm is set at ………………………..TPH. With the flow controller still in manual close the valve. The low alarm is set at ………………………….TPH.
3)
Return the simulator to its designed operating condition in automatic.
4)
Change the set point of FIRC-301 to 120 TPH (step change). Time the fall in the measured value every 10 seconds until the new set point is reached. Complete the enclosed graph from the results you have obtained.
6)
Could the response time be improved? Ask the instructor if he can change the PID settings for a quicker response.
Module No. 9 : Control systems 1
5)
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Module No. 9 : Control systems 1
Unit No. 2 - Practical Tasks
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Unit No. 2 - Practical Tasks
VALUE % OPEN
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Unit No. 2 - Practical Tasks
PRACTICAL TASK 4 TEMPERATURE CONTROL 1)
2)
Make sure the Autodynamics simulator is operating as designed. The VDU screen shows: TI-623
reads
180.0 °C
FR-602
reads
150.0 TPH
FI-601
reads
104.2 TPH
TI-621
reads
25.0°C
TR-622
reads
70.6 °C
FI-603
reads
53.7 TPH
TR-625
reads
81.4°C
TIRC-624
reads
116.7°C
The following questions are to be answered by observation of the screen. The instructor will be simulating process changes to illustrate temperature control a)
Why is the bypass flow rate rising? ------------------------------------------------------------------------------------------
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b)
Why is the bypass flow rate falling?
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What has happened? What could you do to correct the problem?
.
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.
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.
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Unit No. 2 - Practical Tasks
c)
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Unit No. 2 - Practical Tasks
PRACTICAL TASK 5 PRESSURE CONTROL 1)
2)
Make sure the Autodynamics simulator is operating as designed. The VDU screen shows: PI-711 FR-701
reads reads
15.5 kg/ cm2 g 10.2 TPH
HCV-731
is
40% open
HV-731
is
Open
FR-702
reads
10.2 TPH
PIRC-712
indicates
10 kg/cm2 g
PI-713
reads
4 kg/cm2 g
Close HCV-731 slowly until the FAL-701 comes on. The FAL-701 setting is ………………………….TPH. Open HCV-731 slowly until the FAH-701 comes on. The FAH-701 setting is ………………………..TPH. Does the PIRC-712 maintain control over the range from FAH to FAL.
3)
Return the simulator to its designed condition.
4)
Close HV-732. Describe what happens.
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5)
What is the setting of PSV-710
?
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Unit No. 2 - Practical Tasks
PRACTICAL TASK 6 CENTRIFUGAL PUMP CONTROL 1)
2)
Make sure the Autodynamics simulator is operating as designed. The VDU screen shows : HCV --544
is
closed
FIRC-502
reads
125 TPH
PI-513
reads
18 kg/cm2 g
PR-511
reads
4.5 kg/cm2 g
HV-541
is
open
HS-542
is
on
II-551
reads
290 A
PR-512
reads
19 kg/cm2 g
HV-543
is
open
FR-501
reads
125.0 TPH
Reduce the set point on FIRC until the FAL-501 comes on. What is the minimum flow setting for Loop-501 .................. If the FIRC-502 set point must be kept at this low flow level. How can we keep the pump running correctly?
Module No. 9 : Control systems 1
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Return the simulator to its designed position. Open the recycle line valve HCV-544. Describe what happens.
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Unit No. 2 - Practical Tasks
NOTES FOR INSTRUCTOR 1
ON/OFF CONTROL This task is given as an introduction to the Autodynamics simulator. Make sure that all students can operate the keyboard to change displays and manipulate the opening and closing of the valve. For student operation 5, the instructor will set the control desk so that PI-1 11 increases to 155 kg/cm2 with a ramp input set at 3.
2)
MANUAL CONTROL Make sure that all students can remember how to operate the keyboard to change displays and manipulate the opening and closing of the valve. For student operations in part 2 set the control desk so that PI 211 increases to 155 kg/cm2. Step increase is best.
3)
FLOW CONTROL Make sure that all students can remember how to operate the keyboard, change displays and manipulate the opening and closing of the valve. To obtain a reasonable response time for the student operation 4, set the simulator to 0.5 real time.
4)
TEMPERATURE CONTROL Make sure that all students can remember how to operate the keyboard, change displays and manipulate the opening and closing of the valve. This is a task to find if the student understands basic control concepts.
For student operation 2(a), decrease the process flow rate.
Module No. 9 : Control systems 1
For student operation 2(b), lower the inlet coolant temperature. For student operation 2(c), increase the coolant inlet temperature to 35°C and this process flow rate to 170 TPH. 5)
PRESSURE AND PUMP CONTROL
Make sure that all students can remember how to operate the keyboard, change displays and manipulate the opening and closing of the valve. These tasks are given so that the students can try basic manipulations of the process using the simulator keyboard. It is hoped they will understand •
The operation of the PSV at the high high level. Pressure control.
•
The operation of the recycle valve to maintain low output flow rates. Pump control.
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MODULE No. 10 PROCESS CONTROL FUNDAMENTAL
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UNITS IN THIS COURSE UNIT 1
BASIC CONTROL THEORY
UNIT 2
TUNING A CONTROLLER
UNIT 3
INTRODUCTION TO DCS AND PLC
UNIT 4
HONEYWELL TDC 3000 DCS
UNIT 5
FOXBORO IA DCS
UNIT 6
PRACTICAL TASKS
Module No. 10 : Process control fundamental
Unit No. 1 - Basic control theory
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TABLE OF CONTENTS Para
Page
1.0
COURSE OBJECTIVE
3
1.1
INTRODUCTION
4
1.2
PROPORTIONAL CONTROL ACTION
4
1.3
INTEGRAL (RESET) CONTROL ACTION
6
1.4
DERIVATIVE (RATE) CONTROL ACTION
7
1.4.1
Old Type Controller
8
1.4.2
Modern DCS Controller
8
1.5
RATIO CONTROL
10
1.6
CASCADE CONTROL
11
1.7
FEED FORWARD CONTROL
12
1.8
MULTI-VARIABLE CONTROL
13
1.9
ADAPTIVE CONTROL
14
Module No. 10 : Process control fundamental
Unit No. 1 - Basic control theory
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1.0 COURSE OBJECTIVE The student will be able to •
Describe the use of proportional control.
•
Describe the use of integral control.
•
Describe the use of derivative control.
•
Describe the use of combination (P plus I plus D) control
•
Describe the use of cascade control.
•
Describe the use of ratio control.
•
Describe the use of feedforward control.
•
Describe the use of adaptive control.
•
Describe the use of multi-variable control.
Module No. 10 : Process control fundamental
Unit No. 1 - Basic control theory
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Unit No. 1 - Basic control theory
1.1
INTRODUCTION The aim of this unit is to describe the use of proportional, integral and derivative control. The course also introduces the newer methods of control; cascade, ratio, feedforward, adaptive and multi-variable.
1.2
PROPORTIONAL CONTROL ACTION The basic continuous control mode is "proportional control". With proportional control the controller output is algebraically proportional to the error input signal to the controller. The simple block diagram model of the controller shows this.
In this case the controller output is the gain of the controller (K) times the error signal (E), or O/P = KE
Module No. 10 : Process control fundamental
This equation is called the control algorithm. The value of K can be set manually on older pneumatic equipment. On modern DCS systems it is set using a computer programme. The mechanism which adjusts the gain on many industrial controllers is expressed in terms of proportional band (PB). Proportional band is defined as the span of values of the input which corresponds to a full or complete change in the output. This is usually expressed as a percentage and is related to proportional gain by:
In practice, wide bands (high percentages of PB) have low gain and narrow bands have high gain. There are many ways to show the effects of varying proportional band. One example is shown in Figure 1-1.
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Unit No. 1 - Basic control theory
TRAINING MANUAL INSTRUMENTATION
Figure 1-1 Effect Of Proportional Control On Controller Output
Proportional control is quite simple. It is the easiest of the continuous control modes to tune, as there is only one value to adjust. It is very stable and responds quickly to changes.
Module No. 10 : Process control fundamental
However, proportional control has one big disadvantage. At steady state, it shows "offset". This means there is a difference at steady state between the set point, (SP) and the actual value of the Measured Variable (MV).
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Unit No. 1 - Basic control theory
1.3
INTEGRAL (RESET) CONTROL ACTION Reset (integral) action provides a signal which depends on the size of the error signal. It is different from proportional control because it Will continue to cancel any error until the offset is zero. Reset (integral) control action is combined with proportional control action. This combination is called proportional-reset or proportional integral action (PI control). This combination provides a control action which is stable and responds quickly with no offset.
Module No. 10 : Process control fundamental
Figure 1-2 Proportional plus Reset Control Action
Figure 1-2 shows the action of P & I control. The rate at which integral action is applied depends on the reset time adjustment. This is measured in either repeats per minute or minutes per repeat, depending on the manufacturer. The simple diagram below is used to show what this means.
In the diagram above the reset action repeats the proportional action twice in one minute. The reset time is thus either 2 repeats per minute or 0.5 minutes per repeat.
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Module No. 10 : Process control fundamental
Unit No. 1 - Basic control theory
1.4
DERIVATIVE (RATE) CONTROL ACTION Derivative (rate) control action produces an output signal which is proportional to how fast the error signal changes (its rate). This type of control is only used when the loop response is very slow. Using derivative control on a loop which responds to changes quickly is dangerous. The output moves too quickly to a maximum or a minimum and can produce shock waves in the process being controlled. Derivative control action is only used with proportional and integral action. Together, the three control modes provide what is called a Proportional Integral Derivative control action, (PID control). Figure 1-3 shows the effect of PID control for a step change in the error signal.
Figure 1-3 The Output of a PID Controller
The output signal is a combination of the three control actions. Note that the rate adjustment changes how long the derivative signal is applied. Some manufacturers call derivative action "pre-act" as it only produces a signal at the start to quicken the response time. The older types of controller (e.g. Foxboro pneumatic type 43AP or SPEC 200 analog electrical/electronic) combine the PID Control into a single unit which operates on the error signal. Modern microprocessor controllers, however use a lot of PlD control but in different way. The block diagrams below show the difference.
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Unit No. 1 - Basic control theory
1.4.1
The block diagram shows a typical older type PID controller. If the set point is changed the derivative action can cause large and unstable changes in output. So, D is only used for very slow loops.
1.4.2
Module No. 10 : Process control fundamental
Old Type Controller
Modern DCS Controller
The block diagram shows a typical micro-processor based DCS controller. The derivative action is only applied to changes in the measured value. Changes in the set point are not affected by the derivative action. This method provides a better response to process changes and more accurate control. Note that the PID settings are changed using a computer programme. This programme must have an "algorithm" of the controller characteristics.
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Unit No. 1 - Basic control theory
The table below gives a summary of the different types of control action available. ON/OFF CONTROL. Inexpensive Extremely simple Excellent for control of large capacity (volume) systems. Process variable cycles about set point. Control valves are easily worn out. Cannot be used for small capacity systems.
PROPORTIONAL. Simple Good for small capacities. Stable when set up (tuned) correctly. Rapid response. Offset at steady state. Easy to tune.
PROPORTIONAL plus RESET (P & D.: No offset Better response time than reset alone P & 1 can reduce the stability of the loop. The gain may need to be reduced when
Module No. 10 : Process control fundamental
reset is added.
PROPORTIONAL plus RESET plus RATE (PID). Most complex Most expensive Rapid response No offset Difficult to tune Best control if properly tuned.
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Unit No. 1 - Basic control theory
1.5
RATIO CONTROL Ratio control is used when two fluids must be mixed together in a specific ratio. A practical way to do this is to use a standard control system to control the flow on one line. The same transmitter signal is used as a set point for a second controller which controls the flow in a second line. The ratio of one flow rate to the other can be changed by adjusting the gain for proportional band) of the secondary controller. Figure 1-4 shows a typical ratio control system. The air to fuel ratio of the fluid going to the combustion chamber is set at 2:1.
Module No. 10 : Process control fundamental
Figure 1-4 Ratio Control System
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Unit No. 1 - Basic control theory
1.6
CASCADE CONTROL Figure 1-5 is given as an example of cascade control. It shows a chemical mixing vessel. When the two chemicals are mixed they produce heat. Cooling water is passed around the outside of the vessel via spray rings. This keeps the temperature of the reaction constant. The temperature is kept constant using cascade control as follows. The principle of cascade control is to use two controllers. A "master" controller and a "slave" controller. In the drawing the "master" controller compares the temperature of the mixture (TT1) with the setpoint. The "master" controller output signal is used as the set point for the "slave" controller. This compares the "master" output signal with the temperature of the cooling water (TT2). The "slave" output adjusts the cooling water flow valve to maintain the temperature of the mixture at the desired value. The advantage of this type of control is that if there is a change in the temperature of the mixture, the set point driven "slave" will begin corrective action more quickly.
Module No. 10 : Process control fundamental
Cascade control is used mainly on slow reaction processes; in this example, large capacity temperature control.
Figure 1-5 Cascade Control
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Unit No. 1 - Basic control theory
1.7
FEED FORWARD CONTROL Feed forward control is unusual on the older types of control equipment. However, it is becoming popular in µP systems, particularly in the control of large gas-turbine/centrifugal compressor units. Feed forward control is best explained using the following example.
Figure 1-6 Feed Forward Control
Module No. 10 : Process control fundamental
Figure 1-6 shows a liquid being heated by steam in a heat exchanger. The controlled variable is the outlet temperature of the liquid (T2). T2 is maintained at a constant temperature by controlling the flow of steam (F2). The flow control valve for the steam is controlled by a standard controller (FC which gets its "set point" from the feed forward controller. The output signal of the feed forward controller depends on both the temperature (T1) and flow rate (F1) of the liquid as it comes in. This means that any changes at the inlet are detected "before" it affects the outlet temperatures. This means that the response to changes is quicker and so there is closer control of the controlled variable. The operator adjusts the set point on the feed forward controller so that the temperature on the output (T2) is the same as the temperature (T2) of the set point.
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Unit No. 1 - Basic control theory
1.8
MULTI-VARIABLE CONTROL The modern µP controller uses multi-variable control. The controller has mathematical algorithms set into the micro-processor. These provide an output computed from many different inputs . A typical example is the anti-surge controller of a gas compressor. Figure 1-7 below shows a typical UIC system.
Figure 1-7 Multi Variable Control
Module No. 10 : Process control fundamental
The output from the UIC to the recycle valve depends on the inputs from 6 different transmitters. The controller normally uses a P plus I plus D control action. The values of PID are set using a software programme.
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Unit No. 1 - Basic control theory
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1.9
ADAPTIVE CONTROL Adaptive control is a method whereby the gain of a system can be varied depending on the position of the set point. The following shows a simple example of why this is useful in control systems.
Figure 1-8 Level Control of a Separator
Module No. 10 : Process control fundamental
Figure 1-8 shows the level control of a separator. The level in the separator can be set to control at position A or position B. As the level changes the volume of liquid to be removed or added at position A is much greater than what must be removed or added at position B. So, for good response the gain at position A should be greater than the gain at position B. The LIC is µP based. The gain of the controller is programmed by the engineer so that it changes when the set point is changed.
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UNITS IN THIS COURSE UNIT 1
BASIC CONTROL THEORY
UNIT 2
TUNING A CONTROLLER
UNIT 3
INTRODUCTION TO DCS AND PLC
UNIT 4
HONEYWELL TDC 3000 DCS
UNIT 5
FOXBORO IA DCS
UNIT 6
PRACTICAL TASKS
Module No. 10 : Process control fundamental
Unit No. 2 - Tuning a controller
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
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TRAINING MANUAL INSTRUMENTATION
Para
Page
2.0
COURSE OBJECTIVE
3
2.1
INTRODUCTION
4
2.2
WHAT IS GOOD CONTROL
4
2.3
TUNING A LOOP
6
Module No. 10 : Process control fundamental
Unit No. 2 - Tuning a controller
TABLE OF CONTENTS
July 1999- Rev.0
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TRAINING MANUAL INSTRUMENTATION
COURSE OBJECTIVE The student will be able to •
Describe good loop control using simple diagrams.
•
Explain how the PID settings of a controller are set to obtain good control.
•
Describe modern tuning methods.
•
Explain how you can get the best results by co-operating with the panel operator.
Module No. 10 : Process control fundamental
Unit No. 2 - Tuning a controller
2.0
July 1999- Rev.0
Page 3/8
TRAINING MANUAL INSTRUMENTATION
Module No. 10 : Process control fundamental
Unit No. 2 - Tuning a controller
2.1
INTRODUCTION The aim of this unit is to introduce the basics of controller tuning.
2.2
WHAT IS GOOD CONTROL
Figure 2-1 Good Control
Figure 2-1 shows a typical control loop The job of the loop is to keep the measured variable (MV) at the set point. The loop is called a feedback control system. If flow is the process variable, it works as follows. If the flow rate increases the error detector sends a signal to the controller. This signal indicates how much the measured value is more than the set point (MV-SP). The controller then adjusts the correcting unit (CU) so that the MV decreases and the flow rate goes down to the set point.
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If the flow rate decreases to below the set-point, then the error signal is (SP-MV). In this case the controller adjusts the correcting unit (CU) so that the MV increases and the flow rate goes up to the set point. When you are tuning a loop you cannot wait for the variable to change. So, the set point is changed and the MV moves to the new set point. The effect on the control loop is the same. The graph shows the tuning of the three mode controller (PID) to give good control for a step set point change. The response time should be fast and the MV should only go a little over the SP before it stabilises, (small overswing). In some processes it is important to tune the system so that there is no overswing. A slow and smooth response is needed. However, some processes need a fast response and quite big overswings are no problem. The point is you must decide what kind of control is good for each specific loop.
Module No. 10 : Process control fundamental
Unit No. 2 - Tuning a controller
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
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TRAINING MANUAL INSTRUMENTATION
TUNING A LOOP If you must tune a loop it is best to tune it when its on automatic (closed loop). It is possible to tune it on manual (open-loop) but this can be dangerous. The operator may not allow any open loop tuning. For any feedback control system, if the loop is closed (the controller is on automatic), you can increase the controller gain. As you do this, the loop will start to swing more and more. As you continue to increase the gain, you will see continuous cycling (oscillation) in the controlled variable. This is the maximum gain at which the system may be operated before it becomes unstable. The period (time) of these continuous oscillations is called the ultimate period (see Figure 2-2).
Module No. 10 : Process control fundamental
Unit No. 2 - Tuning a controller
2.3
Figure 2-2 Control Loop Response
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TRAINING MANUAL INSTRUMENTATION
1.
Tune out all the reset and derivative action from the controller, leaving only the proportional action. This means you should set T1 equal to infinity and Td equal to zero on the controller (or as close to these values as possible).
2.
Maintain the controller on automatic i.e., leave the loop closed.
3.
Set the gain of the proportional mode of the controller at any value. Then make a disturbance on the process and see what happens. One easy way of making a disturbance is to move the set point for a few seconds and then return it to its original value.
4.
If the response curve from step 3 does not damp out, as in curve A (see Figure 2-2), it means the gain is too high (the proportional band setting is too low). The gain should be decreased by increasing the proportional band setting. Then you repeat step 3.
5.
If the response curve from step 3 stops swinging, as in Curve C (see Figure 2-2), it means the gain is too low (the proportional band setting is too high). The gain should be increased by decreasing the proportional band setting. Then you repeat step 3.
6.
If the response from step 3 cycles continuously, as in Curve B (see Figure 2-2), it means you have best possible gain (optimum proportional band setting). The "ultimate period" of the response curve should be noted. This is the maximum gain at which continuous oscillations are maintained. The ultimate period is written as PU
Module No. 10 : Process control fundamental
Unit No. 2 - Tuning a controller
To determine the maximum gain and the ultimate period, take the following steps:
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Unit No. 2 - Tuning a controller
TRAINING MANUAL INSTRUMENTATION
The values obtained from step 6 are then used to set the PID of the controller. The standard method for setting the values of PID is the Ziegler and Nichols method as follows: Proportional only Set the gain to half the maximum gain (or twice the PB setting) Proportional plus reset. Set the gain to 0.45 of the maximum gain (or 2.22 times the PB setting) Set the reset to read 0.83Pu Proportional plus reset plus derivative Set the gain to 0.6 of the maximum gain (or 1.7 times the PB setting) Set the rest to read 0.5 Pu. Set the rate (derivative) to read 0.13 Pu
Module No. 10 : Process control fundamental
If you follow the above procedure it will produce a reasonably tuned loop. However, it may need to be adjusted during operation. The way the loop responds to real changes in the process must be monitored by the operator. It can take hours for a loop to settle down to normal operational requirements. The operator must decide if any fine tuning is required. On older control loops the response to process changes can be seen on the ink/paper recorder. A modern DCS system displays process variables on the VDU, using what are called "trend" displays. You must ask the operator to show these displays when you check the loop. Normally, it is possible to print these "trend" displays as the information is stored on a hard disc (the historian). You can usually get a copy of what the loop has done for the last 24 hrs.
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UNITS IN THIS COURSE UNIT 1
BASIC CONTROL THEORY
UNIT 2
TUNING A CONTROLLER
UNIT 3
INTRODUCTION TO DCS AND PLC
UNIT 4
HONEYWELL TDC 3000 DCS
UNIT 5
FOXBORO IA DCS
UNIT 6
PRACTICAL TASKS
Module No. 10 : Process control fundamental
Unit No. 3 - Introduction to DCS and PLC
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 1/9
TABLE OF CONTENTS Para
Page
3.0
COURSE OBJECTIVE
3
3.1
INTRODUCTION
4
3.2
INTRODUCTION TO DCS
4
3.2.1
The 5 Level Concept
5
3.2.2
Level Concept Description
6
3.3
PROGRAMMABLE LOGIC CONTROLLERS (PLC)
8
3.3.1
8
Introduction
Module No. 10 : Process control fundamental
Unit No. 3 - Introduction to DCS and PLC
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 2/9
3.0
COURSE OBJECTIVE The student will be able to: •
Draw a simple block diagram of a DCS system.
•
Explain the 5 level DCS concept.
•
Draw a simple block diagram of a PLC system.
•
Explain the use of PLC in field control.
Module No. 10 : Process control fundamental
Unit No. 3 - Introduction to DCS and PLC
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
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Module No. 10 : Process control fundamental
Unit No. 3 - Introduction to DCS and PLC
TRAINING MANUAL INSTRUMENTATION
3.1
INTRODUCTION
The aim of this unit is to introduce, in simple terms, what is meant by DCS and PLC in instrument control systems.
3.2
INTRODUCTION TO DCS Modern control systems in the oil/gas industry now use what is called a Distributive Control System (DCS). This means that the control of a plant (eg. refinery) is split into small units which are distributed around the plant.
Figure 3-1 A Simple DCS System
Figure 3-1 shows a simple DCS system. The plant consists of three separate (distributed) local control units: fractionation, compression and boiler. The loops for each unit are controlled by a local control unit. The information required by the operator is sent by a single cable (data highway) to the central control room. Here, the information is shown on a workstation Video Display Unit (VDU). The operator can adjust set points, Motor Operated Valves (MOV's) etc., from the workstation using the same data highway. There are various manufacturers of DCS’s. Operating Companies use a variety of these systems. The following notes are given as an introduction to the DCS system you might see.
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3.2.1
The 5 Level Concept The layout of a particular DCS depends on the manufacturer. However, all manufacturers make the same type of 5 level DCS as shown in Figure 3-2. Read the diagram from the bottom (level 1) to the top (level 5).
Module No. 10 : Process control fundamental
Unit No. 3 - Introduction to DCS and PLC
TRAINING MANUAL INSTRUMENTATION
Figure 3-2 The 5 Level DCS Concept
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Module No. 10 : Process control fundamental
Unit No. 3 - Introduction to DCS and PLC
TRAINING MANUAL INSTRUMENTATION
3.2.2
Level Concept Description Level 1 This is the field device level. Input devices (transmitters etc.) and output devices (control valves, etc.) are connected to input/output units (I/0 units). I/0 units convert the 4-20mA or digital signals to specially coded signals for the fieldbus. The I/0 units also convert the coded signals on the fieldbus to 4-20mA or digital signals for output control. The fieldbus (data highway) is a cable similar to a co-axial cable (like a television aerial cable). It sends the coded digital signals from level 1 to the control processors at level 2. Level 2 This is the control level. The control processor (CP) uses the data from the fieldbus to control individual control loops. The CP can control more than one loop at a time. The PID settings are placed in the CP using a software program similar to a computer floppy disc.
Figure 3-3 Control Processor with 5 Loops
Figure 3-3 above shows a control processor operating 5 loops (measured variables) at one time. The data on the fieldbus contains all the information for each loop input and output. The CP, using only milliseconds of time, controls each in turn. The PID of each is separately programmed. To the operator it looks as if all the loops are controlled at the same time.
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Unit No. 3 - Introduction to DCS and PLC
TRAINING MANUAL INSTRUMENTATION
Level 3 This is the operator's supervision level. The information which is needed for each loop is displayed on a Video Display Unit (VDU). The operator (supervisor) can adjust the set point, or he can change from manual to automatic etc. using the keyboards on the workstation. Large control systems may have many workstations which display the distributed control units around the plant. Remember that loop control is done from the CP. A fault on a workstation, eg. loss of picture, does not mean the plant has lost control. Level 4 This is the local management control level. The Applications Processor (AP) takes some of the signals from the CP and puts them into a digital code (protocol) so that they can be sent over a higher level data highway. This allows the plant engineer, plant manager, etc. to look at the plant operation remote from the Central Control Room (CCR), e.g. in his office. Normally, you cannot change control operations from this level. It can only display information for management overview. I Level 5
Module No. 10 : Process control fundamental
The group management level. Some signals for the AP are converted so that they can be sent (by microwave link, satellite, etc.) to a distant headquarters. This means that a senior manager, at group level, can view plant operations. The workstation at headquarters can not make changes at plant level. However, the workstation displays up to date information on production operations for planning purposes.
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Module No. 10 : Process control fundamental
Unit No. 3 - Introduction to DCS and PLC
TRAINING MANUAL INSTRUMENTATION
3.3
PROGRAMMABLE LOGIC CONTROLLERS (PLC)
3.3.1
Introduction Programmable Logic Controllers provide electronic switching operations (for emergency shut down procedures, fire and gas alarm systems, etc.). The system is not made to control a loop or a plant. It only provides a switching sequence which is controlled by a software programme. Usually software programmes are written for a PLC which operates an emergency shutdown system, such as fire and gas alarms, starting electric motors and safety circuits etc. These systems can be very large. They require a systems engineer to design the software programme to operate them. However, the following notes are given as an introduction to how these systems work.
Figure 3-4 Simple PLC Shutdown System
Figure 3-4 shows a simple PLC shutdown system. It has an "AND" gate with three input lines. These input lines consists of alarms (either high high or low low), fire detectors (FD) and gas detectors (G D). Alarms or detectors are connected in series to a +24V dc supply. The second "AND" gate has two inputs. One is from the first "AND" alarm circuit gate and the other is from the "ESD" button, which is normally closed. The micro processor controls the shut down circuits (relays, control valves, solenoid valves, etc.) together with an EEPROM. The EEPROM is the memory chip which holds the shut down sequence programme. This programme is put in by the system engineer.
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Under normal working conditions all switches are closed and a "V' digital signal holds the microprocessor so that the "shutdown" devices are in their correct, working position (energised). If a field alarm operates or the ESD button is pressed the "AND" gate output to the µP changes from "V to a "0". The µP now changes the output devices to their "shutdown" positions, using a logic. sequence which comes from the programme in the EEPROM memory. Note that all shutdown systems are made so that they are energised when they are working normally. It is not "safe" to make the system energise for shutdown. An equipment fault or supply problem would not shut the plant down if there was an emergency.
Module No. 10 : Process control fundamental
Unit No. 3 - Introduction to DCS and PLC
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
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UNITS IN THIS COURSE UNIT 1
BASIC CONTROL THEORY
UNIT 2
TUNING A CONTROLLER
UNIT 3
INTRODUCTION TO DCS AND PLC
UNIT 4
HONEYWELL TDC 3000 DCS
UNIT 5
FOXBORO IA DCS
UNIT 6
PRACTICAL TASKS
Module No. 10 : Process control fundamental
Unit No. 4 - Honerwell DTC 3000 DCS
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
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Unit No. 4 - Honerwell DTC 3000 DCS
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para
Page
4.0
COURSE OBJECTIVE
3
4.1
INTRODUCTION
4
4.2
HONEYWELL TDC 3000 DCS
4
4.2.1
Introduction
4
4.2.2
TDC 3000 System Overview
4
HONEYWELL TDC 3000 HARDWARE
7
4.3.1
Introduction
7
4.3.2
The Process and Logic Manager Cabinets
7
4.3.3
UCN and LCN
10
4.3
11
Module No. 10 : Process control fundamental
4.3.4 Control Room Layout
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TRAINING MANUAL INSTRUMENTATION
COURSE OBJECTIVE The student will be able to •
Explain, at block diagram level, the Honeywell TDC 3000 system.
•
Explain the terms used by Honeywell for their DCS equipment.
Module No. 10 : Process control fundamental
Unit No. 4 - Honerwell DTC 3000 DCS
4.0
July 1999- Rev.0
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TRAINING MANUAL INSTRUMENTATION
Unit No. 4 - Honerwell DTC 3000 DCS
4.1
INTRODUCTION The aim of this unit is to introduce the Honeywell TDC 3000 and to relate the TDC 3000 system to the 5 level DCS concept. The unit also explains the terms Honeywell uses for ordinary DCS equipment.
4.2
HONEYWELL TDC 3000 DCS
4.2.1
Introduction The autodynamics simulator which you used when you learned about control systems uses the graphics (pictures and symbols) of the Honeywell TDC 3000 workstation. The displays of PFD's, trends, indicators, alarms etc. are the same as you will see on a real control room VDU. This unit is given to show the layout of the TDC 3000 system which provides the signals seen on the VDU.
4.2.2
TDC 3000 System Overview
Module No. 10 : Process control fundamental
Figure 4-1 is a simplified diagram of the components of the Honeywell TDC (totally distributed control) 3000 DCS. The notes at the side show how the TDC 3000 fits a generalised DCS system. The Honeywell TDC 3000 is a newer version of the older TDC 2000. Therefore, it has extra items not seen on the older systems. The main features are as follows: 1.
Loop control (4-20mA, smart or digital) is carried out by the process manager (PM). The I/0 units for the loops can be placed locally (inside the same cabinet as the PM) or placed at a remote location in the plant. Remote signals are sent using a serial data transmission link (RS422 standard) to the PM.
2.
The switched signals (e.g. alarms, valve positions, fire detectors, gas detectors, etc.) are processed separately using a Logic Manager (LM). The I/0 units for the Logic Manager can be either local or remote in the same way as the PM.
3.
The PM uses a microprocessor to control the loops. This µP can be programmed to set the PID for each loop as required. The µP may also be able to tune the loops automatically for best response. It has an auto-tune facility.
4.
The LM uses a microprocessor to operate the PLC. The Logic Manager µP is programmed by the systems engineer to give the correct shutdown sequences etc.
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5.
Signals needed by -the supervisors on the workstations are transmitted via a Universal Control Network (UCN). This network is not compatible with the workstation local area network, (called a Local Control Network (LCN) by Honeywell). Therefore, a Network Interface Module (NIM) is used.
6.
The LCN data highway provides the signals to...
Module No. 10 : Process control fundamental
Unit No. 4 - Honerwell DTC 3000 DCS
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
•
The operators workstations (VDU and keyboards). These are called Universal Stations (US) by Honeywell.
•
Printers
•
Historian (History Module). This contains a redundant hard disc system which stores operating information so that "trends" can be printed or displayed.
•
Gateways
•
These gateways process signals which can be sent to a management workstation. The diagram shows a gateway which changes LCN data so that it can be sent on an "ETHERNET" high level data LAN. A gateway can be provided to operate a modem. The modem can send signals by radio to the headquarters management if required.
•
Because Honeywell uses its own system for data management (the UCN), other systems have to be connected by special Data Conversion units. These are called Minicop Modules (MM). Typical systems that may be connected to the TDC 3000 are crude metering systems, tank gauging systems, compressor control systems etc.
•
To make the system more reliable many of the units run on what is called a "Redundant" system. This means that there are two units for each operation (e.g. UCN links, NIM, Historian, etc.). If one doesn't work the other takes over automatically.
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Module No. 10 : Process control fundamental
Unit No. 4 - Honerwell DTC 3000 DCS
TRAINING MANUAL INSTRUMENTATION
Figure 4-1 Simplified Block Diagram Honeywell TDC 3000
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Unit No. 4 - Honerwell DTC 3000 DCS
TRAINING MANUAL INSTRUMENTATION
4.3
HONEYWELL TDC 3000 HARDWARE
4.3.1
Introduction The following diagrams are given, with a brief description, to show what the Honeywell TDC 3000 hardware looks like._
4.3.2
The Process and Logic Manager Cabinets Both cabinets are the same size (shown on the LM in Figure 4-2) and are normally placed side by side in the instrument equipment room. The number of each cabinet depends on the installation. LOGIC MANAGER CABINET Figure 4-2 shows the layout of a typical logic manager. The top two racks contain the cards required to make the logic manager and the redundant partner. The bottom two racks contain the 1/0 cards for parallel and serial operation. Parallel operation I/0 cards are used for loops near the LM rack (less than 30m). Serial operation I/0 cards are used for processing serial data coming from remote locations (RS 422 link).
Module No. 10 : Process control fundamental
The cards which are fitted into the slots of each cabinet can be removed easily. They are connected to the system by printed board edge connectors. Normally these cards can not be repaired. The whole card is replaced if it does not work. The broken card is sent back to Honeywell for repair. The LM supplies power at 110V DC and 240V DC. This provides the switching voltages for both input devices (e.g. alarm switches) and output devices (e.g. solenoid valves). PROCESS MANAGER CABINET. Figure 4-3 shows the layout of a typical process manager. The top rack contains the input/output cards (called IOP cards by Honeywell). Racks 2 and 3 contain the redundant process manager cards. The bottom of the cabinet contains the power supplies to operate both the process manager and IOP cards. It also provides the 24V DC needed to drive the loops etc. Note:
July 1999- Rev.0
All loops are connected to the back of the racks through the safety barriers. The Honeywell system is not intrinsically safe.
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TRAINING MANUAL INSTRUMENTATION
Module No. 10 : Process control fundamental
Unit No. 4 - Honerwell DTC 3000 DCS
LOGIC MANAGER CABINET -
Figure 4-2 Logic Manager
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TRAINING MANUAL INSTRUMENTATION
Module No. 10 : Process control fundamental
Unit No. 4 - Honerwell DTC 3000 DCS
PROCESS MANAGER CABINET
Figure 4-3 Process Manager Cabinet
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TRAINING MANUAL INSTRUMENTATION
UCN and LCN The UCN and LCN are both redundant co-axial cables. The nodes (taps) on these cables are specially made so that if they are disconnected the lines are not broken. The other connected units continue working. Each cable has a special terminator (75Ω. This must not be removed as it stops signals which reach the end of the cable from being reflected. Any reflected signals will interfere with processing data and produce errors. Figure 4-4 shows a redundant UCN as an example.
Module No. 10 : Process control fundamental
Unit No. 4 - Honerwell DTC 3000 DCS
4.3.3
Figure 4-4 The UCN Network
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Control Room Layout The universal stations provide the human interface with the TDC 3000 systems. The make up of each individual control room depends on the user's needs. It is made up of what Honeywell call "Optimum Replaceable Units" (ORU). A typical arrangement of a control room is shown in Figure 4-5.
Module No. 10 : Process control fundamental
Unit No. 4 - Honerwell DTC 3000 DCS
4.3.4
Figure 4-5 Supervisors Control Centre
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The following points should be noted on the typical Honeywell TDC 3000 supervisors control room. 1.
Cartridge/Floppy disc drives similar to a PC drive are optional extras. These allow new operating programmes to be added.
2.
The matrix printer is used to print plant operations history for management overview.
3.
Some instrument/process engineers find the old type of pen recorders a convenient way of showing trends. These can be added to the system if required.
4.
The engineer can connect a portable keyboard into the system. This keyboard is used to re-configure the system as required.
5.
The latest Honeywell US has a touch screen facility . To change the display the keyboard is not required. Touch the screen and the display changes as the operator instructs.
6.
The Trackball is the same as an ordinary PC mouse. Rotate the ball and the cursor moves, to the required position on the screen. A trackball is only a mouse upside down.
7.
The electronics modules are contained in what Honeywell call a 5 slot chassis. These 5 slot chassis contain the electronics to drive the US, Historian, Gateways, etc.
8.
The Network Interface Module (NIM), which is fully redundant, is usually housed as shown.
Module No. 10 : Process control fundamental
Unit No. 4 - Honerwell DTC 3000 DCS
TRAINING MANUAL INSTRUMENTATION
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UNITS IN THIS COURSE UNIT 1
BASIC CONTROL THEORY
UNIT 2
TUNING A CONTROLLER
UNIT 3
INTRODUCTION TO DCS AND PLC
UNIT 4
HONEYWELL TDC 3000 DCS
UNIT 5
FOXBORO IA DCS
UNIT 6
PRACTICAL TASKS
Module No. 10 : Process control fundamental
Unit No. 5 - Foxboro IA DCS
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
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Unit No. 5 - Foxboro IA DCS
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para
Page
5.0
COURSE OBJECTIVE
3
5.1
INTRODUCTION
4
5.2
THE BASIC FOXBORO IA SYSTEM
4
5.2.1
Introduction
4
5.2.2
Foxboro IA System Layout.
4
5.2.3
Foxboro IA Hardware
7
TYPICAL FOXBORO IA CENTRAL CONTROL ROOM
10
Module No. 10 : Process control fundamental
5.3
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5.0
COURSE OBJECTIVE The student will be able to •
Explain the terms used by Foxboro in their DCS.
•
Sketch the layout of the Foxboro enclosure.
•
Draw a simplified diagram of the Foxboro DCS and explain what the main components do.
Module No. 10 : Process control fundamental
Unit No. 5 - Foxboro IA DCS
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
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Unit No. 5 - Foxboro IA DCS
TRAINING MANUAL INSTRUMENTATION
5.1
INTRODUCTION The aim of the unit is to introduce the Foxboro Intelligent Automation (Fox IA) DCS. It explains the terms used by Foxboro and shows how IA fits the 5 level concept of a distributed control system.
5.2
THE BASIC FOXBORO IA SYSTEM
5.2.1
Introduction
5.2.2
Foxboro IA System Layout. The Foxboro IA system is one of the most up to date distributed control systems for oil/gas production. The Foxboro system is an improvement on older DCS's as it uses an "Open Industrial System". This means the software used to programme the DCS is industrial standard (UNIX). It can be used to set up both the Foxboro system and equipment from other manufacturers connected to it. It has improved data handling so there is no need for "handshaking". It is also has a system called "Reporting by Exception". This means that measurements are only changed if their values change.
Module No. 10 : Process control fundamental
I
Figure 5-1 shows the layout of a typical Foxboro IA DCS. The levels of control are the same as the Honeywell but the system is simpler at the 1, 2 and 3 level. So, it only needs one data transmission highway. The following points should be noted. 1)
The diagram shows a large Integrated Control System (e.g. control and management of a multi-platform offshore oil field). The supervision level is located on each platform. The area management level consists of a central control room. This is usually on the accommodation platform. It can display information from all the platforms. The group management display is at the headquarters on shore. Data transmission between units can be either by cable using a high level LAN (ETHERNET) or by radio link (e.g. satellite, microwave link, etc.)
2)
The 1/0 conditioning units and the control processors are located in one unit. This can be placed anywhere in the plant.
3)
Signals to and from the control room are placed on what Foxboro call a "Redundant Nodebus". The same cable is used for the control room and field units. If the field units are a long way from the control room a "Nodebus Extender" (NBE) is used. This maintains the voltage level of the transmitted data.
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4) The following processors are connected to the nodebus: AP
-
Applications Processor. This allows data to be processed for the Historian, Printer, etc.
WP
-
Workstation Processor. This allows data to be processed for the workstation, (e.g. VDU, keyboard, mouse/trackball, touch screen, etc.).
Comm
-
Communications Processor. This allows data to be processed for items not using UNIX, (mainly the engineer's PC which uses ASCII)
FDG
-
Foreign Device Gateway Processor. This allows data to be processed so that signals can be received or sent through a modem. The data can be sent to another location, platform, etc.
CBLI
-
Carrier Based LAN Interface. This unit converts the UNIX programming system (protocol) of the nodebus to a protocol for the high level LAN (e.g. Ethernet) and vice-versa. The area management system uses the same equipment as in the control room. Therefore, there must be a CBLI at both ends of the high level LAN.
Module No. 10 : Process control fundamental
Unit No. 5 - Foxboro IA DCS
TRAINING MANUAL INSTRUMENTATION
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Module No. 10 : Process control fundamental
Unit No. 5 - Foxboro IA DCS
TRAINING MANUAL INSTRUMENTATION
Figure 5-1 Foxboro IA DCS Overview
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5.2.3
Foxboro IA Hardware Foxboro combines the first level (field devices) and second level (control devices) into one unit. This unit is called an "Enclosure". It can be placed at any suitable position in the plant. Figure 5-2 shows a Foxboro enclosure in a plant.
Module No. 10 : Process control fundamental
Unit No. 5 - Foxboro IA DCS
TRAINING MANUAL INSTRUMENTATION
Figure 5-2 Foxboro IA Enclosure
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Module No. 10 : Process control fundamental
Unit No. 5 - Foxboro IA DCS
TRAINING MANUAL INSTRUMENTATION
Figure 5-3 Foxboro Industrial Enclosure
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Module No. 10 : Process control fundamental
Unit No. 5 - Foxboro IA DCS
TRAINING MANUAL INSTRUMENTATION
Figure 5-3 shows the inside of an enclosure. The bottom two racks have slots into which the 1/0 units are fitted. These are called Field Bus Modules (FBM) by Foxboro. These FBM's can process any type of field input/output. Some FBM examples are given below.
The top of the enclosure has slots which hold the system processors (called stations by Foxboro). A plant enclosure normally has only CP's and NBE's. The CP's control the area loops and the NBE extends the redundant nodebus to the supervisor's control room.
FBM FBM 01
FUNCTION 0-20 mA INPUT
POINTS 8 AI
FBM 02
THERMOCOUPLE 1mV INPUT
8 A]
FBM 03
RTD INPUT
8 AI
FBM 04
0-20mA INPUT 1 OUTPUT
.4A1/4A0
FBM 10
120 VDC INPUT / OUTPUT SWITCHED
8D1/8DO
FBM 18
SMART TRANSMITTERS
8DA1
AI
ANALOG INPUT
DI
DIGITAL INPUT
AO
ANALOG OUTPUT
DO
DIGITAL OUTPUT
POINTS
Number of inputs/outputs available on each FBM
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TYPICAL FOXBORO IA CENTRAL CONTROL ROOM
Module No. 10 : Process control fundamental
Unit No. 5 - Foxboro IA DCS
5.3
Figure 5-4 Foxboro IA CCR
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TRAINING MANUAL INSTRUMENTATION
Module No. 10 : Process control fundamental
Unit No. 5 - Foxboro IA DCS
Figure 5-4 shows the typical layout of a Foxboro Central Control Room (CCR). A basic description of the system is as follows: 1)
The Central Control Room processes signals from both a remote location, using a CBLI, and from equipment on the main platform where the CCR is located.
2)
Main platform signals are routed through what are called marshalling cabinets. These contain the IS barriers. A Foxboro [A FBM is not intrinsically safe.
3)
The industrial enclosures are the same as the ones described in 5.3.2. However, they need no weather-proofing and Zone 1 protection. These are usually located in a terminal room behind the CCR.
4)
Whether the system is for local supervisory control or CCR control the equipment is the same. It is all run from a Redundant Nodebus connected by either NBEs or CBLIs.
5)
The AP, WP, and COMM processors can be fitted into a station in the enclosure or in slots under the workstations.
6)
The Nodebus Interface (NBI) connects the Nodebus to the engineers PC. This is done so that the engineer does not interfere with operators using the workstations.
7)
The Nodebus has terminators to stop data reflections, (in the same way as the Honeywell TDC 3000).
8)
All Nodebus connectors can be removed without affecting the other units connected to the bus.
9)
Remember that loop control is done by the CP. A fault on the CCR Nodebus will not cause the plant to lose control.
10)
The operator on the workstation normally only supervises the plant operations. However, he can switch a control loop from automatic to manual and perform set point changes.
11)
PID control functions and PLC sequence changes can only be done by the systems technician or engineer. This is usually done via the engineer's PC using a "password" know only to the engineering staff.
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Figure 5-5 shows the typical layout of a Foxboro control room.
Unit No. 5 - Foxboro IA DCS
12)
Module No. 10 : Process control fundamental
Figure 5-5 Foxboro IA Workstations
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UNITS IN THIS COURSE UNIT 1
BASIC CONTROL THEORY
UNIT 2
TUNING A CONTROLLER.
UNIT 3
INTRODUCTION TO DCS AND PLC
UNIT 4
HONEYWELL TDC 3000 DCS
UNIT 5
FOXBORO IA DCS
UNIT 6
PRACTICAL TASKS
Module No. 10 : Process control fundamental
Unit No. 6 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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TABLE OF CONTENTS Para
Page PRACTICAL TASK 1
3
PRACTICAL TASK 2
8
Module No. 10 : Process control fundamental
Unit No. 6 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
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Unit No. 6 - Practical tasks
PRACTICAL TASK 1 Introduction CASCADE CONTROL The Autodynamics simulator will be used to demonstrate cascade control. The process variable to be controlled is the level in a tank. A liquid storage tank is used to store a liquid before it is sent for downstream processing. The liquid has a variable density. Figure PT 1 shows the PFD and P and ID of the process. The normal operating conditions for the loop are: HCV-431 is open 50% to give a flow rate of 100 TPH entering V-400. The cascade loop maintains the level at 50% (half way) for a flow rate of 100 TPH. Alarm settings Above 160 TPH or below 60 TPH.
LAH/L 461
Above 75% or below 25%.
Module No. 10 : Process control fundamental
FAH/L 402
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Module No. 10 : Process control fundamental
Unit No. 6 - Practical tasks
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Figure PT - 1a Process Flow Diagram (PFD).Level Control.
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Module No. 10 : Process control fundamental
Unit No. 6 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
Figure PT 1b P & ID Level Control
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TRAINING MANUAL INSTRUMENTATION
Unit No. 6 - Practical tasks
PRACTICAL PROCEDURE PART 1 1)
Make sure the Autodynamics simulator is operating as designed. Module 4 Level Control. The VDU screen shows: FI-401
READS
100 TPH
FIRC-402
READS
100 TPH
HV-432
OPEN
HCV-431
READS
50%
LIRC-431
READS
50%
PI-412
READS
0.6 kg/cm2 g
1 2)
a) What happens if you adjust the set point of FIRC-402.
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Module No. 10 : Process control fundamental
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b)
Explain the result of part 2 (a)
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1
Place FIRC-402 to manual.
2.
Open HCV-431 so that it reads 60%
3.
Using the manual output control of FIRC-402, try and maintain the level of V-400 at 50%.
4.
Estimate the time taken to bring the level under control.
PART 3 1.
Return the simulator to normal designed condition, with full automatic control.
2.
Increase HCV-431 so that it is 60% open.
3.
Estimate the time taken to bring the level under control.
4.
Does cascade control produce faster response in the system?
Module No. 10 : Process control fundamental
Unit No. 6 - Practical tasks
PART 2
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TRAINING MANUAL INSTRUMENTATION
Unit No. 6 - Practical tasks
PRACTICAL TASK 2 Introduction RATIO CONTROL This task uses the Autodynamic simulator to show the operation of ratio control. Ratio control is used to produce a mixed product for tanker loading. It is possible with the simulation to switch from ratio to cascade operation, using a product analyser. This is also demonstrated. Figure PT 2 gives the VDU display (PFD) and the P & ID for the process. Mixing Operation The mixing operation is used in the in-line blender to mix the available components. The components are blended to meet product demands and product specification. The mixing components for this operation are a heavy product feed and a light product feed. The blended product for loading meets quality specifications using a ratio or cascade control system. Under normal operation the ratio control is used for start up and cascade control for normal running. Ratio control sets the required mixture of heavy and light products. The cascade control is run by an analyser. It makes sure the correct ratio is maintained after the initial settings are made by the ratio control.
Module No. 10 : Process control fundamental
Logic Control This simulation has a PLC system added to provide the following safety function. Block valves HV-526 and HV-528 automatically close when the total volume of the respective trucks reaches 20 m3 (2000 litres). All flows stop, and the high volume alarm comes on. To restart the loading of another tanker the system must be reset by switches HS-531 or HS-532. These switches must be switched "Off" again after resetting. The PLC logic will not allow the block valve (HV-526 or HV-528) to open if the reset is left "On".
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Module No. 10 : Process control fundamental
Unit No. 6 - Practical tasks
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Figure PT-2(a) Product Mixing FPD
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Module No. 10 : Process control fundamental
Unit No. 6 - Practical tasks
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Figure PT-2(b) Product Mixing P & ID
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Unit No. 6 - Practical tasks
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PRACTICAL PROCEDURE Introduction To show how ratio control is done, the simulator exercise 5020 Module 5 will be carried out from the closed down position. The object will be to load a tanker with 20m3 (2000 litres) of SG 0.7 product. Note: The ratio calculation is done as follows (this is done automatically by the loading system).
Then
Let X
=
Quantity of Heavy product.
Let Y
=
Quantity of Light product.
(0.8 x X) + (0.4 x Y) = (X + Y)0.7 0.8X + 0.4 Y = 0.7X + 0.7Y 0.8X - 0.7X = 0.7Y - 0.4Y 0.1X = 0.3Y
Module No. 10 : Process control fundamental
The ratio of heavy to light product is 3:1 1.
The storage tank (slops tank) holds poor quality produce that cannot be sold. HV-526 must not be opened until AIRC-502 reads the correct quality on automatic. When the tanker is fully loaded the system will stop automatically. Make sure you don't open HV-525 until the AIRC-502 is on automatic.
2.
Remember to reset the totalizer after the tanker is loaded. Otherwise the PLC logic will not allow another tanker to be loaded.
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Module No. 10 : Process control fundamental
Unit No. 6 - Practical tasks
PART 1 START UP 1)
OPEN the storage tank block valve (HV-527).
2)
Turn ON HS-51 5 to start the heavy product feed pump (P-f51 5).
3)
With FIC-501 on manual, increase the output of FIC-501 to 10%. This will open the heavy feed valve (FCV-501).
4)
With FIC-503 on manual opens the light product feed valve by increasing the output of FIC-503 to 10%.
5)
Adjust the setpoint of FIC-501 to equal its process variable and transfer the controller to AUTOMATIC.
6)
Check the value at AIRC-502 for the specific gravity of the mixed liquid. If the specific gravity is undesirable, adjust the light flow through FCV-503.
7)
Adjust the ratio of heavy feed to light feed by adjusting the value at HC-20.
8)
Switch HS-529 to RATIO CONTROL, and place FIC-503 in the CASCADE mode.
9)
Continue to gradually increase the setpoint of FIC-501 to design (113.6m3/hr). The ratio controller HC-20 will automatically adjust the light flow through FCV-503. The specific gravity will not change.
10)
Perform a bumpless transfer to place AIRC-502 in control in the following manner:
11)
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a)
Divide the measured variable of FIC-501 by the instrument range.
b)
Multiply that number by the ratio factor set on HC-20.
c)
Convert that number to output percent by multiplying by a factor of 100.
d)
This number becomes the required output setting for AIRC 502.
e)
Adjust the output of AIRC-502 to the number above.
f)
Switch HS-529 to the FEEDBACK condition.
g)
Switch AIRC-502 to AUTOMATIC.
Begin loading tanker no. 1 by putting HV-526 in the OP condition and switching the storage tank block valve (HV-527 CLOSED.
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TRAINING MANUAL INSTRUMENTATION
MODULE No. 11 INSTRUMENT CRAFT PRACTICE
July 1999- Rev.0
UNITS IN THIS COURSE UNIT 1
WORKSHOP SAFETY AND TOOL CARE
UNIT 2
BASIC HAND TOOLS
UNIT 3
TUBING SYSTEMS
UNIT 4
CRIMPING
UNIT 5
PRACTICAL TASKS
Module No. 11 : Instrument craft practice
Unit No. 1 - Workshop safety and tools care
TRAINING MANUAL INSTRUMENTATION
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TRAINING MANUAL INSTRUMENTATION
Unit No. 1 - Workshop safety and tools care
TABLE OF CONTENTS Para
Page
1.0
COURSE OBJECTIVE
3
1.1
INTRODUCTION
4
1.2
WORKSHOP RULES
4
1.3
PERSONAL PROTECTION
6
1.3.1
Body Protection
6
1.3.2
Head Protection
7
1.3.3
Eye Protection
8
1.3.4
Hand Protection
9
1.3.5
Foot Protection
10
CARE OF TOOLS
11
Module No. 11 : Instrument craft practice
1.4
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COURSE OBJECTIVE The trainee will be able to: •
List the general safety rules used in the workshop.
•
Identify the items of personal protection commonly used in the workshop.
•
List the general rules for tool care.
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Unit No. 1 - Workshop safety and tools care
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Unit No. 1 - Workshop safety and tools care
1.1 INTRODUCTION To become a skilled craftsman, a trainee must learn to work safely. He must think of his own safety and the safety of other workers. The safest way of doing the job is the best way. Accidents are caused by careless work habits. They do not just happen, they, are caused. There is always the possibility of accidents when people are working with tools and equipment. These accidents can be costly and painful. Everyone must try to prevent accidents. Managers try to prevent accidents by providing: Workshop rules
2.
Protective clothing and personal safety equipment
3.
Safe tools and equipment
These items alone cannot prevent accidents unless the workers do their part to help. The responsibility of the worker is to: 1 . Follow the workshop rules.
1.2
Module No. 11 : Instrument craft practice
1.
2.
Use protective clothing and equipment.
3.
Use tools and equipment correctly.
WORKSHOP RULES Every workshop has its own safety rules. These rules will vary according to the different equipment in each workshop. It is every trainee's responsibility to learn these rules and then follow them. The purpose of workshop rules is to protect you and your fellow workers. The rules will not protect workers who do not learn and follow them. Although the rules vary from workshop to workshop there are some rules that apply to all workshops. 1.
Know your job: You, should know what you are going to do, how-to do it, and the tools needed before you begin work.
2.
Good housekeeping: Keep your work area neat and tidy. Unused parts, scrap material or tools, left lying around your work area will cause accidents.
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Conduct: A sudden interruption to a worker who is busy doing his job is dangerous; he could hurt himself or another worker.
4.
Clothing The type of clothing needed depends on the job and its dangers. Working in hot areas may require light, loose fitting clothes. Working near rotating equipment means not wearing loose clothes that can be caught. Some other jobs require special protection, such as hard hats, goggles, or safety shoes.
5.
Think safety: Being a skilled worker and knowing all the safety rules does not mean you will be a safe worker. You must think about safety at all times.
6.
Being alert: Although you may be a safe worker yourself, beware of others around you. They may endanger you if they are using machine tools, grinding, or welding.
Module No. 11 : Instrument craft practice
Unit No. 1 - Workshop safety and tools care
3.
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PERSONAL PROTECTION Wearing personal protection is important for developing good work habits. As well as protecting the body, you must protect your head, eyes, hands and feet. Some suitable equipment for personal protection is as follows:
1.3.1 Body Protection For general workshop work a coverall is the safest and most practical form of body protection. Figure 1-1 below shows the safe way to dress in the workshop.
Module No. 11 : Instrument craft practice
Unit No. 1 - Workshop safety and tools care
1.3
Figure 1-1 The Workshop Dress
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Unit No. 1 - Workshop safety and tools care
For extra protection, workers are sometimes required to wear canvas aprons. They are worn in machine shops when using drilling or grinding equipment (see Figure 1-2).
Module No. 11 : Instrument craft practice
Figure 1-2 The Canvas Apron
1.3.2
Head Protection In some work areas it is necessary to wear hard hats. These areas are usually places where work is going on above head height. Long hair is also a problem in the workshop where -there is rotating equipment. If the hair is caught it can cause serious injury.
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Unit No. 1 - Workshop safety and tools care
1.3.3
Eye Protection Eye protection should be worn in any work situation where there are flying objects. Jobs such as grinding, cutting, or machining require eye protection. Welding and furnace operations require tinted safety glasses or goggles to protect the eyes from too much light or flying objects. Safety glasses are similar to regular glasses except that the lenses are strong. Safety glasses only protect your eyes from the front. You can put side shields on the glasses to help protect the eyes from the side. Typical safety glasses are shown in Figure 1-3.
Figure 1-3 Safety Glasses
Module No. 11 : Instrument craft practice
Safety goggles are designed to provide more eye protection because they fit the face better. Goggles are kept in place with a head strap so they do not fall off as easily as safety glasses. The safety lenses are made of clear hard plastic. The lenses allow you to see but prevent flying objects from hitting your eyes. The choice of wearing safety glasses or goggles depends on the task being performed (see Figure 1-4).
Figure 1-4 Safety Goggles
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Unit No. 1 - Workshop safety and tools care
Face shields give full face protection and are good for people who wear ordinary glasses. They allow air to circulate between the face and shield so the glasses do not mist up. When you are working in areas where there are hot flying particles, face masks must be worn (see Figure 1-5).
Figure 1-5 The Face Mask
1.3.4
Hand Protection
Module No. 11 : Instrument craft practice
Gloves should be worn when handling sharp objects, such as sheet metal, casings, machined components and swarf. Gloves must be the correct type for the job, and they must be in good condition. However, gloves should never be worn when operating moving machinery. Most gloves are made of either leather, heavy cloth, canvas, rubber, or plastic. They are designed to prevent cuts from sharp edges, or burns from hot metal. Plastic gloves are worn when handling chemicals.
Figure 1-6 Safety Gloves
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Foot Protection Safety footwear are designed to protect the feet. They must be worn at all times around the plant and in the workshop area. They are reinforced with steel at the toes to prevent the toes from being crushed by falling objects. If you drop a heavy object on your toes it can cause a serious injury and can be very painful. The soles are non-slip and are often reinforced to prevent puncture by sharp objects.
Module No. 11 : Instrument craft practice
Unit No. 1 - Workshop safety and tools care
1.3.5
Figure 1-7 Safety Shoes
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CARE OF TOOLS It is important to make sure that tools are kept in good condition. Worn or damaged tools can cause injuries. A few examples of poorly maintained tools are shown in Figure 1-8.
Module No. 11 : Instrument craft practice
Unit No. 1 - Workshop safety and tools care
1.4
Figure 1-8 Poorly Maintained Tool
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Unit No. 1 - Workshop safety and tools care
The following is a list of general guide lines for the care of tools and equipment. Store tools in the proper place Each tool should have its own special place to be stored when it is not being used. When the job is finished, return the tool to its proper storage place. Putting them away immediately helps prevent them from being lost. Tools lying around the workshop can cause accidents; eg, tripping over an electric drill left on the ground. Care of delicate tools Some tools require special storage because they can be easily damaged. Measuring tools and instrument screwdrivers are good examples. They are soon damaged if they are thrown down or dropped. Take a little time to ensure that these tools are in a safe place when they are not being used. Regular inspection of tools and equipment Even with the best care, tools can become damaged and equipment worn. Some equipment has inspection and maintenance procedures provided by the manufacturer. These inspection procedures should be followed carefully. Small hand tools should be checked regularly. They should be repaired or replaced if they are defective. Electrically powered tools can be particularly dangerous if the cable is damaged; they become a fire hazard. Correct use of hand tools and equipment
Module No. 11 : Instrument craft practice
Using a tool for a job that it was designed for will not damage it. Some tool parts, like hacksaw blades, are replaceable, but the tool itself should not get damaged. If you use tools for other purposes they can be permanently damaged; eg, using wrenches as hammers and screwdrivers as levers. Tools are designed to be held and used in a certain way. Using tools wrongly will make the tool less effective and will cause damage. As an example, some power tools have guards or safety devices. It is very dangerous to remove the guards.
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Unit No. 1 - Workshop safety and tools care
Using the correct size tool Many tools come in different sizes. The size of the tool must suit the job it is to do. Using the wrong size tool is almost as dangerous as using the wrong tool. Using a small screwdriver to turn a large screw, or the wrong sized wrench to turn a nut is dangerous.
Keep tools and equipment clean
Module No. 11 : Instrument craft practice
Very little time is needed to clean your tools when you have finished the job. No one likes dirty tools or equipment. Keeping your tools clean shows you have pride in your work. Dirt and grease can destroy many tools.
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UNITS IN THIS COURSE UNIT 1
WORKSHOP SAFETY AND TOOL CARE
UNIT 2
BASIC HAND TOOLS
UNIT 3
TUBING SYSTEMS
UNIT 4
CRIMPING
UNIT 5
PRACTICAL TASKS
Module No. 11 : Instrument craft practice
Unit No. 2 - Basic hand tools
TRAINING MANUAL INSTRUMENTATION
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TABLE OF CONTENTS Para
Page
2.0
COURSE OBJECTIVE
3
2.1
INTRODUCTION
4
2.2
VICE AND CLAMPS
4
2.3
MEASURING INSTRUMENTS
7
2.4
HAMMERS
9
2.5
PUNCHES
10
2.6
FILES
11
2.7
PLIERS
12
2.8
WRENCHES
13
2.9
DRILLS, TAPS AND DIES
17
2.10
THE POWER DRILL
19
2.11
PNEUMATIC IMPACT WRENCH
20
Module No. 11 : Instrument craft practice
Unit No. 2 - Basic hand tools
TRAINING MANUAL INSTRUMENTATION
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COURSE OBJECTIVE The student will be able to explain the use of the following hand tools. •
Vices and clamps
•
Rules, set squares and protractors
•
Hammers
•
Punches
•
Files
•
Pliers
•
Wrenches
•
Drills, tapes and dies
•
Power drill
•
Pneumatic impact wrench
Module No. 11 : Instrument craft practice
Unit No. 2 - Basic hand tools
2.0
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Unit No. 2 - Basic hand tools
2.1
INTRODUCTION The aim of this unit is to describe and give the uses of the basic hand tools used by the instrument technician.
2.2
VICE AND CLAMPS The Engineer's Bench Vice Bench vices are bolted to the top of the workbench. They are used to hold the workpiece in the right place. The vice should be between 38 to 46 inches from the ground, depending on the workman's elbow height (see Figure 2-1).
Module No. 11 : Instrument craft practice
Figure 2-1 Typical Bench Vice
Often bench vices have a base which can be turned so that the work can be positioned at a required angle. The vice is locked in place with a lock nut. The material is held between the jaws by tightening the screw handle. The jaws are usually serrated to grip the work. Soft jaws should be used to protect work which can be scratched easily. Soft jaws are made from a soft material such as rubber or aluminium. They are fitted over the normal vice jaws.
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Unit No. 2 - Basic hand tools
TRAINING MANUAL INSTRUMENTATION
Another type of hand vice is the machine vice. This is bolted onto the base of the drilling machine to hold small items for drilling (see Figure 2-2).
Figure 2-2 Machine Vice Pipe Bench Vice
Module No. 11 : Instrument craft practice
The pipe vice is another common bench vice which must be bolted to a workbench (see Figure 2-3).
Figure 2-3 Typical Pipe Vice
Pipe can be inserted into the vice by undoing the hook on the side, which allows the frame to open. The pipe is placed on the bottom jaw, then the hook is brought back over and locked in place. The hand wheel is used to make the final tightness of the pipe in the vice. Pipe vices are made in various sizes and styles. The size of the vice depends on the size of the pipe it is to hold. The vice should be large enough to hold the pipe but not large enough to crush it. The pipe vice is used instead of the bench vice for clamping round work. The V-shaped jaws allow more contact with the work. This gives a better grip. July 1999- Rev.0
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Unit No. 2 - Basic hand tools
C Clamp C Clamps do the same job as a vice. They are used to hold the workpiece secure while you are working on it. They are portable but they clamp work to the bench. They are usually used on rough material where scratches are not a problem. C Clamps have four main parts: the frame, the screw, the handle, and the swivel pad (see Figure 2-4). Toolmakers Clamps Toolmakers clamps (parallel clamps) are used on surfaces which must not be damaged. They come in a range of sizes, which depends on the size of the material to be held (see Figure 2-5).
Module No. 11 : Instrument craft practice
Figure 2-5 Typical Toolmaker's Clamp
Figure 2-4 Typical C Clamps
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MEASURING INSTRUMENTS These are the instruments used for marking out a job. Normally the instrument technician will use only three. The steel rule, set square and protractor. The Steel Rule The engineer's precision steel rule is one of the most frequently used measuring tools in the workshop. They are marked with either imperial or metric graduations, or both. They are made from hardened and tempered spring steel. A metric rule usually comes in lengths of 15cm and 30cm. The measuring accuracy of a rule is up to ± 0.2mm. On a metric rule the longest graduation lines are for centimetres. The centimetre lines are divided into ten smaller graduations for millimetres. Some rules have smaller 1/2mm graduations between the mm graduations (see Figure 2-6) below.
Module No. 11 : Instrument craft practice
Unit No. 2 - Basic hand tools
2.3
Figure 2-6 The Steel Rule 1
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Unit No. 2 - Basic hand tools
The Set Square A precision engineer's square is used to ensure the work material is square, and to draw straight lines at 90° to the datum faces. The lines drawn will be parallel to the other datum edge. Squares have a steel blade and handle, which are at 90° to each other. Some squares have measuring graduations on the blades (see Figure 2-7)
Figure 2-7 The Set Square The Protractor
Module No. 11 : Instrument craft practice
The protractor is used to find or measure angles from 0° to 180°. The flat edge of the protractor head is placed on one side of the angle being measured. Then the edge of the rule is placed on the other side of the angle. The angle can be read straight off the dial on the protractor head.
Figure 2-8 The Protractor
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HAMMERS The most common type of hammer is the ball-pein hammer. The main parts of the ball-pein hammer are: the face (the flat striking surface), the pein (the round face), the wedge (used to hold the hammer head to the shaft), the eye (the hole into which the shaft is fitted), and the shaft or handle. The flat face is for hammering, and the pein is for rounding off rivets etc. Soft-faced hammers, or mallets, are used instead of steel hammers when working on machined surfaces or fragile parts. They are used especially for assembling or dismantling parts so that the finished surfaces are not harmed. These hammers are commonly made of plastic, copper, or rubber. Typical examples of these hammers are shown in Figure 2-9.
Module No. 11 : Instrument craft practice
Unit No. 2 - Basic hand tools
2.4
Figure 2-9 Typical Workshop Hammers
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Unit No. 2 - Basic hand tools
2.5
PUNCHES Centre Punch Before any hole is drilled, you must make an indentation with a centre punch, where the hole is to be drilled (this is to stop the drill from slipping). The centre punch is a precision tool so you must be careful to place the point of the punch exactly at the centre of the hole location (see Figure 2-10). Pin Punches
Figure 2-10 The Centre Punch
Module No. 11 : Instrument craft practice
These are specially made tools for the removal of pins etc. A typical example of this type of punch is shown in Figure 2-11. They are usually in sets of about 15 ranging from 1 mm to 10mm.
Figure 2-11 The Pin Punch
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FILES A file is a hand-held cutting tool which is made from good quality tool steel. The blade is hardened, but the tang is soft to take a wooden handle. Files come in all shapes and sizes, e.g. flat, square, round, triangular etc. They are used by the instrument technician for cleaning burrs from tubing, holes etc. A good instrument tool kit will also contain a set of miniature files for use in contact cleaning. A few examples of files found in the workshop are shown in Figure 2-12. VERY HIGH QUALITY INDUSTRIAL STANDARD NEEDLE FILES COATED WITH DIAMOND GRIT ENABLING THEM TO FILE MATERIALS SUCH AS TUNGSTEN CARBIDE, CERAMICS, CARBON, GLASS, HARDENED STEELS ETC. THE SET COMPRISES SIX POPULAR PROFILES: HAND, ROUND , HALFROUND , SQUARE, THREE SQUARE AND TAPERFLAT.
Module No. 11 : Instrument craft practice
Unit No. 2 - Basic hand tools
2.6
Figure 2-12 Typical Workshop Files
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PLIERS Pliers are used for gripping and holding small parts during assembly work. They are also used for bending cables and wires to be connected to instruments. Many types of instrument pliers also have a cutting edge for cutting cables to the correct length. Pliers come in all shapes and sizes. Figure 2-13 shows some common workshop pliers.
Module No. 11 : Instrument craft practice
Unit No. 2 - Basic hand tools
2.7
Figure 2-13 Common Pliers
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TRAINING MANUAL INSTRUMENTATION
Module No. 11 : Instrument craft practice
Unit No. 2 - Basic hand tools
2.8
WRENCHES Wrenches are used to tighten or remove nuts and bolts. They come in many different forms. The following are the ones most commonly found in an instrument workshop. Spanner Wrenches Spanner wrenches are used for fastening and removing nuts and bolts. There are three common types of spanner wrench. They are open ended, ring and adjustable. Figure 2-14 shows examples of these types.
Figure 2-14 Typical Spanners
Note:
July 1999- Rev.0
Don't use an adjustable spanner if the correct size spanner is available. The repeated use of an adjustable spanner will destroy the flat sides on the nut or bolt.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Basic hand tools
Socket Wrench Set Socket wrench sets are made in a wide range of sizes, but all of them have square drives. They are made in both standard and extended length. Extended length sockets are used in restricted places. A set of socket wrenches will include a range of attachments, such as: a reversible ratchet, sliding tee, extension bars, universal joints etc. They are very useful tools because they place the load at 12 points around the nut, and they can be used in places where a spanner can't be used. A typical socket set is shown in Figure 2-15.
Module No. 11 : Instrument craft practice
A
ADAPTER
K
REGULAR 6 POINT SOCKET
B,C,E EXTENSION BARS
L
REGULAR 12 POINT SOCKET
D
SLIDING T HANDLE
M
HEX SOCKET SCREW SOCKET
F
DEEP 6 POINT SOCKET
N
UNIVERSAL 12 POINT SOCKET
G
DEEP 12 POINT SOCKET
0
FLEX HANDLE
H
SPARK PLUG SOCKET
P
FLEX HEAD RATCHET
I
SPEEDER HANDLE
Q
REVERSIBLE RATCHET
J
RATCHET ADAPTER R
UNIVERSAL JOINT
Figure 2-15 Socket Wrench Set
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Torque wrenches are wrenches which are used to tighten bolts to a set tightness. They can be either of the open ended type or have a square drive which fits into a 6/12 point socket similar to a socket wrench set. The wrench has a built in indicator which shows the torque applied in either Ibf-in or N-m. The wrench will start to slip when the set torque is reached so that no more torque can be applied. Remember, torque is the force applied times the perpendicular distance as shown. A typical torque wrench is shown in Figure 2-16.
Module No. 11 : Instrument craft practice
Unit No. 2 - Basic hand tools
Torque Wrenches.
Figure 2-16 Torque Wrench
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Unit No. 2 - Basic hand tools
Allen Wrenches An Allen wrench is an "L" shaped piece of hexagonal tool steel. They are designed for fastening or loosening Allen set screws. A set of Allen wrenches come in a range of metric and imperial sizes. The size is taken across the flats of the-wrench (see Figure 2-17)
Module No. 11 : Instrument craft practice
Figure 2-17 Allen Wrenches
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Unit No. 2 - Basic hand tools
2.9
DRILLS, TAPS AND DIES Twist Drill Twist drills are the cutting tools used to produce holes in most types of material. The drill bits are made from high-speed steel. Standard bits have two helical grooves, or flutes, cut lengthways around the body of the drill. They provide cutting edges, admit cutting fluid, and allow space for the cuttings to escape during drilling. The bit is made so that it can be held by a chuck fitted on the drilling machine. The drilling machine provides the power for making the holes. Figure 2-18 shows a typical twist drill bit.
Figure 2-18 Typical Twist Drill
TAPS
Module No. 11 : Instrument craft practice
Taps are cutting tools used to cut internal threads. They are mad from high-quality tool steel, hardened and ground.
Figure 2-19 Taps
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Unit No. 2 - Basic hand tools
TRAINING MANUAL INSTRUMENTATION
Figure 2-19 shows the main parts of a tap. The shank is the body of the tap. The lands are the cutting edges. The chamfer is the angle at the leading end of the tap. The tap is chamfered to make it easier to start in a drilled hole (taper tap). The flutes are the grooves between the lands. The flutes allow the metal cuttings to fall away from the -lands. The square on the end of the shank is used to attach a tap wrench. Threading a hole with a tap is done by hand, using a tap wrench (see Figure 2-20). When tapping a hole make sure the tap is at right angles (perpendicular) to the hole, otherwise you will make uneven and cracked threads.
Figure 2-20 Tapping with a Tap Wrench Dies
Module No. 11 : Instrument craft practice
The "die" is used to make an external thread on a bar to make a screw. The die is held in a device similar to a tap wrench and the thread is produced by turning the die around the bar being threaded.
Figure 2-21 The Die
Note:
July 1999- Rev.0
You can find the correct tap or die for the hole or screw to be threaded from drill and tap size tables. These must be used to get the correct thread size for the nuts and bolts in use. The tap has a chamfer at the end depending on its use. The taper tap is used to start the thread. The plug tap is used to finish holes that go straight through. The bottom tap for holes that only go some way into the material.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 2 - Basic hand tools
2.10
THE POWER DRILL A power drill rotates the drill bit at a constant high speed. This means holes can be drilled faster and with less effort. A typical workshop power drill is shown in Figure 2-22. Turn the handle to move the rotating drill down through the material to be drilled. Note the safety features which must be used.
Module No. 11 : Instrument craft practice
Figure 2-22 The Power Drill
Portable power drills run on either compressed air or electricity. These are designed to be held in the hand. The trigger switch, or speed control, is used to turn the drill on and off. The motor is in the body of the drill. The drill bit is inserted in the chuck and tightened with the chuck key. This prevents the bit from slipping (see Figure 2-23)
Figure 2-23 The Portable Electric Drill
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PNEUMATIC IMPACT WRENCH An air powered impact wrench uses an air motor and a special clutch. The clutch changes the rotation to a series of fast, high powered impulses. It's used with a socket wrench to turn nuts and bolts. The impulses from the air wrench deliver fast sharp turns to the bolt head. The motor of the impact wrench is reversible. Operating the motor in one direction will tighten the bolt. Operating the motor in the other direction will loosen the bolt (see Figure 2-24).
Module No. 11 : Instrument craft practice
Unit No. 2 - Basic hand tools
2.11
Figure 2-24 Pneumatic Impact Wrench
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UNITS IN THIS COURSE UNIT 1
WORKSHOP SAFETY AND TOOL CARE
UNIT 2
BASIC HAND TOOLS
UNIT 3
TUBING SYSTEMS
UNIT 4
CRIMPING
UNIT 5
PRACTICAL TASKS
Module No. 11 : Instrument craft practice
Unit No. 3 - Tubing systems
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
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Unit No. 3 - Tubing systems
TRAINING MANUAL INSTRUMENTATION
TABLE OF CONTENTS Para
Page
3.0
COURSE OBJECTIVE
3
3.1
INTRODUCTION
4
3.2
TUBING
4
3.3
TUBE BENDING
5
3.4
BENDING TUBING TO SIZE
7
3.5
COMPRESSION FITTINGS
8
3.5.1
Making a Compression Fitting.
9
3.5.2
Remaking a Compression Fitting.
10
3.6
CONNECTORS
11
3.7
THREADS AND TEFLON TAPE
12
3.8
TUBING TOOLS
13
The Tubing Cutter
13
3.8.2
The De-burring Tool
14
TUBING INSTALLATION
15
Module No. 11 : Instrument craft practice
3.9
3.8.1
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COURSE OBJECTIVE The student will be able to : •
State the minimum radius for a tubing bend.
•
State the reasons for sloping process connections (impulse lines) and air supply lines.
•
List the problems which may happen when using compression fittings.
•
Explain the use of the deburring tool
•
Explain when and when not to use "Teflon" sealing tap.
•
Explain the correct tubing fitting to avoid forcing fittings onto an instrument.
Module No. 11 : Instrument craft practice
Unit No. 3 - Tubing systems
3.0
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Unit No. 3 - Tubing systems
3.1
INTRODUCTION The most important mechanical work done by the instrument technician is laying out and connecting up tubing used for: •
Connections from the process to the instrument
•
Pneumatic signal and air supply connections to the instrument.
This unit will explain the basic rules to be followed when carrying out tubing work.
3.2
TUBING Tubing is seamless thin wall pipe. It is made of copper or stainless steel. It is easy to bend using simple hand tools. Normally stainless steel tubing is used throughout the oil/gas industry. This is because it does not corrode easily. Copper tubing is mostly used for water heating systems. It is sometimes used in instrumentation workshops. Low pressure systems, (for example in a training workshop), use plastic tubing which is cheap and easy to use. Tubing comes in standard sizes; either metric or imperial. Tubing layouts should be made all metric or all imperial. They should not be a mixture of both. The table below shows the standard sizes (outside diameter) of instrument tubing.
Imperial (in inches) 1 /8, 3/16, 1 A 3/8, 1 /2, 5/8, 3/4, 7/8, 1
Module No. 11 : Instrument craft practice
Metric (in millimetres) 3, 4, 6, 8, 10, 12, 15, 16, 20, 22 25
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TUBE BENDING When bending thin wall tubing it is important not to kink the tube. The radius of a bend must be at least 2 1/2 times the diameter of the tube as shown in the example in Figure 3-1.
Module No. 11 : Instrument craft practice
Unit No. 3 - Tubing systems
3.3
Figure 3-1 Tube Bending
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Unit No. 3 - Tubing systems
TRAINING MANUAL INSTRUMENTATION
Hand tubing benders are made so that the radius of the bend is correct. The correct use of the hand bender will make good bends automatically. A typical hand tubing bender is shown in Figure 3-2.
Module No. 11 : Instrument craft practice
Figure 3-2 Hand Tubing Bender
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BENDING TUBING TO SIZE There is a good general rule for bending a piece of tubing to the correct length. You should bend it at a point one tubing diameter less than the required length. Figure 3-3 shows where you should bend a 1/2" diameter tube in order for it to reach the required length of 9".
Module No. 11 : Instrument craft practice
Unit No. 3 - Tubing systems
3.4
Figure 3-3 Making a 90° Bend
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Unit No. 3 - Tubing systems
TRAINING MANUAL INSTRUMENTATION
You need practice and judgement to make bends of less than 90°, The smaller the angle, the nearer the bending point to the required length. For example, in Figure 3-3 for a 45° bend, the bending point will be at 8 3/4” Remember it is safer to be too long and cut a bit off the tubing. You must never force a short tube into a connection. Tubing which has been strained to fit is dangerous. It stresses the connectors, which may leak. When it is disconnected it can spring out of place and injure people standing nearby.
3.5
COMPRESSION FITTINGS Instrument tubing connections are done using compression fittings. You must not thread instrument tubing and use a nut to make a connection.
Figure 3-4 Section of a Compression Fitting
Module No. 11 : Instrument craft practice
Figure 3-4 shows a section through a compression fitting. When the nut is tightened the twin ferrules are forced into the tube and against the sides of the connector. They make a metal to metal seal. This seal is very effective if the nut is tightened correctly. It is good for pressures to at least 10,000 psi. There are many manufactures of compression fittings, e.g. Parker, Swagelok etc. Some use only one ferrule (olive) but most use two. The ferrules in different manufacturers' fittings can not be changed with each other. You must -not mix different types of compression fittings.
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Making a Compression Fitting.
Unit No. 3 - Tubing systems
3.5.1
Figure 3-5 Making a Compression Fitting
With reference to figure 3-5. Step 1:
Simply insert the tubing into the Swagelok tube fitting. Make sure that the tubing rests firmly on the shoulder of the fitting and that the nut is finger tight.
Step 2
Hold the connector tightly with a backup wrench. Turn the nut about 1 1/2 turns (from finger tight). This is enough to seal the connection properly.
Module No. 11 : Instrument craft practice
Don't over-tighten the nut
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TRAINING MANUAL INSTRUMENTATION
Remaking a Compression Fitting.
Module No. 11 : Instrument craft practice
Unit No. 3 - Tubing systems
3.5.2
Figure 3-6 Re-making a Compression Fitting
Compression fittings can be re-made many times by the method shown in Figure 3-6. The nut is re-tightened correctly by turning the nut about 1 1/2 turns from finger tight. Don't over-tighten the nut!
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CONNECTORS You must choose the right connectors to connect tubing to an instrument, some plants can use two quite different systems. •
Metric tubing with ISO pipe threads.
•
Imperial (fractional), tubing with National Pipe Threads (NPT)
Manufacturers make connectors to fit all systems. They also. make connectors to connect two different systems. Check which set of connectors are in use on a job site before doing any new tubing work. Figure 3-7 shows a few of the types of connector available (Swagelok).
Module No. 11 : Instrument craft practice
Unit No. 3 - Tubing systems
3.6
Figure 3-7 Some Tubing Connections
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Unit No. 3 - Tubing systems
3.7
THREADS AND TEFLON TAPE Teflon tape is used to seal the gaps between threads. It is only used on "tapered threads". This tape must not be used on threads Of compression fittings. The tape will stop the metal to metal seal being made. This tape must not be used on parallel thread connectors. It will stop the connector from sealing on the bottom of the hole. If Teflon tape is used, make sure the tape is wound in the right direction. When the connector is screwed into the device the tape must be tightened onto the threads, (e.g. anticlockwise on a right hand thread). Put on Teflon tape carefully so that it cannot become loose and block air passages. A typical example of the correct method of putting on Teflon tape is shown in Figure 3-8.
Module No. 11 : Instrument craft practice
Figure 3-8 Applying Teflon Tape
Note:
July 1999- Rev.0
Teflon tape is not as popular as it used to be, particularly on systems running at high temperatures. In some plants special sealing compounds must be used instead of Teflon tape.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 3 - Tubing systems
3.8
TUBING TOOLS A normal tubing tool kit has two special tools; the tubing cutter and the de-burring tool.
3.8.1 The Tubing Cutter
Figure 3-9 The Tubing Cutter
Figure 3-9 shows a hand tubing cutter. The tubing is cut by the cutting wheel. This is rotated, by hand, round and round the tube. The adjustment screw is tightened as the tube is cut. This keeps the wheel in contact with the tube.
Module No. 11 : Instrument craft practice
The tubing cutter is easy to use and cuts tubing better than a hacksaw.
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The De-burring Tool
Unit No. 3 - Tubing systems
3.8.2
Figure 3-10 De-burring Tools
Module No. 11 : Instrument craft practice
A tubing cutter will leave rough edges on the inside of the tube. These rough edges are called burrs. They must be removed before a compression fitting is connected. These burrs can be removed with a round file but the easiest method is to use a de-burring tool. The deburring tool has many cutting edges. The tool is rotated by hand to remove the burrs on the inside of the tube (see Figure 3-10). Make sure the bits of metal are removed after de-burring. Any particles left inside the tubing will quickly block instruments connected to the tubing.
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TUBING INSTALLATION Air lines are usually fitted at a slight slope. This is to make sure that any moisture which collects in the lines does not run into the instrument. The instrument is supplied from the top of the supply line. Moisture collects above the blow down valve. The blow down valve is opened once a day to blow out any moisture collected. Process connections are made differently for gases and liquids. Tubing connections to a gas line are from above. The tubing slopes upwards to the instrument. Tubing connections to a liquid line are from below. The tubing slopes downwards to the instrument. Figure 3-11 shows the above points.
Module No. 11 : Instrument craft practice
Unit No. 3 - Tubing systems
3.9
Figure 3-11 Tubing Installation
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UNITS IN THIS COURSE UNIT 1
WORKSHOP SAFETY AND TOOL CAR
UNIT 2
BASIC HAND TOOLS
UNIT 3
TUBING SYSTEMS
UNIT 4
CRIMPING
UNIT 5
PRACTICAL TASKS
Module No. 11 : Instrument craft practice
Unit No. 4 - Crimping
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 1/6
TRAINING MANUAL INSTRUMENTATION
Para
Page
4.0
COURSE OBJECTIVE
3
4.1
INTRODUCTION
4
4.2
CRIMPING
4
4.3
CRIMP CONNECTORS
6
Module No. 11 : Instrument craft practice
Unit No. 4 - Crimping
TABLE OF CONTENTS,
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TRAINING MANUAL INSTRUMENTATION
COURSE OBJECTIVE The student will be able to: •
Explain the correct method of fitting a conductor into an insulated crimp.
•
Explain why a rachet crimping tool should be used.
Module No. 11 : Instrument craft practice
Unit No. 4 - Crimping
4.0
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Page 3/6
TRAINING MANUAL INSTRUMENTATION
Unit No. 4 - Crimping
4.1
INTRODUCTION The aim of this unit is to explain the insulated crimp connector used in instrumentation.
4.2 CRIMPING This is a method of putting connectors onto electrical/instrument wires so that they can be terminated into a terminal fitting. Crimps come in two basic types, insulated and uninsulated. Insulated crimps are made for small cable sizes. These are the crimps which are normally used by the instrument technician. These crimps come in three standard sizes shown by the colour code.
Module No. 11 : Instrument craft practice
Table 1.
Colour
Metric
Standard Wire Gauge
Red
0.25 to 1.6mm2
22-18 SWG
Blue
.00 to 2.6 mm2
16-14 SWG
Yellow
2.7 to 6.6 mm2
12-10 SWG
Figure 4-1 Insulated Crimp Connection The conductor must fit the crimp terminal within the stated range, (see table 1). You must not use a cable which is too small for the crimp terminal if the cable is too small the crimps will not grip the conductor and the wire can be pulled out. You must not use a cable which is too big for the crimp terminals. If you force a cable which is too big into the crimp terminal, the conductor will be squeezed. This increases the resistance and the connection will get too hot. The crimps are connected to the conductor with a crimping tool. Insulated wires should be crimped twice. The first crimp holds the conductor. The second crimp
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TRAINING MANUAL INSTRUMENTATION holds the cable (conductor and insulation). See Figure 4-1.
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Unit No. 4 - Crimping
TRAINING MANUAL INSTRUMENTATION
Crimping tools should be of the ratchet type. This means that the correct force must be applied before the tool can be released. Figure 4-2 shows a twin crimp ratchet crimping tool. This tool should be used for insulated crimps.
Figure 4-2 Ratchet Crimping Tool
Module No. 11 : Instrument craft practice
This is a ratchet tool for crimping red, blue/black and yellow insulated crimp connectors. The tool has one fixed jaw and one movable jaw which makes the tool easy to use. It may be held in one hand, so the other hand is free to hold the terminal and the wire. The built in ratchet system ensures a complete crimp is made every time. You need much less hand force than with normal tools. When a crimping action has been started the ratchet makes sure the tool cannot be opened until the jaws have completed the crimping action.
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Module No. 11 : Instrument craft practice
Unit No. 4 - Crimping
4.3
CRIMP CONNECTORS There are various types of connector that can be used, e.g. spade, ring, flat tab, receptacle etc. Typical examples are shown in Figure 4-3.
Figure 4-3 Typical Insulated Crimp Connectors with Colour Cod
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UNITS IN THIS COURSE UNIT 1
WORKSHOP SAFETY AND TOOL CARE
UNIT 2
BASIC HAND TOOLS
UNIT 3
TUBING SYSTEMS
UNIT 4
CRIMPING
UNIT 5
PRACTICAL TASKS
Module No. 11 : Instrument craft practice
Unit No. 5 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
July 1999- Rev.0
Page 1/7
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Para
Page
PRACTICAL TASK 1
3
PRACTICAL TASK 2
5
PRACTICAL TASK 3
6
PRACTICAL TASK 4
7
Module No. 11 : Instrument craft practice
Unit No. 5 - Practical tasks
TABLE OF CONTENTS
July 1999- Rev.0
Page 2/7
TRAINING MANUAL INSTRUMENTATION
Unit No. 5 - Practical tasks
PRACTICAL TASK 1 BASIC FITTING EXERCISE Tools Required : Engineer's Rule Engineer's Set Square Scriber, Centre Punch and Hammer Power Drill and Drills Taps and Dies Drill and Tap Tables Test Piece
Job Instructions 1)
Make out the test piece as shown on the diagram.
2)
Centre punch points ABCDEFGH.
3)
Drill
Module No. 11 : Instrument craft practice
A B C D E F G H
with a with a with a with a with a with a with a with a
3.3 mm 4.1 mm 2.05 mm 2.6 mm 7/32" 33/64" 29/64" 17/64"
drill drill drill drill drill drill drill drill
4)
Using a tap and tap wrench (metric) make A and C 4mm and 2.5mm threaded holes.
5)
Using a tap and tap wrench (imperial) make E and G 1/4" and 1 /2" threaded holes.
6)
Using a die and die wrench make screws to fit the tapped holes, i.e. 4mm, 2.5mm, 1/2" and 1/4"
7)
Check that B is the clearance hole for the 4mm screw Check that D is the clearance hole for the 2.5mm screw. Check that F is the clearance hole for the 1/2" screw. Check that H is the clearance hole for the 1/4" screw.
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Unit No. 5 - Practical tasks
TRAINING MANUAL INSTRUMENTATION
Module No. 11 : Instrument craft practice
Test Piece Mild Steel 15mm Plate
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TRAINING MANUAL INSTRUMENTATION
Unit No. 5 - Practical tasks
PRACTICAL TASK 2 DISASSEMBLY AND ASSEMBLY WORK Tools Required
Ring, Open Ended, and Adjustable Spanners Impact Wrench Torque Wrench Job Instructions : 1
Disassemble and assemble control valves, flanges etc. on the instructions of the instructor.
2)
Assemble control valves, flanges etc. on the instruction of the instructor.
Module No. 11 : Instrument craft practice
3)
Remember to tighten a flange using the procedure as shown.
Tighten a little at a time using the sequence shown. Make sure the flange goes down flat, not tipped to one side.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 5 - Practical tasks
PRACTICAL TASK 3 TUBING EXERCISE Tool Required : Tubing bender, tubing cutter and deburring tool Various tubing sizes Various compression fittings Job Instructions : After the instructor has demonstrated the correct connection of tubing to a compression fitting, practice making connections yourself. Making the correct connection of tubing to a compression fitting is not as easy as it seems., When the instructor is satisfied with your fittings carry out the following exercise.
Module No. 11 : Instrument craft practice
Use 4 1/2" NTP 1/4 " tubing, male connectors, one union cross ' and various lengths of A " tubing. Make the layout shown in the diagram.
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TRAINING MANUAL INSTRUMENTATION
Unit No. 5 - Practical tasks
PRACTICAL TASK 4 CRIMPING EXERCISE
Tool Required :
Various conductor sizes (to fit 1.5, 2 and 3.5mm connectors) Various insulated connectors (spade, receptacle, pin, etc.) Terminal strips (e.g. Klippon series) Double crimp - rachet type crimping tools Job Instructions Crimp various conductors With different types of crimp connectors. Practice the crimping technique until the work is satisfactory to the instructor.
Module No. 11 : Instrument craft practice
Connect various conductors to a terminal strip. The layout of the connections will be given by the instructor.
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TRAINING MANUAL INSTRUMENTATION
MODULE No. 12 INTRODUCTION TO PLC
July 1999- Rev.0
UNITS IN THIS COURSE UNIT 1
PLC FUNDAMENTALS
Module No. 12 : Introduction to PLC
Unit No. 1 - PLC Fundamental
TRAINING MANUAL INSTRUMENTATION
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TRAINING MANUAL INSTRUMENTATION
Para
Page
1.0
COURSE OBJECTIVE
3
1.1
INTRODUCTION,
4
1.2
PLC SYSTEMS
4
1.3
RELAY SYSTEMS AND PLC COMPARISON
6
1.4
PLC SYSTEM EXAMPLES
8
1.4.1
The Allen-Bradley System
8
1.4.2
The Dual Redundant Emergency Shut Down PLC System
11
1.4.3
Triple Redundant PLC Systems
13
Module No. 12 : Introduction to PLC
Unit No. 1 - PLC Fundamental
TABLE OF CONTENTS
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Unit No. 1 - PLC Fundamental
1.0
COURSE OBJECTIVE The student will be able to: •
Explain the difference between PLC systems and the older relay systems.
•
Explain the operation of a simple PLC ladder diagram.
•
Explain the function of the Allen-Bradley PLC components.
•
Explain using a block diagram the three basic PLC systems: Single µP PLC
2.
Dual redundant PLC
3.
Triple redundant PLC
Give examples of where the different types of PLC would be used.
Module No. 12 : Introduction to PLC
•
1.
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Unit No. 1 - PLC Fundamental
1.1
INTRODUCTION The aim of this unit is to explain the fundamentals of a Programmable Logic Controller (PLC). It will also act as an introduction to the main part of the course; practical work on a PLC unit.
1.2
PLC SYSTEMS There are various manufacturers of PLC equipment. They all use different methods for sending data and make their diagrams to different standards. This means that the operating companies do not mix the different types of PLC systems. You cannot mix PLC systems when controlling a process. Some of the PLC systems used in the fields are:
Module No. 12 : Introduction to PLC
The most common PLC systems are Allen-Bradley and Modicon which use the ladder diagram method for PLC logic. This system is explained here so that you can practice programming techniques on the Allen-Bradley training equipment available in the workshop.
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Module No. 12 : Introduction to PLC
Unit No. 1 - PLC Fundamental
TRAINING MANUAL INSTRUMENTATION
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TRAINING MANUAL INSTRUMENTATION
Unit No. 1 - PLC Fundamental
1.3
RELAY SYSTEMS AND PLC COMPARISON The ladder diagram used has been explained using relays. Figure 1-2 shows a block diagram of the overall system using relays. It is sometimes called a HARD-WIRED control system. When the control logic is installed it can only be changed manually. Note:
The relay system is still preferred in some safety systems as it is very difficult to check the software operation of a PLC for faults.
Module No. 12 : Introduction to PLC
Figure 1-2 Block Diagram Relay Logic Control
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Unit No. 1 - PLC Fundamental
TRAINING MANUAL INSTRUMENTATION
A PLC collects inputs and distributes outputs in the same way as a relay logic circuit. However, the relays are replaced by a microprocessor which is programmed to provide the switching logic. Figure 1-3 shows a typical PLC block diagram based on Allen-Bradley.
Module No. 12 : Introduction to PLC
Figure 1-3 Block Diagram Programmable Logic Control
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1.4
PLC SYSTEM EXAMPLES
1.4.1
The Allen-Bradley System
Module No. 12 : Introduction to PLC
Unit No. 1 - PLC Fundamental
TRAINING MANUAL INSTRUMENTATION
Figure 1-4 Main Components of Allen Bradley PLC
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TRAINING MANUAL INSTRUMENTATION
Unit No. 1 - PLC Fundamental
Figure 1-4 shows the major components of an Allen-Bradley PLC. It consists of the following units. 1)
Main Processor Unit. •
This units provides the following functions
•
The system µP and RAM.
•
EEPROM memory compartment. An EEPROM is normally added as a back-up to hold the RAM programme in case of system failure.
•
Input conditioning for 10 inputs with status indicators.
•
6 separate outputs with status indicators.
•
Battery compartment with lithium battery. This battery supplies D.C. power to hold the RAM data if the mains supply is lost.
•
Communication reprogrammed.
port
so
that
the
processor
RAM
can
be
The unit is powered by a standard single phase supply WAE: 240V-50 Hz). This supply is connected to the incoming line terminals. 2)
Expansion 1/0 unit. This unit is connected to the main processor unit by a cable. It uses the connections shown to provide increased 1/0's. This provides an extra 10 inputs and 6 outputs. There are status indicators for each 1/0. This unit gets d.c. power from the main processor unit. A d.c. power indicator is provided to show that this unit is powered -correctly. Note: Allen-Bradley also supply a hard-wired relay expansion unit.
Module No. 12 : Introduction to PLC
This unit is used if higher current switching is required. Maximum 2.5A continuous when switching either 240V a.c. or 24V d.c. 3)
Pocket Programmer. This unit has a keyboard and display panel. It is used to programme (configure) the required logic operations. This will be used in the workshop when you try some simple programming techniques.
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TRAINING MANUAL INSTRUMENTATION
a)
Controlling a single process (e.g. pump starting, ship loading sequences etc.)
b)
Larger processes which have no effect on plant safety. (Therefore they must be cheap, e.g. fire detector systems for accommodation units etc.)
c)
As a back-up for a large safety system. (E.g. to operate a shutdown if the emergency shut-down button is pressed or a fire alarm if the "break glass" unit is operated).
Module No. 12 : Introduction to PLC
Unit No. 1 - PLC Fundamental
The Allen-Bradley system described is one of the simple single µP types. It is used for
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The Dual Redundant Emergency Shut Down PLC System The most important feature of an ESD system is that it must only operate when there is a failure in the plant. There are two main problems if the ESD equipment fails. The first problem is the high cost of lost production. The second problem is that if the ESD equipment keeps failing the operations staff by-pass the system in order to keep the plant running. The dual redundant PLC system reduces the chance of an ESD system shutting down the plant because of ESD equipment failure. However, it ensures the plant is shut-down when there is a failure in the plant. Figure 1-5 shows the basic block diagram of a fully redundant PLC system (e.g. the new Allen-Bradley PLC-5 series and ICS).
Module No. 12 : Introduction to PLC
Unit No. 1 - PLC Fundamental
1.4.2
Figure 1-5 Dual Redundant PLC System
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1)
The field inputs are applied to two identical PLC systems in parallel.
2)
The software program for the PLC is applied to both µPs
3)
If the field inputs are correct then the plant operates.
4)
If a field input fails then both PLC systems will detect this. In this case both the output control elements will shut-down the plant.
5)
Because there are two identical PLC systems the chances of a fault on both at the same time is very small. Therefore, a fault on one PLC system will not cause a shut-down because the good system will still hold the output control elements in the correct position.
6)
A faulty unit in the PLC system will indicate it has a fault. Therefore, maintenance can be carried out while the system is still running under the control of the good PLC system.
7)
This type of system uses automatic line checking to ensure the input/output wiring and devices are connected correctly. These systems will be learnt during advanced training at work.
8)
Dual redundant systems are used to control a complete ESD system. They provide a good level of safety at a reasonable cost (e.g. for platform control, oil/gas production units, etc.)
Module No. 12 : Introduction to PLC
Unit No. 1 - PLC Fundamental
OPERATION
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Triple Redundant PLC Systems This system is the latest type of safety system. It ensures the plant only shuts down because of a plant failure but not because of an equipment failure. These systems are expensive. They are only used when the highest safety and reliability is required; e.g. large installations such as refineries, LNG plants etc. An example of the triple redundant PLC is the AUGUST C 300 system. This system is used in the refinery at Umm AI Nar. AUGUST control systems claim that their system is 99.999% guaranteed to shut down the plant ONLY if there is a plant failure.
Module No. 12 : Introduction to PLC
Unit No. 1 - PLC Fundamental
1.4.3
Figure 1-6 Triple Redundant PLC System
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Figure 1-6 shows the basic principle of a triple redundant PLC system. This is sometimes called the 3.2.0 system. The voting unit will keep the plant running if 3 or 2 of the parallel systems are working correctly. It will shut the plant down if only 1 or none of the systems give the correct outputs. The software programme is fed into the three µPs It uses a self checking system so a µP can detect faults in its own system. All the units have fault indicators so that they can be changed while the system continues to work using the good units. All input/output wiring and devices are automatically checked to ensure that they are connected correctly. These systems are very complicated and will be learnt on the job, as it depends on what system the plant uses.
Module No. 12 : Introduction to PLC
Unit No. 1 - PLC Fundamental
TRAINING MANUAL INSTRUMENTATION
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