PCI10202

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Control Loop Elements And Their Contribution To Loop Performance

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Instrumentation File Reference: PCI10202

For additional information on this subject, contact E.W.Reah on 875-0426

Engineering Encyclopedia

Instrumentation Control Loop Elements and Their Contribution to Loop Performance

CONTENTS

PAGE

EXPLAINING THE GENERAL SELECTION CONSIDERATIONS FOR PROCESS INSTRUMENTATION ....................................................... 1 Selection Considerations as Specified by Saudi Aramco Engineering Standard SAES-J-003, Basic Design Criteria ............... 1 Other Selection Considerations......................................................... 6 Primary Elements/Transmitters .............................................. 6 IDENTIFYING THE MOST COMMONLY USED TRANSMITTERS............ 9 Role of Transmitters .......................................................................... 9 Transmitter Types.............................................................................. 9 Transmitter Components ................................................................. 11 Simplified Transmitter Schematics .................................................. 12 Pneumatic Type .................................................................... 12 Electronic Type ..................................................................... 13 Resonant Wire Type ............................................................. 14 DESCRIBING THE MOST COMMON FINAL CONTROL ELEMENTS USED IN PROCESS APPLICATIONS.................................. 18 Control Valve Terminology .............................................................. 18 Valve Body Types............................................................................ 20 Globe Styles.......................................................................... 20 Globe Style Three-Way Valves............................................. 23 Angle Valves ......................................................................... 25 Cage Valves.......................................................................... 26 Butterfly Valves..................................................................... 28 Ball Valves ............................................................................ 30 Eccentric Plug-Type Ball Valves........................................... 31 Control Valve Application Considerations ....................................... 32 IDENTIFYING OTHER CONTROL LOOP ELEMENTS AND THEIR ROLES ........................................................................................... 38 Saudi Aramco DeskTop Standards

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Signal Converters ............................................................................ 38 Current to Pressure Signal Converters, I/P .......................... 38 Pressure to Current Signal Converter, P/I ............................ 39 Analog to Digital, A/D, or Digital to Analog, D/A ................... 40 Volume Booster .................................................................... 41 Valve Positioner .................................................................... 43 GLOSSARY ............................................................................................... 46

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There are various selection considerations involved in the proper choice of instrumentation for a process control loop. The general objective, within some constraints, is to select the best possible instruments for the given application. This should minimize future problems substantially and make the control of the process as easy as possible. The choice of instrument quite often is a compromise or tradeoff between the application requirements and the various attributes and limitations of the hardware available. Selection Considerations as Specified by Saudi Aramco Engineering Standard SAES-J-003, Basic Design Criteria There are design requirements for each type of instrument covered by individual Saudi Aramco standards and specifications. SAES-J-003 is the Engineering Standard which, together with its references, specifications, codes, forms and drawings, covers the requirements for the selection, design and application of process instruments and systems. SAES-J-003 states that the selection of instrumentation should normally take into consideration the following items:

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Some of these terms are straightforward and require no further explanation. Others might benefit from some further explanation. Reliability - Infers the probability that a component, piece of equipment or system will perform its intended function for a specified period of time, usually operating hours, without requiring corrective maintenance. Accuracy - This is one of the most difficult terms to understand due to various interpretations. It is poorly defined and means different things to different people. The term accuracy actually implies inaccuracy or error. The following accuracy related information is from ISA S-51.1, "Standard Process Control Terminology." Accuracy - In process instrumentation, degree of conformity of an indicated value to a recognized, accepted standard value, or ideal value. Measured accuracy - The maximum positive and negative deviation observed in testing a device under specified conditions and by a specified procedure. See Figure 1. Note 1: It is usually measured as an inaccuracy and expressed as accuracy. Note 2: It is typically expressed in terms of the measured variable, percent of span, percent of upper range value, percent of scale length, or percent of actual output reading. Accuracy rating - In process instrumentation, a number or quantity that defines a limit that errors will not exceed when a device is used under specified operating conditions. See Figure 1. Note 1: When operating conditions are not specified, reference operating conditions shall be assumed. Note 2: As a performance specification, accuracy (or reference accuracy) shall be assumed to mean accuracy rating of the device, when used at reference operating conditions.

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Note 3: Accuracy rating includes the combined effects of conformity, hysteresis, dead band and repeatability errors. Refer to the glossary for definition of these terms. The units being used are to be stated explicitly. It is preferred that a + sign precede the number or quantity. The absence of a sign indicates a + and - sign.

OUTPUT MAXIMUM ACTUAL POSITIVE DEVIATION ACTUAL DOWNSCALE CALIBRATION CURVE SPECIFIED CHARACTERISTIC CURVE

HIGH OR POSITIVE PERMISSIBLE LIMIT OF ERROR

ACCURACY RATING

ACTUAL UPSCALE CALIBRATION CURVE

MEASURED ACCURACY

MAXIMUM ACTUAL NEGATIVE DEVIATION LOW OR NEGATIVE PERMISSABLE LIMIT OF ERROR

0

SPAN

INPUT

100%

ACCURACY RATING FIGURE 1

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Accuracy rating can be expressed in a number of forms. The following five examples are typical:

It may help our understanding of accuracy if we look at the term from a process instrumentation and control point of view. What we call accuracy is actually error and can be broken into its components of precision and bias errors. Thus we can describe the accuracy of a device at a point as follows: Accuracy (Error) = Precision Error + Bias Error where, Precision relates to the ability of a measuring device to give or repeat the same reading or output for the same input. Precision is a characteristic of the particular instrument And can not be improved for a given device. If not satisfactory, the only alternative is a more precise instrument, usually at an additional cost. Bias is the difference between the true value as established by the referencestandard reading and the most likely reading by the instrument (average of the instrument's readings). Bias errors are directional and can be calculated in a given application. Accuracy can be improved by adding or subtracting bias errors from the instrument's readings.

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To illustrate these terms let us look at the following example. Assume in this example we throw darts at a target with the following results:

A

B

C

HIGHLY ACCURATE DEVICE, GOOD PRECISION WITH MINIMUM BIAS

PRECISE WITH BIAS ERROR

POOR PRECISION SMALL BIAS ERROR

Looking at the results and relating the example to process instrumentation it is obvious that device (A) is the best to use and device (C) is the worst to use. In process work most of the time we deal with type (B) devices. These are precise or repeatable devices with some bias error. Operators learn to make good product without the need to know the absolute accuracy. They can deal quite well with repeatable information produced by type (B) devices. The only exception is in custody transfer applications (buying and selling) where additional effort is put in to eliminate the bias errors and improve the overall accuracy.

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Other Selection Considerations Primary Elements/Transmitters Range. The region between the limits within which a quantity is measured is the range of that measurement. It is expressed by stating the lower and upper range values. The selected primary element must have sufficient range to measure the controlled variable throughout its operating range. Span. The measurement span is the algebraic difference between the upper and lower range values. The selected primary element must have the required span to cover the entire measurement. Minimum Span . The minimum span of measurement that the primary element can be used to measure within its accuracy rating. The selected primary element must satisfy the minimum span requirement of the controlled variable. Maximum Span . The maximum span of measurement that the primary element can be used to measure within its accuracy rating. The selected primary element must satisfy the maximum span requirement of the controlled variable. Rangeability (Turndown) . In flow applications, rangeability is the ratio of the maximum flow rate to the minimum flow rate within the stated accuracy rating. The selected primary element must have sufficient rangeability or turndown to satisfy the rangeability needs of the controlled variable, i.e. this particular specification helps you to determine at what point during a startup the information of the primary element is accurate enough to switch a loop into automatic operation. Zero Elevation and Suppression . The range at which the zero value of the measured variable is not at the lower range value. (Check Glossary.) The selected primary element has to satisfy the zero value of the measured variable.

RANGE

SPAN

TYPE OF RANGE

0 to 100°C

100°C

Zero based range

20 to 100 in H2O

80 in H2O

Suppressed zero range

- 40 to 120°F

160°F

Elevated zero range

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Response Time - An output expressed as a function of time, resulting from the application of a specified input (step) under specific operating conditions. The response time, in seconds or minutes, gives us information regarding how quickly the primary element responds to a specific input. A good primary element should have a fast (short) response time. An accurate primary element is useless in an industrial application if it takes too long to respond to a process input. Time Constant . This is a specific measure of a response time. It is the time required for a first order system to reach 63.2% of the total change when forced by a step.

INPUT

OUTPUT

t=0

A

INSTRUMENT

A TIME

t=0

A

TIME

STEADY STATE

.632A

0

τ

2τ

3τ

4τ

5τ

t

A = (l - e-t τ )

It takes the output of the device approximately five time constants (5τ) to reach its final steady state value. The time constant τ is a function of the resistance (R) and capacitance (C) associated with the measuring device. τ = RC =Time Constant. Note: For the output of a first-order system forced by a step or an impulse, is the time required to complete 63.2% of the final steady state value for a step input. In higher order systems, there is a time constant for each of the first-order components.

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Characteristic Curve (Input-Output Relationship) . A curve that shows the ideal value of an input-output relationship at steady state. This curve shows the output variable of an element or device as a function of an input variable. The selected primary element forms one of the elements in a loop and its input-output relationship will dictate the overall loop linearity, i.e. if the input-output relationship of the element is linear, then the gain of the element is constant and does not contribute to loop gain non-linearity. On the other hand, if the primary element's input-output relationship is not linear, then its gain is not constant with consequences on overall loop performance. PCI 102.05, Steady State Gains, addresses these issues. Reproducibility . The selected primary elements should be highly reproducible. This means that there should be a closeness of agreement among repeated measurements of the output for the same value of the input made under the same operating conditions over a period of time, approaching from both directions. Reproducibility includes the effects of hysteresis, dead band, drift and repeatability. Noise. In process instrumentation noise is an unwanted component of a signal or a process variable. Some measurements are inherently noisy, subjecting the primary elements to undesirable levels of noise. Noisy process measurements are not desirable and in certain control applications not acceptable. Noise should be eliminated or minimized to acceptable levels, either through noise filtering techniques or better instrument applications techniques. In addition, the measurement span should be selected in such a way that the signal noise to signal span ratio is at an acceptable level, i.e. in boiler drum level applications the span of level measurement is typically 30 in. of water. With this span the signal noise to signal span ratio is usually in a more acceptable level. If the span was 10 in. of water the effects of noise would be three times as much for this span making the noise more difficult to handle.

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Role of Transmitters Sensing a process variable is only the first step in process plant control. The information obtained must be used to make necessary process changes to adjust for external forces, such as increased throughput demands. It is impractical to locate all control instruments in a plant near the process. One operator cannot monitor information scattered around the plant to make wise operating decisions. And, placing an operator at each controller would be an inefficient use of valuable manpower. To complicate matters, it is not easy to bring most measurements to a certain location in their original form. The process fluids could be at high pressure or of dangerous chemicals which would create a hazardous condition in case of rupture or leakage. Another difficulty is regarding the response time of an element with several hundred feet of capillary tubing. The additional dead time or lag produced by this system will not allow effective control. These difficulties are overcome with the introduction of signal transmission systems. This allows the centralization of control operations since signals can be sent longer distances and are of a uniform standard value. Transmitter Types Transmitters as purchased from vendors usually consist of two major components. The body of the transmitter (bottom works) contains a measuring element (transducer) that converts process conditions or parameters into motion, force or some other parameter. The transmitter head (top works) contains pneumatic, electronic, or microprocessor-based components to convert the transducer signal into a standard signal suitable for transmission to other locations (centralized) in the plant. These conversions allow the process parameters to directly relate in a known function (linear or square root, for example) to the output signal.

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Smart transmitters are essentially the same as conventional transmitters, except that the microprocessor-based transmitter head has certain additional capabilities. This may include remote calibration checks, self diagnostics, configuration and reranging with a hand-held communicator (at the transmitter, in a field junction box, or from the control room). In process instrumentation most transmission systems are either pneumatic or electronic devices working with analog (continuous) signals. Recent introduction of digital transmitters provide an alternative and might change the picture in the future. The schematic below shows the use of a differential pressure transmitter using an orifice as a primary device to determine flow in a pipeline. The transmitter converts the d/p into a usable signal and transmits it to a remote controller. The controller manipulates a control valve in the pipeline to adjust the flow rate.

PROCESS TRANSMITTER

r FIC

FT

1 SIGNAL TRANSMISSION LINES

I

FLOW INDICATING CONTROLLER

P

PRIMARY ELEMENT

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Transmitter Components Transmitter top works consist of three basic components. These are: 1. Detector - The function of the detector is to detect the process signal input or primary element output. 2. Amplifier - The function of the amplifier is to amplify the detected signal. 3. Negative Feedback - The function of negative feedback is to balance (stabilize) the mechanism allowing it to accept changes in the primary element's output. These three components are shown below as part of a simplified pneumatic transmitter.

d = .0006''

RESTRICTOR

NOZZLE

AMPLIFIER OR RELAY

TYPICALLY 20 PSI AIR SUPPLY

FLAPPER AIR

DETECTOR

FEEDBACK BELLOWS ( STABILIZER ) PROCESS SIGNAL INPUT (PRESS,TEMP, FLOW, LEVEL )

AIR SUPPLY TO AMPLIFIER AND DETECTOR OUTPUT ( 3 - 15 PSI ) ( 20 - 100 kPa ) INPUT TO CONTROLLER

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Simplified Transmitter Schematics Pneumatic Type The actual force-balance pneumatic transmitter's simplified schematic may look like this:

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Electronic Type The simplified scheme of an electronic force-balance differential pressure transmitter may look like this:

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Resonant Wire Type The resonant frequency of the oscillator circuit changes as a function of wire tension. The wire in this circuit is represented by the resistance.

VIBRATING WIRE PRESSURE TRANSDUCER DIAPHRAGM BACKING PLATE SI OIL FILL

WIRE ATTACHED TO DIAPHRAGM HERE

WIRE ATTACHED TO BACKING PLATE HERE LOW PRESSURE SIDE

HIGH PRESSURE SIDE

OSCILLATOR

OSCILLATOR CIRCUIT

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It is impossible for manufacturers to make transmitters customized to meet exactly the user process conditions regarding range and span. This is not practicable and the price most likely not acceptable to the user. Instead, manufacturers make instruments to serve a broad range of conditions and applications. The user or manufacturer, with appropriate information regarding the span and range of measurements, must make adjustments to make sure the instrument meets the process requirements. This act of ascertaining outputs of the transmitter corresponding to a series of values of the quantity which it has to transmit is called calibrating the instrument. In ordering the transmitter the user, with manufacturer's assistance, must make sure the particular device has the correct specifications to meet the job requirements. Accuracy, rangeability, area classification, minimum and maximum spans, and process static pressures are some of the specifications of interest. The calibration procedure usually involves the following adjustments: Zero Adjustment - Adjust a zero screw so that at 0% measurement input the output is 0% (usually 4 mA DC or 3 psi). Span Adjustment - Adjust the span of the instrument so that at 100% measurement input, the output is 100% (usually 20 mA DC or 15 psi). In newer transmitters this could be the end of the calibration procedure. In some older transmitters, however, the zero and span adjustments interact, so both adjustments have to be checked iteratively until making a change in one adjustment does not affect the other. A second concern while calibrating is to check the transmitter's linearity by putting a 50% input to see if the output goes to 50%. A non-linear transmitter requires either further calibration, in the case of older, motion-balance type pneumatic transmitters (angularity adjustment), or requires troubleshooting, in the case of forcebalance pneumatic transmitters which are inherently linear. Most electronic transmitters are either inherently linear or have built-in linearization functions.

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20

TYPICAL TRANSMITTER CALIBRATION CURVE

16

OUTPUT

mA dc

An Actual Transmitter Curve Might Look Like This

12

" Ideal " Transmitter curve

8 NOTE: 4 mA = " LIVE " ZERO

4 0% 0 PSI 0°F

25%

50% 100 PSI 500°F INPUT

75%

100% 200 PSI 1000°F

Transmitters can be calibrated to be either direct acting or reverse acting. The preferred choice in most applications is to have a direct acting transmitter. This is much more comfortable for the operator since a change of output directly relates to a change of input. If the transmitter output increases with an increase in the measured process parameter, the transmitter is direct acting. If the output decreases with an increasing process parameter, the transmitter is reverse acting. Keep in mind that the transmitter, as one of the elements of the loop, contributes to achieving a negative feedback loop requirement of an odd number of reverse acting elements.

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The gain of an element as you recall is a vector made up of steady state and dynamic components. G T = KT GT ∠∅T Transmitters are fast acting devices with not much of a dynamic gain concern, that is, GT ∠∅T ≈ 1 ∠0°. This implies that the transmitter gain is essentially a steady state concern. PCI 102.06 gives a thorough treatment of this subject. The steady state gain of the transmitter was defined as the change of output divided by the change in input at a steady state. The gain of the transmitter could have dimensions and is the slope of the input-output relationship as shown below.

K T = SLOPE OF CURVE = CONSTANT = ² OUT ( 20 - 4 ) mA 16 mA = = ² IN ( 200 - 0 ) °F 200°F

20 mA 100%

OUTPUT

4 mA

OR KT =

0% 0

INPUT

200°F

100% INPUT SPAN

=

100% 200°F

=

1% 2°F

If the transmitter is recalibrated to a new span of measurement, i.e. 0 to 100°F range and a span of 100°F. The gain will change as follows: 100% 100% 1% KT = = = INPUT SPAN 100°F °F This gain is twice as much as the previous gain and will double the loop gain with the potential of making the loop unstable. The controller gain must be backed-off to accommodate the additional transmitter gain and maintain loop stability. Remember that the open loop gain is the product of all the elements' gains forming the loop and it dictates the loop response. GL = GT × GV × GP × GC

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There are various selection considerations regarding the choice of the final controlling element used in regulating the supply to the process. Most control loop problems evolve from and are somehow related to the final controlling element selected and sized for the particular application. On some processes the final controlling element could be a damper or the speed of a motor but by far the most common final controlling element is the control valve. The control valve is by far the weakest element in the loop. There have been tremendous advances through the introduction of microprocessors to improve the performance of the transmitters and process computers. These innovations have had little effect on the the valve performance. There are various considerations involved in the selection of the valve for the particular application. Before we get into the selection considerations let us look at the common control valve types along with the terminology. Control Valve Terminology A control valve is an engineered variable flow restriction, by means of which flow rate, pressure, temperature, liquid level, and composition are maintained at desired values in the process. The input signal to the control valve is the output signal from a controller. The control valve is constructed such that the stem lift (plug position) is related to the input signal. The relationship between stem position and the area open for flow is called the valve characteristic. This relationship is extremely important in determining the suitability of a given valve for a given service, and therefore receives much attention from control engineers and valve manufacturers.

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The following diagram illustrates control valve terminology.

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The valve, as seen, can be divided into two major portions. The upper portion forms what is referred to as the valve actuator, motor, or operator. It is usually a spring loaded diaphragm actuated by the controller output either directly in pneumatic applications, or through current to pneumatic transducers (I/P) in electronic or digital applications. The bottom portion of the valve (the valve body) contains a valve plug which is positioned by the diaphragm actuator. The valve body-plug combination is essentially a variable restriction or orifice. The valve characteristic depends on the shape of the opening between the valve body and plug and the pressure drop variations caused by the flow. Valve Body Types The two general categories of control valves are linear actuated and rotary actuated. Globe, angle and cage valves are linear actuated, while butterfly and eccentric plug and ball valves are of the rotary type. Globe Styles The most common control valve body style is the conventional globe type, although it is declining in popularity in favor of the cage and rotary valves. New installations use fewer globe styles because of economic and performance advantages of these other control valves. The globe control valve can be either single or double-seated.

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Single-Seated - Single-seated valves are usually employed when tight shut-off is required, or in sizes of 1 inch and smaller where the unbalanced forces acting on the valve stem is unimportant as a factor in actuator selection. Tight shutoff in this case usually means that the maximum expected leakage is less than 0.01 percent of the maximum valve Cv when subjected to an air test with 50 psi upstream and 0 psig downstream pressure. Single-seated valves may have a top or top-and-bottom guided construction; that is, the valve stem is guided within the lower portion of the valve bonnet, or top closure, and on the bottom of the body. Single-seat design also allows a somewhat higher flow capacity than top-and-bottom guided valves for a given orifice size. Saudi Aramco Engineering Standard SAES-J-700 specifies that single-seated globe valves shall be used in shut-off service and gas compressor recycle service.

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Double-seated - Double-seated valve is generally top and bottom guided. Here practical leakage approaches 0.5 percent of the rated Cv because it is nearly impossible to close the two ports simultaneously; particularly, if thermal expansion causes additional distortion after the valve is installed. However, the advantage of double-seated construction lies in the reduction of required actuator forces, because the hydrostatic effects of the fluid pressure acting on each of the two seats, tend to cancel out opposite forces. Double-seated valves have upper and lower ports of different diameters (to allow withdrawal of the smaller plug through the larger port). This contributes to an unbalanced force condition which must be corrected by the actuator. Complete cancellation of these forces is not possible because of the hydrodynamic effects of the fluid that passes the plug contour at high velocity. Fluid passing the lower seat (which tends to close the plug) has a further tendency to "suck" the plug into the seat, thereby creating a dynamic imbalance between this force and the differential pressure acting across the upper-plug area.

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Globe Style Three-Way Valves Three-way valves are a design extension of a typical double-seated globe valve. Here a distinction is made between a valve used for diverting service and one used for mixing or combining service. A typical three-way valve, as shown, has a modified double-ported body with the lower plug seating opposite from the normal shut-off position. Either a direct or a reverse actuator is used, again depending upon the desired fail-safe action. Note that the plug has additional rib guiding in the orifice to compensate for the omission of the lower guide post. The body has an internal bridge to separate the right hand and the lower outlet. A diverting valve might be used for a heat exchanger bypass where the heating medium enters Port "C." Part of the fluid leaves Port "U" to bypass the exchanger. The remaining portion of the fluid then goes to the heat exchanger, through Port "L," to heat an independent process stream and then rejoins the bypass fluid stream from Port "U."

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The drawing below illustrates a three-way valve used for combining or mixing service. Here, two separate fluids entering Port "L" and Port "U," respectively, are combined in a desired ratio and leave through the common Port "C." The ratio between the amount of fluid coming through either Ports "U" or "L" is determined by the plug position. An upward movement of the plug decreases the flow passing Port "L" and at the same time increases the flow area for Port "U." Note again that skirt guiding is employed to provide additional guiding in line with the upper guide post. Saudi Aramco Engineering Standard SAES-J-700 specifies that the flange for the common port shall be marked "common" or "open".

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Angle Valves Angle valves generally are single-seated valves with special body configurations. This allows them to additionally satisfy the need of an elbow to suit specific piping designs. The streamlined interior passage of angle valves tends to prevent an accumulation of solids on the body wall. This type of valve has been used for coking hydrocarbons and high pressure-drop service. The underlying thought was to keep turbulence that is created by the throttling process away from the valve's internal parts. However, in doing this, the energy conversion takes place in the downstream pipe, which may lead to severe pipe vibration and noise. Another characteristic is the relatively high pressure recovery obtained with the streamlined flow pattern. High pressure recovery means a low cavitation index and occurrence of cavitation under low or moderate pressure drops with liquid media. Angle valves may also be used in cases where the piping layout does not allow the installation of a globe valve and may be used for handling certain erosive fluids, such as abrasive catalyst material. In the latter, a discharge into the outlet pipe (with flow direction tending to close the plug) prevents erosion of the inside of the valve housing. If cavitation or flashing is inevitable, an angle valve may be arranged to discharge directly into a vessel or other enlarged fluid volume, thus avoiding damage to internal valve parts. An additional advantage of the angle valve is the self-draining feature which is of value when handling certain dangerous fluids such as corrosive or radioactive liquids. This allows for the process fluids to be totally drained between process steps or prior to start of the next batch. Saudi Aramco Engineering Standard SAES-J-700 specifies the use of only singleseated angle valves.

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Cage Valves Cage valves are a variation of the single-seated globe valve and the most often specified process control valve due to substantial advantages in typical globe valve applications. An important economic advantage is lower cost over the globe valve for the same flow capacity. So called "top entry" or cage valves have the advantage of trim removal. The drawing below shows a typical top entry valve with balanced, singleseated trim. Valves of this type usually have streamlined body passage to permit increased flow capacity. The inner valve parts, often referred to as "quick change trim," can easily be removed after removing the bonnet because of the absence of internal threads. The cage and seat ring are sometimes designed as one piece. The valve plug then assumes the form of a flat disk, The valve flow characteristic is achieved by shaped windows usually cast in the circumference of the cage, The valve plug can also be made as a piston having a hollow internal passage that permits the fluid pressure to communicate to both sides of the plug. A sliding seal, located in a groove near the top of the piston seals the upper plug area against the outlet portion of the valve. This balanced design, as shown below, tends to cancel out the hydrostatic forces acting on the plug and leads to a great reduction in the required actuator forces. However, as in double-seated valves, a substantial increase in seat leakage must be tolerated. As mentioned previously, the biggest advantage of the top entry valve is the ease of maintenance. The valve trim can be replaced, if necessary, without removing the valve body from the line. This is one of the reasons why this particular valve type has gained wide acceptance since its recent introduction.

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CAGE VALVE SINGLE SEAT BALANCED

The inherent flow characteristics of the valve can be changed by using cages with different openings. This allows inherent flow characteristics such as linear, equal percentage and quick opening.

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Instrumentation Control Loop Elements and Their Contribution to Loop Performance

Butterfly Valves The most common type of rotary valve used for control is the butterfly valve shown below. The typical application range is in sizes from 3 inches up to 72 inches, for low or moderate pressures, or on unusual applications involving large flows at high static pressures but with limited pressure drop. A typical butterfly valve, in throttling service, is normally limited to a 60° opening. In all metal construction, the vane-to-body clearance in the closed position gives a leakage flow that is generally equivalent to the leakage of double-seated valves (0.5 percent of the rated Cv). Many design modifications are available which minimize leakage, including angle-seating and the use of piston rings. Because of the simple body design, elastomer inserts can be adapted to give a tighter seal within the temperature limits of the insert material. Typical linear materials are Neoprene, Buna-N, or for high temperature and special chemical services, Viton A. Actuator force requirements are dictated by a combination of two factors: the friction load imposed by side loading on the shaft due to the differential pressure, and the dynamic torque induced by the flow around the vane. The dynamic torque acts in the closing direction and, in the conventional design, reaches a maximum at about 70 degrees open. In designs recently introduced, special shaped vanes (Fisher-Fishtail and Masoneilan Mini-Torque) are used to reduce the dynamic torque and to permit 90 degrees operation for increased capacity on certain installations. Another design modification involves an eccentrically mounted vane to permit, for example, tight closure on cryogenic fluids using a seal of Teflon or Kel-F, which may be mounted either within the body or on the circumference of the vane. The most common body design is the flangeless "wafer" type for bolting between line flanges. ANSI body ratings are used but the valves are also rated for maximum pressure drop in the closed position and in the 60° open position. This valve does not have standard ISA or API face-to-face dimensions. Saudi Aramco Engineering Standard SAES-J-700 specifies that butterfly valves in hydrocarbon service shall be flange or lug-type design. Wafer-type design butterfly valves shall be used in non-hydrocarbon service only. The shaft shall be of an one piece type and of Manufacturer's standard material.

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Design of the rotary shaft and its supporting shaft bearings are important in assuring the success of butterfly valves in control service. Bearing materials range from reinforced Teflon for very low to moderate temperatures and pressures, up to combinations of the hard-facing alloys that permit use at temperatures well above 1000°F. For many years, outboard bearings were specified almost exclusively. These are located outside the rotary packing box and were not in contact with the process fluid. This simplified the bearing design, but resulted in eccentric loading on the packing. The recent trend has been to use inboard bearings because of the availability of wear resistant and corrosion-resistant materials. A variety of bearing materials are available compatible with different process fluids. Properly selected, the butterfly valve, in sizes 3 inches and above, generally offers the advantages of simplicity, low cost, light weight, and space saving, in combination with good flow control characteristics. For moderate temperatures and pressures, the elastomer-lined valve includes the possibility of tight shutoff. In selecting the valve, consideration must be given to the possibility of cavitation on liquid flow due to the high pressure recovery coefficient; the possibility of damage due to water hammer with fast closing in liquid service; and compensation for the effect of pipe reducers in computing the valve capacity because of the high basic Cv rating.

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Instrumentation Control Loop Elements and Their Contribution to Loop Performance

Ball Valves Another common rotary shaft valve is the ball valve. Ball valves use a full sphere or a portion of a spherical plug that controls the flow of fluid through the valve body. Although initially designed for manual operation requiring tight shut off, their durability along with their ability to handle high-capacity flows has made them a good candidate for certain process applications. Ball valves are supplied in two basic types, throttling ball valve supported entirely by seats and the characterized ball valve (hollowed out spherical segment) that turns on two short shafts similar in design to those of a butterfly valve. The inherent flow characteristic of the full ball valve is equal percentage while the characterized ball is normally linear. Throttling ball valves are rated for high pressure drops to 3,000 psi and characterized ball valves are rated considerably lower to 300 psi. Ball valves have a high flow capability for a given size. Consequently, ball valves are often about onehalf the size of the pipe they are installed in. While reduced size is an economic advantage, it should be noted that this valve reaches choked flow sooner and is prone to cavitation problems at lower delta p's since it is a high recovery valve. Ball valves of larger size than required by flow calculations may be needed to avoid cavitation due to the inherent high recovery of the valve. The major problem area with ball valves concerns the seating surface of the ball or segment and the surface of the ball itself. Particles entrained in the fluid stream can become lodged between the ball and its seating surface causing rapid wear or erosion of special coatings applied to the ball to prevent fluid corrosion. A practical method of limiting ball surface erosion is to use chrome plating on the throttling ball or contoured ball. This also improves its tight shutoff capability.

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Instrumentation Control Loop Elements and Their Contribution to Loop Performance

Eccentric Plug-Type Ball Valves

PLUG CENTER

SEAT

N PE O 50°

CLOSED

SHAFT CENTER

ECCENTRIC PLUG - TYPE CONTROL VALVE

The ball valve can be characterized by using a segment of the ball. The plug (ball segment) operates on an eccentric path as shown above. The popularity of this valve has increased substantially over the last few years. A typical version of this valve manufactured by Masoneilan, Inc. is shown above. It is supplied without flanges and is mounted between pipe flanges. Face-to-face dimensions are non-standard and should be handled for mounting purposes like the butterfly valve. The normal rotary stroke is 50 degrees and an essentially linear flow characteristic is obtained with a Cv rating similar to a high capacity cage valve. Operating torque is low due to the eccentric motion of the spherical face. The valve is suitable for flow in either direction. Seating action is positive and sliding seal problems associated with conventional ball valves is eliminated. The rotating plug is constructed of silicon carbide and is unaffected by erosive materials that might be encountered in most services. One disadvantage is that the valve must be removed from the piping to replace the seat. The low cost of this valve type for a given flow capacity, when compared with more conventional designs, is undoubtedly responsible for its widespread use. An extra advantage of high rangeability adds to the incentive for eccentric plug valve application.

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Control Valve Application Considerations Saudi Aramco Engineering Standard SAES-J-700 states the following regarding the design and application of control valves. Control valves shall not be used as emergency shutdown (ESD) valves nor as emergency isolation valves (EIV). Control valve positioning may be included in ESD or EIV logic. (e.g. for venting or draining equipment) when failure of the control valve would not increase the severity of the emergency situation. Selection of control valve design shall be based on application, operating conditions, installation requirements, and economic considerations. Refer to SAES-L-008, Selection of Valves, for general valve selection criteria. The following valve designs may be considered: globe valves (2- and 3-way), angle valves, ball valves, butterfly valves, axial flow valves and rotary plug valves. Design guidance may be obtained from the Saudi Aramco Design Practices Manual and the references listed therein. Mandatory design requirements are listed in the following sections. The following is a list of some of the selection considerations regarding the choice of the valve. Most of the relevant terms are thoroughly discussed in PCI 103. Our interest is to look at a few select items that affect loop performance. Valve Body Material End Connections and Ratings Valve Trim Valve Trim Material Valve Actuator (Motor) Size Maximum Permissible Valve Noise Level Valve Action: Direct or Reverse Valve Fail-Safe Consideration: Action Desired on Air Failure Inlet and Outlet Pipeline Sizes and Schedules

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Instrumentation Control Loop Elements and Their Contribution to Loop Performance

Flow Capacity Required: Minimum and Maximum Flows Sizing, Cvmax, Cvmin Pressure Information on the Valve Maximum Outlet Pressure Minimum Outlet Pressure Maximum Inlet Pressure Minimum Inlet Pressure Pmin and Pmax Across the Valve Pressure Drop at Normal Flow Pressure Drop at Tight Shutoff Valve Shelf (Inherent) Characteristics Installed Valve Characteristic Rangeability (Shelf and Installed) Dynamic Considerations Cavitation Flashing Fluid Properties: Temperature, Viscosity, Specific Gravity, Type of Fluid Valve Auxiliaries Valve Hysteresis Valve Positioners Power Loss

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Instrumentation Control Loop Elements and Their Contribution to Loop Performance

Let us investigate some of these items as they relate to control loop concerns. Control Valve - A final controlling element, through which a fluid passes, which adjusts the size of flow passage as directed by a signal from a controller to modify the rate of flow of the fluid. Valve Action - The valve is one of the elements in the loop and its action direct or reverse affects the choice of controller action for negative feedback. Pmin / Pmax Ratio - This pressure ratio across the valve varies due to the maximum and minimum flow rate across the valve. To calculate this ratio it is necessary to know (calculate) the maximum and minimum inlet and outlet pressures. This ratio along with the selected shelf (inherent) valve characteristic provides the installed valve characteristic. Flow Characteristic - Relationship between flow through the valve and percent rated travel as the latter is varied from 0 to 100 percent. This is a special term. It should always be designated as either inherent flow characteristic or installed flow characteristic. Flow Coefficient, CV - Is a capacity coefficient which is defined as the number of U.S gpm of 60°F water which will flow through a wide-open valve with a constant pressure drop of 1 psi across the valve. The flow coefficient is experimentally determined for each style and size of valve by the valve manufacturer. It relates the actual flow to CV in a liquid flow application as follows: Q = CV ∆P G where: Q = Capacity in gpm CV = Valve flow coefficient ∆P = Pressure differential in psi G = Specific gravity of liquid

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Instrumentation Control Loop Elements and Their Contribution to Loop Performance

Inherent Flow Characteristic - Flow characteristic when constant pressure drop is maintained across the valve, or (∆Pmin / ∆Pmax) = 1.

100

INHERENT FLOW CHARACTERISTIC CURVES PERCENT FLOW

80

QUICK OPENING LINEAR

60 40 20

0

EQUAL PERCENTAGE

20

40

60

80

100

PERCENT OF STEM POSITION

Inherent Linear Flow Characteristic - An inherent flow characteristic which can be represented ideally by a straight line on a rectangular plot of flow versus percent rated travel. (Equal increments of travel yield equal increments of flow at a constant pressure level.) Inherent Equal Percentage Flow Characteristic - An inherent flow characteristic which for equal increments of rated travel, will ideally give equal percentage changes of the existing flow.

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Instrumentation Control Loop Elements and Their Contribution to Loop Performance

Installed Valve Characteristic - This is the actual input-output characteristic of the valve. The slope of this curve dictates the gain of the valve at that particular operating point and loop performance depends on this actual characteristic. The curves change shape as a function of the valve plug characteristic and the ∆Pmin / ∆Pmax ratio across the valve. The installed valve characteristic of an inherent linear characteristic valve shifts towards a quick opening installed characteristic as ∆Pmin / ∆Pmax