Reciprocating Compressor

Module No. 6 :Module Compressors No. 6 : & Compressors Turbines & Turbines

Unit No. 2 - Reciprocating Unit No. 2 - Reciprocating compressors compressors

UNITS IN THIS COURSE UNIT 1

AN INTRODUCTION TO COMPRESSORS AND TURBINES

UNIT 2

RECIPROCATING COMPRESSORS

UNIT 3

CENTRIFUGAL COMPRESSORS

UNIT 4

TURBINES

TABLE OF CONTENTS Para Page 2.0

COURSE OBJECTIVES

3

2.1

BASIC PRINCIPLES

4

2.1.1

5

2.2

How a Reciprocating Compressor Works

CAPACITY CONTROL

8

2.2.1

Valve Unloaders

8

2.2.2

Compressor Speed

10

2.2.3

Clearance Pockets

11

2.2.4

Hot Gas Bypass

13

2.3

CONTROL OF SUCTION PRESSURE

13

2.4

AUTOMATIC SHUTDOWN SYSTEMS

14

2.0

COURSE OBJECTIVES This course describes the different types of compressors and turbines found in process plants. The course progresses in Units from basic terminology to the principles of operation of reciprocating and centrifugal compressors and steam and gas turbines. On completion of the course the student will be able to: •

Explain the basic terminology used in describing compressor and turbine operations.

•

Explain the functions and classifications of compressors and turbines in process plants, and identify the different types of prime movers used to power compressors.

•

Identify suction scrubbers and explain how they work.

•

Explain the basic principles of operation of reciprocating compressors including capacity control methods, control of suction pressure, and Page 1/15

Unit No. 2 - Reciprocating compressors

automatic shutdown systems.

2.1

•

Explain the basic principles of operation of centrifugal compressors including capacity control methods, surge control, and automatic shutdown systems.

•

Explain the basic principles of operation of gas turbines, steam turbines, and turbo-expanders.

BASIC PRINCIPLES Reciprocating compressors can be either single acting machines or double acting machines. A reciprocating compressor works by a piston moving within a cylinder. If the piston compresses gas on one stroke only, then the compressor is called 'single acting. If the piston compresses gas on both the forward and the backward strokes, then the compressor is called 'double acting.

Module No. 6 : Compressors & Turbines

Figure 2-1 Single Cylinder Reciprocating Compressor A single cylinder reciprocating compressor has the following parts: •

A crankshaft to change the rotary movement of the prime mover into reciprocating movement. The crankshaft works in conjunction with the crosshead.

•

A connecting rod to connect the crankshaft to the crosshead.

•

A crosshead to work in conjunction with the crankshaft to convert rotary movement into reciprocating movement.

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Unit No. 2 - Reciprocating Unit No. 2 - Reciprocating compressors compressors

•

A distance piece placed between the prime mover and the compressor. The distance piece is really a housing which joins the prime mover to the compressor.

•

A piston rod to connect the crosshead to the piston.

•

A piston to work within the cylinder to compress the gas.

•

Piston rings to form a-seal between the piston and the cylinder wall.

•

A cylinder which contains the gas which is to be compressed.

•

A suction valve (or suction valves) to permit gas to enter the compressor cylinder.

•

A discharge valve (or discharge valves) to permit gas to leave the cylinder after compression.

•

A head end clearance pocket can be installed on the head end of, the compressor. The head end is the end opposite to the end the piston rod passes through. The clearance pocket allows the Operator to vary the capacity of the compressor. Clearance pockets can be fixed or variable volume and controlled either manually or automatically.

If the compressor has more than one cylinder most of these component parts will be found for each cylinder. 2.1.1 How a Reciprocating Compressor Works

Module No. 6 Module : Compressors No. 6 : Compressors & Turbines & Turbines

Figure 2-2 shows a reciprocating compressor cycle. At the start position (Figure 2-2 A) the piston has reached the end of its travel towards the head end. The suction and discharge valves are both closed. In Figure 2-2B the piston has started its backward stroke. The backward stroke may also be called the suction stroke or the induction stroke. Backward stroke will be used throughout this course. As the piston travels down the cylinder, the pressure in the cylinder decreases. When the pressure outside the cylinder is greater than the pressure inside the cylinder the suction valve opens. Gas is drawn. into the cylinder as the piston continues on its backward stroke. Figure 2-2C shows the piston at the end of the backward stroke. The cylinder is now full of gas. The pressure in the cylinder is such that both the suction and discharge valves are closed.

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Unit No. 2 - Reciprocating compressors

Figure 2-2 Reciprocating Compressor Cycle As the piston starts on its forward stroke (Figure 2-2D) the pressure of the gas in the cylinder is increased. The pressure in the cylinder increases until it is greater than the pressure in the discharge pipeline. The discharge valve opens and the compressed gas is released to the discharge pipeline.

Module No. 6 : Compressors & Turbines

As the piston comes near the end of its forward stroke the pressure in the cylinder gets less. When the pressure in the cylinder falls to less than the pressure in the discharge pipeline, the discharge valve closes. The compressor has now completed one full cycle and is once again at the start position as shown in Figure 2 - 2A.

If the compressor is a double acting type exactly the same sequence of events takes place as for a single acting compressor. The sequence takes place on both sides of the piston. This means that when the piston is on its backward stroke gas is being drawn into the cylinder on one side of the piston. At the same time, gas is being compressed and discharged on the other side of the piston. On the forward stroke of the piston Page 4/15

the same thing happens, but the other way around. The gas previously drawn into the cylinder is now being compressed. At the same time gas is being drawn into the cylinder on the other side of the piston.

Unit No. 2 - Reciprocating compressors

The piston rod passes through one end of the compressor cylinder. This end of the cylinder is called the drive end. Anything which takes place on this side of the piston is said to take place on the drive side. The other end of the cylinder is called the head. end. Anything which takes place on this side of the piston is said to take place on the head side. Because gas is compressed on both sides of the piston, the strokes of the piston are only called the forward stroke and the backward stroke.

Module No. 6 : Compressors & Turbines

Figure 2-3 Double Acting Compressor Backward Stroke

2.2

CAPACITY CONTROL The output of a compressor must be controlled (regulated) to provide enough gas for the system it supplies. The most common control point is the discharge pressure of the compressor. For reciprocating compressors four methods of capacity control are available: •

Valve unloaders,

•

Controlling the speed of the compressor,

•

* Variable or fixed volume clearance pockets,

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•

* Hot gas bypass.

Module No. 6 : Compressors & Turbines

Unit No. 2 - Reciprocating compressors

The type of capacity control chosen depends on the size of the compressor, the type of prime mover used, and the amount and range of control ~ required. It may sometimes be necessary to combine two types of capacity control to get the result wanted. 2.2.1 Valve Unloaders When a reciprocating compressor is running normally the suction and discharge valves open and close in a set sequence (order) as the piston moves backwards and forwards in the cylinder. Special equipment can be fitted to the compressor to hold the suction valve (or valves) open. This equipment is known as a valve unloader. Valve unloaders can be operated manually or automatically. Valve unloaders are usually used only on constant speed compressors. They are always fitted to the suction valve (or valves) of the compressor. When a valve unloader is in use it holds the suction valve open. On the backward stroke of the piston, gas is drawn into the cylinder. The same gas is pushed out of the cylinder through the open suction valve on the forward stroke of the piston. With nothing to hold the gas in the cylinder there is no increase in the pressure of the gas.

Figure 2-4 Manual Valve Unloaders Look at the single cylinder, double acting compressor arrangement shown in Figure 2-4. Manual valve unloaders are fitted to each of the suction valves. If neither of the valve unloaders is used, the compressor is running at 100% capacity. With one of the valve unloaders used the compressor only works on one stroke of the piston, not on both strokes. The compressor is running at 50% capacity. If both valve unloaders are in use the compressor is running at 0% capacity which is also called 'no load'.

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Large reciprocating compressors are often started up on 'no load'. This means that the prime mover does not have to do as much work as it would do if there was a load on the compressor. This increases the life of the prime mover.

Module No. 6 : Compressors & Turbines

Unit No. 2 - Reciprocating compressors

Valve unloaders can also be controlled automatically. Figure 2-5 shows an automatic valve unloader fitted to the drive end of a single cylinder, double acting reciprocating compressor. In this case, the control signal to load or unload the suction valve comes from the compressor discharge pipeline. The unloader mechanism is operated by an air signal from a pilot. The pilot receives a supply of instrument air and a separate signal from the compressor discharge pipeline. At the pre-set compressor discharge pressure the pilot opens to supply instrument air to the unloader mechanism on the suction valve.

The unloader mechanism includes a diaphragm. As the instrument air enters the chamber above the diaphragm, the diaphragm is forced downwards. Below the diaphragm are rods which rest on the suction valve. As the diaphragm moves downwards, so do the rods. This makes the suction valve open and this unloads that side of the cylinder which, in this case, is the drive end. When the compressor discharge pressure falls below the pre-set level the pilot valve closes, shutting off the instrument air supply to the Unloader. The diaphragm rises due to springs place , beneath it. The rods come off the suction valve and the suction valve closes. The drive end of the cylinder is once again loaded.

Figure 2 - 5 Automatic Valve Unloader

2.2.2 Compressor Speed As the speed of a compressor is increased its capacity also increases. So, as the compressor speed is decreased its capacity also decreases. Page 7/15

Unit No. 2 - Reciprocating compressors

Using compressor speed to control capacity is limited by the type of prime mover driving the compressor. The speed of internal combustion engines, usually diesel engines, can easily and accurately be controlled either manually or automatically. The speed range is usually between 100% and 50% of rated speed.

Steam turbines and gas turbines can also be speed controlled although it is more usual to consider these prime movers as constant speed machines. Most reciprocating compressors run at low to medium speeds. Steam and gas turbines are high speed drivers. When a steam or gas turbine is used as the prime mover for a reciprocating compressor the output speed of the turbine is normally reduced using gears or a V-belt drive. This drive arrangement is known as the transmission. Variable speed transmissions are available but are not commonly seen in process plants. A variable speed transmission works like the gearbox of a car. Variable speed electric motors are expensive. For this reason it is unusual for them to be used as prime movers for capacity control by compressor speed. 2.2.3 Clearance Pockets Capacity control using a clearance pocket is one method which can be used on constant speed reciprocating compressors.

Module No. 6 : Compressors & Turbines

At the end of each compression stroke some gas is left in a clearance space inside the cylinder. The clearance space is made up of spaces in the valve recesses and the space that there is between the piston and the cylinder end at the finish of the stroke. (See Figure 11 - 19.)

Figure 2-6 Clearance Space The capacity of a reciprocating compressor can be controlled by changing the amount of clearance in the cylinder. To do this a

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Module No. 6 Module : Compressors No. 6 : Compressors & Turbines & Turbines

Unit No. 2 - Reciprocating Unit No. 2 - Reciprocating compressors compressors

clearance pocket is used The clearance pocket increases the amount of clearance in the cylinder. The amount of clearance can be controlled and so the capacity of the compressor can also be controlled. Normally a clearance pocket is fitted to the head end of the compressor cylinder. Sometimes clearance pockets can be fitted to each end of the compressor cylinder. Clearance pockets can be fixed volume or variable volume. Examples of both types are shown in Figure 2-7.

Figure -2-7-.Clearance Pockets

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2.2.4

Hot Gas Bypass A compressor fitted with hot gas bypass capacity control has a bypass line running from the discharge of the compressor to the suction of the compressor. The bypass line includes a valve which is usually controlled automatically.

Unit No. 2 - Reciprocating compressors

Capacity control of this type gives a very wide range of control. However, the compressor must be operating at full load and full capacity all the time. It is also necessary for the gas being returned to the suction side of the compressor to be cooled to normal suction temperature. If this is not done, compressor discharge temperatures will be too high. 2.3

CONTROL OF SUCTION PRESSURE Control of the suction pressure of a reciprocating compressor is particularly important when the compressor forms part of a continuous process. Imagine a compressor Which receives gas from one process plant. The purpose of the compressor is to increase the pressure of the gas so it can be sent to another process plant for further processing or storage. The process plant which receives the gas from the compressor is set up to handle the gas at a certain temperature and pressure, and at a certain flow rate. If the suction pressure at the compressor falls below the normal operating range the temperature of the gas leaving the compressor will be higher than the normal range. It may be unacceptable to the downstream process plant. If the suction pressure at the compressor rises above the normal operating range, both the flow of gas and the discharge pressure of the gas will be higher than the normal range. These increases may also be unacceptable to the downstream process plant. Also, when the compressor suction pressure is higher than normal, the compressor and its prime mover are overloaded. This may cause the compressor or its prime mover to fail. If this happens the upstream process plant may have to shut down because it can not get rid of its gas. The downstream process plant may also have to shut down because it is not receiving gas.

Module No. 6 : Compressors & Turbines

Control of reciprocating compressor suction pressure is done in a similar way to capacity control, which has been described in Paragraph 2.2. Clearance pockets, compressor speed and hot gas bypass can all be used to control compressor suction pressure. 2.4

AUTOMATIC SHUTDOWN SYSTEMS A compressor is an expensive piece of equipment. If, for example, the compressor is not being supplied with enough lubricating oil it will be damaged. Repairing the damage will cost money. Also, with the compressor out of service for repair, upstream or downstream processes could be affected. For this reason, compressors are fitted with automatic shutdown systems. These systems detect when something is going wrong and shut down the compressor before damage can occur. Although this could still affect upstream or downstream processes, the cost is far less than repairing a damaged compressor.

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Unit No. 2 - Reciprocating compressors

Listed below are some of the usual automatic shutdown systems fitted to a reciprocating compressor. The numbers in brackets refer to the instruments shown on Figure 2-8. 1.

Low lubricating oil Pressure (1) is detected by a low pressure sensor. The low pressure sensor is usually located on the lubricating oil supply line.

1.

Low lubricating oil level (2) is detected by a low level sensor fitted on the lubricating oil sump.

1.

High vibration (3 and 4) is detected by instruments called vibration monitors. The vibration monitors can be fitted to various places on the compressor or its prime mover.

1.

High liquid level in the suction scrubber (5) is detected by a high level sensor fitted on the suction scrubber. A high liquid level in the suction scrubber could cause liquid to be carried over into the compressor.

1.

High differential pressure across the lubricating oil filter (6) is detected by a high differential pressure sensor fitted on the filter. A high differential pressure across the lubricating oil filter shows that the filter is dirty and requires cleaning. If the filter is not cleaned it will lead to a low lubricating oil pressure.

1.

High temperature of the discharged gas (7, 8 and 9) is detected by high temperature sensors. The sensors may be fitted on the compressor discharge line or on the discharge line from each stage of the compressor.

1.

Overspeed (10) is detected by an overspeed sensor which is usually fitted to the prime mover of the compressor.

1.

Low suction pressure (11) is detected by a low pressure sensor fitted in the compressor suction line.

1.

Packing failure (12) is detected either by measuring the volume of gas venting from the seal area or by measuring the temperature of the vented gas. An increase in either volume or temperature will indicate packing failure.

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All of these sensors are linked to the shutdown arrangements for the compressor. The usual method of shutdown is to cut off the supply of fuel, steam or electricity to the prime mover of the compressor.

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Figure 2 -8 Automatic Shutdown Systems

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