Petroleum Gas Compression workbook 2.pdf

POL Petroleum Open Learning

Petroleum Gas Compression Part of the Petroleum Processing Technology Series

OPITO

2

THE OIL & GAS ACADEMY

Petroleum Gas Compression - Unit 2 - Reciprocating Compressors

Petroleum Open Learning

(Part of the Petroleum Processing Technology Series)

Contents

Page

Training Targets

2.2

Introduction

2.3

Section 1 – Basic Theory

2.4

Operating Principles Capacity and Compression Ratio Compressor Performance

Section 2 – Design and Construction

Cylindars Pistons and Piston rings Compressor Valves Stuffing Box Crankshaft, Connecting Rod, Crosshead and Piston Rod

Section 3 – Auxiliary Systems Cooling System Lubrication System Suction and Discharge Piping System Drive Coupling

2.29

training targets for you to achieve by the end of the unit

test yourself questions to see how much you understand

check yourself answers to let you see if you have been thinking along the right lines

activities for you to apply your new knowledge

Section 4 – Operation of Reciprocating Compressors A Typical Gas Compression System Alarm and Shutdown Systems The Main Operational Checks on a Reciprocating Compressor

Check Yourself – Answers

2.17

Visual Cues

2.37 summaries for you to recap on the major steps in your progress

2.44

2.


Introduction


Training Targets When you have completed Unit 2 of the Petroleum Gas Compression series you will be able to : •

Explain the basic operating principles of a reciprocating compressor.

•

Describe the construction of a reciprocating compressor.

•

Explain the function and operation of the principal components of a reciprocating compressor.

•

Describe the layout and operation of the auxiliary systems associated with a reciprocating compressor.

•

Explain a basic reciprocating compressor alarm and shutdown system.

•

List the common operating checks carried out on a reciprocating compressor.

2.


Introduction


As I previously explained, the compressor requirements of an oil or gas production system are dependent upon many variables. Each system will have its own characteristics and show detailed design differences.

In this Unit, we will be looking at the construction and operation of typical reciprocating compressors.

You saw in Unit 1, that compressors can be classified as either continuous flow or positive displacement machines. The reciprocating compressor is the most common of the positive displacement type. This is the one we are going to concentrate on in this unit.

The Unit is divided into four sections:

Reciprocating compressors are designed to operate over a wide range of capacities and pressures. Small portable machines may be adequate for the delivery of small volumes, at pressures of, say, 1.5 bar. Large industrial units may be required to deliver several thousand cubic metres per hour, at pressures approaching 1000 bar.

Section 1 covers the basic operating theory of reciprocating compressors. In Section 2, we will look at the design and construction of a typical machine. Section 3 will describe a range of auxiliary equipment and in Section 4, we will be looking at basic compressor operations.

2.


Section 1 - Basic Theory Operating Principles All positive displacement compressors operate by : • creating a low pressure space into which gas may flow • closing the entrance to this space


When the air pressure in the cylinder is greater than the pressure in the bicycle tyre, the air flows into the tyre. A small non-return valve on the bicycle tyre prevents the air from flowing out of the tyre back into the cylinder. The whole cycle may now be repeated. All reciprocating compressors work in a similar way to the bicycle pump.

• displacing the enclosed gas with a mechanical device (thereby increasing the pressure) • opening the exit to the space, allowing the compressed gas to leave The simplest form of reciprocating compressor in common use is the Bicycle Pump. In this type of compressor, a small washer, the piston, is pushed back and forth inside a tube which is called the cylinder. As the piston moves backwards it creates a low pressure space inside the cylinder. The washer is then distorted and allows outside air to flow past it into the cylinder.

Figure 1 on the next page shows the main components of a reciprocating compressor. Take a look at the Figure and try to become familiar with the names of the various parts.

When the piston reaches its furthest point of backward travel, the washer again flexes, and seals the gap between the piston and cylinder. Now, when the piston moves forward, the volume in the cylinder is reduced and the air is compressed.

2.


2.


You can see from Figure 1 that the flow of gas through the compressor is controlled by valves. These act as non-return valves to permit flow in one direction only. These valves are often called ‘check valves’. They are positioned in the inlet (suction) and outlet (discharge) of the compressor. Gas enters the cylinder through the suction valve and leaves through the discharge valve, • Suction valves open when the cylinder pressure is lower than the pressure of the gas to be compressed • Discharge valves open when cylinder pressure is higher than the pressure of the system into which the gas is to be discharged Reciprocating compressors are classified as : Single Acting or Double Acting

Let’s look at the way in which each of these work. First the single acting compressor. Figure 2 shows the flow of gas through this type of machine.

2.


The section of the cylinder nearest the crank is called the crank end. The section of the cylinder furthest away from the crank is called the head end. Only the space at the head end of the cylinder is used for compression. In the single acting compressor, the back stroke is the suction, or intake stroke. The forward stroke is the compression or discharge stroke.

In a double acting compressor there is a suction stroke and a discharge stroke each time the piston moves either backwards or forwards. This is illustrated in Figure 3.

2.


The back stroke is the compression stroke at the crank end of the cylinder and the suction stroke at the head end of the cylinder. As can be seen, these roles are reversed during the forward stroke. Now, why not try the following Test Yourself:

Test Yourself 2.1 Is a bicycle pump a single or double acting compressor?

You will find the answer to Test Yourself 2.1 on Page 2.44

2.


Figure 4 is a more detailed drawing of a double acting reciprocating compressor. Study the drawing for a while and identify its components.

2.


Capacity and Compression Ratio Compressors are used to increase the pressure of gases and transfer these compressed gases to a higher pressure system. The volume of gas moved by the compressor in a given period of time is called its capacity. But, as the gas is being compressed during transfer, its volume is reducing, and we need to be careful at what point we measure this capacity. Capacity is measured as the volume of gas entering the compressor in a given time period. The amount by which a compressor increases the pressure of the gas is called the compression ratio. it is defined as : Discharge Pressure Suction Pressure For example, if the suction pressure is 10 bara and the discharge pressure is 40 bara, the compression ratio is 40/10 or 4. It is usually expressed as 4 : 1 or 4 to 1.

Activity Take a bicycle pump and, without connecting it to a bicycle tyre, pump it ten times. Now put your hand on it and test the temperature. What do you notice? After you have done this, connect the pump to a bicycle tyre and pump it another ten times.

When pumping before connection to the tyre, you will notice no temperature increase. This is because you are displacing air into the atmosphere without increasing its pressure. After connecting to the tyre, however, you should have noticed a sharp increase in temperature as the pressure in the tyre increases.

What do you notice about the temperature this time? Repeat this a few times while the pump is still connected to the tyre. After every ten strokes, check the temperature of the pump by feeling it with your hands.

2.10


The activity you have just completed demonstrated a basic fact regarding compressors. When a gas is compressed, its temperature increases. The higher the compression ratio, the larger the temperature increase. This temperature increase has a detrimental effect on both the efficiency of the compressor and its mechanical reliability. Because of these considerations, the temperature rise is restricted to certain limits (typically about 200°C, although higher temperatures may be experienced). One of the basic means of limiting the temperature rise is to limit the compression ratio to about 6 to 1. If the required final discharge pressure cannot be met by this compression ratio, then compression is carried out in a number of stages. Machines capable of doing this are called multi-stage compressors. They have coolers to reduce the temperature of the gas between each stage.

Test Yourself 2.2 Diesel engines are classed as ‘compressionignition’ engines - in other words, the heat generated by compression of the fuel/air mixture also ignites this mixture. In a particular diesel engine, the compression ratio is 20:1. Air is taken in from the atmosphere at a pressure of 1 bara. What will be the pressure of the air/fuel mixture in the engine cylinder when maximum compression is reached?

I suggest you have a go at the following Test Yourself, to underline the points covered above.

You will find the answer to Test Yourself 2.2 on Page 2.44

2.11


Compressor Performance The performance of a reciprocating compressor can be represented by a pressure/volume (PV) diagram. One of these is shown in Figure 5, which illustrates the relationship between the cylinder pressure of a compressor and the cylinder volume enclosed by the piston, for a single-acting compressor.

2.12


Horizontal distance in the PV diagram represents a change in volume produced by the movement of the piston in the cylinder. Vertical distance on the diagram represents a change of pressure in the cylinder caused by the movement of the piston.

When the cylinder pressure drops slightly below the suction pressure (at point B), the suction valve opens. Curve AB on the PV diagram represents the pressure fall and volume increase as the piston begins to move back on the suction stroke.

As the piston moves back and forth in the cylinder, the volume and pressure in the cylinder changes. We will now use the PV diagram to follow these changes. The diagram shows a complete compression cycle, consisting of one backward stroke and one forward stroke.

The opening of the suction valve is represented on the PV diagram by point B.

Point A represents the end of the compression stroke and we shall use this as our starting point. (The piston is designed so that it cannot touch the end of the cylinder. The small space which is left between the piston and the end of the cylinder is called the clearance space. At the end of every stroke there is a small amount of gas left in the clearance space.) As the piston begins to move back in the cylinder, on the suction stroke, the gas remaining in the cylinder expands. As the gas expands, the pressure in the cylinder decreases.

As the piston moves further back in the cylinder, gas flows in through the suction valve. This is represented on the PV diagram by the line from B to C. The end of the suction stroke is represented by point C. At this point, the piston reverses its direction and begins the compression stroke. As soon as the piston begins to move in the opposite direction the gas begins to be compressed. Cylinder pressure rises above suction line pressure and the suction valve closes. As the piston continues to move forward in the cylinder the gas pressure increases and, at point D, the gas is compressed to a level slightly higher than the pressure of the gas in the discharge system. At this point the discharge valve opens. For the rest of the stroke, D to A, gas is forced out through the discharge valve and into the high pressure discharge system.

2.13


Test Yourself 2.3 The following statements are in the wrong order. Place them in their correct sequence, starting with No.1:

1.

Piston begins suction stroke.

.... 1 ....

8. Gas remaining in cylinder expands and discharge valve closes.

............

2.

Gas flows from cylinder into discharge line.

............

9. Suction valve closes.

............

3.

Piston reverses direction at end of suction stroke.

............

10. Gas in cylinder is compressed to above suction line pressure.

............

4.

Cylinder pressure rises above discharge line pressure. ............

11. Discharge valve opens.

............

5.

Cylinder pressure falls below suction line pressure.

............

12. Suction valve opens.

............

6.

Gas flows into cylinder from suction line.

............

7.

Piston reverses direction at end of discharge stroke.

............ The answers to Test Yourself 2.3 will be found on Page 2.44

2.14


Summary of Section 1 In this first Section of Unit 2, we have looked at the basic theory of operation for positive displacement compressors. We started by looking at how a reciprocating compressor operates, and compared this operation to that of a bicycle pump. The main components of a reciprocating compressor were described, and you identified these on a simple line diagram.

You saw that reciprocating compressors can be classified as either :

Using diagrams, you looked at the flow of gas through single acting and double acting compressors.

Next, we went on to consider compressor capacity and compression ratio. We saw how the flow of gas through the compressor could be represented by a pressure/volume diagram.

• single acting, or • double acting

We will now go on to Section 2, which examines the design and construction of reciprocating compressors. But first, by way of a little revision, try Test Yourself 2.4.

2.15


Test Yourself 2.4 Indicate whether the following statements apply to a single-acting, or double-acting reciprocating compressor, or both.

Single- Acting

Double- Acting

1.

Only the space at the head end of the cylinder is used for compression.

6.

The suction valve opens every second stroke.

2.

There is a suction and a discharge stroke each time the piston travels the length of the cylinder.

7.

There is a suction and a discharge valve at each end of the cylinder.

3.

A suction valve is open during each suction stroke.

8.

The forward stroke is the compression, or discharge stroke

4.

During each stroke (forward and backward) a discharge valve is open.

5.

The back stroke is the suction, or intake stroke.

Single- Acting

DoubleActing

You will find the answers to Test Yourself 2.4 on Page 2.45 2.16


Section 2 - Design and Construction In this Section, we are going to have a look at the principal components of a reciprocating compressor. We will see how they are constructed and exactly what they do. I have listed below the components which we will consider in the section :


The piston is fitted with piston rings, which we will look at later. Because these will cause wear, the cylinders are commonly lined with a smooth bored liner, which can be replaced when it becomes worn. Figure 6 is a drawing of a cylinder, liner, piston and piston rings.

• Cylinders • Pistons and piston rings • Compressor valves • Stuffing box and packing • Crankshaft, connecting rod, crosshead and piston rod Take another look at Figure 1 on Page 2.5. See how many of the components listed above you can identifythey are not all labelled ! When you have done that, we will look at each item on the list in turn.

Cylinders You have seen in previous illustrations that the cylinder in a reciprocating compressor is considerably more substantial than a bicycle pump. However, it is still basically a tube in which a piston slides back and forth.

2.17


Wear occurs where the piston rings rub against the liner. To avoid this wear forming a ‘shoulder’ or ‘step’, counter bores are machined into the liner. (Look at Figure 6 again). A counterbore is a small increase in cylinder bore diameter, made just above the point at which the end piston rings stop and reverse direction.

Cylinder Cooling

Liners are usually pressed or expanded into place in order to avoid slippage which could result in knocking and excessive wear.

On smaller machines, the cooling is done by blowing air across fins which are attached to the outside of the cylinder. Most reciprocating compressors, however, use a liquid cooling system.

Cylinder Lubrication In low pressure/low temperature applications, the cylinders may not require lubrication. In this case, the pistons may be fitted with self-lubricating piston rings, made of nylon or teflon. However, in most compressor applications, cylinder lubrication is required to prevent excessive overheating or wear. In such situations, a boundary layer lubrication system is usually installed. This injects small droplets of oil into the cylinder, to be distributed by the movement of the piston rings. This type of lubrication prevents the formation of an oil mist in the gas leaving the compressor.

On compressors with lower compression ratios, the cylinders may not require cooling. In most cases, however, the temperature rise across the machine requires that the cylinders are cooled.

The cylinders are surrounded by cooling jackets, through which a coolant solution is circulated. This solution is usually a mixture of water and glycol, which also acts as an anti-freeze agent.

You can see the cooling jacket round the cylinder in Figure 7, on the next page.

2.18


The coolant fluid is circulated to prevent localised hot spots and to take away unwanted heat generated by compression. This removal of unwanted heat improves compressor efficiency.

2.19


Pistons and Piston Rings Pistons are most commonly made from a solid casting. The piston rod, often made of stainless steel, is tapered where it passes through the piston. It is then secured against the shoulder by a lock nut. This is illustrated in Figure 8.

To prevent or minimise gas leakage between the piston and the liner, piston rings are provided to make a seal. They fit into grooves cut in the side wall of the piston. The piston rings also serve to carry some heat from the piston to the cylinder wall. The clearance between piston and cylinder wall must be: • small enough to prevent the back-flow of gas across the piston • small enough to permit adequate support of the piston rings • large enough to prevent the rings from sticking to the cylinder and causing excessive friction All piston rings are designed to wear more rapidly than the cylinder liner, which should be true and free from scores.

2.20


Figure 9 shows two types of piston ring.

Compressor Valves

All valves have certain features in common: • a valve seat which provides a pressure tight gas seal.

There are several types of valve used in reciprocating compressors. There is no significant difference between suction and discharge valves, and they both operate in a similar manner.

• a valve plate or other device to seal across the valve seat. • a spring or other mechanism to hold the valve plate on the seat in the closed position. •

a cover to contain the springs and prevent the plate from moving too far.

A typical valve is shown in Figure 10.

During operation the rings must move out against the cylinder to effect a ring-to-wall seal, and the gaps in the rings allow them to do this. The sealing effect is aided by the piston and rings expanding out towards the cylinder wall as the compressor reaches operating temperature. 2.21


The valve plates are in the form of rings connected by webs and are held lightly against the seat by a set of small leaf or coil springs. To open the valve, the gas must overcome the pressure of the gas behind the plate and the light tension of the springs. Any tendency of the valve to slam or flutter can often be controlled by changing the tension of the valve springs.

Remember that a suction valve is properly installed when you can depress the plate in towards the centre of the cylinder, and a discharge valve is installed properly when you can depress the plate away from the centre of the cylinder. It must be emphasised that any loose material such as screws or nuts falling into a cylinder can cause very severe damage. Hence compressor valves are installed with through-bolts, lockscrews or jackbolts to hold the valve assembly together.

Stuffing Box In order to prevent leakage of compressed gas from the cylinder past the piston rod, some form of seal is required. The most common type of seal is the stuffing box. The stuffing box consists of a series of seal elements each containing a pair of seal rings. Figure 11, on the next page, shows the arrangement of a seal element with a type of seal ring known as the TR type.

Compressor valves are among the most important parts of a reciprocating compressor and the following points should always be born in mind: 1.

A worn or damaged valve allows gas to leak back.

2.

When a valve leaks, the gas returning through the valve is hotter. Valve leakage can often be detected by an increase in temperature at the valve.

3.

The sudden, chilling effect of cold liquid on a hot valve can break the valve plate. Hence the requirement for liquid-free gas in the compressor.

4.

Dirt or frozen deposits can foul or damage a valve and prevent it from seating properly.

2.22


The TR type seal element consists of two rings which are distinguished from each other as follows • the internal ring (T) is fitted first and has tangential cuts • the external ring (R) is fitted last and has radial cuts

The T ring haste function of preventing gas leakage. The R ring protects the T ring and helps to dissipate heat. The two rings are assembled with staggered cuts and a dowel (not shown) provides for their correct positioning.

2.23


Another type of stuffing box seal element is the TT type as shown in Figure 12. In this, both rings have tangential cuts.

The ends of the seal ring segments, in both the TR and TT types, are not in contact. This allows them to compensate for the progressive wear of the rings by gradually moving closer together. A spiral spring, assembled on the groove drawn round the edge of each ring, keeps the segments together. 2.24


Figure 13 is a drawing of a typical Stuffing Box.

The series of sealing elements are held in position by the stuffing box end cover. This is secured by long stud bolts. Piping for the entry and exit of lubricating oil, and the venting of gas are built into the end cover.

Crankshaft, Connecting Rod, Crosshead and Piston Rod The drive motor (either an electric motor or an internal combustion engine) imparts a rotary motion to the drive shaft. This is converted to reciprocating motion by the crankshaft, connecting rod and crosshead.

Crankshaft The crankshaft is made of forgeable carbon steel, machined throughout. It is provided with a single crank and is suitably counterweighted to limit the dynamic load on the foundation. The crankshaft ends are equipped with bearings of the bush type. They are fitted on the crankcase sidewalls.

2.25


The crankshaft, connecting rod, crosshead and piston rod are shown in Figure 14.

2.26


Connecting Rod

Crosshead

This is made of high strength pressed steel.

The crosshead connects the piston rod to the connecting rod of the crankshaft. It is equipped with shoes which permit it to slide back and forth within the crosshead guides. (See Figure 15)

Both ends of the connecting rod are equipped with heavy duty sleeve bearings. Figure 15 shows the connecting rod at the crosshead.

The connecting rod is moved by the crankshaft. As the crankshaft rotates, the connecting rod reciprocates.

Piston Rod Piston rods are usually made of stainless steel. They are accurately ground and have no taper within their length of travel. The piston rod screws into the crosshead and is secured in place by a locking device. A slinger ring prevents oil from the crankcase being carried out by the piston rod and reaching the cylinder. It is installed on the piston rod, as you can see in Figure 14.

Now that you are familiar with the components of a typical reciprocating compressor, have a go at the following Test Yourself before moving to the next Section. 2.27


Test Yourself 2.5

Summary of Section 2

1.

What is the purpose of a counterbore in a liner?

In this Section, we have looked at the component parts of a reciprocating compressor.

2.

Why are liners pressed or expanded into place?

You will have noted how the piston is lubricated and how the cylinders are cooled.

3.

How does a boundary layer lubrication system work ?

4.

How does a cooling system improve the efficiency of the compressor?

5.

What are the two functions of piston rings?

6.

Which components convert rotary motion to reciprocating motion?

You will find the answers to Test Yourself 2.5 on Page 2.45

We saw how the space between the cylinder liner and the piston is sealed by the piston rings.

The construction of compressor valves was described, and how they operate to maintain the flow of gas through the compressor. We have looked at the different types of seal used in the stuffing box, and how the stuffing box prevents gas from escaping from the compressor along the piston rod. You saw how the crankshaft, crosshead and connecting rod convert the rotary motion of the driver to the reciprocating motion required by the compressor.

In the next Section, we will take a look at the auxiliary systems which are used with reciprocating compressors.

2.28


Section 3 - Auxiliary Systems In this Section, we will be looking at a number of auxiliary systems which are associated with reciprocating compressors. These are: • Cooling System • Lubrication System • Suction and Discharge Piping System • Drive Coupling

Cooling System You saw earlier that, as gas is compressed, its temperature increases. The compressor’s cooling system removes some of the heat generated by compression (heat of compression) and also protects the piston and cylinder from becoming overheated. Figure 16 is a simple line drawing of a compressor cooling system, cooling the cylinder and stuffing box.


The coolant solution (usually a mixture of water and an antifreeze, such as glycol) is circulated through the cylinder coolant jackets. This prevents the formation of local hot spots, and provides for an even distribution of heat. The heat is carried away from the cylinder by the coolant, in a closed loop thermosyphon system. • The cylinder jackets are connected by pipes to an expansion tank, which allows expansion of the coolant solution as it heats up during compressor operation. (This tank is provided with a vent and a level gauge.) • A thermosyphon effect is obtained when the coolant is warmed by the heat from the compressor: the cold (and therefore heavier) coolant flows from the bottom of the tank to enter the bottom of the cylinder jacket, while hotter (and therefore lighter) coolant is displaced from the cylinder jacket back to the expansion tank. The return line to the tank is near the top, but below the liquid level. • The warm coolant loses heat from the sides of the tank to the atmosphere and, when cold, falls to the bottom of the tank. • As long as the circuit is kept full of coolant, the coolant will keep flowing around the system. This limited circulation system gives adequate cooling for a process compressor handling high pressure, low temperature gas.

2.29


Lubrication System

2.30


Figure 17 is a line drawing of a typical lubrication system which supplies lubricating oil to the following parts of the compressor : • Crank mechanism • Piston rod packing • Crosshead The lubricating oil forms a surface film which reduces friction and, therefore, wear between the moving compressor parts.

Oil from the crankcase sump first passes through the coarse strainer. This strainer is removable so that it can be cleaned. The oil is then drawn into the pump suction. The pump increases the pressure of the oil and discharges it to the oil cooler. From the cooler outlet the oil flows, via fine filters, to two separate lubrication systems: • crosshead and stuffing box • crankshaft frame

The lubricant also has a cooling function. Some of the heat generated by friction is carried away by the lubricating oil. The lubrication system supplies filtered oil at the required pressure and temperature to the compressor frame. The most common form of lubrication is a forced feed type. Here, the oil is pumped under pressure to the required parts. The pressure is supplied by means of an electric motor driven pump. A standby pump is usually provided in order to achieve uninterrupted operation. This can be seen in Figure 17. The lubricating oil is collected and stored in the crankcase sump. The sump is equipped with a heater, level sight glass, coarse strainer and a drain.

2.31


Suction and Discharge Piping System Figure 18 shows a typical piping system for a single stage reciprocating compressor.

2.32


A liquid knockout drum is installed in the suction piping to remove any entrained liquid from the process gas. A drain is provided to allow any accumulated liquid to be removed.

Such pulsations may also cause starvation on the suction side of the compressor. The effective capacity of the cylinder may be reduced by as much as 25% by operating without a suction volume bottle.

The knockout drum is one of the most important items of equipment in the piping system. Liquids are incompressible fluids and, if they enter the compressor, even in very small amounts, they could cause the cylinder to rupture.

The capacity of a suction volume bottle is normally not less than seven times the total cylinder capacity for all cylinders served. The bottles are usually located close to their cylinders.

To save space, volume bottles can be replaced by pulsation dampeners. The most common pulsation dampener is the baffle type. Figure 19 shows a typical baffle type pulsation dampener.

In addition, cold liquid mist entering a hot compressor can seriously damage the suction valves. A strainer is fitted in the suction piping downstream of the knockout drum. This strainer is normally installed for start-up purposes to prevent hard pieces of scale, welding beads, etc., left over from construction and maintenance, from entering the compressor and causing damage. The suction piping transfers the process gas to the inlet of the compressor via a suction volume bottle. The purpose of the suction volume bottle is to act as a reservoir which damps down pulsations in the inlet gas. Such pulsations are due to intermittent flow through the compressor and, if they happen to match the natural frequency of vibration in the pipework, can cause serious damage.

2.33


The gas leaves the compressor via a discharge volume bottle. The purpose of the discharge volume bottle is to prevent excessive momentary discharge pressures or pulsations. Large discharge pulsations can result in severe overloads to the compressor and pipework and may also reduce effective cylinder capacities. Again, the discharge volume bottle may be replaced by a pulsation dampener. All volume bottles are equipped with drains and pressure taps for checking pressure losses. They must be located so that they can be easily removed for inspection or possible repair. The compressor discharge piping transfers the compressed gas to the process equipment. A non-return valve is fitted in the discharge pipe close to the compressor. The function of the non-return valve is to prevent high pressure gas from the downstream process equipment flowing backwards through the compressor when it is not operating. (The compressor discharge valves should prevent the backflow of gas through the compressor. The non-return valve is fitted as an added safeguard.) A block valve is also fitted to the discharge of the compressor. This valve is used for compressor isolation. The discharge block valve should always be opened before the compressor is started up.

A compressor vent line is fitted on the compressor side of the discharge block valve. The compressor vent line is used: • to depressurise the compressor after shutdown

A compressor bypass line is sometimes used to transfer discharge gas back into the compressor suction piping and reduce the efficiency/capacity of fixed speed machines.

• to purge the compressor of flammable gas before maintenance

Drive Coupling

• to purge the compressor of air before start-up

Reciprocating compressors are normally driven by an internal combustion engine or an electric motor, which is connected to the crankshaft by means of a drive shaft and a direct coupling.

For start-up and maintenance purposes, the most common purge gas is nitrogen. The purge gas is: • injected into the suction line • allowed to flow through the compressor • vented from the system via the vent line During start-up, the vent line is also used to purge the nitrogen from the compressor casing with the gas which is to be compressed. On a typical oil production platform, the vent line is routed to the platform flare system. The compressor casing is protected against excessive pressure by a pressure relief valve fitted in a branch pipe which is connected to the compressor discharge line. To prevent accidental isolation of this relief valve, it is always fitted on the compressor side of the discharge block valve.

A direct coupling will only accommodate small inaccuracies in the alignment of the drive and crankshaft - both the motor and the compressor must be accurately positioned to achieve an acceptable alignment. This is usually ensured by using a common base for the driver and the compressor. This common base is called a bed·plate. The bed-plate is accurately machined to ensure that it is level, and the two machines are positioned by the use of dowels. A small clearance is maintained between the two halves of the coupling to avoid imposing any end thrust on the motor bearings.

2.34


The rotational direction of the crankshaft is important and the motor rotation should be checked to make sure that it matches that of the machine, before the two are coupled together.

You have now completed Section 3 of this Unit on reciprocating compressors, which dealt with the auxiliary systems. The following Test Yourself will help to reinforce your understanding of the topics covered.

Test Yourself 2.6 1.

Why do we mix glycol with the water in the cooling system?

2.

What makes the water circulate through the cooling system?

3.

Where is the lubricating oil collected and stored?

4.

What is the most common form of lubrication for a reciprocating compressor stuffing box?

5.

Why is there always a liquid knockout drum installed in the suction piping ?

6.

Are liquids compressible?

7.

On a typical oil production platform where would you expect the compressor vent line to lead to ?

8.

How do we ensure that the driver and compressor are accurately aligned?

You will find the answers to Test Yourself 2.6 on Page 2.46

2.35


Summary of Section 3 In this Section on auxiliaries, we have looked at : • the compressor cooling system • the lubrication system • the suction and discharge piping system • the driver coupling

You saw how the cooling system supplied cooling liquid to the cylinders and stuffing box. Use of the thermosyphon effect to achieve circulation of this cooling liquid was also explained. We then looked at the lubrication system and saw how the lubricating oil was stored, filtered and then pumped to the crank mechanism, piston rod packing and crosshead.

The suction and discharge piping system was examined. We saw how the knockout drum prevented liquid from entering the compressor. Volume bottles (or pulsation dampeners) were used to reduce pulsations caused by the intermittent flow of gas into and out of the compressor. We noted the use of the vent line and saw how the pressure relief valve was always positioned on the compressor side of the discharge block valve.

Finally we looked at the driver coupling and saw how the use of a common bed-plate for the driver and the compressor reduced the problems of alignment.

In the next Section, we will look at the operation of a typical gas compression system using reciprocating compressors, together with alarm and shutdown systems and some of the main operational checks.

2.36



Section 4 - Operation of Reciprocating Compressors In this, the final Section of the reciprocating compressor unit, we will be looking at the operation of the compressor.

I have divided the Section into the following topics : • a typical gas compression system • alarm and shutdown systems

A Typical Gas Compression System Figure 20 is a line drawing of a separation and gas compression system which uses reciprocating compressors.

• the main operational checks on a reciprocating compressor

2.37


It is intended to illustrate a typical reciprocating compressor installation, but is not meant to represent any specific plant. Follow this illustration using the description below. From the diagram you can see that:

• After treatment the gas is passed through the 2nd stage suction knockout drum before being fed into the 2nd stage of the two stage compressor. • Here, the gas is compressed to meet the requirements of a gas-lift system or of sales gas.

• Low-pressure gas from the 2nd stage of an oil/gas separation system is • Note that, in this two stage system, both passed through a booster compressor compressors are driven by the same motor. suction knockout drum.This drum removes any entrained liquids from the gas before it is fed into the booster compressor. (The suction Alarms and Shutdown Systems knockout drum is sometimes referred to as the suction scrubber.) We should now look at how we control the compression process and how we protect the • The booster compressor increases the equipment from damage. pressure of the gas from the 2nd stage separator to that of the 1st stage. Generally, all process controls are designed to inform • After passing through the booster compressor the gas is cooled before it joins with gas from the 1st stage separator. • The combined gas stream is then passed through another suction knockout drum to remove any entrained liquids before being fed into the suction of the 1st stage of a two stage compressor. • This 1st stage increases the gas pressure to a level which allows it to be treated, say, in a gas liquids recovery plant.

A minor process disturbance maybe any process variable (temperature, pressure, level, flow, etc.) which is too high or too low. This will not be a dangerous situation, but it has the potential to become dangerous if not attended to. For example: • Suppose there is a low liquid level in the coolant tank of a compressor. There is no immediate danger of overheating. If the operator reacts quickly to top up the tank with coolant, the immediate problem is solved. (Clearly, however, the operator must find out why the coolant level fell in the first place.) When this type of disturbance occurs, the control system will generate an alarm. The setting of the alarm status usually gives the operator sufficient time to react and correct the problem before the situation becomes dangerous.

the operator automatically if anything goes wrong. Process control systems normally work on two levels: • minor process disturbances • major process disturbances, or emergency incidents

Major Process Disturbances or Emergency Incidents A major process disturbance may be any process variable which is so high or low that the system has reached a potentially dangerous condition.

2.38


One example may be where the low coolant level problem described above has not been dealt with : • The low-level alarm from the coolant tank has already warned the operator of a minor problem. A temperature measuring device in the remaining coolant will have warned him that the coolant temperature was rising. If he failed to react to these two minor alarms then, before the coolant started to boil, the control system would generate a shutdown and alarm. The shutdown and alarm sequence will automatically shut down the compressor safely, to prevent damage to the equipment. An emergency incident may be any situation which would cause immediate danger to the system being controlled. It may be directly related to the system, or have nothing at all to do with it : • An example of an emergency incident which is directly related to the system being controlled would be where fire or smoke had been detected in the immediate area. • An example of an incident not directly related would be where there was a failure of a utility system, such as instrument air.

In both cases, there is an immediate danger to the process, and the control system would generate a shutdown and alarm which would automatically shut down the compressor to prevent damage to the equipment. In both minor and major process problems, the alarm normally consists of a flashing light and a ‘beeper’ which draws the operator’s attention to the problem. It is called an audio/visual alarm system. The flashing light normally lights up behind a glass plate which has the number and name of the particular alarm written on it. The beeper is normally common to all the alarm systems. The alarm light will continue to flash and the beeper to ‘beep’ until the operator ‘acknowledges’ the problem by pressing a button. When the problem has been ‘acknowledged’ in this way, the beeper stops sounding and the light stops flashing but stays alight. This reminds the operator that the problem still exists. The light will not be extinguished until the problem is resolved and the alarm has been re-set.

We will now take a closer look at the gas compression system shown in Figure 20. We can see that there are a number of suction knockout drums. If the liquid level were too high in any of these suction knockout drums, an alarm would be sent to the main control room. If the operator failed to stop the liquid level rising any higher then, before the liquid was carried over into the compressor, where it would cause damage, the shutdown and alarm would be activated by a highhigh level switch which responds to a high-high liquid level in the knockout drum. The control system would then : • shut down the compressor • give an alarm to the operator • indicate that the compressor had been shut down because of a high-high level in a particular suction knockout drum.

2.39


In addition to these high level alarms and high-high level shutdown and alarms, the following alarms and shutdowns are fitted to most reciprocating compressors : Low lube-oil pressure alarm and low-low lube oil pressure shutdown and alarm If the lubricating oil pressure is too low, then the compressor will not be lubricated properly and excessive wear, or even a piston seizure, may result. High vibration alarm and high-high vibration shutdown and alarm If the compressor vibrates too much, this indicates excessive wear, poor alignment or incorrect operation. Excessive vibration will damage bearings, valves, pistons and cylinder walls. High temperature alarm and high-high temperature shutdown and alarm on the coolant system If the coolant system gets too hot, it will be unable to cool the compressor effectively, and damage to the pistons and cylinders will result.

High temperature alarm and high-high temperature shutdown and alarm on the lubricating oil system

High discharge temperature alarm and high-high discharge temperature shutdown and alarm

If the lubricating oil gets too hot, it will become less viscous and will be unable to lubricate the bearings and pistons effectively.

If the gas discharge temperature is too high then damage may occur to the compressor, either because the lubricating oil becomes too thin, or the temperature rating of the downstream pipework is exceeded.

The following alarms and shutdown and alarms are fitted on the piping into and out of the compressors: Low suction pressure alarm and low-low suction pressure shutdown and alarm If the suction pressure is too low, then the compressor cannot achieve the discharge pressure required. High discharge pressure alarm and high-high discharge pressure shutdown and alarm If the discharge pressure is too high, then the pressure rating of the equipment maybe exceeded, or the compression ratio, and therefore the gas discharge temperature, will rise.

The driver which is driving the compressor will also be fitted with its own alarm, and shutdown and alarm, system. This system is normally tied into the compressor system and is classed as a local alarm or local shutdown and alarm, because it operates in conjunction with the compressor, without being installed on it. In addition to all the shutdown and alarms which may be fitted to the compressor, its adjacent pipework and its driver, there maybe other emergency situations which will shutdown the compressor. A prime example of such a condition would be a fire in the compressor area. Under these conditions, it would be unwise to keep the compressor running and therefore it would be shut down by a fire and gas alarm and shutdown system.

2.40


The Main Operational Checks on a Reciprocating Compressor

• the liquid is unable to flow through the discharge valve fast enough to reduce the pressure

Two examples of inhibit alarm conditions are:

Having looked at how a reciprocating compressor system is controlled and shut down, we now need to consider how this system should be operated.

• the pressure continues to rise until the engine stalls (or the cylinder head blows off!)

A limit switch on the discharge valve from the compressor

The ‘golden rules’ for operating a reciprocating compressor are as follows :

Another type of alarm maybe fitted to the compressor, called an inhibit alarm. Inhibit alarms are fitted to prevent the compressor from being started under certain conditions but, once the compressor is running, the inhibit alarms will not stop the compressor.

Before the compressor can be started, this switch may need to be in the ‘ON’ position, showing that the valve is fully open. If an operator were foolish enough to close the discharge valve after the compressor had been started, the compressor would shut down because of a high-high pressure condition, not because the switch had been moved to the ‘OFF’ position. A low temperature switch fitted to the lubricating oil tank If the lube-oil is too cold at start-up, then it would be too viscous to circulate around the compressor and protect the bearings. The compressor is therefore inhibited from starting until the lube-oil reaches a minimum temperature. Once the compressor is running, however, the lube-oil will be heated by the compressor and its temperature should not fall.

Check that the suction and discharge pipelines are lined up correctly

Before Starting the Compressor

We must make sure that the compressor has an uninterrupted supply of gas to the suction and that, after compression, the gas is able to flow away from the compressor to its intended destination.

Check that the compressor is purged of all air

Check that dependent systems are operational

If the compressor is not completely purged of air then it may act as a ‘compression-ignition’ engine (for example, a diesel). This means that, when the first compression stroke occurs, the heat of compression may ignite the air/gas mixture in the cylinder and an explosion will occur.

Before starting the compressor, we need to be sure that it is not going to shut down because of a lack of gas, because the main driver has run out of fuel, or for other reasons not directly related to the compressor itself.

Check that the suction line is free from liquids Liquids are incompressible. If there is liquid in the cylinder when the piston starts a compression stroke:

Check that the discharge valve is fully open This ensures that pressure built up in the compressor is allowed to flow away without interruption.

• the pressure rises rapidly

2.41


Check that the discharge relief valve Is operational This must be checked very carefully. If any piping in the system is wrongly aligned, or if any of the highhigh pressure shutdown systems are not working, then it is this valve which provides us with adequate protection against a pipeline rupture or damage to the compressor. Check that the lubricating oil system is operating correctly

Check that the cooling system is operating correctly We should check that: • there is sufficient coolant in the tank • any coolant added to the system is of the correct type and concentration • pumps, where fitted, are running or ready to run when the compressor is started

We should check that: • there is sufficient lube-oil in the tank • any lube-oil added to the system is of the correct type and grade • pumps, where fitted, are running or ready to run when the compressor is started

Check that no current alarm or shutdown conditions exist (including inhibit alarms) Even if the compressor controller allowed us to start up the compressor with a high liquid level in the suction knockout drum, it would be unwise to do this. If the level increased as we started, the compressor would be shut down by the high-high level condition.

When the Compressor is Running Check that the pressures, levels, flows and temperatures are within operational limits These checks must be made frequently, say, at least once every two hours. They form the bulk of a typical operator’s working day. The successful operation of any process will depend on repeated checks of this nature, to ensure that nothing is amiss with the system, or with the equipment. Get to know the characteristics of each compressor set Each compressor set has its own particular operating characteristics. These characteristics consist, not only of data which can be measured (by reading gauges, level indicators, and so on) but of less ‘scientific’ information such as the noise made by the equipment. The operator should know when the machine ‘sounds right’. Each compressor makes a different noise and, with practice and familiarity, a change in this noise can be the first warning that something is going wrong. If you are Involved in compressor operations you should become completely familiar with the equipment under your control. The specific operating procedures should be followed and safe working practices adopted at all times.

2.42


Test Yourself 2.7 1.

In Figure 20, where does the Booster Compressor take gas from?

2.

In Figure 20, what is the produced gas finally used for?

3.

Would a high-high level in a compressor suction drum be classed as a minor process disturbance?

4.

What normally happens when the operator ‘acknowledges’ an alarm?

5.

What does ‘high vibration’ indicate in a compressor?

6.

Why are inhibit alarms fitted, and what makes them different from other alarms?

Summary of Section 4 In the final Section of this Unit on reciprocating compressors, we have concentrated on the operation and control of the system. The Section was split into three parts :

• In the first part we looked at a typical gas compression system using reciprocating compressors. I described each component for you, and how the system overall was operated and controlled. • We then went on to look at the various alarms, and shutdown and alarms which would be incorporated into such a system. We saw why each particular alarm and shutdown was fitted and what it was there to protect. • Finally, we reviewed the main operational checks which we would expect to make on a reciprocating compressor system. We saw why the checks were made and what action the operator was expected to take.

Now that you have completed Section 4, you have come to the end of Unit 2 of the compression programme. I must emphasise once again that this unit is not meant to take the place of specific manufacturers guidelines or operating Instructions. It is intended to give you a good basic grounding in the design, construction and operation of reciprocating compressors.

Now go back to the Training Targets and satisfy yourself that you have met these targets.

You will find the answers to Test Yourself 2.7 on Page 2.46 2.43


Check Yourself - Answers

Check Yourself 2.1 A bicycle pump is a single acting compressor.

Check Yourself 2.3 The steps should be in the following order: 1.

Piston begins suction stroke. 9.

Suction valve closes.

8.

Gas remaining in cylinder expands and 4. discharge valve closes.

Cylinder pressure rises above discharge line pressure.

Check Yourself 2.2

5.

Discharge valve opens.

20 bara

12.

Cylinder pressure falls below suction 11. line pressure. 2. Suction valve opens.

6.

Gas flows into cylinder from suction line.

3.

Piston reverses direction at end of suction stroke.

Piston reverses direction at end of discharge stroke.

10.

Gas in cylinder is compressed to above suction line pressure.

7.

Gas flows from cylinder into discharge line.

2.44


Check Yourself 2.4 1.

Only the space at the head end of the cylinder is used for compression.

2.

There is a suction and a discharge stroke each time the piston travels the length of the cylinder.

3.

A suction valve is open during each suction stroke.

4.

During each stroke (forward and backward) a discharge valve is open.

5.

The back stroke is the suction, or intake stroke.

6.

The suction valve opens every second stroke.

7.

There is a suction and a discharge valve at each end of the cylinder.

8.

The forward stroke is the compression, or discharge stroke.

Check Yourself 2.5 Single- Acting

DoubleActing

a a a a a

a a a a

a

a

1.

To prevent the formation of shoulders in the liner

2.

To avoid slippage of the liner, resulting in knocking and excessive wear

3.

A boundary layer lubrication system injects small droplets of oil into the cylinder. The oil is distributed as a thin layer by the movement of the piston rings

4.

The cylinder cooling system improves compressor efficiency by removing unwanted heat of compression

5.

To prevent or minimise gas leakage between the piston and the liner

To carry some of the heat from the piston to the cylinder wall

6.

The crankshaft, crosshead and connecting rod assembly 2.45


Check Yourself 2.6

Check Yourself 2.7

1.

So that the water will not freeze in cold weather

1.

From the second stage of the oil/gas separation system

2.

The thermosyphon effect. Warm (lighter) water rises, cold (heavier) water sinks

2.

As lift gas and/or as sales gas

3.

In the crankcase sump

4.

The most common form of lubrication is a drip feed type

3.

No. It is a major problem. If the level gets any higher, the liquid may enter the compressor and cause damage. A shutdown and alarm will be generated

5.

To remove any entrained liquid from the process gas and prevent the possibility of serious damage due to liquids entering the compressor

4.

The beeper stops sounding and the light stops flashing but stays alight to remind the operator that the problem still exists

6.

They are generally considered to be incompressible

5.

It is a sign of excessive wear, poor alignment or incorrect operation

7.

The flare system

8.

By mounting them on a common bed-plate

6.

Inhibit alarms are fitted to prevent the compressor from being started under certain conditions. Once the compressor is running, the inhibit alarms will not stop the compressor

2.46

Petroleum Gas Compression workbook 2.pdf

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