petroleum gas compression workbook 2.pdf

POLPetroleum Open Learning

OPITO

THE OIL & GAS ACADEMY

Petroleum GasCompression

Part of thePetroleum Processing Technology Series

2

Contents Page

TrainingTargets 2.2

Introduction 2.3

Section1BasicTheory 2.4OperatingPrinciplesCapacityandCompressionRatioCompressorPerformance

Section2DesignandConstruction 2.17CylindarsPistonsandPistonringsCompressorValvesStuffing BoxCrankshaft,ConnectingRod,CrossheadandPistonRod

Section 3 Auxiliary Systems 2.29CoolingSystemLubricationSystemSuctionandDischargePipingSystemDriveCoupling

Section4OperationofReciprocatingCompressors 2.37ATypicalGasCompressionSystemAlarmandShutdownSystemsTheMainOperationalChecksonaReciprocatingCompressor

CheckYourselfAnswers 2.44

VisualCuestrainingtargets for you to achieve by the end of the unit

testyourself questions to see how much you understand

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

activities for you to apply your new knowledge

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

PetroleumGasCompression-Unit2-ReciprocatingCompressors(Part of the Petroleum Processing Technology Series)

Petroleum Open Learning

2.

TrainingTargets

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.

PetroleumGasCompression-Unit2-ReciprocatingCompressors

IntroductionPetroleum Open Learning

2.2

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

The Unit is divided into four sections:

Section1 covers the basic operating theory of reciprocating compressors.

InSection2, we will look at the design and construction of a typical machine.

Section3 will describe a range of auxiliary equipment

and

in Section4, we will be looking at basic compressor operations.

Each system will have its own characteristics and show detailed design differences.

You saw in Unit1, that compressors can be classified as either continuous flow or positivedisplacement machines. The reciprocatingcompressoris the most common of the positivedisplacement type. This is the one we are going to concentrate on in this unit.

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, .5 bar. Large industrial units may be required to deliverseveral thousand cubic metres per hour, at pressures approaching 000 bar.

As I previously explained, the compressor requirements of an oil or gas production system are dependent upon many variables.


IntroductionPetroleum Open Learning

2.

OperatingPrinciplesAll positive displacement compressors operate by :

creating a low pressure space into which gas may flow

closing the entrance to this space

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 BicyclePump. In this type of compressor, a small washer, thepiston, 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 toflow past it into the cylinder.

When the piston reaches its furthest point of backward travel, the washer again flexes, and sealsthe gap between the piston and cylinder.

Now, when the piston moves forward, the volume in the cylinder is reduced and the air is compressed.

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.

Figure1 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.


Section1-BasicTheoryPetroleum Open Learning

2.


2.5

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 theinlet(suction)andoutlet(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 :

SingleActing

or

DoubleActing

Lets look at the way in which each of these work. First thesingleactingcompressor.Figure2 shows the flow of gas through this type of machine.


2.

The section of the cylinder nearest the crank is called the crankend. The section of the cylinder furthest away from the crank is called the headend. 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 doubleactingcompressor there is a suction stroke and a discharge stroke each time the piston moves either backwards or forwards.

This is illustrated in Figure3.


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 TestYourself:

TestYourself2.1

Is a bicycle pump a single or double acting compressor?

You will find the answer to TestYourself2.1on Page 2.


2.

Figure4is a more detailed drawing of a double acting reciprocating compressor.Study the drawing for a while and identify its components.


2.

CapacityandCompressionRatioCompressors 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 compressionratio. it is defined as :

Discharge PressureSuction Pressure

For example, if the suction pressure is 0 bara and the discharge pressure is 0 bara, the compression ratio is 0/0 or . It is usually expressed as : or to .

ActivityTake 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.

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.

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.


2.0

The activity you have just completed demonstrated a basic fact regarding compressors. Whenagasiscompressed,itstemperatureincreases. The higher the compression ratio, the larger thetemperature 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 tocertain limits (typically about 200C, although higher temperatures may be experienced).

One of the basic means of limiting the temperature rise is to limit the compression ratio to about to .

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 calledmulti-stagecompressors. They have coolers to reduce the temperature of the gas between each stage.

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

TestYourself2.2

Diesel engines are classed as compression-ignition 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:. Air is taken in from the atmosphere at a pressure of bara.

What will be the pressure of the air/fuel mixture in the engine cylinder when maximum compression is reached?

You will find the answer to TestYourself2.2 on Page 2.


2.

CompressorPerformanceThe performance of a reciprocating compressor can be represented by a pressure/volume(PV)diagram.One of these is shown in Figure5,which illustrates the relationship between the cylinder pressure of a compressor and the cylinder volume enclosed by the piston, for a single-acting compressor.


2.2

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.

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 compressioncycle,consisting of one backward stroke and one forward stroke.

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 clearancespace. 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 thecylinder expands.

As the gas expands, the pressure in the cylinder decreases.

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

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

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.

TestYourself2.3

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

. Piston begins suction stroke. .... ....

2. Gas flows from cylinder into discharge line. ............

. Piston reverses direction at end of suction stroke. ............

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

5. Cylinder pressure falls below suction line pressure. ............

6. Gas flows into cylinder from suction line. ............

. Piston reverses direction at end of discharge stroke. ............

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

. Suction valve closes. ............

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

. Discharge valve opens. ............

2. Suction valve opens. ............

The answers to TestYourself2.3 will be found on Page 2.


2.

SummaryofSection1In this first Section of Unit 2, we have looked at the basic theory of operation for positivedisplacementcompressors.

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 :

singleacting,

or

doubleacting

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

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

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 TestYourself2.4.


2.5

TestYourself2.4

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

Single- Double- Acting Acting

. 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.

. A suction valve is open during each suction stroke.

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

5. The back stroke is the suction, or intake stroke.

Single- Double- Acting Acting

. The suction valve opens every second stroke.

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

. The forward stroke is the compression, or discharge stroke

You will find the answers toTestYourself2.4on Page 2.5


2.

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 :

Cylinders

Pistons and piston rings

Compressor valves

Stuffing box and packing

Crankshaft, connecting rod, crosshead and piston rod

Take another look at Figure on Page 2.5. See how many of the components listed above you can identify- they are not all labelled ! When you have done that, we will look at each item on the list in turn.

CylindersYou 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.

The piston is fitted with pistonrings,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.

Figure6 is a drawing of a cylinder, liner, piston and piston rings.


Section2-DesignandConstructionPetroleum Open Learning

2.

Wear occurs where the piston rings rub against the liner. To avoid this wear forming a shoulder or step, counterbores are machined into the liner. (Look at Figure 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.

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

CylinderLubricationIn low pressure/low temperature applications, the cylinders may not require lubrication. In this case, the pistons may be fitted with self-lubricatingpistonrings,made of nylon or teflon.

However, in most compressor applications, cylinder lubrication is required to prevent excessive overheating or wear. In such situations, a boundarylayerlubrication 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.

CylinderCoolingOn 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.

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.

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 Figure7, on the next page.


2.

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.

To prevent or minimise gas leakage between the piston and the liner, pistonringsare 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.

PistonsandPistonRingsPistons are most commonly made from a solid casting. The pistonrod, often made of stainless steel, is tapered where it passes through the piston. It is then secured against the shoulder by alocknut. This is illustrated in Figure8.


2.20

All valves have certain features in common:

a valve seat which provides a pressure tight gas seal.

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 Figure10.

Figure 9 shows two types of piston ring.

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.

CompressorValvesThere 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.


2.2

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.

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

. 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.

. 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.

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

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.Figure11, on the next page, shows the arrangement of a seal element with a type of seal ring known as the TR type.

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.


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.


2.2

Another type of stuffing box seal element is the TT type as shown in Figure12. 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.


2.2

Figure13 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,ConnectingRod,CrossheadandPistonRodThe 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.

CrankshaftThe crankshaft is made of forgeable carbon steel,machined throughout. It is provided with a singlecrank and is suitably counterweighted to limit thedynamic 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,connectingrod,crossheadandpistonrod are shown inFigure14.


2.2

ConnectingRodThis is made of high strength pressed steel.

Both ends of the connecting rod are equipped withheavy duty sleeve bearings. Figure15shows theconnecting rod at the crosshead.

CrossheadThe crosshead connects the piston rod to theconnecting rod of the crankshaft. It is equipped withshoes which permit it to slide back and forth withinthe crosshead guides. (See Figure 5)

The connecting rod is moved by the crankshaft. Asthe crankshaft rotates, the connecting rodreciprocates.

PistonRodPiston rods are usually made of stainless steel.They are accurately ground and have no taperwithin their length of travel.

The piston rod screws into the crosshead and issecured in place by a locking device.

A slinger ring prevents oil from the crankcase beingcarried out by the piston rod and reaching thecylinder. It is installed on the piston rod, as you cansee in Figure .

Now that you are familiar with the components of atypical reciprocating compressor, have a go at thefollowing TestYourselfbefore moving to the nextSection.


2.2

TestYourself2.5

. What is the purpose of a counterbore in a liner?

2. Why are liners pressed or expanded into place?

. How does a boundarylayer lubrication system work ?

. How does a coolingsystem improve the efficiency of the compressor?

5. What are the two functions of pistonrings?

. Which components convertrotarymotion to reciprocatingmotion?

You will find the answers toTestYourself2.5on Page 2.5

SummaryofSection2

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

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

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

The construction of compressor valves was described, and how they operate to maintain the flowof gas through the compressor.

We have looked at the different types of seal used in the stuffing box, and how the stuffing boxprevents 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 reciprocatingcompressors.


2.2

CoolingSystemYou saw earlier that, as gas is compressed, its temperature increases. The compressors cooling system removes some of the heat generated by compression (heat of compression) and also protects the piston and cylinder from becoming overheated.

Figure16 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 closedloopthermosyphonsystem.

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.

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


Section 3 - Auxiliary SystemsPetroleum Open Learning

2.2

LubricationSystem


2.0

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 anddischarges 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

Figure17 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.

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 .

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.

SuctionandDischargePipingSystemFigure18shows a typical piping system for a single stage reciprocating compressor.


2.2

A liquidknockoutdrum 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.

The knockout drum is one of the most important items of equipment in the piping system. Liquids are incompressible fluids and, iftheyenterthecompressor,eveninverysmallamounts,theycouldcausethecylindertorupture.

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 suctionvolumebottle.

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.

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 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 pulsationdampeners.

The most common pulsation dampener is the baffle type.

Figure 19 shows a typical baffle type pulsation dampener.


2.

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

DriveCoupling

Reciprocating compressors are normally driven by an internal combustion engine or an electric motor, which is connected to the crankshaft by means of adriveshaftand a directcoupling.

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 bedplate.

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.

The gas leaves the compressor via a dischargevolumebottle. The purpose of the dischargevolume 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 pulsationdampener.

All volume bottles are equipped with drains and pressure taps for checking pressure losses. Theymust 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-returnvalveis 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.)

Ablockvalve is also fitted to the discharge of the compressor. This valve is used for compressor isolation. Thedischargeblockvalveshouldalwaysbeopenedbeforethecompressorisstartedup.

A compressor ventline is fitted on the compressor side of the discharge block valve. The compressor vent line is used:

to depressurise the compressor after shutdown

to purge the compressor of flammable gas before maintenance

to purge the compressor of air before start-up

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 pressurereliefvalvefitted 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.


2.

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 of this Unit on reciprocating compressors, which dealt with the auxiliary systems. The followingTestYourself will help to reinforce your understanding of the topics covered.

TestYourself2.6

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

2. What makes the water circulate through the cooling system?

. Where is the lubricating oil collected and stored?

. 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 ?

. Are liquids compressible?

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

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

You will find the answers to TestYourself2.6 on Page 2.


2.5

You saw how the cooling system supplied cooling liquid to the cylinders and stuffing box. Use of the thermosyphoneffect 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 knockoutdrumprevented liquid from entering the compressor. Volumebottles(or pulsationdampeners) 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-platefor 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.

SummaryofSection3In this Section on auxiliaries, we have looked at :

the compressor cooling system

the lubrication system

the suction and discharge piping system

the driver coupling


2.

ATypicalGasCompressionSystemFigure20 is a line drawing of aseparation and gas compression system which uses reciprocating compressors.

I have divided the Section into the following topics :

a typical gas compression system

alarm and shutdown systems

the main operational checks on a reciprocating compressor

In this, the final Section of the reciprocating compressor unit, we will be looking at theoperation of the compressor.


Section4-OperationofReciprocatingCompressorsPetroleum Open Learning

2.

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:

Low-pressure gas from the 2nd stage of an oil/gas separation system is passed through a booster compressor suction knockout drum.This drum removes any entrained liquids from the gas before it is fed into the booster compressor. (The suction knockout drum is sometimes referred to as the suctionscrubber.)

The booster compressor increases the pressure of the gas from the 2nd stage separator to that of the st stage.

After passing through the booster compressor the gas is cooled before it joins with gas from the st 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 st stage of a two stage compressor.

This st stage increases the gas pressure to a level which allows it to be treated, say, in a gas liquids recovery plant.

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.

Note that, in this two stage system, both compressors are driven by the same motor.

AlarmsandShutdownSystemsWe should now look at how we control the compression process and how we protect the equipment from damage.

Generally, all process controls are designed to inform the operator automatically if anything goes wrong. Process control systems normally work on two levels:

minor process disturbances

major process disturbances, or emergency incidents

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.

MajorProcessDisturbancesorEmergencyIncidentsA major process disturbance may be any process variable which is so high or low that the system has reached a potentially dangerous condition.


2.

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 shutdownandalarm. 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 relatedwould 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 shutdownandalarmwhich 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 operators attention to the problem. It is called an audio/visualalarmsystem.

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 beepertobeepuntiltheoperatoracknowledgestheproblembypressingabutton.

When the problem has been acknowledged in this way, the beeper stops sounding and the light stops flashing butstaysalight. This reminds the operator that the problem still exists. The light will notbeextinguished 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, beforetheliquidwascarriedoverintothecompressor, where it would cause damage, the shutdownandalarmwould be activated by a high-highlevelswitchwhich 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.

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 :

Lowlube-oilpressurealarmandlow-lowlubeoilpressureshutdownandalarm

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.

Highvibrationalarmandhigh-highvibrationshutdownandalarm

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.

Hightemperaturealarmandhigh-hightemperatureshutdownandalarmonthecoolantsystem

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.

Hightemperaturealarmandhigh-hightemperatureshutdownandalarmonthelubricatingoilsystem

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

The following alarms and shutdown and alarms are fitted on the piping into and out of the compressors:

Lowsuctionpressurealarmandlow-lowsuctionpressureshutdownandalarm

If the suction pressure is too low, then the compressor cannot achieve the discharge pressure required.

Highdischargepressurealarmandhigh-highdischargepressureshutdownandalarm

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.

Highdischargetemperaturealarmandhigh-highdischargetemperatureshutdownandalarm

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 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 localalarmorlocalshutdownandalarm, 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 alarmandshutdownsystem.


2.0

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

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

BeforeStartingtheCompressorCheckthatthecompressorispurgedofallair

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.

Checkthatthesuctionlineisfreefromliquids

Liquids are incompressible. If there is liquid in the cylinder when the piston starts a compression stroke:

the pressure rises rapidly

Another type of alarm maybe fitted to the compressor, called an inhibitalarm. 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.

Two examples of inhibit alarm conditions are:

Alimitswitchonthedischargevalvefromthecompressor

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 oiltank

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.

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

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

Checkthatthesuctionanddischargepipelinesarelinedupcorrectly

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.

Checkthatdependentsystemsareoperational

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.

Checkthatthedischargevalveisfullyopen

This ensures that pressure built up in the compressor is allowed to flow away without interruption.


2.

CheckthatthedischargereliefvalveIsoperational

This must be checked very carefully. If any piping in the system is wrongly aligned, or if any of the high-high 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.

Checkthatthelubricatingoilsystemisoperatingcorrectly

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

Checkthatthecoolingsystemisoperatingcorrectly

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

Checkthatnocurrentalarmorshutdownconditions 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.

WhentheCompressorisRunningCheck that the pressures, levels, flows and temperaturesarewithinoperationallimits

These checks must be made frequently, say, atleastonceeverytwohours. They form the bulk of a typical operators 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.

Gettoknowthecharacteristicsofeachcompressorset

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.

IfyouareInvolvedincompressoroperationsyoushouldbecomecompletelyfamiliarwiththe equipment under your control. The specific operatingproceduresshouldbefollowedandsafeworkingpracticesadoptedatalltimes.


2.2

TestYourself2.7

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

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

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

. 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?

You will find the answers toTestYourself2.7 on Page 2.

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 , you have come to the end of Unit 2 of the compression programme.Imustemphasiseonceagainthatthisunitisnotmeanttotaketheplaceofspecific 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.

SummaryofSection4In the final Section of this Unit on reciprocating compressors, we have concentrated on the operation and control of the system.


2.

CheckYourself2.1A bicycle pump is a single acting compressor.

CheckYourself2.220 bara

CheckYourself2.3The steps should be in the following order:

. Piston begins suction stroke. . Suction valve closes.

. Gas remaining in cylinder expands and . Cylinder pressure rises above discharge discharge valve closes. line pressure.

5. Cylinder pressure falls below suction . Discharge valve opens. line pressure.

2. Gas flows from cylinder into discharge line.2. Suction valve opens.

6. Gas flows into cylinder from suction line. 7. Piston reverses direction at end of discharge stroke.

. Piston reverses direction at end of suction stroke.

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

CheckYourself-AnswersPetroleum Open Learning

2.

CheckYourself2.4 Single- Double- Acting Acting

. 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.

. A suction valve is open during each suction stroke.

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

5. The back stroke is the suction, or intake stroke.

. The suction valve opens every second stroke.

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

. The forward stroke is the compression, or discharge stroke.

CheckYourself2.5

. To prevent the formation of shoulders in the liner

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

. 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

. 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

. The crankshaft, crosshead and connecting rod assembly


2.5

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CheckYourself2.6. So that the water will not freeze in cold weather

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

. In the crankcase sump

. The most common form of lubrication is a drip feed type

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

. They are generally considered to be incompressible

7. The flare system

. By mounting them on a common bed-plate

CheckYourself2.7. From the second stage of the oil/gas separation system

2. As lift gas and/or as sales gas

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

4. The beeper stops sounding and the light stops flashing butstays alight to remind the operator that the problem still exists

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

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.

Petroleum GasPetroleum Gas Compression workbook 2