petroleum gas compression workbook 4.pdf

POLPetroleum Open Learning

OPITO

THE OIL & GAS ACADEMY

Petroleum GasCompression

Part of thePetroleum Processing Technology Series

4

Contents Page

TrainingTargets 4.2

Introduction 4.3

Section1OtherTypesofPositiveDisplacementCompressor 4.4 RotaryScrewCompressors LobeCompressors SlidingVaneCompressors LiquidRingCompressors

Section2OtherTypesofContinuousFlowCompressor 4.11 AxialFlowCompressors MixedFlowCompressors FluidicCompressors Ejectors

CheckYourself-Answers 4.18

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

PetroleumGasCompressionUnit4-OtherTypesofCompressor(PartofthePetroleumProcessingTechnologySeries)

Petroleum Open Learning

4.1

TrainingTargets

When you have completed Unit 4 of the Petroleum Gas Compression series you will be able to :

List the main types of compressor used in the petroleum industry other than centrifugal and reciprocating compressors

Describe the construction and operation of four types of rotary positive displacement compressors

Describetheconstructionandoperationoftheaxialandmixedflowofdynamiccompressors

Describetheconstructionandoperationoftheejectortypefluidiccompressor

Explain the principal uses of the compressors listed


4.2

TestYourself4.1Test a typical compressor family tree, naming the types of compressor in each main branch.

YouwillfindtheanswerinCheckYourself4.1on page 4,18

In Units 2 and 3 of this compressor programme we concentrated on two types of compressor :

reciprocating

centrifugal

These are the common types of compressor found in petroleum production operations.

However, they are by no means the only cornoressors in use. You will remember from Unit 1 that we classifiedcompressorsintoanumberofdifferentcategories and represented this as a family tree

Check your recall of Unit 1 now by trying the followingTestYourself question.

From the family tree you can see that we must consider a few more machines to complete our study of compressors.

In this Unit, therefore, we will tie up the loose ends in the programme on compression by taking a brief look at theseothertypes.

I do not intend to go into nearly so much detail as I did in Units 2 and 3. In fact, we will limit ourselves to just two sections.

In Section1 we will look at the basic construction and operating principles of another type of positive displacement compressor, namely the rotary compressors. (In the family tree, we also included the diaphragm type of reciprocating machine. However, I do not intend to say anything further about this compressor.)

Section2 will concentrate on the other continuous flowmachines.Thesearetheaxial and mixed flow dynamic and the fluidic types.

PetroleumGasCompressionUnit4-OtherTypesofCompressor

IntroductionPetroleum Open Learning

4.3

Figure 1(a) is a plan view and Figure 1(b) is a simplifiedendviewofthemachine.

You will remember from the family tree that the two branches of the positive displacement arm led to the reciprocating machines and the rotary machines. In this section we will be having a look at the rotary types. These include:

screw type

lobe type

vane type

liquid ring type

Lets now look at each of these in turn.

RotaryScrewCompressorsThe doublerotaryscrew type of compressor is the most common form of rotary compressor used in the oil and gas industry. In this machine, two intermeshing screws are used to trap and compress a gas in the space between the screws.

The double rotary screw compressor is a positive displacement machine which delivers a constant volume at variable pressures. It has a single stagecompression ratio of approximately 4 to 1.

Take a look at Figures1(a) and 1(b) which show, in simplifiedform,arotaryscrewcompressor


Section1-OtherTypesofPositiveDislacementCompressorPetroleum Open Learning

4.4

The compressor casing contains two precision machined, helical screw rotors. One of these rotors has splines, giving it a male profile. These splines mesh with grooves in the other, female, rotor. The rotors are usually of small diameter. This small diameter allows shaft speeds of up to 20 000 rpm.

The screws rotate together in opposite directions.The splines and grooves mesh to form a series of pockets which travel one following the other, towards the discharge end of the machine, The volume in each pocket reduces, thereby increasing the pressure. as it moves from suction to discharge,

In most designs the rotors require no lubrication within the compressor chamber and can produce an oil free compressed gas,

Rotary screw compressors are often found in compressed instrument air service,

LobeCompressorsLobe compressors are often used where large volumes of relatively low pressure gas are required,

Figure2shows how atwinlobecompressorworks andhowthegasflowsthroughthecompressor.

The casing of the compressor encloses two rotors, each of which has a Figure of eight shape. At each endofthefigureofeightisalobe.Thesespeciallyshaped rotors are geared together externally to ensure correct meshing. They rotate in opposite directions within the casing.

The incoming gas is trapped between the compressor casingandthetipsofthelobes.Asthelobesarefittedclose together, no gas can escape backwards between the lobes themselves. The trapped mass of gas is then pushed forwards towards the delivery end of the compressor, with no reduction in volume.

Think for a moment about this process and then do the following TestYourself

TestYourself4.2 What is the fundamental difference in method of compression, between a lobe type compressor and the other two positive displacement compressors we have looked at up to now, ie, the reciprocating type and the screw type rotary?

YouwillfindtheanswerinCheckYourself4.2 on page 4.19


4.5

From the answer to this Test Yourself you can see that the back pressure in the delivery pipework determines the outlet pressure of this compressor.

The machine we have just been looking at is a two lobe compressor. However, lobe compressors may be found with two, three or four lobes per rotor.

Because of these low pressure applications often referred to as blowers.

Lobe compressors can be found on petroleum croductlon installations as Fuel Gas compressors, or as Booster compressors and other lower pressure applications.

SlidingVaneCompressorsSlidingvane compressors are designed to supply a constant volume and pressure of gas.

Figure3 shows the basic construction of a slidingvanecompressor. Familiarise yourself with the components, then go through the following description of how the machine works.

Within the casing of the compressor is mounted a rotor or drum. The drum is designed to rotate eccentrically within the casing.

The drum carries a number of vanes. These are free to slide back and forth within slots machined into the rotor. As the drum rotates. the vanes are thrown out by centrifugal force so that they contact the casing. Sometimes spring loading assists in this.


4.6

The sliding vane compressor operates by trappingthe gas in a succession of pockets whose volume isgradually reduced. The pockets are formed by theinside of the casing, the outside of the drum and apair of sliding vanes.

Gas enters the casing through the inlet nozzle andis trapped in a pocket where the vanes are at theirmaximum extension. As the drum rotates thevanes are pushed back into the drum as thedistance between casing and drum decreases. This then reduces the volume of the pocket. Thepressure of the gas within the pocket increases untilthe volume of the pocket is at its minimum. At thispoint the gas is discharged through the deliverynozzle.

Sliding vane compressors are very prone to damageand breakdown if any liquids or solid particles enterthe gas stream. The vane tips erode and gas canthen leak backwards, past these tips.

They are occasionally found in Heating, Ventilationand Air Conditioning (HVAC) Systems, In thisservice are used both as air blowers and asrefrigerant compressors.

LiquidRingCompressorsTheliquidringcompressoris more commonlycalled a vacuumpump, It is used extensively ininjection water treatment plants, It is capable ofcreating a vacuum which assists in thede-oxygenation of water.

The unit consists of a cast iron casing whichcontains a multi-bladed impeller. The impeller ismounted on a shaft supported at each end onbearings.Theshaftisfittedeccentricallytothecasing so that the blades of the impeller are nearerto the casing at one point of the rotation.

Figure4, on the next page, shows diagramaticallythe construction of a liquid ring compressor.


4.7


4.8

The liquid ring compressor operates in a similarmanner to the sliding vane type of compressor. Theessential difference is the way in which the reductionin volume of the pockets is achieved.

You will remember that this reduction in pocketvolume is accomplished, in a sliding vanecompressor, by the vanes being pushed back intothe drum. The liquid ring compressor uses anadvancing ring of water to create the same effect.

Look at Figure 4 and see how this works.

Water is injected into the unit and centrifugal forcecauses it to form a liquid ring which is pinned to theinside of the casing.

Air is drawn a pocket formed between theimpeller blades and the inner surface of the waterring. This occurs where the blades are furthest fromthe casing and the pockets have their greatestvolume. As the impeller rotates, the blades getnearer to the casing wall. The ring of wateradvances into the pocket, and pocket volume isreduced. This increases the pressure of the air inthe pocket The compressed air / water mixture isthen ejected at the discharge port.

Have a look at Figure 4 again, and try to visualisewhat is going on inside the compressor,

SummaryofSection1

In this section you have seen how a selection of rotary compressors work. These were:

screw type

lobe type

vane type

liquid ring type

You saw the lobe type of compressor was the only one which did not trap gas and then reduce its volume. In fact the trapped gas in such a machine is pushed towards the discharge at an almost constant volume.

We did not go into a great deal of detail regarding the construction of rotary compressors. However, Section 1 should have given you an idea of how these machines work and what their main applications are.

Before moving to Section 2, have a go at the following TestYourself to check yourunderstanding of Section 1.


4.9

TestYourself4.3Which type of positive dlsplacernent compressor do the following statements refer to?

1. The male splines mesh with the female grooves.

2. Springs assist in throwing the vanes outward

3. These compressors are for low pressure applications and are sometimes called blowers

4. The piston moves back and forth within a cylinder

5. The drum is mounted eccentrically within the casing.

6. This compressor is more commonly called a vacuum pump.

7. Thetworotorshaveafigureofeightshape.

8. The small diameter of the rotors allows shaft speeds of up to 20 000 rpm.

YouwillfindtheanswersinCheckYourself4.3on page 4.19


4.10

Thecontinuousflowbranchofthefamilytreewas divided into dynamicmachines and fluidic machines. Unit 2 of the Compressor Series concentrated on one of the dynamic machines, namely the centrifugal compressor. In this section we will have a brief look at the other dynamic types of compressor, the axial flow and mixed flow compressors.

Finallywewillconsiderjustonetypeoffluidicmachine,that is the ejector.

Once again we will not go into any great detail but will only cover the basic construction and operation of each unit.

AxialFlowCompressorsLikecentrifugalcompressors,axialflowcompressorsare dynamic machines. They impart kinetic energy to the gas by increasing its velocity. This kinetic energy is then converted into pressure energy by allowing the gas to slow down.

Inthecaseofanaxialflowcompressor,theflowofgas stays parallel (axial) to the shaft. The simplest typeofaxialflowcompressoristhepropeller.Airisdrawninto the propeller from the front and thrown backwards through the propeller.

Inthecaseofanaxialflowcompressoralargenumberof blades, called rotorblades, are attached to an impeller.Theremaybeasmanyasfifteenormoreimpellers attached to a shaft. The impellers and the shaft are contained within a casing.

Therotorbladesrunbetweenfixedblades,calledstators, mounted inside the compressor casing.

Figure5 shows the component parts of an axial flow compressor

As the impellers rotate, the shaped rotor blades inducethegastoflowacrossthem,paralleltotheshaft.

The rotor blades accelerate the gas towards thestator blades. The shape of these stationary bladescauses them to act as diffusers, and slow down thegas. This converts kinetic energy into pressureenergy. The stator blades then direct the gastowards the next set of rotor blades. and so on.

Themostcommonuseforaxialflowcompressorsinthe oil and gas is as inlet air compressorson gas turbines. In this service they are ideally suited to provide the large amounts of low pressure air which are required to enable the engine to function.

Figure 6 : On the next page shows the Axial Compressor, in a Gas Turbine layout


Section2-OtherTypesofContinousFlowCompressorPetroleum Open Learning

4.11


4.12

MixedFlowCompressorsMixed Flow compressors utilise both centrifugal andaxialflowswithinthesamecompressorcasing,

All centrifugal compressors have anelementofmixedflow.Inthesemachines,thegasflowsaxiallyalongtheshaftintotheinletofeachimpeller before being accelerated by centrifugalforce.

Mixedflowcompressorsutiliseaformofaxialflowblades called inletguidevanes installed upstreamofthefirstcentrifugalimpeller.Theseguidethegasintotheeyeofthefirstimpellerinthemostefficientdirection. The inlet guide vane often has a variablepitch facility which allows the operator to control thedirectionofthisinletflow.

Before we move on to have a look at Fluidioc compressors, try the following TestYourselfquestion about dynamic compressors of the axial and mixed flowtypes.

TestYourself4.4

List 5 errors which appear in the following sentences.

Axialflowcompressorsaredynamicmachines.Theyimpartpotentialenergytothegasbyincreasingitsvelocity.Thisenergyistoconvertedpressureenergyasthegasissloweddown.Theflowofgasstays parallel to the shaft. A number of stator blades are attached to the impeller. They rotate between fixedbladesmountedinsidethecompressorcasing.Rotorbladesactasdiffuserstoslowdownthegasafter it has been accelerated. Axial compressors are often used as inlet air compressors on gas turbine installations.

Mixedflowcompressorsutiliseaxialflowandreciprocatingflowwithinthesamemachine.Compressorswhicharedesignatedmixedflowutiliseinletguidevanestoguidethegasintotheaxialflowimpeller,

YouwillfindtheanswersinCheckYourself4.4 on page 4.19


4.13

FluidicCompressorsInFluidiccompressors,acarrierfluidisacceleratedinordertotransferitsenergytoanotherfluid.

Fluidic compressors have no pistons, valves, rotors or any other moving parts.

Theyarerelativelyinefficientbutareoftenfavouredfor use with corrosive materials or in inaccessible situations,

Ejectors(alsocalledEductors) and DiffusionPumpsarethetwomostcommontypesoffluidiccompressor. However, because diffusion pumps are seldom found in petroleum producing operations, we will not be considering them in this Unit.

EjectorsEjectors (or Eductors) are names used to describeequipment which operates to the BernoulliPrinciple.

Bernoulli was a scientist who discovered that there is arelationshipbetweenpressureandvelocityofafluidasitflowsacrossarestriction.

Take a look at the two diagrams in Figure7.

Figure7(a)showsfluidflowingacrossarestriction.

Figure7(b) illustrates this relationship in the form of agraph. Asthefluidentersthethepressurefallsand the velocityincreases. This occurs as

pressureenergy is convertedkineticenergy.

At the exit from the restriction, the reverseoccurs, as you can see.


4.14

Figure8 shows a restriction built a of equipment called an ejectorThis restriction, similar to the one shown in Figure 7, is called a venturitube.

Afluid,calledthedriving fluid, is pumped, at high velocity, through the venturi tube of the ejector viaan inlet nozzle. The underside of the ejector is connectedtothegasinlet.Asthedrivingfluidentersthe venturi tube, its velocity increases and its pressure falls. This reduction in pressure pulls gas from the gas inletandentrainsitinthedrivingfluid.This can create a vacuum at the gas inlet and, in fact this type of equipment can also be called a vacuumpump.

Asthemixtureofdrivingfluidandgasleavestheventuri tube, its velocity falls again and the pressureincreases to that of the discharge pipework.

Ejectors have many applications in petroleumproduction. For example:

To increase the vacuum which is applied to the de-aeration tower of a water treatment facility (in conjunction with a liquid ring compressor)

To pull gas into a water stream in order to create a foam in water clean-up facilities


4.15

SummaryofSection2In this short section we have had a look at some of the other compressors which fall into thecontinuousflowcategory.Thesewere:

axialflowcompressors

mixedflowcompressors

ejectors

Yousawthatthefirsttwoweredynamicmachines.Theyworkinasimilarmannertothecentrifugalcompressor which we considered in Unit 2 of this Compression Series

Theaxialflowcompressorusesrotorbladeswhichactasimpellerblades,andstatorbladeswhichactasdiffusers.Theflowofgasthroughsuchamachineisparalleltotheshaft,hencethenameaxialflow.

Theejector,however,worksinadifferentwayandhasnomovingparts.Itreliesonadrivingfluidcreatingalow pressure zone. This type of equipment is often used in water injection and produced water applications. Further units in the Petroleum Processing Technology Series will explore these applications more fully.

Now,beforeleavingtheUnit,trythefinalTestYourselfquestion.


4.16

TestYourself4.5Decide to which type of compressor and which branch of the family tree (positive displacement or continuous flow)thefollowingcompressorcomponentsbelong.Ihavedonethefirstoneasanexample.

1. pistonrod reciprocatingcompressor positivedisplacement

2. splined rotors ............................................. .......................................

3. inlet guide vane ............................................. .......................................

4. crosshead ............................................. .......................................

5. venturi tube ............................................. .......................................

6. water ring ............................................. .......................................

7. figureof8rotor ............................................. .......................................

8. drum and vanes ............................................. .......................................

9. dry gas seal ............................................. .......................................

10. stator blades ............................................. .......................................

YouwillfindtheanswersinCheckYourself4.5on page 4.20


4.17

CheckYourself4.1Sketch a typical compressor tree, naming the types of compressor in each main branch.

Your sketch will look something like the family tree shown opposite.

(YouwillrememberthatyoufirstsawthisdiagraminUnit 1 on Page 1.26).


4.18

CheckYourself4.2With the piston and screw types, thecompressor traps a mass of gas and reducesits volume, to achieve compression, The lobetypecompressorsimplypushesafixedvolumeof gas into a higher pressure discharge.

CheckYourself4.31. screw type

2. vane type

3. lobe type

4. reciprocating type

5. vane type

6. liquid ring type

7. lobe type

8. screw type

CheckYourself4.41. Kineticenergy is imparted not potential energy

2. Rotorblades not stator blades are attached to the impeller

3. Statorblades act as diffusers, not rotor blades

4. Mixedflowcompressorsuseaxialflowand centifrugal flow, not axial and reciprocating.

5. Inletguidevanes guide the gas into the firstcentrifugal impeller nottheaxialflow impeller.


4.19

CheckYourself4.5

1. piston rod reciprocating compressor positive displacement

2. splined rotors screw type positive displacement

3. inletguidevane mixedflow continuousflow

4. crosshead reciprocating positive displacement

5. venturitube ejector continuousflow

6. water ring liquid ring positive displacement

7. figureof8rotor lobetype positivedisplacement

8. drum and vanes sliding vane positive displacement

9. drygasseal centrifugal continuousflow

1O. statorblades axialflow continuousflow


4.20

Petroleum GasPetroleum Gas Compression workbook 4

petroleum gas compression workbook 4.pdf

Documents