petroleum gas compression workbook 1.pdf

POLPetroleum Open Learning

OPITO

THE OIL & GAS ACADEMY

Petroleum GasCompression

Part of thePetroleum Processing Technology Series

1

Visual Cues

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

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

Contents Page

Training Targets 1.2 Introduction 1.3

Section 1 - Compressor Applications 1.4

Section 2 - Basic Principles of Compression 1.15 Pressure - Volume Relationship Temperature - Volume Relationship The Combined Gas Law Energy

Section 3 - Types of Compressor 1.26 The Compressor Family Tree Positive Displacement Compressors Continuous Flow Compressors Compressor Selection

Check Yourself - Answers 1.32

Petroleum Gas Compression - Unit 1 - An Overview(Part of the Petroleum Processing Technology Series)

Petroleum Open Learning

1.1

Training Targets

When you have completed Unit 1 of the Petroleum Gas Compression series you will be able to :

List the main uses of compressors in the petroleum producing industry.

Perform simple calculations involving pressure, temperature and volume relationships.

Explain how energy conservation principles are applied to compressor technology.

Define compression ratio and compressor capacity.

Describe the compressor family tree.

List the factors which influence compressor selection.


1.

The oil may then be pumped to a refinery, or a terminal for onward transportation.

The gas which is separated from the oil may be transported by pipeline for sale. It can also be used on site for a number of other applications. For instance:

it may be injected back into the reservoir to help maintain the pressure there

it could be used in wells to assist them to flow, using a technique known as gas lift

it may be used as fuel on the plant or platform

some of its constituents may be removed as a liquid in a gas liquids recovery plant

But, however the gas may be used, it invariably will be at too Iow a pressure when it leaves the separation system. In order to transport the gas, or allow it to do useful work, it will usually need compressing to a higher pressure. This requires the use of some kind of gas compression plant.

This unit is the first in a series which covers the subject of gas compression. In this one, we will have a look at the basic principles of compression. The second unit in the series will cover reciprocating compressors, the third will concentrate on centrifugal machines, whilst the fourth will be dealing with other types of compressor.

I have called this unit an overview. It is necessary to introduce the subject in some detail before concentrating on specific types of machine. Therefore, in the unit, we will cover material which is common to all compressor technology.

The unit is divided into 3 sections, and we will belooking at them in the following order:

Compressor applications -In this section we will consider the reasons for, and uses of, compressors in a petroleum producing operation

Basic principles of compression - Here we will look at the underlying theory of gas compression. In this section, you will also be introduced to some of the terms and expressions commonly used in compression technology

Types of compressor - This section will concentrate on the different types of compressor in common use and their suitability or otherwise for specific applications

In the petroleum industry, the two main hydrocarbon components dealt with are crude oil and natural gas. These substances are produced together from the underground reservoir in varying proportions.

At the surface, the oil and gas are separated from each other. These two streams are then further processed independently, where necessary.

Petroleum Gas Compression - Unit 1 - An OverviewIntroduction


1.3

In this section we will consider typical compressor applications on an offshore production platform. Of course, what we cover would be applicable to an onshore plant, but the emphasis here will be offshore

Lets look first at a simplified flow diagram for an oil and gas processing system.Figure 1 shows such a diagram

Petroleum Gas Compression - Unit 1 - An OverviewSection 1 - Compressor Applications


1.

Reservoir pressure

Ratios and gas volumes produced (the field Gas Oil Ratio - G.O.R.)

Pressure requirements of platform gas facilities

The system I am using as an example in this unithas two stages of separation operating at 17 barand 1. bar respectively.

You will see that, after gathering the well fluids together, the first part of the process is a separation system. Here the oil, water and gas which are produced from the reservoir are separated from each other. (This process is covered in detail in the Oil and Gas Separation Unit, which is also part of our Petroleum Processing Technology Series of open learning programmes.) After separation, the crude oil is treated if necessary, metered and pumped away for further processing.

The produced water is cleaned, and then disposed of.

But we are particularly interested in what happens tothe gas. Let us look a little more closely at this. Wewill trace the gas flow through the various facilities,and will build up a simple picture of the gas processoperation.

A separation system may consist of a number ofseparators working in series. They operate atsuccessively lower pressures. The actual number ofvessels and their operating pressures will depend onvariables such as :


1.

Reservoir pressure

Ratios of oil and gas volumes produced (the field Gas Oil Ratio - G.O.R.)

Pressure requirements of platform gas facilities

The system i am using as an example in this unit has two stages of separation operating at 17 bar and 1. bar respectively.

In order to recombine the gas from each stage forfurther use, the pressure of the gas leaving the ndstage of separation must be increased to the pressure of the 1st stage. A compressor is therefore required to do this.

Figure 3 develops the gas flow diagram to includethis compressor.


1.

After compression from nd to 1st stage separator pressure, all the gas is now at a pressure of 17 bar. However, in our example, the gas will require drying {dehydrating} and will have some of its constituents liquefied in a gas liquids recovery plant. This requires the gas to be at an even higher pressure. A further stage of compression istherefore required at this point. In our hypothetical process plant the pressure is raised from 17 bar to 8 bar.

Figure 4 shows this further stage of compression and shows, in outline, the gas liquidsrecovery system.


1.7

The residual gas, after dehydration and gas liquids recovery, will be used for three things:

gas lift

gas export

gas re-injection into the reservoir

In order to export the gas from the offshore location to a terminal onshore a much higher pressure is nowrequired at the platform. Similarly, extra pressure is needed to inject the gas into the well for gas lift. At thispoint in our example, therefore, the pressure is raised to 170 bar by further compression.

Figure 5 shows this.


1.8

Even at this high pressure, we would not be able to re-inject the gas into the reservoir. The actual pressurerequired to do this would depend on a number of things such as :

reservoir depth

reservoir pressure

type of reservoir rock

In our example, we need a pressureof, say, 08 bar. So, yet anotherstage of compression is required.

Figure 6 completes our simplediagram of gas flow on a platform.


1.

We have just been considering the gas flow on an oil production platform. Of course, the gas which is produced from the reservoir may not be associated with oil. We may be looking at a gasfield.

In the early days of production from a typical gas field, the pressure of the gas at the surface will be sufficient to transport it to shore. As the life of the field progresses, however, the natural pressure of the reservoir declines. A point is reached where this pressure is no longer sufficient to transport the gas to shore.

When this happens, it is necessary to install gas compression plant on the platform.

We have just been looking at the compression of natural gas from the reservoir. However, there are a number of other applications of compression which you could come across in petroleum producing operations. Lets consider some of these:

Compressed Air SystemsCompressed air is required on a platform to supplythe following :

instrument air

works air

inert gas generation

The instrument air system requires a plentiful supplyof clean, dry air at an adequate pressure.

A works air system is required to drive tools and equipment around the platform. It may not be quite so demanding in terms of air dryness, but a plentiful supply must be maintained.

Inert gas is required on the platform for purging equipment and providing a blanket of non-flammable gas in certain vessels. The gas used is nitrogen, which is manufactured from atmospheric air on the platform. The inert gas generator requires a supply of air at a suitable pressure.

In all cases, the pressure required may be in theregion of, say, 10 bar.

A typical air compression package might consist ofthe following items:

air prefilter

air compressor

wet air receiver

air drier

instrument air receiver

A schematic layout of such a system is shown inFigure 7.


1.10


1.11

Refrigeration SystemsThe gas liquids recovery system, which I referred to earlier, requires gas to be chilled to a fairly low temperature, i.e. -30C or less. One way of doing this uses a refrigeration system.

A refrigeration system which employs a compressor is the vapour compression cycle process. In this process, the refrigerant in the form of a vapour is first compressed, then condensed to a liquid. This is followed by expansion over a valve, and evaporation, to achieve the necessary cooling effect. The process then starts again.

It is not my intention to look at the process in any detail in this unit. However, Figure 8 illustrates it in outline.


1.1

Portable Air CompressorsIt may be necessary, particularly during the construction of a platform, to have access to compressed air in locations where there is no supply. In this case portable compressors may be employed.

HVAC BlowersOn an offshore platform, the accommodation will require a heating, ventilation and air conditioning (HVAC) system. This will necessitate the use of low pressure/high volume compressors, which are often referred to as blowers.

Test Yourself 1.1 The following statements are either true or false. Indicate with a tick in the box provided which statements belong in which category. If the statement is false, correct it.

TRUE FALSE

1. Oil and gas are separated from each other in separators which always operate at high pressures.

. Some of the components of a gas stream leaving a separator can be liquified.

3. Excess gas on a platform is disposed of by flaring.

. The pressure required to inject gas into a reservoir depends on the amount of gas liquids which have been recovered.

5. Compressed air is only required on a platform for inflating tyres on a helicopter.

You will find the answers in Check Yourself 1.1 on page 1.32


1.13

Summary of Section 1

In Section 1, I have tried to show you some applications of gas compression in petroleum processing.

In the section, you saw that gas which is separated from oil is usually at too Iow a pressure to be transported or to do useful work. Compression facilities are required to raise the pressure of the gas for:

gas liquids recovery

gas lift

gas export

gas injection

You also saw that compressors are required for instrument and works air, and for refrigeration purposes.

Im sure that you could think of a few more applications of compressors, but the ones I have just described are the major ones in petroleum production operations.

You now have an overall impression of the way in which compressors are used. In Section 2 we will move on to look at some of the basic scientific principles which affect the way in which compressors work.


1.1

You saw in Section 1 of this Unit how important thegas compression facilities are in a petroleumproducing operation. In this section we are going tolook at some of the basic principles of compression- some of the rules which govern how gases behavewhen they are subjected to changes in pressure,temperature and volume.

We will also look briefly at the application of energyconservation principles to gas compression.

Finally, I will introduce some of the terms andexpressions which are commonly used incompressor technology.

Lets start by having a look at the relationshipbetween the pressure and volume of a gas.

You should be aware that the following relationshipsapply to what is often referred to as an ideal gas.

In the real world, however, gases are not ideal andtheir behaviour departs from the ideal situation.

The magnitude of this deviation depends upon thenature of the gas, and the actual pressures andtemperatures involved. More advanced calculationscan account for these deviations - for example, bythe use of compressibility factors.

However. for our purposes, in this CompressorProgramme we are assuming ideal conditions.

Pressure - Volume RelationshipThe basic law which relates pressure and volume ina gas is known as Boyles law. This law statesthat:

At a constant temperature, the volume of agiven mass of gas is inversely proportional toits absolute pressure

This may be written as an equation, as follows:

P1V1 = PV

In this equation:

P1 is the initial pressure in absolute units

V1 is the initial volume

P is the final pressure in absolute units

V is the final volume

Two things should be noted here:

the process has to be at a constant temperature. Any process which takes place

at a constant temperature is known as an isothermal process

the pressure in this relationship has to be the absolute pressure or the pressure above absolute zero pressure. Absolute pressure means the pressure read on a gauge, plus the pressure of the atmosphere :

absolute pressure = gauge pressure +atmospheric pressure

For example, when using bar as the unit ofpressure:

bara = barg + atmospheric pressure

For our purposes we will take the atmosphericpressure to be 1 bar, although the exact figureis 1.013 bar.

The inverse relationship in Boyles Law means thatif the volume of the gas is reduced, the pressureincreases. Similarly, if the volume is increased thepressure is reduced.

This relationship can be shown in a simpleillustration.

Figure 9 overleaf shows this

Petroleum Gas Compression - Unit 1 - An OverviewSection 2 - Basic Principles of Compression


1.1

We can use the Boyles Law equation to determinethe change in volume of a gas with changes inpressure and vice versa. I will show you a workedexample first. Then try Test Yourself 1.2 out foryourself :


1.1

EXAMPLE 1 BOYLES LAW CALCULATION

If 100 litres of gas is compressed from 10 barg to 1 barg, what will be its volume after this compression at constant temperature.

First remember to convert the pressures to absolute units,

P1 = 10 + 1 = 11 baraP = 1 + 1 = 1 baraV1 = 100 litresV = The volume after compression

Boyles Law states

P1 V1 = P V

We must rearrange this equation

ie

V = P1V1 P

V = 11 x 100 = 8.7 litres 1

Test Yourself 1.2

Using Boyles Law

a) 300 litres of gas at 2 barg is compressed to barg at constant temperature. What will be its new volume?

b) The volume of a mass of gas at 0 barg is reduced from 1800 litres to 00 litres at constant temperature. What will be its final pressure?

You will find the answers inCheck Yourself 1.2 on page 1.33

Now lets look at the relationship betweentemperature and volume.


1.17

Temperature - Volume RelationshipThe basic law in this relationship is Charless Law.

This states that :In a constant pressure process, the volume of a given mass of gas is directly proportional to its absolute temperature.

Once again, we are dealing in absolute values - this time absolute temperature.

The absolute temperature is the temperature above absolute zero.

Absolute zero is the lowest point on the absolute temperature scale, which is measured in units called Kelvin (K) in the S1 system. In Imperial system the units are known as Rankine (oR)

On the Celsius temperature scale, absolute zero (OoK) is at -273.15C. (A convention, which is by no means universally applied, is to omit the term degree when using absolute temperature units).

On the Fahrenheit scale, absolute zero (OoR) is at -.7F

In most practical situations sufficient accuracy is achieved by using 73 as the conversion factor between Celsius and Kelvin, and 0 between Fahrenheit and Rankine. So, to quote an absolute temperature in Kelvin, when we are working with Celsius units, we add 73 to the temperature in Celsius.

i.e. 100C = ( 100 + 273 )= 373 Kelvin ( absolute)

Check that you understand this by doing the followingTest Yourself

Test Yourself 1.3 State the following temperatures in absolute units.

1. 0C

. 100C

3. 0C


Note that this process is at constant pressure,Such a process is called an isobaric process.

The Charless Law relationship means that, ifthe temperature of a fixed mass of gas isincreased at a constant pressure, its volumewill also increase.

Again a simple drawing, Figure 10, on thenext page, shows this.


1.18

As before, the relationship can be written as anequation for calculation purposes:

V1 V T1 = T

This equation can be used to calculate the changein volume with changes in temperature and viceversa. Again, a worked example is given first. TestYourself 1.4 then gives you further practice at usingCharless law.


1.1

EXAMPLE 2

CHARLESS LAW CALCULATION

If 100 litres of gas is heated from 0C to 80C at constant pressure, whatwill be its final volume?

Again, remember to use absolute units. V1 = 100 litresT1 = 0 +73 = 313 KT = 80 +73 = 33 KV = Final volume.

Charless law states

V1 VT1 = T

rearrange the equation

V = V1T T1

V = 100 x 33 = 113 litres 313

Test Yourself 1.4

a) The temperature of 50 litres of gas is raised from 1C to 38C at constant pressure. What will be the volume of the gas at 38C?

b) 800 litres of gas at a temperature of C is reduced ISOBARICALLY to a volume of 700 litres. What will be the final temperature?



1.0

The Combined Gas LawThe two laws of Boyle and Charles can be combinedto allow us to relate all three variables of pressure,volume and temperature.

This combined gas law may be written as:

P1V1 PV T1 = T

You will remember that the relationships we havejust been looking at apply to what is called a perfectgas. In reality, however, gases are not perfect, butreal. They do not behave exactly as you wouldexpect from these (perfect) gas laws. For practicalpurposes, however, we can use these gas laws toperform basic calculations.

We have just seen how pressure and temperaturealter as we reduce the volume of a fixed mass ofgas. But what does this mean as far ascompression is concerned?

The fact that the pressure will increase as thevolume of a gas is reduced may suggest to us howwe could make a simple compressor. Look back toFigure on page 1.1. What does the series ofsimple drawings remind you of ?

It reminds me of a bicycle pump. The piston withinthe cylinder is reducing the volume of gas. You willremember from Boyles Law that this reduction involume will increase the pressure.

If, in Figure , we :

remove the compressed gas from the cylinder when the piston is at the bottom of its stroke

refill with low pressure gas as the piston moves upwards again

we have a simple reciprocating compressor.

This is, in fact, what a bicycle pump is. We will lookat the construction and operation of reciprocatingcompressors in Unit of this series on compressors.

Now lets look at another scientific concept, namely,energy.

EnergyAlthough the word energy is very commonly used, itis difficult to define precisely. We could say that aperson has energy if he or she has the capacity todo things or influence events. In science, a systemhas energy if the objects in the system can do thingsand possibly affect other objects.

Consider for a moment the following example.

Supposing you were standing next to a cricket ballwhich is resting on the ground. The ball isnt doingmuch or affecting anything else. If you picked it upthen threw it at the nearest window, however, itwould certainly be doing something. It would alsobe affecting the window. If you picked it up thendropped it on your toe, it would affect your toe.

The point of this, is that you would have given theball energy by lifting it, and then throwing it.These are two forms of energy.

Lifting the ball gives it potential energy. From itsposition in your hand, it was then able to fall andaffect your toe.

Throwing the ball gave it energy of motion. This iscalled kinetic energy.

There are many other forms of energy and I havelisted some in the following table.


1.1

FORM OF ENERGY EXAMPLESPotential a weight lifted above the ground

Kinetic any moving object

Chemical gas, oil, coal, etc. They can be burnt to provide heat energy

Magnetic available when two magnets repel or attract each other

Electric available from an electric socket

Heat a tank of hot water has more energy than one full of cold water

Pressure a vessel at high pressure has more energy than one at a low pressure

TABLE 1. SOME FORMS OF ENERGY

The list above is not exhaustive and there are other forms of energy.

Test Yourself 1.5

What forms of energy are indicated by the following:

1. A separator operating at 0 bar and 80C.

. A pallet suspended from a crane hook.

3. A bottle of propane gas.

. An object being pumped along a pipeline.

. A boiler full of steam.



1.

This is a fundamental principle of science - the principle of conservation of energy.

This principle can be applied in the design of equipment which is used to change one form of energy into another.

Imagine having a machine which can rapidly speed up a mass of gas. Energy would be added to the gas in the form of kinetic energy. This energy would have come from the conversion of some of the energy being used to drive the machine. Now, if the gas is rapidly slowed down, its kinetic energy is reduced. But the total energy in the gas must remain the same, so the kinetic energy must be converted into some other type of energy. In a compressor the conversion is to pressure energy and some heat energy.

This is the principle of operation of a centrifugalcompressor, which is the subject of Unit 3 of thiscompressor series.

Lets move on now to look at some terms andexpressions which you need to become familiar withduring your investigation of gas compression.

Compression RatioA compressor must have the capability to take ingas at a certain pressure and deliver it at a higherpressure. The relationship between a compressorsintake and delivery pressure is known as itscompression ratio.

For instance, supposing a compressor takes in gasat a pressure of 0 bara, and delivers it at 0 bara.The ratio between delivery and intake pressure is0 / 0 = 3.

In this case, the compression ratio of the machineis 3. Note how, once again, the units of pressureare absolute units, i.e., bara.

If an identical compressor takes in gas at 30 baraand delivers it at 0 bara its compression ratio isalso 3, i.e. 0/30.

This ratio can be used to make comparisonsbetween compressors of different types.

Make sure you understand this concept bytrying the following Test Yourself.

Test Yourself 1.6

a) A compressor takes in gas at 20 bara and delivers it at 70 bara. What is its compression ratio?

b) The same machine running under the same conditions takes in gas at 0 bara. What will be the delivery pressure?



1.3

Compressor CapacityWhen considering the performance of differentcompressors, it may be useful to compare theamount of gas compressed in a certain time foreach.

The volume of gas passing through the machine in agiven period of time is called the compressorcapacity.

The gas, however, is being compressed as it flowsthrough, and its volume is being reduced. We mustbe careful, therefore, Where we measure the volumeflowing through, or capacity.

The compressor capacity is defined as thevolume of gas compressed and delivered perunit time, expressed in terms of inlet conditionsof temperature and pressure.

The most common type of compressor is an aircompressor which takes in air from the atmosphere.Because of this, gas compressor capacity is oftenquoted in terms of air at conditions of atmosphericpressure and 15C.

You will often see compressor capacity referred toas the Free Air Delivered (F.A.D.)

A Compressor can be described as having an FADof 30m3 per hour at 7 bara. This means that it takesin 30m3 of air per hour at atmospheric conditionsand delivers it at 7 bara.

Mass Flow RateA compressor requires some form of driving motoror engine. It is important to know the powerrequirements of such a machine for a givencompression unit. This power depends on the massof gas compressed per unit time rather than thecapacity of the compressor. So another term youmay come across is that of mass flow rate. It isquoted in units of pounds per second or kilogramsper second.


1.

Summary of Section 2In this section we have had a brief look at some basic principles of compression.

First we considered the basic gas laws which relate pressure, volume and temperature.You saw that the relationship between pressure and volume at constant temperaturecan be expressed by the equation:

We next considered the principle of energyconservation. Here you saw that there are manyforms of energy including kinetic ( the energy ofmotion) and pressure energy. You saw that energycannot be destroyed, but only converted into anothertype. I pointed out that gas can be compressed byincreasing its kinetic energy, then converting thisenergy into pressure energy.

Finally we noted some compression terms.These were:

compression ratio - the ratio of the discharge to the inlet pressure of a compressor

compressor capacity - the volume of gas compressed per unit time, expressed in terms of inlet conditions of temperature and pressure

mass flowrate - the actual mass of gas compressed per unit time

In the next section we will go on to look at some different types of compressor in common use

P1V1 = PV

This is known as Boyles Law.

Charless Law relates volume and temperaturewhen the pressure is constant:

V1 V T1 = T

You also saw that these equations can bemerged into the Combined Gas Law.


1.

Up to now in this Overview Unit you have seen thatthere is a considerable requirement for compressionplant in a petroleum producing operation. You havealso looked at the basic principles of gascompression. But what kind of machine is used toachieve the required increase in gas pressure? Inthis section we are going to look at the various typesof compressor which are available, and for whatapplication they are most suited.

The Compressor Family TreeCompressors can be classified into a number ofcategories according to the way they work. Thiscompressor family tree is shown in Figure 11.

Petroleum Gas Compression - Unit 1 - An OverviewSection 3 - Types of Compressors


1.

You can see from Figure 11 that all compressors fallinto one of two main groups:

Positive Displacement/Intermittent Flow Compressors (these are commonly known as Positive Displacement Compressors and we will use this term throughout the remainder of this Unit)

Continuous Flow Compressors

We will start our look into the compressor family treeby talking about positive displacement compressors.

Positive DisplacementCompressorsA positive displacement compressor works on theprinciple of pushing a gas from a vessel by partially,or completely displacing its internal volume.

This is usually achieved by mechanical means or,less frequently, by a second fluid.

Because the vessel is alternately emptied andrefilled the flow is intermittent. The intermittentflow into and out of the compressor causes thepressure to pulsate on both the inlet (suction) andoutlet (discharge) sides.

Positive displacement compressors will developsufficient pressure to overcome any resistance toflow and the operational limits are essentiallydetermined by the driver power and the strength ofthe compressor parts.

From the family tree we can see that positivedisplacement compressors fall into two types. Theyare:

Reciprocating Compressors

Rotary Compressors

Reciprocating CompressorsReciprocating compressors play a very importantrole in the oil and gas industry and for this reasonUnit 2 of the compressor series is dedicated tothem specifically.

Reciprocating compressors come in all shapes andsizes and fall into two types:

piston type

diaphragm type

The action of the fluid-transferring parts is the samein each. A piston or diaphragm is made to pass, orflex, back and forth in a chamber.

In the more complex types of compressor, thechamber is equipped with valves on the inlet andoutlet to control the flow of the gas being compressed.

The operation of these valves is linked to :

the motion of the piston or diaphragm

the rise and fall of the pressure in the chamber

Rotary CompressorsRotary compressors have a variety of uses in the oiland gas industry.

In this type, the displacement of the fluid is producedby the rotation of one or more elements within astationary housing.

The most common types of rotary compressor foundin the oil and gas industry are the:

screw compressor

lobe compressor

sliding vane compressor

liquid ring compressor

These compressors will be explained further in Unit 4of this series.


1.7

Continuous Flow CompressorsFrom the compressor family tree we can also seethat the second group of compressors are theContinuous Flow Compressors.

In these compressors the movement imparted to thegas is continuous and constant. Continuous flowcompressors fall into two types, which are:

Dynamic Compressors

Fluidic Compressors

Dynamic CompressorsDynamic compressors have a system of elements(called impellers) which are arranged on a shaft.The impellers rotate with the shaft and impartenergy to the gas by increasing its velocity.

The amount of energy which is imparted to the gasby a dynamic compressor is mainly determined by :

the design of the impellers

the number of impellers used

the speed at which the impellers rotate

the density of the gas which is being compressed

There may be as few as one impeller, or as many astwenty or more impellers, on a shaft. The shaft maybe rotated at speeds which exceed 30 000 rpm.

When the gas leaves each impeller it is allowed toslow down. As this happens, kinetic energy isreplaced by pressure energy. You will rememberthis from Section of this unit.

Dynamic compressors are classified according tothe manner in which the gas flows through thecompressor. Within this category are:

Centrifugal Compressors - where, in each stage, the gas flows radially outwards

Axial Flow Compressors - here, the gas flows along the line of the shaft

Mixed Flow Compressors - a combination of centrifugal and axial types

Centrifugal Flow compressors (commonly referred to as centrifugal compressors) are dealt withcomprehensively in Unit 3 of the Compressor Series

Axial and Mixed Flow compressors will be coveredin Unit 4.

Fluidic CompressorsFluidic compressors, including the Ejector andDiffusion Pump types, will be covered in Unit 4.

Before moving on, have a go at the following TestYourself question.


1.8

Test Yourself 1.7Indicate whether the following compressors are positive displacement machines orcontinuous flow machines.

Positive displacement Continuous flow

1. Double acting reciprocating compressor

2. Mixed flow compressor

3. Axial compressor

. Screw type compressor

. Sliding vane compressor

. Centrifugal compressor


Compressor SelectionAs you have just seen, we can choose from avariety of compressors in order to perform the taskof raising the pressure of a gas. The choice ofcompressor for a particular application will often bebased on the two factors which we looked at inSection :

compression ratio

capacity

However, many other factors may influence thischoice. Some of these I have listed below:

nature of the gas - hot or corrosive gases may restrict the choice because of the requirement for special sealing or lubricating systems, or special materials used in construction

reliability - for continuous running applications

costs - not only the initial capital costs, but service and maintenance costs may have to be considered

power availability - the power available to drive the compressor could influence the choice of machine


1.

Of course, compression ratio and capacity are of critical importance when choosing a machine.Figure 12 shows typical pressure and capacity ranges over which various types of compressor usually operate. You should note that the Figure shows very approximate ranges and some compressors may be capable of operating outside the ranges indicated.


1.30

Summary of Section 3In section 3 we looked at the compressor family tree and at the different groups and types of compressor.

We then discussed the different compressors which may be found on an oil production facility. The different characteristics which placed them within certain groups and types were examined. We then looked at each type of compressor found within each group.

Finally, we considered, briefly, compressor selection.

You have now completed this Overview Unit in the Petroleum Gas CompressionSeries. Unit 2 of the Series will examine Reciprocating Compressors in detail.

Now use Figure 1 to answer the following TestYourself question.

Test Yourself 1.8

From Figure 12 decide what type of compressorwould be suitable for the following applications.

1. Delivering 8 m3 / hour at 30 bar

. Delivering 1,000 m3 / hour at 70 bar



1.31

Check Yourself 1.11. FALSE Separators are operated at various pressures depending upon - reservoir pressure - ratios of oil and gas produced - pressure requirements on the platform - number of stages of separation.

. TRUE

3. TRUE

. FALSE The pressure required depends on - reservoir depth - reservoir pressure - type of reservoir rock.

. FALSE Compressed air is required on a platform for - instrument air - works air - inert gas generation

Check Yourself - AnswersPetroleum Open Learning

1.3

Check Yourself 1.31. 313 Kelvin

. 373 Kelvin

3. 73 Kelvin

Check Yourself 1.2

Using Boyles Law

P1V1 = PV

and remembering to work in absolute units.

a) P1 = +1 = 3 bara V1 = 300 litres P = +1 = bara V = ?


V = P1V1 P

V = 3 x 300

= 10 litres

b) P1 = 0 + 1 = 1 bara V1 = 1800 litres V = 00 litres P = ?


P = P1V1 V

P = 1 x 1800 00

= 3 bara

= 3 - 1

= barg


1.33

Check Yourself 1.4a) Using Charless Law

V1 V T1 = T

and remembering to use absolute temperature values.

V1 = 0 litresT1 = 1 + 73 = 8T = 38 + 73 = 311V = ?


V = V1T T1

V = 0 x 311 8

= 3.8 litres

b) Using Charless Law

V1 V T1 = T

and remembering to use absolute temperature values.

V1 = 800 litresT1 = + 73 = 38V = 700 litresT = ?


T = VT1 V1

T = 700 x 38 800 = 87K

= 87 - 73

= 1oC


1.3

Check Yourself 1.51. Pressure and heat energy.

. Potential energy.

3. Pressure and chemical energy.

. Kinetic energy

. Pressure and heat energy.

Check Yourself 1.6a) Compression Ratio = delivery pressure intake pressure

= 70 bara = 3. 0 bara

b) Delivery Pressure = intake pressure x compression ratio = 0 x 3. = 10 bara


1.3

Check Yourself 1.71. Positive displacement. 2. Continuous flow.

3. Continuous flow.

. Positive displacement.

. Positive displacement.

6. Continuous flow.

Check Yourself 1.81. Reciprocating compressor.

. Centrifugal compressor.


1.3

Petroleum GasPetroleum Gas Compression workbook 1Petroleum GasPetroleum Gas Compression workbook 1

petroleum gas compression workbook 1.pdf

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