Compression Fundamentals

8/12/2019 Compression Fundamentals

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Compression Fundamentals

INTRODUCTION TO GAS PLANT

Compression and treatment of gaseous hydrocarbons for an oil and gasprocess facility will depend upon the requirements and export specification forthat site.

In order to maximise the recovery of these hydrocarbons various processes ofgas compression, dehydration and natural gas liquefaction are employed.

Also where applicable, a fuel gas system is installed and made available tothe various users.

Throughout the training module your understanding will be checked as youprogress through the units.

This will be managed by asking questions and by prompting you to highlightvarious areas within the schematics.

inally at the end of the module you will be requested to answer a set ofmultiple choice!multiple selection questions aimed at assessing theunderpinning knowledge gained.

TRAINING AIM

The aim of this training module is to provide you with the necessaryinformation and knowledge required in understanding hydrocarbon gascompression systems.

The specific knowledge gained from studying the theoretical principles andpractical applications involved can then be used towards developing an overallunderstanding of process operation and control.

This in turn will lead towards safe, efficient and economic operation of plantand equipment

TRAINING OBJECTIVES

"n completion of the module, participants will be able to#

$nderstand the principles ofCompressor %esign Criteria

&xplain how compressors are


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designed and si'ed for the dutyrequired

&xplain how compression ratios

are calculated based onabsolute temperature andpressure

1.0 INTRODUCTION

(efore we discuss gas compression and the different types of compressors in

detail, we need to consider some of the necessary design fundamentals.

The design of compressor equipment used in the oil and gas industry is basedupon the fact that hydrocarbon liquids and gases possess behaviour patternsthat remain constant.

That is to say, for a given temperature, methane gas will generate a givenpressure under all known processing systems.

2.0 ABSOLUTE PRESSURE


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Almost all calculations in the processing ofnatural gas require the use of pressure andtemperature readings measured in absoluteterms.

In the case of pressure readings we must takeinto consideration the pressure imposed by theatmosphere, which we know is ).*) bar +).-lbs per square inch +psi.

A normal pressure gauge fitted to a vessel,records only the pressure contained within thevessel. To obtain absolute pressure readings wemust add the pressure imposed by the airsurrounding the vessel, which is ).*) bar+).-psi

As an example, assuming a gauge reading of )* bar +)- psi, to gain anabsolute reading we must add ) bar +).- psi, which gives an absolutereading of )) bar +)/).- psi.

"ne more example to test your knowledge.

Assuming that a vessel is reading * on the pressure gauge, what would be itspressure reading in absolute terms0

The answer of course is# * 1 ) bar +).- psi 2 1 bar (14.7 psi)

3auge readings are indicated thus# barg.+gauge

Absolute readings are indicated thus# bara.+absolute

3.0 ABSOLUTE TEMPERATURE


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Absolute 'ero is the minimum temperature that can be achieved.It is the temperature at which the motion of particles constitutingmatter would be at rest.

In the case of the temperature scale that we refer to as

Centigrade or Celsius, we are aware that the free'ing point ofwater is * degrees and the boiling point is )** degrees, butclearly, temperatures exist well below and above this range.

"ur normal temperature gauges will record reasonably lowtemperatures and also very high upper range temperature, butwhat is absolute temperature0

In order to account for the extremes of temperature that arepossible, an additional scale, relating to the centigrade scale, hasbeen devised. This scale, which is called the 4elvin scale, records

temperatures down to absolute 'ero. (y reference to the 4elvinscale on the right you will observe that the free'ing point of wateris 5-64, compared to * degrees on the centigrade scale.

1.1 INTRODUCTION

7atural gas is defined as a mixture ofhydrocarbon gases and associated impurities.

There is no one mixture or composition thatcan be referred to as natural gas. &ach gasstream produced has its own composition.&ven two gas wells from the same field mayhave different compositions.

7atural gas sold to the normal consumer willconsist mainly of a pure gas called methaneand small amounts of ethane and propanemixed with it.
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1.1 INTRODUCTION

2.0 THREE STATES OF MATTER

2.1 INTRODUCTION

Almost all matter can exist in one of three states namely Solid, Liquid orGaseous.

7atural gas is defined as a mixture ofhydrocarbon gases and associated impurities.

There is no one mixture or composition thatcan be referred to as natural gas. &ach gasstream produced has its own composition.&ven two gas wells from the same field may

have different compositions.

7atural gas sold to the normal consumer willconsist mainly of a pure gas called methaneand small amounts of ethane and propanemixed with it.


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8e are all aware that water can and does exist as ice, water and vapour, withthe state +or phase dependent upon the temperature that exists.

At normal room temperature water exists as a liquid and will remain so unlessthe temperature is raised to 100C (1F)or lowered to 0C (!F), at whichpoints the liquid will "#ange p#ase to eit#er $apour or i"e respe"ti$el%.

Temperature therefore plays an important part in the phase change operation,but what of the pressure effect in this operation0

3.0 MOLECULAR WEIGHT

3.1 DEFINITION

9olecular weight +98 is the sum of the individual atomic weights in a

molecule.

or example, the molecular weight of water +:5" is ); because the atomicweight of each of the hydrogen atoms is ) and the atomic weight of oxygen is)/.

i.e. 1& ' 1.

The hydrocarbon gases that we will be discussing throughout thispresentation are composed of carbon and hydrogen atoms, which haveatomic weights of 1 and1respectively.


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It therefore follows that methane, having four hydrogen atoms and one carbonatom, will have a molecular weight of )/. The second gas is of course ethane,which is made up of two carbon atoms and six hydrogen atoms. Themolecular weight is therefore +/ x ) 1 + * 1 2 6*.

entane, which can be in vapour form at appropriatetemperatures and pressures, will have a molecular weight of ++ * 1 1 +)5 x

) 2 -5.

8e can see that methane, having fewer atoms of carbon and hydrogen thanethane will therefore be lighter in actual weight and, in turn, we accept thatpropane and butane are again, in turn, heavier. It is reasonable therefore torefer to light and heavy gases, with methane being the lightest and butanebeing the heaviest.


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apour pressure is the pressure e*ertedbythe vapour of a substance when both itsliquid and its $apour are in equilibrium.

&quilibrium is established when the rate of

evaporation of the liquid is equal to the rateof condensation of the vapour.

8hen a liquid enclosed within a vessel orother container is heated to its boiling point,the vapours given off will occupy all theavailable space. As vaporisation or boilingcontinues, the number of molecules in thevapour space will increase and cause anincrease in pressure.

The pressure exerted by the vapour or gasis due to the impact of its componentmolecules against the confining walls of thecontainer.


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All materials exhibit a definite vapour pressureto a greater or lesser degree atany temperature above absolute 'ero.

"ne additional fact#

The magnitude of the vapour pressure within an enclosed vessel does not

depend upon the amount of liquid contained in the vessel. As long as someliquid is present, a true vapour pressure will result.

The surface area of the liquid also plays little or no part in the formation of avapour pressure, but is dependent entirely upon the maximum potentialenergies of attraction, which must be overcome in vaporisation.

.0 PARTIAL PRESSURE

%altonBs law of partial pressuresstates that#

The total vapour pressure for mixtures is the sum of the vapourpressures of each individual component.
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apour pressurecreated by one pure liquid will not affect the vapour pressureof a second pure liquid when the liquids are nonreacting, and the liquidsand!or vapours are mixed within the same system.

8hen behaving ideally, there is complete indifference on the part of eachcomponent to the existence of the others.

%uring the compression of any gas other than a pure gas, the principles ofpartial pressure are at work. This is true even in normal /.;D barg air

compression for power purposes, because there is always some water vapourmixed with the intake air and the compressor must handle both components.

Actually, air is itself a mixture of a number of components, including oxygen,nitrogen, argon etc, and its total pressure is the sum of the partial pressures ofeach of those components.

:owever, because of the negligible variation in the composition of dry airthroughout the world, it is considered as, and will hereafter be treated as, asingle gas with specific properties of its own.

!.0 BOILING POINT

The boiling points of the various gases are given in the >hysical ConstantsCharts as follows and we can see that there is a wide variation between them.
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The boiling point of our lightest gas, methane, is given as )/).=)EC +5=;./DE, while butane at the same pressure will boil at a temperature of*.=)EC +6)E.

The boiling points shown in the tables are for pure liquid gases.

(oiling point temperatures of mixtures occur over a range of values, whichdepend on the concentration of the mixture components, as well as thepressure.The boiling point of each liquid is related to the pressure applied tothe liquid. Any increase in applied pressure will increase the boiling point ofthe liquid.

Fet us look at the boiling point of water, which we know to be )**EC +5)5E

at ).*) bara +).-psia.

If we increase the pressure above the liquid phase to /.;D bara +)**psia,then a temperature of )/EC +65;E would be required to boil the water.Conversely, if we lower the pressure below atmospheric, then a lowertemperature would be required to boil the water.

This principle is applied in crude oil refineries, where atmospheric residuefrom the crude distillation units is passed to a vacuum distillation unit, where itcan be boiled at very low temperatures.

".0 FREE#ING POINT OF HYDROCARBONS


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All substances, including natural gases, can exist in any one of three forms@solids, liquids or vapours.

8e accept the simple fact that butane can exist as a liquid or a vapourbecause we are able to see this on any normal gas plant, but to imagine that it

can exist in the solid state stretches our imagination somewhat. It is truenevertheless and our >hysical Constants Charts show that this can and willhappen, if the temperature is reduced to )6;.6=EC +5)-.*=E atatmospheric pressure.

9ethane, on the other hand, will need atemperature reduction to );5.=EC +5D/./E to change phase to solid atatmospheric pressure.

8e can see from the above thatchanging phase from the solid form tothe liquid or vapour form requires aninput of heat.

Changing phase from the vapour form to a liquid can be achievedeither by a temperature reduction or indeed a pressure increase.

.0 PRESSURE$ VOLUME$ TEMPERATURE ANDPOWER RELATIONSHIP

In any compressor, a predictable pressure, volume, temperature and powerchange occurs. A working knowledge of how each relates to the other isnecessary in order to understand compressor operations.

The following general statements apply to a gas being compressed#

>ressure?ises

olume%ecreases

Temperature?ises

>ower is required


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APPENDI% A& THE GAS LAWS

An awareness of the gas laws whichgovern the changes in state of gasesmust be thoroughly appreciated if aclear understanding of gascompression and gas processing isto be obtained.

%uring this course a great deal isdiscussed on the question of energyand its uses, and an understandingof the first law of thermodynamicswill prove useful later.

8hen you click on the next button you will see the various gas laws shown,click on each one to view it in more detail. 8hen you are finished, click on thenext button to move onto the assessment.

Gas Compression

TRAINING AIM

The aim of this training module is toprovide you with the necessaryinformation and knowledge requiredin understanding hydrocarbon gascompression systems.

The specific knowledge gainedfrom studying the theoreticalprinciples and practical applicationsinvolved can then be used towardsdeveloping an overallunderstanding of process operationand control.

This in turn will lead towards safe, efficient and economic operation of plantand equipment.
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TRAINING OBJECTIVES

"n completion of the 3as Compression module, participants will be able to#

$nderstand the principles ofCompressor %esign Criteria

Accurately describe compressortypes, their function and control


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This category of compressor utilises theeffects of centrifugal force to increase gaspressure. These machines operate atrelatively high speed.

Centrifugal compressorshave fewer movingparts than the reciprocating type and haveproved to be very reliable throughout theindustry.

The advantages, disadvantages andoperating parameters of each type ofcompressor will be dealt with later in thetraining module.

"ther types of compressorinclude#


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"ther applications include#

The compression of propane or freon gases in refrigeration systems

Instrument and plant air compressors

.0 THE MECHANICAL COMPONENTS OF ARECIPROCATING COMPRESSOR

?eciprocating Compressor :eads comprise five maGor components asfollows#

The cylinder

The piston and piston rings

The piston rod

The suction and discharge valves

The packing

These maGor compressor assembly components are shown in the diagrambelow, move your mouse over the diagram to highlight the components andclick on the highlighted component to see more information.
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1.0 THE MECHANICAL COMPONENTS OF ARECIPROCATING COMPRESSOR

?eciprocating Compressor :eads comprise five maGor components asfollows#

The cylinder The piston and piston rings

The piston rod

The suction and discharge valves

The packing

These maGor compressor assembly components are shown in the diagrambelow, move your mouse over the diagram to highlight the components andclick on the highlighted component to see more information.

2.0 COMPRESSOR ELEMENTS

?eciprocating Compressors elements and stages

&very compressor is made up of one or more basic elements. A singlestagecompressor comprises a single element or a group of elements in parallel.
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9any compression problems involve conditions beyond the practicalcapability of a single compression stage.

Too great a compression ratiocauses excessive discharge temperature andother design problems. It may therefore become necessary to combine

elements or groups of elements in series to form a multistage unit, in whichthere will be two or more stages of compression.

In order to avoid excessive temperature rise the gas is frequently cooledbetween stages to reduce the temperature and volume entering the nextstage.

Hou should note that each stage is an individual basic compressor withinitself. It is si'ed to operate in series with one or more additional basiccompressors and even though they may operate from the same powersource, each unit is a separate compressor.

3.0 INTERSTAGE COOLING

The principles of interstage cooling apply to both centrifugal and reciprocatingcompressors.
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In both types of compressor it is necessary to introduce stages ofcompression in order to gain the necessary compression ratio withoutexceeding the design limits of discharge temperature.

(y staging the flow of gas through a number of compressor units, we candivert the gas flow through a cooling section to rid ourselves of unwantedheat.

There are additional advantages to be gained from interstage cooling, namelya reduction in gas volume, which in turn reduces the amount of horsepowerrequired.
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(y reference to thecompression ratio chart, ratioand the amount of gas beingcompressed determinehorsepower requirements.

4.0 CAPACITY AND POWER

Hou may recall that a reciprocating compressor consists of a driver and one ormore compression cylinders, with the driver being the energy input device
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D'-+A/6 C'()*+,,'* 78 F9+: V'-(+ C-+*/+ P'/;+,

The main purpose of the compressor is to raise the pressure of the gas, butnature decrees that heat is also transferred.

As we know, there are a number of different formats in use with positivedisplacement compressors, such as singleacting, doubleacting etc, but themost commonly used machine in the industry today is in the doubleactingmachine.

This type of unit compresses gas on both strokes of the piston.

5.0 SUCTION PRESSURE CONTROL

The control of gas suction pressure to the machine is the key to steady stateoperating conditions, and a number of different methods are used to achievethis.

S)++: C'*'-


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7ot all machines are fitted with a means of controlling the speed, but thosethat are have a definite advantage. Capacity, as we have said before, relatesto a number of factors, not least is the number of strokes the machine makesper minute. 8ith strokes relating to speed, it follows that if we reduce thespeed, we reduce the strokes and thus, reduce the capacity required.

C-+*/+ P'/;+ A:


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alve lifters or unloaders are used, in the main, to unload the machine forstarting and stopping purposes. This will ensure that undue stress is notplaced upon the machine during these critical periods.

.0 METHODS OF COMPRESSOR LOADING

There are four main methods of loading reciprocating compressors.

The loading procedures adopted by operators will fall into one of the fourmethods described in this section, or a combination of them. :owever thebest procedure for one system may not be the best for another due to thedifferent designs.

The simplest, most reliable and safest method of loading a compressor is@B=),, : D,/8*6+ C8+/; V->+ M+8': '? L':6@.

:ere there is no danger of exceeding the rated piston rod loads and nodanger of excessive horsepower peaks above the normal operatinghorsepower. It can be used for any application and the rate of loading can beestablished by the rate of closing of the bypass valve. In addition to this,method is the simplest for automatic sequence loading or manual operation,

as each block valve is opened or closed individually.


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!.0 COMPRESSOR LUBRICATION AND

COOLING SYSTEMS

Corre"t lubri"ation o t#e "ompressor "omponents, su"# as t#e pa"-ingand t#e piston, is possibl% t#e most important parameter in t#eoperation o a "ompressor.

If these and other items are not lubricated then they will become hot andultimately sei'e, distort or fail.

%ue to advances in lubricants and material technologies the amount oflubricating oil that a compressor consumes has significantly decreased inrecent times.

or example, piston rings and packing are often constructed from plastics. Asolid substance, molybdenum disulphide, which has very good lubricatingproperties, is used to impregnate the materials, which in turn reduces wear.

".0 THE CONNECTING ROD AND CROSSHEAD

The next consideration is how the piston is made to move.

The compressor cylinders may be supported in one of two ways#

They can be fixed directly on an engine or electric motor

They can be fixed to a frame and then driven by an engine or motor

8hichever method is employed, a

crosshead and connecting rod arerequired to transfer motion to thecompressor piston from the drivershaft.

The piston rod is fixed to one endof the crosshead and to the otherend is fixed one end of theconnecting rod. The other end ofthe connecting rod is attached to

the crankshaft of the drive unit.


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7ote that the crosshead and crankshaft are lubricated utilising a separatelubrication system to that employed for the compressor cylinder.

To provide the necessary straight line piston and rod motion the crossheadcomponents must be manufactured to fine tolerances and correctly installed.

.0 THE FRAMETYPE COMPRESSOR

8hen the cylinder section of a compressor is fixed on a frame, the frameconstitutes a part of the overall compressor.

The graphic above shows the principal components of a frametypecompressor. The frame housing is usually constructed from steel or cast ironand is equipped with ribs to adequately strengthen the unit against thestresses and strains imposed during the compression process.

The compressor cylinders are attached to the frame, which also incorporateshatches for maintenance purposes.

rametype compressors are available in a large range of si'es from singlecylinder lowpower units to )*cylinder highpower units of more than)*,***k8 +)6)* :>.

The crankshaft is located in the centre of the frame which is supported along

its length by bearings to ensure that it is kept as straight as possible.


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10.0 MAINTENANCE PROGRAMMES

The primary obGective of any maintenance programme is to minimisemaintenance expenses and to maximise gas throughput.

The three different types of maintenance programmes are#

Failureaintenan"e

&quipment is repaired or replaced when the unitceases to perform its assigned function.

/re$enti$eaintenan"e

(ased on experience, repairs at assigned intervalsmay be considered in order to avoid more expensivefailures.

/redi"ti$eaintenan"e

?epairs are initiated as predicted by equipment suchas the compressor vibration analysis and bycollection of operating data on each unit.

The following are some areas where these operating parameters arereviewed to determine when a maGor overhaul or repairs are required#

Crankcase pressure

9anifold pressure

Compression pressure

Condition of oil

Crankcase oil consumption

Crankcase inspection

Total hours run

11.0 COMPRESSOR VALVE FAILURE

alve failure can generally be classified as resulting from the followingcauses#

8ear and fatigue

oreign materials

Abnormal mechanical action

12.0 OPERATING AND MAINTENANCE

RECORDS


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14.0 SAFETY PRECAUTIONS

8e must always remain alert to the possibility of inGury to personnel engagedin the monitoring and repair of gas compressor units.

(efore maintenance programmes are commenced, full and properisolation of the system should be implemented and documented.9aintenance work should only proceed under the appropriate permitto work and company isolation practice.

>oints to note are as follows#

1. The compressor should be

electrically isolated followingestablished procedures.2. >ressure should be vented

from the compressorcylinders. If extremelyflammable or poisonous gasis being compressed, thecompressor cylinder shouldbe purged with nitrogen andthen depressurised.

3. All compressor pipeworkshould be isolated asrequired by companyisolation practices andprocedures.

%uring recommissioning of the equipment#

All compressor pipework and cylinders in service should be purgedt#oroug#l% it# nitrogen and lea- testedprior to restarting the

equipment. A serious explosion could result from mixinghydrocarbons and air which has been left in the system.

1.0 INTRODUCTION

1.1 C+*?6- C'()*+,,'*,


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Centrifugal compression equipment can be typified by the use of the followinggeneral systems#

Fans loers Compressors

The difference between the various systems lies in the degree of compressionachieved by each type of unit. Typicaly these are #

ans usually compress large volumes of gas to low discharge

pressures in the order of only 0.07 barg (1 psig) (lowers or boosters discharge at pressures up to about .+ barg(!&

psig) Compressors discharge at pressures above + barg (!& psig)

2.0 THE MECHANICAL CONSTRUCTION OF ACENTRIFUGAL COMPRESSOR

2.1 THE CASING

There are two types of casings used to house centrifugal compressors#

The hori'ontal split casing

The vertical split casing or the barrel casing.

2356389L S/L58 C9S5G

8#e #ori:ontal split "asingis made in two halves and bolted together. Thistype of casing is used where the pressures involved are from low to mid range.

This type of compressor is comparatively easy to strip down for maintenancepurposes and they are in common use throughout the industry.


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82; 9;L C3/;SS3The barrel compressor or vertical split compressor is rather more complex indesign.

They consist of a barrel with no hori'ontal Goints, into which is positioned theimpeller housing. The impeller is held in position by a series of bolts at bothends of the barrel. To remove the impeller, the bolts at both ends of the barrelmust be removed and the impeller withdrawn from the barrel, before any workcan take place on the impeller housing. The vertical split compressor is a#ig#


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5.0 LUBRICATION

All rotating machinery will have a lubrication system installed for protection ofthe equipment.

In the case of compressors, however, the lubrication system may well becombined with the seal oil system that we have Gust examined. 8hat iscertain, however, is that operating pressures within each system will vary tosome degree and we should look at the systems as individual components.

Fubricants perform many functions as follows#

rovide a degree of sealing action

?emove the heat generated from friction

?emove wear material, dirt and debris

>rotect the metal surfaces from corrosion

9ost lubricants are refined from crude oil, but many have additives added inorder to meet the needs of rust prevention and corrosion control.

All of the above means, of course, that selection of a lubricating oil for aparticular Gob must be precise. 8e cannot throw any type or grade of oil intoour machines and expect good, longlasting results. The lubricating oilselected for a machine is the one that best suits the needs of that machineand should be used exclusively for it.

Certain oils are not refined from crude oil, but are made from syntheticmaterials. These synthetic oils are used in very highpressure machineswhere operating conditions may be such that normal oils would degrade.

C9>8535t is $ital t#at t#e "orre"t oil or t#e s%stem is ala%sused.


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.0 COMPRESSOR OPERATING CURVES

&ach compressor will have been designed to deliver a given amount of gas ata given discharge pressure, providing certain conditions are met. Theseconditions relate to a constant suction pressure and temperature, and feedgas that has a reasonably constant density.

Another important factor applying to centrifugal compressors is that ofconstant speed of rotation.

7ote that the two main types of compressor drive are#

ixed speed electric drive +which can be fitted with a variable speedcoupling

3as turbine variable speed units

8hen the conditions of pressure, temperature, gas density and speedchanges, then the conditions through the compressor unit will change and thedesign output will not be achieved.

If the conditions change drastically, then the stability of thecompressor is at risk.

The capacity characteristics of the compressor therefore relate to suctionpressure, temperature and flowrate, desired discharge conditions of pressure,temperature and flowrate, the density of the gas and the compression ratioacross the machine.

!.0 GAS DENSITY

8e have already said that gas density is an important factor in compressordesign and is therefore a factor, which can upset all our calculations if itshould change to any great degree.

The compression ratio, which we have discussed in detail, is dependent upona number of factors@ among them being the density of the gas that we wish tocompress.

or example, air is approximately =* heavier than natural gas.
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If we fed air into our compressor instead of natural gas at the same conditionsof pressure and temperature, the compression ratio would be =* higher thanfor natural gas. The compressor driver would also require =* more power tocompress the air.

:owever in most compression situations, the density will not change to anymarked degree, and should it be expected to do so, a control system can beintroduced to control the density at any given point.

Circumstances have arisen where gas from a irst stage ofseparation +lighter gas has been fed directly into the low pressurecompressors which normally take gas from a t#ird stage ofseparation +heavier gas The result of the above has been a greatdeal of surging in thelo


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.0 LIQUID NOCOUT AND SALT REMOVAL

L: '/;' V+,,+-,

It is very important that liquids are prevented from entering centrifugal gascompressors where they could cause serious damage to the impellers,diaphragms and casing.

Fiquids, carried forward in the gas stream, can be in the form of vapour mist,small droplets or indeed larger liquid slugs. The situation can be aggravated

during plant startup, shutdown or upset conditions when the likelihood ofliquids being carried forward in the gas stream is even greater.

&rosion of the compressor components due to liquid bombardment will resultin a loss of efficiency and vibration problems, as the impellers becomeunbalanced. In extreme cases, this could result in failure of the impeller unitand the casing.

Fiquid slugs, due to their density and virtual incompressibility may producesudden and severe effects such as#

A reduction in compressor speed, causing the driver unit to labour oreven stall

&xcessive strain on the gearbox of the unit

(ending of the compressor shaft with resultant casing failure, due tothe sudden and uneven forces which are set up

lant.

In it you have learned about the construction and mechanical components ofboth main types of compressor, namely#

?eciprocating Compressors and


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Centrifugal Compressors

Hou have also learned about the various control systems installed to ensuresafe operating parameters for both types of machine.

Hou should now appreciate the importance of gas compressor sealingsystems, both BwetB and BdryB along with the subsystems required to operatethem safely.

Hou should now understand why lubrication and cooling of the moving parts isvital to continued operation of the units.

Above all never forget the importance of, and the need for, properly designedplant and equipment so that production of oil and gas is conducted safely andefficiently.

inally, we hope that you have enGoyed this module and in order to finish ityou are required to complete a set of questions that are designed to assessand underpin the knowledge you have gained.

Gas =e#%dration

1.0 TRAINING AIM

The aim of this training module is toprovide you with the necessary

information and knowledge required tounderstand gas dehydration systems.

The specific knowledge gained fromstudying the theoretical principles andpractical applications involved can thenbe used towards developing an overallcomprehension of process operation andcontrol.

This in turn will lead towards safe,

efficient and economic operation of plantand equipment.

1.2 TRAINING OBJECTIVES

"n completion of the 3as Compression module participants will be able to#


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$nderstand the principles of%ehydration


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acceptable levels. 8ater removal is achieved by dr%ing orde#%drating theproduced gas stream.

The term =e#%drationdescribes the process for removing water from gas orhydrocarbon liquid in order to meet the specifications mentioned above.

Another factor in deciding the amount of water to be removed is determinedby that stipulated in the sales spe"ii"ationof the contract.

2.0 GAS DEHYDRATION

%ehydration of hydrocarbon gas requires a Jdriving forceJ which will causewater to leave the vapour phase and condense as a liquid so that it can beremoved from the gas.

8ithin our industry, the following methods have been used to accomplishcondensation of water vapour within the saturated gas.

1. 9bsorption with liquid desiccantssuch as gl%"ol or met#anol.2. 9dsorption with solid desiccants such as alumina, sili"a gel, "al"ium

"#loride or mole"ular sie$e.

3. Cooling below initial dewpoint by either expansion or externalrefrigeration.4. Compression followed by cooling.5. Chemical reaction.

"f the above methods the first two namely Absorption and Adsorption are theones most commonly used and therefore we will examine both of thesemethods in detail.

The first thing we need to learn is the difference between Absorption andAdsorption techniques

2.1 THE ABSORPTION PROCESS

The mechanism ofater $apour absorptionin a liquid desiccant involves adriving force, which causes the vapour to leave the gas and condense. Thisdriving force must be greater than the opposing force, which tends to makethe condensed liquid vaporise, otherwise absorption will not occur.
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%ehydration is accomplished by redu"ing t#e tenden"% o a liquid to$aporise or by in"reasing t#e dri$ing or"e, which causes vapourcondensation.

In gas dehydration the main driving force is the dieren"e in t#e partial

pressure o ater in t#e gas p#aseand the partial pressure o ater int#e liquid p#ase. This technique of Absorption %ehydration is usedextensively to remove water vapour from a gas stream.

2.2 THE ADSORPTION PROCESS

The term 9dsorptiondescribes any process wherein molecules from a gasare held on the surface of a solid by surface forces.

The exact mechanism of adsorption is difficult to define. "ne theory is thatadsorption occurs due to liquefaction of a component in the vapour phase andits retention by "apillar% a"tionin the exceedingly fine pores of an adsorbingsolid.

Another theory advanced is that adsorption is due to "#emi"al "ombinationof a component in the vapour phase with the ree balan"e o atomson the

surface of the solid.

A combination of both phenomena is most likely.

The adsorption process then, is one in which a solid material will selectivelyremove a particular component from a fluid and retain that component on itssurface.

2.3 THE FUNDAMENTAL PROCESS

In time the dessicant itself will become saturated with water which reduces itscapacity for further adsorption.

Therefore the dessicant bed will require to be BregeneratedB by BvaporisingB thewater which has condensed on the dessicant granules. This is normallyachieved by causing hot dry +regeneration gas to flow upwards through thebed. This enables the removal of contaminants adsorbed near the inletwithout flushing them through the entire bed.
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The regeneration gas is heated to about 5=KC above final regeneration bedtemperature desired +normally in the region of 6)*KC.

In order to maintain continuous process operation, this type of system usuallyhas at least two beds of dessicant, one being used to dry the gas, while the

other is being regenerated. The cycle bed temperature profile can be seen inthe accompanying schematic.

7ote# The adsorption cycle will be discussed in detail later in this trainingmodule.

3.0 GLYCOL DEHYDRATION

3lycol dehydration is usually the most economic process to meet a givendewpoint specification and therefore is commonly used within the industry.3lycol equipment is not difficult to operate and maintain and can easily beautomated for virtually unattended operations.

A common way of stating the quantit% o ater $apour in a natural gasstream is in terms of its ater depoint.

%ewpoint is defined as the temperature at which vapour begins to condenseinto liquid.


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&xport specifications will be stipulated, but in general terms 7atural gaspipeline specifications require that the gas contains no more than /*ppm+parts per million of water. This corresponds to a water dewpoint ofapproximately *EC at /Dbara. %ewpoint depression is the difference in ECbetween the inlet gas temperature and the water dewpoint temperature of the

outlet gas.

Therefore, a gas at 6;EC and /Dbara must have a dewpoint depression of6;EC to meet pipeline specifications.

%ewpoint depression is accomplished by dehydration.

3lycols can be used with sour or acid gases but obviously certainprecautions must be taken since the gases are absorbed into theglycol.

3.1 ABSORPTION AND REGENERATIONPROCESS

Fets now examine the absorptionand regenerationprocesses in detail.

A,'*)'


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3lycol +usually T&3 and wetgas are brought into intimatecontact in a contactorvessel ortower, often referred to as theB3lycol ContactorB. The

contactor may consist of aseries of trays housing bubblecaps, or in other cases containstructured packing internalswithin the column depending ondesign requirements.

The pure @lean@ gl%"ol isallowed to enter the contactorcolumn at the top and cascadesover the structured packing, or

down through the glycol trays,absorbing water vapour fromthe gas as it passes downthrough the column.

The wet gas enters near thebase of the column and as itrises up through the vessel,more and more water isabsorbed by the counter

flowing glycol until at the top ofthe vessel the gas exits at thecorrect specification of watercontent.

The amount of dehydration achieved is dependent on#

glycol circulation rate +litres per kilogram of water in the gas

lean glycol concentration number of trays or depth of packing in the contactor and

contact temperature.

3.2 FOAMING

"ne of the most serious problems encountered is oamingo t#e gl%"ol.
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oaming increases glycol losses and reduces plant capacity and henceefficiency. &ntrained glycol will carry over the top of the contactorwith thegas, and the foaming will cause poor contact between the gas and the glycol,decreasing the drying efficiency.

3lycol may foam with light hydrocarbon liquids, some corrosion inhibitors andcorrosion products, salt and finely divided suspended solids. 3lycol may alsofoam due to excessive turbulence and high liquid to vapour contact velocities.

Although defoamers may in some cases be effective, they do not solve thebasic problem. oaming problems require individual attention, the best curebeing the proper care of the glycol solution. The most important measures toreduce these problems are effective gas cleaning upstream of the glycolsystem and good filtration of the circulating glycol solution.

9onitoring p: content will provide information on whether the glycol is too

a"idi" ("orrosion problems)or to al-aline (s"aling problems).

4.0 GLYCOL DEHYDRATION PLANT

The maGor pieces of equipment in a glycol dehydration plant are as follows#

Contactor lash tank

ilters

?egenerator

&ach of these items of process equipment is discussed separately prior toconsidering an overall dehydration system.
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4.1 THE CONTACTOR

Contactorswill normally contain between and )5 bubble captrays +althoughsome manufacturers prefer valve type trays. 3ood contactor design withrespect to tray spacing, tray liquid levels etc is imperative for efficientoperation.

(ubble caps provide a high degree of gas to liquid contact. The wet gasstream flows upward through the vessel JcontactingJ the glycol solution flowingdownwards. 8ater present in the gas is absorbed by the glycol. %ry gaspasses out of the top of the contactor through a heat exchanger. The gascools the incoming hot glycol stream which in turn increases its absorption

efficiency.


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>roduced solids cause fouling, foamingand plugging and are best removedby filters placed in the rich glycolstream.

3lycol filtration generally consists of two stages#

1. Solids remo$al, these filters are designed to accomodate for full flowon a duty!standby basis.

2. Gl%"ol Carbon iltersremove contaminants by adsorption. The

contaminants may include glycol degradation products, chemicals andlubricants etc. These filters normally operate on a Bside streamprincipleB.

4.4 THE GLYCOL REGENERATORRECONCENTRATOR

In the regenerator the water absorbed by the glycol is removed by boiling atrelatively high temperatures in the region of 5*= KC.
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The regenerator unit is made up of the following main components#


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>ositive displacement pumps are normally used and it must be rememberedthat the pumps handle a fluid that is frequently dirty and corrosive. Cylindercorrosion, pump cup or ring wear and sticking or plugged valves must bedetected as early as possible and corrected to maintain process efficiency.

The pump rates should be commensurate with the gas volume beingprocessed. In general, glycol flowrates of 1+ to 40 litres per -ilogram oater to be removed are required. "ne of the most common sources of glycol

loss is at the circulating pump packing glands. If a pump leaks upwards of1litre per da%, the packing needs replacing.

. GLYCOL TO GAS HEAT E%CHANGER

Fean glycolfrom the circulating pump!s (temperature 0C to 10C)flows tothe contactor towerthrough a glycol to gas heat exchanger where the heat ispassed to the dry gas stream from the contactor.
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The glycol to gas heat exchanger may be in coil form as depicted in theschematic.


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3lycol required per day =5** x 5= 2 1!0000 litres or1!0 m!

Gl%"ol "ir"ulation rate )6* ! 5 2 +.417m!D #our

5. Fet us assume that a hot dry stripping gasis required in a regenerationsystem to produce a lean glycolpurity in excess of D;.=.

Buestion# %etermine if stripping gas will be required to achieve an outlet gasdewpoint of


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8here the highest possible dewpoint depression is required an adsorptionprocess using a solid or dry desiccantmay be the most effective method. (yfar the most widely used adsorbent in service at present is known as amolecular sieve.

8ith the molecular sieve a large surface area is presented for adsorptionwhich, because of the si'e and structure of the molecular pores, holds thewater on its surface.

There are a number of commercially available desiccants used for gasdehydration. These desiccants can be reactivated or regenerated so that theycan be used through many cycles of adsorption and reactivation.

8ith dry desiccant dehydration very low dewpoints can be obtained, and it isnot uncommon to achieve a resultant residual water vapour in the outlet gasof less than -g per m!by this method. In a normal application this may

well correspond to a dewpoint of < 40C.

5.1 REGENERATION CYCLE

8hen the desiccant bed is completely saturated with water the gas stream is

switched to the second tower and the desiccant in the first tower isregenerated.
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A split stream +about )* of the total from the main gas flow passes througha heater and flows +opposite to the normal flow direction through the watersaturated desiccant bed. The gas heats up the bed and vaporises the water.

?egenerator gas and water enter the gas cooler where the water condenses

and is separated from the gas stream in the separator. The gas from theseparator is then recombined with the main gas stream.

ollowing the regenerating cycle the bed has to be cooled down before it canbe switched back to adsorption@ this is termed as conditioning the bed.

Conditioning of the bed is achieved by continuing to flow the splitstream tothe first tower, but bypassing the heater and flowing the unheatedregeneration gas through the bed.

.0 SEPARATION OF HEAVY HYDROCARBONS

%epending on the composition of the gas in question, cooling it for thepurpose of dehydration, could possibly bring the pressure and temperature ofthe gas within the twophase region of the phase envelope.

This could result in the condensation not only of water, but also of a streamconsisting mainly of heavy hydrocarbons. 8here this does occur it isnecessary to separate and recover the liquid hydrocarbons.

The reasons for separating out liquid hydrocarbons can be summarised asfollows#
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8o a$oid "ondensation o t#ese liquids during subsequent#andling o t#e gas

8o meet a sales gas (or users) #%dro"arbon depoint

spe"ii"ation 8o upgrade t#e produ"t $alue o t#e gas b% separating premium

"omponents su"# as liquid petroleum gas (L/G)

8hatever the reason for condensing and separating liquid hydrocarbonssimultaneously with water, this must be considered when selecting acombined system.

!.0 RETROGRADE CONDENSATION

Instead of following the usual pattern of liquid changing into gas whilereducing the pressure, at certain temperature ranges heavier hydrocarbonswill condense out of natural gas during pressure reduction and may,depending on the temperature bracket, evaporate again during furtherpressure reduction.

This phenomenon, common to most composite natural gases, is calledretrograde "ondensation. An explanation of this phenomenon can be seen

in the phase diagram of a hydrocarbon mixture, sometimes called JphaseenvelopeJ.
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The Bbubble point "ur$eB is represented by the line AC and the Bdepoint"ur$e@by the line (C. The point where these curves meet is known as thecritical point.

9t t#is point t#e properties o t#e liquid and t#e $apour #a$e be"ome

identi"al and t#e% are no longer distinguis#able. At any combination ofpressure and temperature within the envelope AC( the system consists oftwo phases. At conditions to the right of the dewpoint curve the system will beall vapour and to the left of the bubble point curve all liquid.

The diagram indicates the phase changes that must occur when the pressureand the temperature of a system are varied.

".0 HYDRATES

A hydrate is a solid formed by the physical combination of water moleculesand hydrocarbon gas molecules contained +particularly those of methane.

:ydrates are members of a group of chemical compounds called clathrates, aterm used to describe compounds which exist in a stable condition but are notthe result of the chemical combination of all the molecules present.

:ydrates resemble snow in appearance although they can vary fromextremes of a Gellylike mush to solid ice.

%epending on the physical conditions prevailing, hydrates may form attemperatures ell abo$e t#e ree:ing point o ater. ?emember that ice isa crystalline structure made up of only water molecules and forms at *oC.

".1 PREDICTION OF HYDRATE FORMATION

9ost of the gas produced in the offshore oil and gas industry is saturated withwater vapour. 7atural gas systems being primarily methane are thereforeprone to hydrate formation.

:ydrates will form at temperatures well above the free'ing point of water andthe temperature at which a hydrate forms depends upon the pressure in thestream. Aspressure in"reases, thetemperature at #i"# #%drates ormalso in"reases.
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The graph illustrated shows, in the presence of free water, the temperaturesat which hydrates form in natural gas as a function of pressure.

It should be noted that composition of the gas can significantly affect thehydrate temperature and that this curve is an approximation for a common

gas.

As already mentioned nonhydrocarbon impurities such as #%drogensulp#ide and "arbon dio*ide will tend to accelerate hydrate formation.

".2 PREVENTION AND REMOVAL OF HYDRATES

:ydrates can be prevented or minimised, by using two basic methods asfollows#


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M+8':1

emo$e t#e ater from the gas stream using some form ofdehydration procedure. This is usually carried out by contacting thegas with a glycol liquid that will absorb the water from the gasstream.

M+8':2

>se a #%drate in#ibitor methanol +methyl alcohol or glycol canbe inGected into a gas stream to suppress hydrates and also to lowerthe free'ing point of the water present. This approach is analogousto the use of antifree'e in a car radiator. In this approach the wateris not absorbed and removed, it is merely protected from free'ingand inhibited from forming hydrates.

=epoint "onditioning, mentioned in section -, is another method ofpreventing hydrate formation.

:ere the process stream is subGected to conditions of temperature andpressure beyond those that will be met during normal operations. This willcondense out free water +and unwanted hydrocarbon liquids in a controlledsituation.

Assume as an example that a gas pipeline may operate at a minimumtemperature of 'C.

Then by subGecting the gas to a temperature of say )EC, it is assured that allliquid JknockoutJ that could occur at 15EC will have been achieved under

controlled conditions prior to the gas entering the pipeline.

Therefore the risk of dewpoint being reached during normal operations of thepipeline has been removed.

".3 INJECTION OF HYDRATE INHIBITOR

A hydrate inhibitor will be inee"ti$e if it is not mixed with the water at theprecise moment water condensation occurs.If an inhibitor is flowing along thebottom of a pipeline and condensation of water is occurring on the walls of theline, hydrate formation will not be prevented.

8hen methanolis used as a hydrate inhibitor, it vaporises and becomes anintegral part of the gas. As the gas cools the methanol condenses and mixeswith the water as it also condenses, thus providing protection.

8hen glycol is used as the hydrate inhibitor only a very small amount of glycolwill actually go into the vapour phase. To be effective it is essential that
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turbulent flow conditions exist to ensure the presence of glycol at the pointwhere wet gas is cooled to its hydrate forming temperature.

It is therefore of vital importance that good mixing occurs at the point ofinhibitor inGection. InGection upstream of a choke or pressure control valve will

cause the inhibitor to be thoroughly mixed as pressure reduction and violentagitation occur.


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This is often the case with offshore operations where concern forcorrosivity and hydrate formation in subsea pipelines is ofparamount importance.

8hen you click on the next button you will be able to answer a questionrelating to this section of the course. 8hen you have answered this question,click on the next button to move onto the assessment.

GL e"o$er% erigeration

NGL R+/'>+*= : R+?*6+*'

T*6 A(

The aim of this training module is toprovide you with the necessaryinformation and knowledge required inunderstanding 73F +7atural 3as Fiquidrecovery systems and industrial

refrigeration systems.

The specific knowledge gained fromunderstanding the theoretical principlesand practical applications involved canthen be used toward developing anoverall understanding of processoperation and control.

This in turn will lead towards safe,efficient and economic operation of

plant and equipment.

NGL R+/'>+*= : R+?*6+*'

T*6 O+,


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"n completion of the 73F!?efrigeration 9odule participants will be able to#

$nderstand the principles of 73F

?ecovery

&xplain the operation of a Turbo&xpander

%escribe the turbo expanderprocess

Accurately describe the principles

of refrigeration

rocesses employed to extract 73F usually concentrate the composition inthe ethane!propane!butane component range.

The 73F product may be further fractionated at the onshore terminal toproduce ethane and propane rich products. The propane rich fraction, when
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mixed with butanes, is sold asLiquid /etroleum Gases(L/G).

Two main processes are used for the recovery of 73F#

The absorption process. The condensation process.

2.0 REFRIGERATION

2.1 INTRODUCTION

9echanical refrigeration is the process of lowering the temperature of asubstance, either in the solid, liquid or gaseous state, below that of itssurroundings. In practical terms, most commercial refrigeration systems aregenerally required to cool the substance below ambient temperature.

The refrigeration process is essentially a physical mechanism. Thismechanism embodies special aspects of heat transfer, the conversion of workinto heat +or the reverse and the operation of heat engines for energyconservation and energy transfer.

?efrigeration technology is extensively used in the oil refining, petrochemicaland natural gas industries.

In these industries hydrocarbon liquids and gases are chilled in processesused for product purification, separation at low temperature fractionation andvery low temperature liquefaction of natural gas for storage and shipment inliquid form to overseas marketing areas.

3 S((*=

Hou have now finished the trining section of this moduel on %ehydration,7atural 3as Fiquefaction +73F and ?efrigeration. In it you have learnedabout the processes involved and the principle of operation.

Hou have also learned where each of the systems is employed within the

overall process and how they are used to maximise production.


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8ithe regard to dehydration systems, you now have the knowledge toappreciate the difference between A(

Compression Fundamentals

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Transcript of Compression Fundamentals