Intro to Compressor and Turbines

7/29/2019 Intro to Compressor and Turbines

1/20

UNITS IN THIS COURSE

UNIT 1 AN INTRODUCTION TO -COMPRESSORS AND TURBINES

UNIT 2 RECIPROCATING COMPRESSORS

UNIT 3 CENTRIFUGAL COMPRESSORS

UNIT 4 TURBINES

TABLE OF CONTENTS

Para Page

1.0 COURSE OBJECTIVES 3

1. 1 COMPRESSOR AND TURBINE TERMINOLOGY 41.1.1 Compression Ratio 41.1.2 Compression Heat 41.1.3 Cooler 51.1.4 Capacity 61.1.5 Positive Displacement 71.1.6 Reciprocating 81.1.7 Stroke 81.1.8 Velocity 91.1.9 Dynamic 91.1.10 Kinetic Energy 9

1.1.11 Centrifugal 101.1.12 Suction 101.1.13 Discharge 111.1.14 Multi-stage 111.1.15 Labyrinth Seal 131 1 16 Bypass 131.1.17 Pressure Differential 141.1.18 Thrust 151.1.19 Surge 151.1.20 Recycle 161.1.21 Prime Movers 161.1.22 Couplings 17

1.2 WHY COMPRESSORS ARE USED 18

1.3 CLASSIFICATION OF COMPRESSORS 19

1 .4 PRIME MOVERS 201.4.1 Electric Motors 201.4.2 Steam Turbines 211.4.3 Gas Turbines 221.4.4 Internal Combustion Engines 23

1.5 SUCTION SCRUBBERS 24

1.0 COURSE OBJECTIVES

Mo

duleNo.

6:Compressors&

Turbines

UnitNo.

1-IntroductiontoCompressors&

Turbines

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

Page 1/24


2/20

This course describes the different types of compressors and turbines found inprocess plants. The course progresses in Units from basic terminology to theprinciples of operation of reciprocating and centrifugal compressors and steamand gas turbines. On completion of the course the student will be able to:

Explain the basic terminology used in describing compressor and turbine

operations.

Explain the functions and classifications of compressors and turbines inprocess plants, and identify the different types of prime movers used topower compressors.

Identify suction scrubbers and explain how they work.

Explain the basic principles of operation of reciprocating compressorsincluding capacity control methods, control of suction pressure, andautomatic shutdown systems.

Explain the basic principles of operation of centrifugal compressors includingcapacity control 'Methods, surge control, and automatic shutdown systems.

Explain the basic principles of operation of gas turbines, steam turbines, andturbo-expanders.

1. 1 COMPRESSOR AND TURBINE TERMINOLOGY

Some special words are used when talking about compressors and turbines.The following terminology will be explained in this Unit:

Compression Ratio Suction

Compression Heat , Discharge

Cooler Multi-stage

Capacity Labyrinth Seal

Positive Displacement Bypass

Reciprocating Pressure Differential

Stroke Thrust

Velocity Surge

Dynamic Recycle

Kinetic Energy Prime Movers

Centrifugal Couplings

1.1.1 Compression Ratio

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

Page 2/24


3/20

The compression ratio is the ratio between the compressor dischargepressure and the compressor intake pressure. Because it is a ratio there areno units of measurement.

If the suction pressure is 15 psig and the discharge pressure is 90 psig thenthe compression ratio is 6: 1. That is, 90 divided by 15.

What is the compression ratio for a compressor with pressure of 10 psig anda discharge pressure of 40 psig ?

1.1.2 Compression Heat

There. are relationships between the pressure, the volume and thetemperature of a gas. If one of them changes, at least one of the other twomust also change. The changes are expressed as two Laws of Physicscalled Boyle's Law and Charles' Law.

Boyle's Law states that the volume ofa given mass of gas varies inversely as itspressure if the temperature remains constant. What this means is that as one goesup, the other one comes down. If the volume gets bigger, the pressure gets smallerand if the volume gets smaller, the pressure gets bigger. Remember that thetemperature must remain the same as the other two change.

Charles' Law states that the volume of a given mass of gas varies directly with itstemperature if the pressure remains constant. What this means is that as thetemperature of a gas goes up, the volume will also go up. If the temperature comes

down, the volume will also come down. Remember that the pressure must remainthe same for this to be true.

Therefore, when a compressor does its job the heat of the gas increases. In fact,the increase in temperature can be very large. The increase in temperature isusually not wanted so coolers are used to lower the temperature.

1.1.3 Cooler

As a gas is being compressed its temperature is increased. There are severalreasons why this increase in temperature is not wanted. One of the main reasons

is that it reduces the efficiency of the compressor. That is, it makes the compressordo more work for the same result.

The temperature of the gas can be reduced by using a cooler. If the compressorhas more than one stage, the gas may be cooled as it passes from one stage tothe next. In this case the cooler is called an intercooler.

If the compressor has three stages, intercoolers may be fitted between the first andsecond stages and between the second and third stages. These intercoolers take

the name of the stage of compression they are cooling. The intercooler betweenthe first and second stages is called the first stage intercooler. The one betweenthe second and third stages is called the second stage intercooler.

It may only be necessary to cool the gas as it leaves the compressor. In this casethe cooler is called an aftercooler.

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

Page 3/24


4/20

The very large compressors used in process plants are often fitted with intercoolersand aftercoolers.

There are many different types of intercooler and aftercooler. One which is quitecommon in process plants is called a fin-fan cooler. The hot gas from thecompressor passes through piping which has fins fitted to it. The fins increase

surface area to improve cooling. Air is blown across the piping by one or morefans. Although this is ordinary air at normal temperature the compressed gases arevery hot so even ordinary air will cool them down.

Coolers may also cool a compressed gas using a liquid. The piping from thecompressor passes through a vessel which completely surrounds the piping. Liquidpasses around in the space between the compressor pipi ng and the wall of thevessel.

Figure 1 -1 Coolers

1.1.4 Capacity

The capacity of a compressor is the amount of gas moved in a given time. The unitsof measurement for capacity are a volume divided by a time. Examples of capacityunits are cubic metres per minute (m3/min) or cubic feet per minute (ft 3/min).

ModuleNo.

6:Compressors&Turbine

s

UnitNo.

1-Introduction

toCompressors&Turbines

Page 4/24


5/20

1. 1. 5 Positive Displacement

Positive displacement takes place when a solid object is introduced into the spaceoccupied by a gas. When this happens, the gas is forced out of whatever iscontaining it.

A familiar exam pie of this is the hand pump used to inflate footballs, tyres and otherair-filled equipment. One of these is shown in Figure 1-2.

Figure 1 -2 Positive Displacement

The hand pump works like this. The piston can be considered as a solid objectbecause no gas (in this case air) can get past it as it moves downward. With thepiston at the top of the cylinder the cylinder is filled with air. As the piston movesdownwards the air in the cylinder is pushed out of the cylinder, through theconnecting pipe and into the football or tyre. The pressure of the air increases asthis is done (in accordance with Boyle's Law).

As the piston moves upwards a new volume of air is drawn into the cylinder. In thisvery simple type of positive displacement compressor the new air is drawn roundthe edges of the piston. Part of the piston is made of a flexible material (oftenleather) to allow this to happen.

1.1.6 Reciprocating

Reciprocating means movement in two directions but in only one plane. The planemay be horizontal with the movement backwards and forwards or vertical with the

movement up and down.

A piston inside a cylinder can only move in one plane but in two directions as shownin Figure 1-3.

Mo

duleNo.

6:Compressors&

Turbines

UnitNo.


Turbines

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

Page 5/24


6/20

Figure 1 -3 Reciprocating Movement

1.1.7 Stroke

The stroke of a reciprocating compressor is the distance the piston moves within the

cylinder. This may not be the full length of the cylinder.

When the piston moves in the direction which will draw gas into the cylinder thisstroke has two names. It may be called either the suction stroke or the inductionstroke. When the piston moves in the direction which will push gas out of thecylinder this is known as the compression stroke.

If the piston compresses gas only on one stroke then the compressor is said to besingle acting. If the piston compresses gas on both strokes the compressor is saidto be double acting. Figure 1-4 shows single acting and double acting pistons.

Figure 1-4 Single and Double Acting Compressors

Page 6/24


7/20

1.1.8 Velocity

The word velocity is often used to mean speed. However, velocity is more than thatand the correct meaning of the word is important.

Velocity is a measure of the rate of motion of an object expressed as the rate of

change of its position in a particular direction with time. The units ofmeasurement for velocity are a distance divided by a time. Commonly used unitsare metres per second (m/s), feet per second (ft/s), and kilometres per hour (km/h).

1.1.9 Dynamic

Dynamic means concerned with energy or with forces that produce motion(movement).

A dynamic compressor is a continuous flow machine in which the mechanical actionof rotating vanes or impellers adds velocity and pressure to the flowing gas.

1.1.10 Kinetic Energy

Kinetic energy is the energy a body has because of its motion. Kinetic energy isused in centrifugal compressors to increase the pressure of the gas.

1.1.11 Centrifugal

Centrifugal means moving outwards from the centre. Centrifugal force is a forcecaused by spinning. This force is used in centrifugal compressors.

Figure 1 -5 shows the effect centrifugal force had on an object. Although it is difficultto imagine, gas can be considered as an object so centrifugal force can be applied

to it as well as a solid object.

Figure 1 -5 Centrifugal Force

Figure 1-5 shows a shaft. At the top of the shaft is an arm with a small ball on theend of it. The arm is attached to the shaft by a simple hinge.

Mo

duleNo.

6:Compressors&

Turbines

UnitNo.


Turbines

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

Page 7/24


8/20

The left-hand picture shows the shaft stationary. There is no centrifugal force so thearm hangs straight down. The right hand picture shows what happens when theshaft rotates. Centrifugal force makes the arm and the ball swing out to be at rightangles (90) to the shaft.

1.1.12 Suction

Suction is the side of the compressor at which gas enters the compressor. If thecompressor is being used to compress air, it will draw in air from around itself. Thesuction pressure will be the same as atmospheric pressure. If the compressor ispart of a process, the suction pressure will be the same as the pressure of theprocess upstream of the compressor.

Although the word 'suction' is commonly used, it does not necessarily mean thatthe gas entering the compressor has a negative pressure, (less thanatmospheric pressure). Compressors may be used in a process where thesuction pressure is less than atmospheric pressure. Used like this, thecompressor may be called an exhauster. The word 'induction' is sometimes usedin place of suction, particularly with reciprocating compressors.

1.1.13 Discharge

Discharge is the side of the compressor at which gas leaves the compressor. Innormal operation the discharge pressure will always be higher than the suctionpressure. If it is not, something is wrong either with the compressor or in theprocess itself.

1.1.14 Multi-stage

Multi-stage means that the compressor has more than one stage of compression.Both reciprocating compressors and centrifugal compressors can be multi-stagemachines.

In a multi-stage reciprocating compressor the gas which is to be compressedenters the first stage of the compressor. The pressure of the gas is increased inthe first stage cylinder. The gas leaves the first stage cylinder and goes to thesuction of the second stage of the compressor. The pressure of the gas isincreased more in the second stage cylinder. Leaving the second stage cylinderthe gas may be sent to further stages or to the discharge of the compressor.

In a centrifugal compressor it is the number of impellers which determines howmany stages there are. At each impeller the pressure of the gas is increased.

For reciprocating and centrifugal compressors there are design limitations on thenumber of stages. One of the main limitations is the temperature of the gas. Asthe gas passes through each stage, both its pressure and its temperatureincrease. Cooling between stages helps reduce the temperature of the gas butthis may not be enough for good compression in later stages.

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

Page 8/24


9/20

Figure 1 -6 Multi-Stage Compressors

1.1.15 Labyrinth Seal

A labyrinth seal is one form of seal used in compressors. Labyrinth seals will notstop leakage completely but they do reduce it a lot. For this reason labyrinth sealsare more commonly used as seals between stages in a compressor. As shaft sealsthey are usually used together with another form of seal.

In a labyrinth sea[ several grooves are machined on the inner surface of the seal.The teeth between two adjacent grooves are machined to sharp knife edges and fitvery closely to the shaft. Sometimes matching grooves and teeth are machined onthe shaft to mesh with those on the seal.

A labyrinth seal works by changing the direction of flow of the gas many times as

the gas passes towards the outer edge of the seal. Each time the gas changesdirection the pressure drops a little. By the time the gas reaches the outer edge ofthe seal its pressure has been reduced almost to that of the pressure on theoutside of the seal.

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

Page 9/24


10/20

There is a possibility of the seal making contact with the shaft of the compressor.For this reason the seal is made of softer metal than that of the shaft. If contact ismade the softer metal of the seal will wear away instead of the expensive shaftbeing damaged.

Figure 1-7 Labyrinth Seal

1.1.16 Bypass

A bypass is an arrangement of pipes and valves which allows a gas or liquid to flowaround something instead of through it. Two bypass arrangements are shown inFigure 1-8. One bypass is round a compressor, the other is round a control valve.,

Figure 1-8 Bypass Arrangements

There are several reasons why bypass arrangements may be included in the designof a process plant.

It may be necessary to bypass equipment in the plant or whole sections of the

plant for the plant to be properly started up. This could include the need forreverse flow of the process fluid to pressurise the plant or to introduce purgegas to the plant.

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

Page 10/24


11/20

The equipment fitted with bypass arrangements may not need to be in serviceall the time. A typical example of this is the equipment used to introducemonitoring devices called pigs into pipelines.

A bypass arrangement may allow a continuous process to continue if theequipment fitted with the bypass arrangement fails.

A bypass will allow a piece of equipment to be removed from service formaintenance without the need to shut down the entire process.

1.1.17 Pressure Differential

A pressure differential is the difference between two pressures. It is often used toindicate that a machine is not running as well as it should be.

The pressure differential is usually the difference between the inlet pressure of amachine and the outlet pressure of the same machine.

An oil filter for example, has a higher inlet pressure than its outlet pressure. Whenthe filter is new, the pressure differential is small. As the filter does its job andcollects more and more dirt from the oil passing through it, the outlet pressure fallsso the pressure differential gets larger. When the pressure differential reaches acertain point it is time to clean the filter or replace it with a new one.

1.1.18 Thrust

Thrust occurs only in centrifugal compressors, not in reciprocating machines. Duringthe normal operation of a centrifugal compressor the discharge pressure is higher

than the suction pressure. This causes a force to act through the shaft andimpellers of the compressor in a discharge-to-suction direction. As the force actsthough the shaft, it is an axial force.

The most common way to reduce thrust in centrifugal compressors (to anacceptable level) is by using a balance piston. A balance piston does not stopthrust entirely. The remaining thrust force is handled by a thrust bearing.

1.1.19 Surge

Surge is a condition which affects only centrifugal compressors. For every speed ina centrifugal gas compressor there is a certain capacity below which the operation

of the gas compressor becomes unstable.

Surge is caused by a reduction in the volume of gas passing through thecompressor together with an increase in the compression ratio. The point at whichthis happens is called the surge point. As the compressor approaches the surgepoint the pressure differential between the suction and discharge pressuresincreases. The pressure differential reaches the point where the compressorcannot cope with it. The flow pattern of the gas through the impellers collapses.

The high pressure gas at the edge of each impeller flows back to the eye of theimpeller and builds up the suction pressure. This allows the compressor to startworking properly again. If the conditions do not change, the surge point will onceagain be reached and the series of events will be repeated. This cycle of eventshappens very, very quickly.

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

Page 11/24


12/20

Surge can cause serious damage to a compressor. The frequency of the surgedetermines the amount of damage caused. A large volume of gas in the compressordischarge will cause low frequency surge with high damage to the compressor. Asmall volume of gas in the compressor discharge will cause high frequency surgewith low damage to the compressor.

A gas compressor can be brought out of surge by any of the following:

reducing the speed of the compressor,

increasing the flow through the compressor,

reducing the compression ratio by using a recycle loop (kick back) to putdischarge gas into the suction.

Most large capacity compressors are fitted with an automatic anti-surge system. Themain feature of an anti-surge system is the recycle line. The recycle line allows gasfrom the discharge side of the compressor to be recycled to the suction side of thecompressor. The amount of gas being recycled must be controlled otherwise the

gas would just go round in a loop. The recycle line is fitted with a flow control valve.The flow control valve is controlled by signals from instruments on another part ofthe compressor. Surge control is fully described in Unit 3 of this course.

1.1.20 Recycle

To recycle is to send something back to where it came from. In gas compressors arecycle line is part of the surge control system. Gas from the discharge side of thecompressor is recycled to the suction side of the compressor.

1.1.21 Prime Movers

Prime movers are the machines which drive compressors. Prime movers can beelectric motors, steam turbines, gas turbines or internal combustion engines suchas a diesel engine.

The prime mover is separate from the compressor. The two machines are joinedtogether by a coupling.

1.1.22 Couplings

Couplings are used to connect the shaft of a compressor to the shaft of its primemover. The two shafts are lined up as closely as possible to each other, butuneven heating of materials and equipment and variations in loading can causesome misalignment. The coupling permits a slight misalignment of the two shaftswithout damage to either shaft.

The two most common types of coupling used on gas compressors are the gearcoupling and the flexible diaphragm coupling.

Gear couplings are commonly used in high speed, high load compressors. Theywork by allowing their parts to slide or otherwise move relative to one anotherwith no deliberate bending or flexing.

A set of gear teeth inside the coupling mesh with matching teeth on the shaft of

the compressor. At the other end of the coupling another set of gear teeth insidethe coupling mesh with matching teeth on the shaft of the prime mover. The setsof teeth are free to move-within each other.

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

Page 12/24


13/20

Small gear type couplings are often grease packed for lubrication purposes.Large, high speed gear couplings are more usually lubricated with a circulatedlubricating oil system.

Flexible diaphragm couplings are commonly used on compressors which havesteam or gas turbine prime movers because distortion due to heat is large.

The flexible diaphragm coupling transmits power from the prime mover to thecompressor using a solid, rigid hub on each machine. A flexible metal diaphragmis bolted to the outer edge of each hub and a spacer connects the twodiaphragms.

All misalignment is handled by the flexing of the thin webbed diaphragms. Asthere are no rubbing parts in the coupling, there is no need for the coupling to belubricated. Although this type of coupling can handle large angles ofmisalignment it can only accept small changes in backwards and forwards (axial)movement.

Figure 1-9 Shaft Couplings

1.2 WHY COMPRESSORS ARE USED

The pressure of a gas is important for transporting the gas and for processing orstoring it. Gas arriving at a processing plant, or already in the plant, may not be atthe correct pressure for what is to happen to it next. If the pressure of the gas is too

high it can be reduced easily. If the pressure of the gas is too low a compressormust be used to increase it.

The main reasons for increasing the pressure of the gas are as follows.

To transport the gas through a pipeline. The longer the pipeline, thehigher the gas pressure needs to be.

To make the pressure correct for processing.

To make the pressure correct for storage at high pressure.

To turn the gas into a liquid. The small compressor in a refrigerator does

this. When the liquid is allowed to turn back into a gas, this change causescooling.

ressors&Turbines

UnitNo.

1-Introductiont

oCompressors&Turbines

UnitNo.

1-Introd

uctiontoCompressors&Turbines

Page 13/24


14/20

To produce combustion or a chemical reaction.

To provide a source of energy, for example compressed air to driveair-driven tools or motors, or for use by instruments.

1.3 CLASSIFICATION OF COMPRESSORS

Figure 1 - 10 shows how compressors are classified. In this course onlyreciprocating and centrifugal compressors will be covered because these are themost common types of compressors found in process plants.

A reciprocating compressor is classified as a positive displacement machine. Thatis, a solid object displaces gas from inside the machine and in so doingcompresses the gas to increase its pressure. In a reciprocating compressor this isdone by one or more pistons moving in one or more cylinders.

If the piston compresses only in one direction of the piston (one stroke) thecompressor is called 'single acting'. If the piston compresses on both the forwardand back strokes the compressor is called 'double, acting'.

Centrifugal compressors compress gas by moving the gas and increasing its speedwithin the compressor. This means that one form of energy is added to the gas andthen that energy is changed to pressure energy. For this reason centrifugalcompressors are said to be dynamic machines.

Figure 1-10 Compressor Classification-

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

Page 14/24


15/20

1.4 PRIME MOVERS

In process plants the four main types of prime movers which are used are:

electric motors,

steam turbines,

gas turbines,

internal combustion engines.

The selection of a suitable prime mover depends on a number of factors.

Power requirements may be considered on their own or together with availablespace. For equal power ratings a turbine (steam or gas) will occupy less spacethan an internal combustion engine.

Cost is an important consideration. The initial cost to buy the machine must be

considered together with the operating and maintenance costs over the lifetime ofthe machine.

* The availability of fuel or steam and how easy it is to supply it to the prime Movermust be considered. If a process plant has a surplus of gas it would make sense touse gas turbine prime movers whenever possible. If the process required a supplyof low pressure steam then steam turbine prime movers could be used to reducethe pressure of steam from the steam generators of the plant.

Last, but not least, safety must be considered. If the prime mover must be located ina hazardous area where there is a possibility of an explosion, a steam turbinewould be the first choice.

1.4.1 Electric Motors

Electric motors used as prime movers may be either direct current (DC) oralternating current (AC) machines.

A DC motor would normally be used where the speed of the prime mover has to bechanged over a wide range of speeds. Unfortunately, DC motors are more complexthan AC machines and they are also very expensive when compared to most ACmotors. For these reasons AC motors are more common in process plants.

The most commonly used AC motor in process plants is called a polyphase

induction motor. It is often referred to as a squirrel cage motor because of thedesign of the rotor. This type of motor is strong and simple in construction. Thespeed of the motor depends on the frequency of the electrical supply. As thefrequency of the electrical supply is not easy to change, this type of motor isconsidered to be a constant speed motor.

If the speed of an AC motor needs to be controlled, a motor with a wound rotor mustbe used. This type of motor is more complex than a squirrel cage motor, so it isalso more expensive.

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

Page 15/24


16/20

Figure 1 -11 Typical Process Plant Electric Motor

1.4.2 Steam Turbines

Steam turbine prime movers are easily recognised by the heavily insulated steamsupply pipes which lead to them.

Steam turbines may be single stage or multi-stage machines. In a multi-stage steamturbine the number of stages is set by the number of rotor discs (or wheels) on theturbine shaft. For each rotor disc there is a matching set of stator discs which looksimilar to the rotors but are fixed in position. The purpose of the stator discs is todirect the steam on to the rotor discs on the shaft.

When one or more of the following conditions exist in a plant, a steam turbine primemover would be used (if the plant has a high pressure steam supply).

Low pressure steam is needed for plant processes.

Explosive atmospheres are present in the surrounding area.

Variable speed drives are needed.

High speed operation is needed.

The surrounding area is very hot or damp

1.4.3 Gas Turbines

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

Page 16/24


17/20

Figure 1-12 Gas Turbine

Gas turbines are very noisy machines. For this reason gas turbine prime movers areoften located inside an acoustic enclosure to reduce the noise level. In this case,

all an Operator will see is a building about the size of a small Portacabin. If anOperator needs to enter the acoustic enclosure ear protectors must be worn.

A gas turbine can be recognised (whether it has an acoustic enclosure or not) bythe large diameter, heavily insulated exhaust pipeline leading from the turbine. Gasturbine exhaust gas can have a temperature of 550C (1022F). This very hot gasmay be used for heating purposes somewhere else in the process plant.

It is important to note that within a gas turbine there is another air compressor.

This is also driven by the turbine and is used to compress air for use within theturbine. This compressor is actually part of the gas turbine and it should not beconfused with the centrifugal compressor for which the gas turbine is the primemover.

The events which occurcontinuously inside a gas turbine are as follows.

Compression.Atmospheric air is compressed in the compressor part of thegas turbine.

Combustion. Gas or liquid fuel is added to the compressed air and ignited.

Expansion. The combustion gases are expanded, creating energy. Theexpanding gases pass through the turbine part of the gas turbine. Theenergy of the gases is used to turn the rotor of the gas turbine.

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

Page 17/24


18/20

Exhaust. Exhaust gases are led away from the gas turbine.

Some gas turbines are called 'dual fuel'. This means that the turbine can run oneither gas fuel or a liquid fuel such as diesel oil.

1.4.4 Internal Combustion Engines

An internal combustion engine is an engine which burns fuel inside the engine.The fuel must be made to burn (to be ignited). Two methods are used to do this,a spark generated by electricity or the heat created by compressing air in thecylinder before the fuel is admitted.

These methods form two classifications for internal combustion engines, sparkignition and compression ignition. Engines using petrol (gasoline) are sparkignition engines. Engines using diesel oil are compression ignition engines.

In process plants diesel engines are preferred over petrol engines. The mainreason for this is safety. Petrol engines need electricity to generate the spark.Also, the fuel they use is much more volatile than the fuel used by dieselengines. Volatile means that the fuel is very easy to ignite and therefore an extradanger in a process plant.

Internal combustion engines are not very common in process plants. They areusually used to drive small sized machines or machines which need to beportable.

Internal combustion engines have two advantages over other prime movers. Theyare self contained and do not rely on a supply of electricity, gas or steam, and theyare very easy to start using several methods including hand cranking. For these

reasons diesel engines are often fitted to emergency equipment such as anemergency fire water pump.

1.5 SUCTION SCRUBBERS

Most gases contain small amounts of liquid. Often this liquid is in droplets so smallthat it cannot be seen by a human eye. During the process, two or more smalldroplets combine to form larger droplets. Two or more of the larger dropletscombine until there is definitely a liquid present.

Liquids do not compress and if liquid enters a compressor serious damage will be

caused. The final defense to remove all traces of liquid from a gas is the suctionscrubber. A suction scrubber is fitted into the compressor suction pipeline as near tothe compressor inlet as possible.

ModuleNo.

6:Co

mpressors&Turbines

Un

itNo.

1-IntroductiontoCo

mpressors&Turbines

Page 18/24


19/20

Figure 1-13 Suction Scrubbers

Page 19/24


20/20

There are many different designs of suction scrubbers although the way inwhich they work is similar, Figure 1-13A shows a very basic suctionscrubber. This type of suction scrubber has a simple design but it is veryefficient. It is common in process plants. Figure 1-13B shows a morecomplex design which also has filter elements.

Look at Figure 1-13A. Gas enters the scrubber at the mid point of the verticalwall of the vessel. Inside the scrubber, at the gas entry point, is an inletdiverter. The purpose of the inlet diverter is to help separate liquid dropletsfrom the gas.

Inside the vessel further separation of liquid from gas takes place. This iscaused by gravity. The liquid droplets are heavier than the gas so they willfall out of the gas stream. The droplets collect in the bottom of the vesseland are removed from there under level control.

The gas passes out of the top of the vessel through a mist extractor. The mistextractor removes any very fine particles of liquid which may still be in thegas. Although there are many different designs of mist extractor, one incommon use consists of a wire mesh cage filled with steel wool. The steelwool allows the gas to pass through it freely but traps any very smalldroplets of liquid. The very small droplets join together (coalesce) until theyare large enough to fall from the mist extractor to the bottom of the scrubber.

Now look at Figure 1-13B. This suction scrubber has filter elements built intoit and is divided into two sections, the gravity section and the filter section.

Gas enters the scrubber at the filter section and passes through the cartridgefilter elements. The filter elements have the advantage that they will removesolid material from the gas as well as liquids. Any solids will be trapped inthe filter elements. A lot of the liquid will be removed from the gas on theoutside of the filter elements and will collect in the bottom of the filter sectionof the scrubber. The liquid will be removed from the bottom of the filtersection of the scrubber under level control.

The gas passes from the filter section of the scrubber to the gravity section.The gravity section works in the same way as the simpler scrubber shown inFigure 1 -13A. There is an inlet diverter where the gas stream enters thegravity section and a mist extractor where it leaves it. Liquids removed fromthe gas collect in the base of the gravity section and are removed underlevel control.

Modu

leNo.

6:Compressors&T

urbines

UnitNo.

1-Introd

uctiontoCompressors&T

urbines

Documents

Intro to Compressor and Turbines