Petroleum Gas Compression Workbook 4.pdf

POL Petroleum Open Learning

Petroleum Gas Compression Part of the Petroleum Processing Technology Series

OPITO

4

THE OIL & GAS ACADEMY

Petroleum Gas Compression – Unit 4 - Other Types of Compressor

Petroleum Open Learning

(Part of the Petroleum Processing Technology Series)

Contents

















Page

•

Training Targets

















4.2

Visual Cues

•

Introduction

















4.3

training targets for you to achieve by the end of the unit

•

Section 1 – Other Types of Positive Displacement Compressor 4.4 Rotary Screw Compressors

test yourself questions to see how much you understand











Lobe Compressors Sliding Vane Compressors Liquid Ring Compressors

•

Section 2 – Other Types of Continuous Flow Compressor Axial Flow Compressors

4.11

•

Check Yourself - Answers

4.18







Mixed Flow Compressors Fluidic Compressors Ejectors













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

4.1

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

•

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

•

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

•

Describe the construction and operation of the axial and mixed flow of dynamic compressors

•

Describe the construction and operation of the ejector type fluidic compressor

•

Explain the principal uses of the compressors listed

4.2

Petroleum Gas Compression – Unit 4 - Other Types of Compressor

Introduction

In Units 2 and 3 of this compressor programme we concentrated on two types of compressor : • reciprocating • centrifugal These are the common types of compressor found in petroleum production operations.

Test Yourself 4.1 Test a typical compressor family tree, naming the types of compressor in each main branch.

However, they are by no means the only cornoressors in use. You will remember from Unit 1 that we classified compressors into a number of different categories and represented this as a family tree

Check your recall of Unit 1 now by trying the following Test Yourself question.

You will find the answer in Check Yourself 4.1 on page 4,18

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From the family tree you can see that we must consider a few more machines to complete our study of compressors. In this Unit, therefore, we will tie up the loose ends in the programme on compression by taking a brief look at these other types. I do not intend to go into nearly so much detail as I did in Units 2 and 3. In fact, we will limit ourselves to just two sections. In Section 1 we will look at the basic construction and operating principles of another type of positive displacement compressor, namely the rotary compressors. (In the family tree, we also included the diaphragm type of reciprocating machine. However, I do not intend to say anything further about this compressor.) Section 2 will concentrate on the other continuous flow machines. These are the axial and mixed flow dynamic and the fluidic types.

4.3

Petroleum Gas Compression – Unit 4 - Other Types of Compressor

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Section 1 - Other Types of Positive Dislacement Compressor You will remember from the family tree that the two branches of the positive displacement arm led to the reciprocating machines and the rotary machines. In this section we will be having a look at the rotary types. These include:

Take a look at Figures 1(a) and 1(b) which show, in simplified form, a rotary screw compressor

Figure 1(a) is a plan view and Figure 1(b) is a simplified end view of the machine.

• screw type • lobe type • vane type • liquid ring type Let’s now look at each of these in turn.

Rotary Screw Compressors The double rotary screw type of compressor is the most common form of rotary compressor used in the oil and gas industry. In this machine, two intermeshing screws are used to trap and compress a gas in the space between the screws. The double rotary screw compressor is a positive displacement machine which delivers a constant volume at variable pressures. It has a single stage compression ratio of approximately 4 to 1.

4.4

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

The incoming gas is trapped between the compressor casing and the tips of the lobes. As the lobes are fitted close together, no gas can escape backwards between the lobes themselves. The trapped mass of gas is then pushed forwards towards the delivery end of the compressor, with no reduction in volume.

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

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

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

Test Yourself 4.2

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

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

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

Figure 2 shows how a twin lobe compressor works and how the gas flows through the compressor.

The casing of the compressor encloses two rotors, each of which has a Figure of eight shape. At each end of the figure of eight is a lobe. These specially shaped rotors are geared together externally to ensure correct meshing. They rotate in opposite directions within the casing.

You will find the answer in Check Yourself 4.2 on page 4.19 4.5

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From the answer to this Test Yourself you can see that the back pressure in the delivery pipework determines the outlet pressure of this compressor. The machine we have just been looking at is a two lobe compressor. However, lobe compressors may be found with two, three or four lobes per rotor. Because of these low pressure applications often referred to as blowers. Lobe compressors can be found on petroleum croductlon installations as Fuel Gas compressors, or as Booster compressors and other lower pressure applications.

Sliding Vane Compressors Sliding vane compressors are designed to supply a constant volume and pressure of gas.

Figure 3 shows the basic construction of a sliding vane compressor. Familiarise yourself with the components, then go through the following description of how the machine works.

Within the casing of the compressor is mounted a rotor or drum. The drum is designed to rotate eccentrically within the casing. The drum carries a number of vanes. These are free to slide back and forth within slots machined into the rotor. As the drum rotates. the vanes are thrown out by centrifugal force so that they contact the casing. Sometimes spring loading assists in this. 4.6

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The sliding vane compressor operates by trapping the gas in a succession of pockets whose volume is gradually reduced. The pockets are formed by the inside of the casing, the outside of the drum and a pair of sliding vanes. Gas enters the casing through the inlet nozzle and is trapped in a pocket where the vanes are at their maximum extension. As the drum rotates the vanes are pushed back into the drum as the distance between casing and drum decreases. This then reduces the volume of the pocket. The pressure of the gas within the pocket increases until the volume of the pocket is at its minimum. At this point the gas is discharged through the delivery nozzle.

Liquid Ring Compressors The liquid ring compressor is more commonly called a vacuum pump, It is used extensively in injection water treatment plants, It is capable of creating a vacuum which assists in the de-oxygenation of water. The unit consists of a cast iron casing which contains a multi-bladed impeller. The impeller is mounted on a shaft supported at each end on bearings. The shaft is fitted eccentrically to the casing so that the blades of the impeller are nearer to the casing at one point of the rotation.

Sliding vane compressors are very prone to damage and breakdown if any liquids or solid particles enter the gas stream. The vane tips erode and gas can then leak backwards, past these tips. They are occasionally found in Heating, Ventilation and Air Conditioning (HVAC) Systems, In this service are used both as air blowers and as refrigerant compressors.

Figure 4, on the next page, shows diagramatically the construction of a liquid ring compressor.

4.7

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4.8

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The liquid ring compressor operates in a similar manner to the sliding vane type of compressor. The essential difference is the way in which the reduction in volume of the pockets is achieved. You will remember that this reduction in pocket volume is accomplished, in a sliding vane compressor, by the vanes being pushed back into the drum. The liquid ring compressor uses an advancing ring of water to create the same effect. Look at Figure 4 and see how this works. Water is injected into the unit and centrifugal force causes it to form a liquid ring which is pinned to the inside of the casing. Air is drawn a pocket formed between the impeller blades and the inner surface of the water ring. This occurs where the blades are furthest from the casing and the pockets have their greatest volume. As the impeller rotates, the blades get nearer to the casing wall. The ring of water advances into the pocket, and pocket volume is reduced. This increases the pressure of the air in the pocket The compressed air / water mixture is then ejected at the discharge port.

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

Summary of Section 1 In this section you have seen how a selection of rotary compressors work. These were: • screw type • lobe type • vane type • liquid ring type You saw the lobe type of compressor was the only one which did not trap gas and then reduce its volume. In fact the trapped gas in such a machine is pushed towards the discharge at an almost constant volume. We did not go into a great deal of detail regarding the construction of rotary compressors. However, Section 1 should have given you an idea of how these machines work and what their main applications are.

Before moving to Section 2, have a go at the following Test Yourself to check your understanding of Section 1.

4.9

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Test Yourself 4.3 Which type of positive dlsplacernent compressor do the following statements refer to? 1.

The male splines mesh with the female grooves.

2.

Springs assist in throwing the vanes outward

3.

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

4.

The piston moves back and forth within a cylinder

5.

The drum is mounted eccentrically within the casing.

6.

This compressor is more commonly called a vacuum pump.

7.

The two rotors have a figure of eight shape.

8.

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

You will find the answers in Check Yourself 4.3 on page 4.19 4.10

Petroleum Gas Compression – Unit 4 - Other Types of Compressor

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Section 2 - Other Types of Continous Flow Compressor The continuous flow branch of the family tree was divided into dynamic machines and fluidic machines. Unit 2 of the Compressor Series concentrated on one of the dynamic machines, namely the centrifugal compressor. In this section we will have a brief look at the other dynamic types of compressor, the axial flow and mixed flow compressors. Finally we will consider just one type of fluidic machine, that is the ejector. Once again we will not go into any great detail but will only cover the basic construction and operation of each unit.

Axial Flow Compressors Like centrifugal compressors, axial flow compressors are dynamic machines. They impart kinetic energy to the gas by increasing its velocity. This kinetic energy is then converted into pressure energy by allowing the gas to slow down.

In the case of an axial flow compressor a large number of blades, called rotor blades, are attached to an impeller. There may be as many as fifteen or more impellers attached to a shaft. The impellers and the shaft are contained within a casing. The rotor blades run between fixed blades, called stators, mounted inside the compressor casing. Figure 5 shows the component parts of an axial flow compressor

As the impellers rotate, the shaped rotor blades induce the gas to flow across them, parallel to the shaft. The rotor blades accelerate the gas towards the stator blades. The shape of these stationary blades causes them to act as diffusers, and slow down the gas. This converts kinetic energy into pressure energy. The stator blades then direct the gas towards the next set of rotor blades. and so on. The most common use for axial flow compressors in the oil and gas is as inlet air compressors on gas turbines. In this service they are ideally suited to provide the large amounts of low pressure air which are required to enable the engine to function.

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

In the case of an axial flow compressor, the flow of gas stays parallel (axial) to the shaft. The simplest type of axial flow compressor is the propeller. Air is drawn into the propeller from the front and thrown backwards through the propeller.

4.11

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4.12

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Mixed Flow Compressors Mixed Flow compressors utilise both centrifugal and axial flows within the same compressor casing, All centrifugal compressors have an element of mixed flow. In these machines, the gas flows axially along the shaft into the inlet of each impeller before being accelerated by centrifugal force. Mixed flow compressors utilise a form of axial flow blades called inlet guide vanes installed upstream of the first centrifugal impeller. These guide the gas into the eye of the first impeller in the most efficient direction. The inlet guide vane often has a variable pitch facility which allows the operator to control the direction of this inlet flow. Before we move on to have a look at Fluidioc compressors, try the following Test Yourself question about dynamic compressors of the axial and mixed flow types.

Test Yourself 4.4 List 5 errors which appear in the following sentences. Axial flow compressors are dynamic machines. They impart potential energy to the gas by increasing it’s velocity. This energy is to converted pressure energy as the gas is slowed down. The flow of gas stays parallel to the shaft. A number of stator blades are attached to the impeller. They rotate between fixed blades mounted inside the compressor casing. Rotor blades act as diffusers to slow down the gas after it has been accelerated. Axial compressors are often used as inlet air compressors on gas turbine installations. Mixed flow compressors utilise axial flow and reciprocating flow within the same machine. Compressors which are designated mixed flow utilise inlet guide vanes to guide the gas into the axial flow impeller,

You will find the answers in Check Yourself 4.4 on page 4.19

4.13

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Fluidic Compressors In Fluidic compressors, a carrier fluid is accelerated in order to transfer its energy to another fluid. Fluidic compressors have no pistons, valves, rotors or any other moving parts. They are relatively inefficient but are often favoured for use with corrosive materials or in inaccessible situations, Ejectors (also called Eductors) and Diffusion Pumps are the two most common types of fluidic compressor. However, because diffusion pumps are seldom found in petroleum producing operations, we will not be considering them in this Unit.

Ejectors Ejectors (or Eductors) are names used to describe equipment which operates to the Bernoulli Principle. Bernoulli was a scientist who discovered that there is a relationship between pressure and velocity of a fluid as it flows across a restriction. Take a look at the two diagrams in Figure 7. Figure 7(a) shows fluid flowing across a restriction. Figure 7(b) illustrates this relationship in the form of a graph.

As the fluid enters the the pressure falls and the velocity increases. This occurs as pressure energy is converted kinetic energy.

At the exit from the restriction, the reverse occurs, as you can see. 4.14

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Figure 8 shows a restriction built a of equipment called an ejector This restriction, similar to the one shown in Figure 7, is called a venturi tube.

A fluid, called the driving fluid, is pumped, at high velocity, through the venturi tube of the ejector via an inlet nozzle. The underside of the ejector is connected to the gas inlet. As the driving fluid enters the venturi tube, its velocity increases and its pressure falls. This reduction in pressure pulls gas from the gas inlet and entrains it in the driving fluid. This can create a vacuum at the gas inlet and, in fact this type of equipment can also be called a vacuum pump. As the mixture of driving fluid and gas leaves the venturi tube, its velocity falls again and the pressure increases to that of the discharge pipework. Ejectors have many applications in petroleum production. For example: • To increase the vacuum which is applied to the de-aeration tower of a water treatment facility (in conjunction with a liquid ring compressor) • To pull gas into a water stream in order to create a foam in water clean-up facilities

4.15

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Summary of Section 2 In this short section we have had a look at some of the other compressors which fall into the continuous flow category. These were: • axial flow compressors • mixed flow compressors • ejectors You saw that the first two were dynamic machines. They work in a similar manner to the centrifugal compressor which we considered in Unit 2 of this Compression Series The axial flow compressor uses rotor blades which act as impeller blades, and stator blades which act as diffusers. The flow of gas through such a machine is parallel to the shaft, hence the name axial flow. The ejector, however, works in a different way and has no moving parts. It relies on a driving fluid creating a low pressure zone. This type of equipment is often used in water injection and produced water applications. Further units in the Petroleum Processing Technology Series will explore these applications more fully. Now, before leaving the Unit, try the final Test Yourself question.

4.16

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Test Yourself 4.5 Decide to which type of compressor and which branch of the family tree (positive displacement or continuous flow) the following compressor components belong. I have done the first one as an example. 1.

piston rod



reciprocating compressor



positive displacement

2.

splined rotors

.............................................

.......................................

3.

inlet guide vane

.............................................

.......................................

4.

crosshead

.............................................

.......................................

5.

venturi tube

.............................................

.......................................

6.

water ring

.............................................

.......................................

7.

figure of 8 rotor

.............................................



.......................................

8.

drum and vanes

.............................................

.......................................

9.

dry gas seal

.............................................

.......................................

10.

stator blades

.............................................

.......................................

You will find the answers in Check Yourself 4.5 on page 4.20 4.17

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Check Yourself 4.1 Sketch a typical compressor tree, naming the types of compressor in each main branch. Your sketch will look something like the family tree shown opposite. (You will remember that you first saw this diagram in Unit 1 on Page 1.26).

4.18

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Check Yourself 4.2

Check Yourself 4.3

Check Yourself 4.4

With the piston and screw types, the compressor traps a mass of gas and reduces its volume, to achieve compression, The lobe type compressor simply pushes a fixed volume of gas into a higher pressure discharge.

1.

screw type

2.

vane type

1.

Kinetic energy is imparted not potential energy

3.

lobe type

2.

Rotor blades not stator blades are attached to the impeller

4.

reciprocating type

5.

vane type

3.

Stator blades act as diffusers, not rotor blades

6.

liquid ring type

4.

Mixed flow compressors use axial flow and centifrugal flow, not axial and reciprocating.

7.

lobe type

8.

screw type

5.

Inlet guide vanes guide the gas into the first centrifugal impeller not the axial flow impeller.

4.19

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Check Yourself 4.5 1.

piston rod

reciprocating compressor

positive displacement

2.

splined rotors

screw type

positive displacement

3.

inlet guide vane

mixed flow

continuous flow

4.

crosshead

reciprocating

positive displacement

5.

venturi tube

ejector

continuous flow

6.

water ring

liquid ring

positive displacement

7.

figure of 8 rotor

lobe type



positive displacement

8.

drum and vanes

sliding vane

positive displacement

9.

dry gas seal





centrifugal







continuous flow

1O.

stator blades



axial flow







continuous flow

























4.20