Petroleum Gas Compression Workbook 3.pdf

POL Petroleum Open Learning

Petroleum Gas Compression Part of the Petroleum Processing Technology Series

OPITO

3

THE OIL & GAS ACADEMY

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Petroleum Gas Compression - Unit 3 - Centrifugal Compressors (Part of the Petroleum Processing Technology Series)

Contents

Page

•

Training Targets

3.2

•

Introduction

3.3

•

Section 1 – Basic Theory

3.4

•

Section 2 – Design and Construction

3.11













Operating Principles of a Centrifugal Compressor

Compressor Casing Rotating Assembly Bearings Diffusers and Diaphragms Compressor Seals

•

Section 3 – Auxiliary Systems









Seal Oil System Lubrication System Cooling System Drivers and Couplings

•

Section 4 – The Operation of Centrifugal Compressors







•



Performance Characteristics Operating Problems, Alarms and Shutdowns The Main Operational Checks on a centrifugal Compressor

Check Yourself – Answers

3.32

3.42

3.55

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

test yourself questions to see how much you understand

check yourself answers to let you see if you have been thinking along the right lines

activities for you to apply your new knowledge

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

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Training Targets When you have completed Unit 3 of the Petroleum Gas Compression series you will be able to:

• Explain the basic operating principles of a centrifugal compressor.

• Describe the construction a centrifugal compressor.

• Explain the function and operation of the principal components of a centrifugal compressor.

• Describe the layout and operation of the auxiliary systems associated with a centrifugal compressor.

• Explain a basic centrifugal compressor alarm and shutdown system.

• List the common operating checks carried out on a centrifugal compressor.

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Petroleum Gas Compression - Unit 3 - Centrifugal Compressors

Introduction

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In Unit 1 of this Compressor programme, you saw that a centrifugal compressor is a member of the dynamic branch of the continuous flow family of compressors. Dynamic compressors increase the pressure of gas in a different way to that of the positive displacement machines. A reciprocating compressor reduces the volume of a trapped mass of gas to increase its pressure. A dynamic machine, however, uses the principle of energy conservation to achieve pressure increase.

There are two main types of dynamic compressor, and these are : • centrifugal (radial flow) compressors • axial flow compressors

The Unit is divided into four sections. Section 1 covers the basic operating theory of a centrifugal compressor.

We are only going to look at centrifugal compressors in this unit. However, in Unit In Section 2, we will look at the design and construction of a typical machine. 4 of the compressor programme, we will look at axial flow machines in a little more detail. In Section 3, we will concentrate on the auxiliary equipment Centrifugal compressors are large capacity, continuous flow machines with a very smooth output. When run at their optimum speed and loading, they are vibration and free and have few moving parts. Section 4 looks at centrifugal compressor performance and operations. They are capable of delivering very large volumes of gas. In days gone by, they tended to be used for lower pressure applications than reciprocating compressors. However, modern machines are capable of delivering gas at pressures in excess of 700 bar.

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Petroleum Gas Compression - Unit 3 - Centrifugal Compressors

Section 1 - Basic Theory Operating Principles of a Centrifugal Compressor In the Introduction I said that a dynamic machine uses the principle of energy conservation to increase the pressure of a gas. This means that one type of energy is converted to another. In this case it is the energy a gas has due to its velocity which is converted into pressure energy. The compression of gas in a centrifugal compressor is a two part process. • In part one, gas enters the machine and it is speeded up or accelerated.This increases the energy of the gas by giving it kinetic energy or energy of motion. A certain amount of pressure energy is also added at this time. • In part two, the gas is rapidly decelerated. This converts kinetic energy into more pressure energy.

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The compressor consists of an impeller mounted on a shaft which can be rotated at high speeds. This assembly is enclosed in a casing which incorporates a diffuser. Also built into the casing are the suction and discharge ports, i.e. the inlet and outlet of the compressor. We will be looking at the components of a compressor in much more detail in Section 2. For the moment we will concentrate on this rather simple machine. Look again at Figure 1 and follow the flow of gas through the compressor from the inlet or suction port to the outlet or discharge port. Gas enters the compressor through the suction port and is directed to the inlet or eye of the impeller. As the impeller rotates, the gas is forced to rotate with it, causing the following effects : Centrifugal force causes the gas to flow from the eye to the outside or rim of the impeller. As the rim of the impeller is travelling faster than the eye, the gas speeds up as it moves outwards.

Look at Figure 1 which shows a simple centrifugal compressor with its main components.

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Figure 2 shows the effect in graphical form.

Again, this effect is shown graphically in Figure 3.

The impeller is fitted with blades which act rather like airplane wings. These create a lift force which helps to force the gas from the low pressure at the eye to the higher pressure at the rim.

These two forces, the centrifugal force and the lift force, accelerate the gas and raise the pressure. This is the first part of the process. 3.

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When the gas leaves the impeller at the rim, it enters the diffuser. This part of the compressor is a flow channel. It is designed in such a way that the flow area is gradually increased along its length.

As the gas flows through the diffuser, the increase in flow area causes the gas to slow down. This reduction in velocity means that the gas loses its kinetic energy. Figure 5 shows this reduction in velocity.

The diffuser shown in Figure 1 is called a volute diffuser. A volute is an increasing spiral shape, as illustrated in Figure 4. You will recognise the shape if you look at shellfish such as whelks or winkles.

Because the total energy of the gas cannot be reduced, the fall in kinetic energy must be compensated for by a rise in some other type of energy. In this case the pressure energy of the gas is increased. 3.

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The next Figure, 6, shows, in graphical form how the pressure of the gas increases as it flows through the diffuser.

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So, you have now seen how the two parts of the process produce an overall pressure increase from the inlet to the outlet of the compressor. Figure 7 shows the overall process from inlet to outlet, again in graphical form.

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The compressor we have just been looking at is called a single stage compressor. It has one impeller and one diffuser. It will have a capability of increasing the pressure of gas by a fixed amount. You will remember from Unit 1 of the compressor series, that this increase is known as the compression ratio. To remind yourself of compression ratio, have a go at the following simple Test Yourself question.

Test Yourself 3.1 a) If a compressor takes in gas at a pressure of 25 psia and delivers it at a discharge pressure of 90 psia, what is its compression ratio? b) If a compressor has the same compression ratio as the one in part (a) and it takes in gas at 30 psia, what is its discharge pressure?

You will find the answers in Check Yourself 3.1 on page 3.55

The compression ratio which can be achieved largely depends on the centrifugal force which is developed by the impeller. This in turn depends on three things: • speed of rotation • impeller shape • impeller diameter There is a practical limit to some of these. For instance, the diameter of the impeller and its speed of rotation will determine how fast the rim of the impeller is travelling. This cannot be allowed to exceed the speed of sound in the gas it is compressing. If it does so, serious damage to the machine may result as the sound barrier is broken. In order to achieve greater pressure increases, multiple impellers and diffusers can be fitted inside one casing. Each impeller and diffuser unit is called a stage and such a machine is called a multi-stage compressor. In a multi-stage compressor, the gas passes through an impeller, then a diffuser, is then directed to the eye of the next impeller, and so on.

Now, before moving on to the next section, have a go at the following Test Yourself question.

Test Yourself 3.2 Of the following 10 items, only 5 belong to a centrifugal compressor. Indicate with a tick in the box provided which items are centrifugal compressor components. Impeller Diffuser Piston Rod Cylinder Shaft Crank Casing Blades Cross Head Clearance Pocket

Each stage is, in effect, a compressor in its own right. In the case of a five stage compressor there are actually five compressors within one casing. You will find the answers in Check Yourself 3.2 on page 3.55 3.

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Summary of Section 1 In this Section on the basic theory of centrifugal compressors, you saw that the principle of energy conservation is used to raise the pressure of gas. You saw that, within the compressor, the gas is first accelerated to give it kinetic energy and some pressure energy. It is then slowed down and the kinetic energy is converted into more pressure energy. We looked at a simple compressor made up of the following parts: • impeller • shaft • diffuser • casing • inlet port • outlet port It was pointed out that a single impeller has restrictions which can limit the amount of pressure increase it can produce. In order to achieve greater pressure increases, multi-stage compressors should be used. In the next Section we will look in more detail at the components of a typical multi-stage centrifugal compressor.

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Petroleum Gas Compression - Unit 3 - Centrifugal Compressors

Section 2 - Design and Construction

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In this Section we are going to look at the component parts of a centrifugal compressor. We will see how they are constructed and what their function is in the operation of the machine. I have listed below the components which we will consider. These are: • casing • rotating assembly - consisting of





shaft







impeller







balance piston

• bearings • diffusers and diaphragms • seals

Before we proceed, take a look at Figure 8 overleaf, and identify these components. 3.11

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Of course, a compressor has to be powered by something. The motor which powers the compressor is called the main driver or simply, the driver. In this Section we will not concern ourselves with the construction or operation of the driver. However, centrifugal compressors are orientated, or lined up, relative to the main driver, so it is worth identifying the orientation at this point. The end of the compressor nearest to the driver is called the drive end or inboard end. The end of the compressor furthest from the driver is called the non-drive end or outboard end. Let’s move on to the components now.

Compressor Casing The compressor casing is used to house the component parts of the compressor. It may be either horizontally split or vertically split, depending upon the design and application.

Horizontally Split Casings Figure 9 shows a compressor with a horizontally split casing. The casing is made in two halves which are then bolted together along a horizontal join.

The internal assembly of the compressor may be removed only after the two halves of the compressor casing have been unbolted and lifted apart.

Horizontally split compressors are mainly used in lower pressure service. 3.13

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Vertically Split Casings A vertically split casing consists of a barrel shape with end caps bolted onto each end of the barrel. The internal assembly of the compressor may be withdrawn from the non-drive end of the casing after the end plate has been removed. Vertically split casing compressors are often called barrel compressors. They are mainly used in high to medium pressure service. Figure 10 shows a vertically split compressor casing.

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Rotating Assembly You will remember that this assembly consists of a shaft, impeller(s) and a balancing piston. Let’s take a look at each of these components in turn.

The Shaft The compressor shaft is the heart of the centrifugal compressor. It carries the impellers and balance piston and they all rotate together within the compressor casing. Figure 11 indicates the basic shape of the compressor shaft and the relative positions, on the shaft, of the various components.

To minimise vibration, the rotating assembly is finely balanced and not allowed to run near any critical speeds. Every rotating assembly has a number of speeds, where it will reach a peak of vibration. These speeds are called the critical speeds. Critical speeds can be calculated as the compressor is designed and built. The normal running speed of a compressor is set to avoid being near a critical speed. If any rotating equipment is run at a critical speed, damage can be caused to bearings within seconds as the vibration causes metal to metal contact. It is normal practice to go through critical speeds as quickly as possible when accelerating the compressor to its normal running speed. 3.15

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The Impeller

The impellers provide the driving force for the gas as it flows through the compressor. They are fixed to the compressor shaft and rotate within the compressor casing. The impellers impart kinetic energy to the gas by increasing its velocity within the compressor casing. Impellers are available in a variety of designs such as open, semi-open or closed, and the different types are shown in Figure 12.

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The open type impeller consists of a number of blades attached to a shaft. It rotates within the casing of the compressor and the gas is constrained between the open impeller blades and the casing wall. The semi-open impeller, in addition to the blades, has a disc or hub to which the blades are attached. The hub stiffens the assembly, and helps confine the gas to the blade area. The closed type of impeller is the most common in large compressor applications, and this is the one we will concentrate on. It has blades of course, and also a hub. At the front of the impeller, however, the blades are attached to a cover or shroud.

You will remember from Section 1, that the impeller cannot be rotated at too great a speed - a speed which would result in the outer circumference of the impeller exceeding the speed of sound in the gas it is compressing. This is due to the possibility of damage being caused as the sound barrier is broken and turbulence is created within the casing. Remember also that, to overcome this restriction, a multi-stage compressor may be used to achieve a given compression ratio. To illustrate this further, if each stage has a compression ratio of 1.35 then a five stage compressor would have an overall compression ratio of 6.75.

The Balance Piston

The shroud also confines the gas to the blade area and provides stiffening.

The last component of the rotating assembly is the balance piston.

Closed impellers are made of forged steel. The blades may be welded to both the disc and the shroud. Alternatively, the blades may be machined from a solid disc and then welded to the shroud.

In a compressor the pressure at the inlet is obviously less than the pressure at the outlet. This difference in pressure across a compressor acts on the impellers and shaft to create a thrust force. The force tends to push the rotating assembly towards the inlet, or suction end, of the compressor. A force pushing along the line of the shaft is called an axial force.

Figure 13 shows a closed impeller.

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In Figure 14, we can see how the thrust force which is generated will be imposed upon a single impeller.

If nothing was done about this, severe damage could be done to the machine as the rotating assembly tried to move axially within the casing. One way to minimise rotating assembly movement is to install thrust bearings in the compressor. We will be looking at how these bearings work shortly. On a large compressor, the amount of thrust force developed is extremely high. If a thrust bearing was the only device installed to prevent thrust forces from damaging the compressor, then the bearing would probably be bigger than the compressor itself. To reduce the effects of the thrust forces and hence reduce the thrust bearing requirement, a balancing piston is fitted to the shaft. It is designed to reduce the thrust forces to an acceptable level. Figure 15 shows a typical balancing (or balance) piston assembly which is fitted to the high pressure end of the rotating assembly.

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The balance piston is a solid drum which is attached to, and rotates with the shaft. The high pressure gas at the discharge of the compressor, acts on the inboard side of the drum. Low pressure gas from the suction side of the compressor is fed to the outboard side of the drum. The pressure differential across the drum is maintained by having seals between the drum and the casing. Figure 16 shows how the low pressure gas from the suction side is fed to the outboard side of the drum via a balancing line.

The pressure differential across the drum produces an axial force which opposes the thrust exerted by the unbalanced forces acting on the impellers. 3.19

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Bearings The rotating assembly of a compressor needs to be supported within the casing and allowed to rotate freely. This means that some form of bearing is required. The bearings used for this application are usually referred to as the main bearings or journal bearings. Also, as you have already seen, a thrust bearing is required, but let’s look at the journal bearings first. Look back to the drawing of the shaft. It is Figure 11 on Page 3.15. You will see that there are just two locations for journal bearings. They are at the extreme outboard and inboard ends of the shaft. The bearings can be of several different types, but the one I will describe is known as a tilting pad journal bearing. It is a development of a simple sleeve type journal bearing which is shown in Figure 17.

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The sleeve type journal bearing consists of a simple housing forming a sleeve around the shaft. As the shaft rotates, it causes the film of oil to form a wedge which holds the shaft and housing apart.

The tilting pad type bearing is a development of this. It has a number of pads which are located on fixed pivots attached to the stationary housing. Figure 18 shows the arrangement of the pads in the bearing. As the shaft rotates, the film of oil again forms a wedge between shaft and pad. In addition, the tilting pads give the bearing self aligning properties. These compensate for any slight misalignment of shaft and bearing. They also tend to distribute evenly the loads which are created when the rotating assembly is spinning. And now, let us take a look at thrust bearings.

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You have already seen how the balancing drum helps to reduce the thrust forces to acceptable levels. Any residual thrust, however, must be taken up by a thrust bearing. A typical thrust bearing is shown in Figure 19 and is called a pivoting pad thrust bearing.

The bearing has a collar which is fitted to and rotates with the shaft. Located in recesses machined into a fixed or non-rotating thrust surface, are a number of metal shoes or pads. The collar rotates against the pads which are free to pivot. An oil wedge forms between the collar and the pads, as in the tilting pad journal bearing. Figure 19 shows a thrust bearing which is capable of taking up thrust in one direction only. Most thrust bearings, however, can take up thrust axially in both directions. This requires two fixed or stationary shaft thrust surfaces containing two sets of pads.

Diffusers and Diaphragms From Figure 8 we can see that the diaphragms and diffusers are non-rotating parts of the compressor. From Section 1, you will remember that gas leaving the impeller at the rim enters a flow channel called the diffuser. In the simple, single stage compressor which I used as an example, the diffuser was in the shape of a volute. In a multistage compressor, having a volute after each stage would be unwieldy. Therefore, in this case, the flow channels are formed by having diaphragms, which form part of the casing, and separate the stages. The adjacent walls of individual diaphragms form a diffuser passage.

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Figure 20 demonstrates the layout of the diaphragms and diffusers in a horizontally split. multi-stage centrifugal compressor.

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The diaphragms also guide the gas through return passages to the eye of the next impeller. Although not shown in Figure 20, the diaphragms also carry labyrinth seals which prevent the back flow of gas along the shaft from the high to the low pressure sides of the impellers. We will now have a closer look at seals.

Compressor Seals Compressor seals can be divided into: • internal seals, which are designed to prevent the movement of gas within the compressor casing • external seals, which are designed to prevent the escape of gas from the compressor casing to the atmosphere.

Internal Seals Let us consider internal seals first. Labyrinth seals are the most common form of internal seal. They consist of a series of teeth, across which the gas would have to flow, in order to escape from a high pressure area into a low pressure area. Figure 21 shows a labyrinth seal and how it works.

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In a labyrinth seal the teeth are most commonly machined into sleeves which are attached to the casing or diaphragm, and are a very close fit with the revolving shaft. They operate by maintaining a minimum gap between the shaft and the seal, and creating turbulence in the gas as it passes across each seal surface. This turbulence converts pressure energy into heat energy, and pressure is therefore lost across each stage of the labyrinth. The teeth of the labyrinth are machined into soft alloys of aluminium or lead. In order to minimise the gap between shaft and seal they are often allowed to rub against the rotating element when being bedded in. The number of teeth on the labyrinth will vary from as few as two to well over 20. The number used will depend upon: • the type of gas being compressed • the level of sealing required • the differential pressure across the seal

The type of seal selected depends upon the job it has to do. However, because of the imperfect sealing nature of a labyrinth seal, it is never used as the external shaft seal when compressing flammable or dangerous gases. Typical uses of labyrinth seals in a centrifugal compressor are : • as interstage seals to prevent the flow of high pressure gas from the tip of the impeller to its eye

The most common devices used for external sealing are liquid film seals. Liquid film seals are commonly used as external seals on high pressure, heavy duty compressors as they provide complete sealing capabilities. There are, however, other types of seal in use. You will come across carbon ring seals, mechanical contact seals and increasingly, dry gas seals. In this Unit, we will concern ourselves only with the liquid film seal and the dry gas seal.

Figure 22 on the next page shows a liquid film seal • to maintain the differential pressure across the and the way in which it works. balance piston • to control the escape of compressor gas into the sealing system of an oil film seal • to control the loss of lubricating or seal oil along a shaft

External Seals Now let us look at external seals. When flammable or dangerous gases are being compressed it is important that the gases do not escape from the compressor. To prevent this from occurring, the gap between the compressor shaft and the compressor casing, at each end, is sealed.

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Liquid Film Seals The liquid film seal uses oil as the sealing medium. Oil is pumped between the shaft and two tight fitting sleeves which are anchored to the compressor casing. The two sleeves form an inboard sealing element and an outboard sealing element. The oil flowing across the outboard sealing element does not come into direct contact with the compressed gas and is returned directly to a seal oil reservoir. The oil flowing across the inboard sealing element comes into contact with the compressed gas. Some of the gas may dissolve in the oil, and the oil is therefore routed to a de-gassing system before being returned to the seal oil reservoir. The liquid film seal has no touching parts in its assembly. It is therefore not prone to wear, and is very reliable.

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Dry Gas Seals The dry gas seal is a recent development which is becoming increasingly popular as an external compressor seal. Figure 23 illustrates the construction and operation of this type of seal. The seal consists of : • a rotating tungsten ring, attached to the compressor shaft • a static carbon ring, attached to the casing The static carbon ring is pushed towards the rotating tungsten ring by a set of coiled springs. When the compressor is at rest, the two faces touch to form a gas tight seal. Each contact face is machined to a high degree of flatness so that this sealing effect can be achieved.

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A series of spiral grooves are cut into the face of the tungsten ring, as shown in Figure 24. When the shaft begins to rotate, gas is pulled into these grooves, and forced towards the centre of the ring. A tiny area of high pressure gas is created at the inside end of each groove, called a sealing dam.

Pressure of gas in the sealing dams forces the carbon ring away from the tungsten ring, against the coiled springs. At this point, the two surfaces are no longer in contact and, therefore, no frictional heat is being generated. This means that no cooling is required under normal running conditions.

Now have a go at the following Test Yourself question.

If we were handling a non-flammable gas, some of the gas being compressed would be used to create the seal. The small amount of leakage through the seal could then be vented. Figure 23, however, shows the seal arrangement used for flammable or toxic gases. A labyrinth seal is mounted between the gas being compressed and the dry gas seal. Nitrogen is injected into this space at a pressure slightly higher than the gas being compressed. Some nitrogen may leak back into the compressor, but no toxic or flammable gas can escape across the dry gas seal to atmosphere. When used in this way, the nitrogen is called a buffer gas. Dry gas seals give a reliable level of sealing without the ancillary equipment associated with liquid film seals- tanks, pumps, filters, de-gassers, and so on. Apart from cost savings, the weight of this equipment could be as high as seven or eight tons. This will be a very important factor on an offshore production facility, and the use of dry gas seals in this environment is expected to increase.

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Test Yourself 3.3 Read through the following statements and fill in the missing words from the list given below: 1.

In a . . . . . . . . . . . . . . . . . . . .. split casing, the casing is made in two halves which are bolted together.

2.

At a rotating assembly’s............................... speed it will reach a peak of vibration.

3.

In a closed impeller the blades are attached to both the................... and the.........................

4.

The thrust force acting on the rotating assembly is taken up by two items. They are the ..................... and the ...................................

5.

The ........................guide the gas through the return passages to the eye of the next impeller.

6.

The most common internal seal is the............................seal.



LIST OF WORDS



HUB, DIFFUSERS, CRITICAL, SHROUD, THRUST BEARING, LABYRINTH, BALANCING PISTON, HORIZONTALLY.

You will find the answers in Check Yourself 3.3 on page 3.55

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Summary of Section 2 In the Section we have just worked through, we looked at the component parts of a centrifugal compressor. These items are : • casing • rotating assembly (shaft - impeller-balance piston) • bearings • diffusers and diaphragms • seals You saw that the casing can be horizontally split or vertically split and I illustrated the difference between the two.

From there we moved on to look at the rotating assembly which consists of a shaft, one or more impellers and a balancing piston. The impellers are mounted on the shaft and rotate with it. You saw that there are a number of basic types of impeller open, semi-closed and closed. The balance piston is also mounted on the shaft and its function is to reduce the thrust forces which can act on a centrifugal compressor. I pointed out that it does this by using a pressure difference across it to counteract the main thrust. Bearings were the next thing we considered. We looked at journal bearings and thrust bearings. Journal bearings are used to support the rotor and allow it to rotate freely. Thrust bearings are used to prevent movement of the shaft in an axial direction. In both cases the tilting or pivoting pad type bearing was illustrated.

Next we looked at diaphragms and diffusers. These are the non-rotating parts of the compressor which reduce the velocity of the gas leaving the impeller, thus raising its pressure. They also guide the gas from the outlet of one impeller to the inlet eye of the next. Finally we considered seals. You saw that they are either internal or external seals. The labyrinth types are commonly used as internal seals. Liquid film seals are most often used for external sealing purposes, although dry gas seals are becoming increasingly popular. Both types were considered in detail. You should be aware, however, that other designs maybe used as external seals.

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Before moving on to Section 3, where we will be looking at auxiliary systems, take some time to try the following Test Yourself.

Test Yourself 3.4 State whether the following components are part of :

1.

Inboard element

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

a)

the rotating assembly

2.

Labyrinth teeth

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

b)

the sealing system

3.

Impeller rim



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

c)

the casing and non-rotating assembly

4.

Diffuser



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

5.

Thrust collar



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

6.

Diaphragm



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

7.

De-gasser



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

8.

Journal bearing

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

9.

Balancing piston

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

10.

Inlet port

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



You will find the answers in Check Yourself 3.4 on page 3.56 3.31

Petroleum Gas Compression - Unit 3 - Centrifugal Compressors

Section 3 - Auxiliary Systems In this, the third section of the Unit, we will be looking at the auxiliary systems associated with centrifugal compressors. These are: • seal oil system • lubrication system • cooling system • driver and coupling

Seal Oil System

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Figure 25 also shows how the oil flows through the seals. Note how part of the oil in each seal flows outwards past the outboard sealing element. This oil does not come into contact with gas and, therefore, can flow directly back to the seal oil reservoir. The oil which flows inwards past the inboard sealing element becomes contaminated with gas. In order that this oil can be used again it must be cleaned. It is therefore routed to a de-gassing system before returning to the reservoir. (We will talk about the reference line very shortly).

Let’s start with the seal oil system. As we saw in the last Section, high pressure, heavy duty compressors are often fitted with liquid film seals. These are designed to prevent any gas from leaking to the atmosphere from the shaft ends. Look at Figure 25. This is a simplified view of a compressor which shows the liquid film seals at each end of the shaft. Each seal will be the same type as the one illustrated in Figure 22 on Page 3.26.

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The system which supplies the oil to the seals consists of the following items. • a seal oil reservoir • one main and one auxiliary seal oil supply pump • filters • coolers • a header tank • seal oil traps (de-gassing system) Take a look at Figure 26 which shows the seal oil system in simplified form.

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The oil is pumped from the reservoir via the filters and coolers to the header tank. A pressure reference line takes gas from the seals and feeds it to the header tank. Variations in this gas pressure at the seals are, therefore, mirrored in the tank. However, the oil pressure at the seal must always be greater than the gas pressure there, otherwise gas will escape. This is achieved by always positioning the header tank above the compressor, and maintaining the seal oil at a fixed level in the tank with a level controller. The seal oil pressure, therefore, will always exceed the gas pressure at the seal by an amount equal to the static head of oil. From the header tank the oil flows to the seal. As you have just seen, some of the oil then flows outwards between the shaft and the outboard sealing element to the reservoir. The rest flows across the inboard sealing element, to the reservoir via the de-gasser Follow carefully the layout of the seal oil system in Figure 26 and ensure that you are familiar with the components and the method of operation. As you look at Figure 26, remember the comment I made on Page 28 about the weight of a liquid film seal system - that it could easily reach several tons. It will be clear why lighter systems, such as dry gas seals, often prove more popular in an offshore environment.

Lubrication System We can now move on to the lubrication system. All rotating machinery must have a lubrication system of some form or another. Compressors are no exception. The lubricant, in this case oil, performs the following functions: • separates moving parts • removes heat generated by friction • reduces metal wear • protects metal surfaces from corrosion

consider the two as separate systems. A typical lubrication system would consist of the following components : • an oil reservoir • a main and auxiliary pump • coolers • filters The system layout is shown in Figure 27, overleaf. Take a look at this now and identify the components.

The lubrication system normally provides a flow of oil to the journal and thrust bearings of the compressor. In addition, it provides lubrication for the main driver, gear box and other accessories. Most lubricating oils, usually abbreviated to lube oils, are refined from crude oil. However, to give them their special properties, chemical additives may be mixed with them. Each compressor will have a lube oil which is specified for that particular machine. Care must be taken to ensure that no other lubricant is used, in order to prevent the possibility of damage to the compressor. In some compressors, the lubrication system may be combined with the seal oil system which we have just been looking at. Here, however, we will

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The lube oil is pumped from the reservoir through coolers which reduce the temperature of the oil to its correct operating value. From there it passes through filters which remove any dirt particles. The cooled, clean oil then flows under pressure to each bearing through pipework, grooves and channels. After performing its lubricating job, the oil flows back to the reservoir under gravity. The system pressure is maintained by having a pressure controller in the line between the filters and the compressor. Note that a header tank is incorporated into the system. Under normal running conditions the tank is kept topped up with oil from the main feed line. A small amount of oil will overflow back to the reservoir. If the main lube oil pumps fail, the compressor will shut down. When this happens, the bearings are lubricated using a gravity feed from the header tank. This ensures that there will be no damage caused to unlubricated bearings during the time that the compressor is rolling to a halt. Before moving on to the next part of Section 3, in which we will look at cooling systems, try Test Yourself 3.5.

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Test Yourself 3.5 Indicate if the following statements are true or false. If false give the correct answer. a)

The external seals of a high pressure compressor are usually labyrinth seals.

b)

In a seal oil system a pressure reference line takes gas from the seals and feeds it to a header tank.

c)

In a seal oil system the oil which comes in contact with the gas is routed to the reservoirs via the filters.

d)

Lubricating oil helps to remove heat generated by friction.

e)

In a typical lube oil system the filters remove dirt particles from the oil.

You will find the answers in Check Yourself 3.5 on page 3.56

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Cooling System Compression generates heat. It is often necessary to cool the compressed gas for safe and efficient operation of the plant. In many installations, there may be a number of compressors working in series. If the hot, compressed gas from the first compressor were passed directly to the suction of the next machine, overheating and damage could occur. In such a situation, an aftercooler may be installed downstream of each compressor. Figure 28 shows a typical aftercooler for a centrifugal compressor, and its position in the system.

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Large compressors will consist of two or more sets of impellers mounted on a common shaft. In order to reduce the temperature increase within such a machine, the gas may be discharged after the first set of impellers, cooled, and directed to the suction of the next set. In this case, the gas passes through an intercooler as it flows between the two sets of impellers. This arrangement is shown in Figure 29.

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Drivers and Couplings In the final part of this Section, I want to have a brief look at drivers and couplings. Centrifugal compressors used in oil and gas field operations are normally driven by gas turbines or electric motors. These are connected to the compressor by means of drive shafts and couplings. Some drivers, which rotate at high speeds, may be coupled directly to the compressor. However, it is often necessary to incorporate some kind of speed increasing gearbox between driver and compressor. Couplings are used to connect two shafts together and transmit the rotary motion of one to the other. Great care is taken to align the driver shaft to the compressor shaft. However, due to variations in loading and possible uneven heating of materials and equipment, small misalignment conditions can occur. Because of this a flexible coupling is required. You should not misunderstand the word ‘flexible’. A compressor coupling may be over 10 inches in diameter, and appear to be far from ‘flexible’ when handled.

In some instances, it may be necessary for the compressor to run at a variable speed, even though the driver may be a constant speed machine such as an electric motor. In these instances, a variable speed fluid coupling may be fitted between the main driver and the gearbox to permit this. In this short piece on drivers and couplings I have not tried to describe the components in detail. However, it is important that you know the terminology associated with them.

Before I summarise this Section, attempt Test Yourself 3.6.

Test Yourself 3.6 In the following list of components, some belong to the compressor auxiliary systems, some do not. If not, to which system do they belong? 1.

Impeller

2.

Lube oil cooler

3.

Coupling

4.

Driver

5.

Balancing piston

6.

Diaphragm

7.

Intercooler

8.

De-gasser

9.

Reservoir

10.

Thrust collar

You will find the answers in Check Yourself 3.6 on page 3.56 3.40

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Summary of Section 3 In this Section on auxiliaries, we have looked at seal oil systems, lubricating systems, cooling systems, drivers and couplings. Starting with seal oil systems, you saw that the oil is pumped from a reservoir, through coolers and filters, to a header tank. The level in the tank maintained the correct oil pressure on the seal. Some of the oil, which becomes contaminated with gas, is cleaned in a degassing system before being used again. The rest of the oil goes straight back to the reservoir. Turning our attention to the lubricating system, you saw that the system is very similar to the seal oil system. Again, coolers and filters are used and the oil is continually circulated round the system and back to a reservoir. The lube oil not only lubricates the compressor bearings, but also the driver and gear box. A cooling system may be required to reduce the temperature of the gas for safe and efficient operation. You saw that this system may take the form of an intercooler, or an after cooler. Finally, in the Section, we had a brief look at drivers and couplings. The compressors may be driven by electric motors, or gas or steam turbines. They may be directly coupled or be driven via a gear box. Couplings connect the shafts of the driver and compressor. These couplings are called flexible couplings which are capable of taking up any small misalignments between the shafts. In the final Section of this Unit on centrifugal compressors, we will take a look at compressor operations.

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Petroleum Gas Compression - Unit 3 - Centrifugal Compressors

Petroleum Open Learning

Section 4 - The Operation of Centrifugal Compressors In this, the final Section of the centrifugal compressor unit, we will be looking at the operation of the compressor. I have divided the Section into the following topics: • compressor performance characteristics • operating problems, alarms and shutdowns • operational checks Let’s start by considering the performance characteristics.

Performance Characteristics The performance of a centrifugal compressor can be shown on a set of operating curves. These are graphs prepared individually for each compressor. They show the range of flows, heads, efficiencies and speeds within which a particular compressor is capable of operating. In other words, they indicate the performance of the compressor under different operating conditions. Figure 30 is a simple graph which shows a single operating curve for a specific compressor.

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You can see that: • the vertical axis gives the actual compression ratio (or head) as a percentage of the design compression ratio • the horizontal axis gives the inlet flow (capacity) as a percentage of the design capacity The solid line curve which passes through the design point shows the relationship between head and inlet flow when the machine is running at 100% of its design speed. The design point for any compressor is the point at which the machine is :

You will notice three other lines on Figure 30, marked as: • optimum efficiency • approximate surge limit • stonewall (choke) line We will look at all of these in some detail later. In fact, the performance of a compressor is usually expressed by a family of curves. The shape and position of the curves depend on a number of things. These include the design, size, speed and the number of impellers.

• running at 100% of its design speed • compressing 100% of the design capacity or inlet flow • producing 100% of the design compression ratio

A typical set of curves is shown in Figure 31, overleaf.

At this point the compressor is operating at 100% efficiency. As you can see from the operating curve, any changes to the speed, flow or pressure will remove it from this point.

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In Figure 31, the curves represent the relationship between head and flow for a number of different speeds. The efficiency of the machine at various heads, flows and speeds is indicated in the graph as a series of ellipses representing lines of constant efficiency. You will also see that optimum efficiency at varying speeds is represented by a single line running through the ellipses. The machine is capable of operating at conditions anywhere within the envelope of the curves. This means that the compressor is able to operate at conditions away from its design point, but at a reduced efficiency. The boundaries of this envelope to the left and right are labelled approximate surge limit and stonewall (choke) line. I will have more to say about these lines shortly. The upper boundary of the envelope is determined by the maximum speed at which the impeller is capable of rotating. This in turn depends on its size and construction. The lower boundary is not really a problem for the compressor. If the surge and stonewall lines were extended down, they would meet at the point of zero flow and zero head. So the compressor could operate at much lower speeds. This lower limit is usually determined by the minimum speed at which the driver can be operated. 3.44

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Let’s now look at the two boundaries of the compressor performance curves shown in Figures 30 and 31. These are the approximate surge limit and stonewall. First of all, surge limit.

Compressor Surge The surge limit is the minimum flow for a given speed at which the compressor can maintain stable operation. At inlet flow rates to the left of this line, the operation of the machine becomes unstable. We can analyse a compressor characteristic curve to see how surge in a compressor occurs. Take a look at Figure 32 which shows a curve of discharge pressure against inlet flow for a constant speed. You should note that I have deliberately exaggerated the shape of this curve in order to make the following explanation of surge more easily understood.

Imagine that the compressor is running at constant speed at Point 1 on the curve. This means that it will be delivering a certain pressure (P) with a corresponding inlet flow (F). Everything is normal. Supposing, however, that there is a sudden increase in downstream resistance which reduces the inlet flow to Point 2 on the curve. Don’t forget that, if the compressor is running at a constant speed, the pressure and flow values must lie on the curve. At this point the compressor will continue to operate in a stable manner. Even though there is a pressure increase in the downstream pipework, the compressor discharge pressure has risen to overcome this. Now look at Figure 33

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Supposing, now, that the compressor is operating at Point 3 on the curve which is right at the peak. Now, if there is a reduction in flow due to an increase in downstream resistance, the operating point moves to Point 4. This point is in a region where the compressor actually produces less pressure than at Point 3. The machine now cannot produce the pressure necessary to overcome the downstream resistance. When this condition occurs, the flow momentarily reverses its direction so there is no forward flow. The operating point now moves to Point 5.

In major cases of surge there are complete reversals of flow which result in a massive shunting of the rotating assembly. Each cycle may occur over twice per second and on every cycle the whole rotating assembly (which may weigh in excess of two tons) shunts inside the compressor casing as the thrust forces change direction across the compressor. Each shunt can throw enormous stresses on thrust bearings, journal bearings, seals and shaft couplings.

The effects of compressor surge are well known and predictable. Because of this, all high pressure compressors are fitted with anti-surge control loops which prevent the situation from occurring. The anti-surge control loops operate by controlling the flow of gas through the compressor to a pre-set minimum.

The ultimate result of compressor surge is the rapid backward and forward movement of the rotating assembly, and the resulting vibration may:

With no flow through the machine, the discharge pressure is reduced. This then allows the gas flow rate to build up again towards Point 6.

• destroy internal labyrinth seals

But, at that flow rate, the pressure delivered by the machine is less than that required to overcome the downstream resistance. The operating point then moves along the curve towards Point 3 again and, once beyond there, the cycle is repeated.

• seize the rotating assembly in its bearings

The cycling I have just described causes oscillations of the gas flow in the compressor and pipeline, which is known as surge.

• shatter the impellers and/or the compressor casing

• destroy journal and/or thrust bearings

• stall the main driver • shatter the shaft coupling and/or gear box

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Figure 34 is a drawing of a simple anti-surge controller which re-cycles gas from the discharge of the compressor back to the suction line.

You can see from the drawing that the anti-surge controller is connected to a control valve. The valve is in a pipeline which is connected between the compressor suction and discharge lines outside the machine. The controller senses and compares the rate of gas flow into the compressor, and the pressure rise across it. If the flow rate at a given pressure rise is less than the setting of the controller, the control valve is opened. This allows enough gas to flow from the discharge to the suction of the compressor to keep the suction flow rate above the surge value. Note that the recycled gas is cooled in a heat exchanger. This is to prevent a rapid increase in temperature as hot discharge gas is fed to the suction and further heated as it is compressed again. In view of the damage which may result from surge, most centrifugal compressors are fitted with an approaching surge alarm and an anti-surge shutdown, in addition to an anti-surge controller. These systems work as follows : The anti-surge controller should normally prevent surge from occurring. If it fails to operate, then the approaching surge alarm will be activated. The approaching surge alarm will alert the operator to the fact that the anti-surge controller has not managed to rectify the situation. If the situation is not corrected then the anti-surge shutdown is activated. The anti-surge shutdown device is set to activate just before the point at which the compressor will enter surge. When this safety device operates, the compressor will be stopped. 3.47

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Stonewall or Choke

Operating Problems, Alarms and Shutdowns

You will remember that the second boundary line on the compressor performance graph was called the stonewall (choke). Figures 30 and 31 both show this. Stonewall is the point at which the flow rate through the compressor approaches the speed of sound in the gas it is compressing. As this occurs, the shock waves generated result in a choking effect, which prevents the gas from building pressure. The symptoms that the compressor is approaching a stonewall condition are a rapidly falling discharge head coupled with very little extra flow. Stonewall limits the maximum flow which the machine can achieve at a given speed.

Test Yourself 3.7 a)

List 3 possible consequences of surge in a compressor.

b)

What measurements are compared by an anti-surge controller?

c)

What determines the lower speed limit of a centrifugal compressor?

Just like any other machinery or process plant, compressors are protected from malfunction or damage by instrumentation systems. These systems will generate alarms and compressor shutdowns if dangerous situations should arise. Let’s have a look at some of these situations now.

Excessive Compressor Speed If the compressor is being driven by an electric motor, rotational speed is normally fixed and normal running speed cannot be exceeded. If the compressor is being driven by a steam or gas turbine, normal running speed could be exceeded which might result in severe damage to the system. In these cases the compressor, and the main driver, are normally protected by : • a speed governor on the main driver which is set to control the compressor at a maximum of 100% normal running speed

Before moving on, have a go at Test Yourself 3.7.

• an electronic speed sensor on the main driver which will shut down the compressor if its rotational speed exceeds 105% of normal running speed You will find the answers in Check Yourself 3.7 on page 3.57 3.48

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• a mechanical trip which flies out from the shaft The results are varied but may include: of the main driver and trips the fuel supply, • The compressor may slow down or stop and if the rotational speed of the compressor the main driver may stall exceeds 110% of normal running speed

Ingress of Liquids Ingress of free liquids into the compressor can be extremely dangerous. The liquids could be in the form of mist or droplets, or in the form of larger slugs. If liquids enter a centrifugal compressor in mist or droplet form, there may be erosion of the impellers, diaphragms and casing.

• The strain on the gear box may cause it to fail or burst • The compressor shaft may be bent by the sudden and uneven forces which occur. This bending of the compressor shaft may often result in a compressor casing failure • The seal system may fail due to an over pressuring of the system

This erosion may create a loss of efficiency, or problems of vibration as the impellers become unbalanced.

The most common result is that considerable damage to the compressor’s shaft, shaft coupling, impellers and casing will occur.

In extreme cases, the erosion will result in both impeller and casing failure.

In order to prevent liquids entering the machine, all centrifugal compressors are fitted with suction knock-out drums. These are pressure vessels located in the pipework leading to the suction of the compressor. They are designed to separate any free liquid from the suction gas stream.

If the liquids are in the form of slugs when they enter the compressor then, as liquids are both incompressible and denser than gases, the effects are likely to be sudden and dramatic.

The liquid which is separated from the gas, accumulates in the bottom of the drum. If this liquid level rises beyond a certain point, there is a danger of it being carried over with the gas. To prevent this, a low liquid level is maintained in the vessel by level control instrumentation. Any further rise in level, perhaps through a level control malfunction, will trigger an alarm at a pre-set value. If the level still continues to rise, the instruments will cause a shutdown of the compressor plant.

Figure 35, on the next page, shows a simple layout of compressor and knock out drum.

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Vibration If the compressor starts to vibrate, the rotating impellers could rub against the casing. This could damage the machine, and in severe cases, cause catastrophic failure. The vibration could be caused by surge, corrosion products being deposited in the compressor, erosion of impellers, or misalignment of the driver and compressor. To protect the compressor, vibration monitors are installed at various points on the machine. At predetermined vibration levels, the monitors will initiate alarms, followed by shutdowns.

Surge We looked at the problem of surge a little earlier in this Section. Check that you remember what protects the machine in the case of surge by attempting Test Yourself 3.8.

Test Yourself 3.8 Describe with the aid of a simple sketch an anti-surge controller.

temperature alarms and shutdowns. In addition to the alarms and shutdowns which I have just described, other alarms may be fitted to the compressor. These are called inhibit alarms. They are fitted to prevent the compressor being started under conditions which may be detrimental to the machine. Once the machine is running, however, the inhibit alarm will not cause a shutdown. I have included below, as examples, a few inhibit alarm situations : • An inhibit which will not allow the compressor lube-oil pump to start until the lube-oil temperature is at a pre-set level.

You will find the answer in Check Yourself 3.8 on page 3.57

• An inhibit which will not allow the compressor to start until the lube-oil temperature is at a pre-set level. • An inhibit which will not allow the compressor to start until the lube-oil reservoir is full.

Lube Oil Pressure and Temperature

• An inhibit which will not allow the compressor to start unless the suction valve is open.

If the pressure on the lube oil system is too low, the compressor will not be lubricated properly. Excessive wear on bearings would then occur. To prevent this, pressure sensors initiate an alarm, followed by a shutdown, if the pressure falls below a set value. Similarly, if the lube oil gets too hot, it will become too thin and lose its lubricating properties. Temperature sensors will initiate high lube oil

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The Main Operational Checks on a Centrifugal Compressor

Check that the valves in the suction and discharge pipelines and the anti-surge system are in the correct position, i.e., open or closed.

We have looked at how the system is controlled, and what alarm and shutdown systems are commonly installed. Now we can look at how the compressor should be operated.

• any lube-oil added to the system is of the correct type and grade

The golden rules for operating a centrifugal compressor are:

We have to make sure that the compressor has an uninterrupted supply of gas to the suction. We also have to make sure that the gas is able to flow away from the compressor to where it is intended to go. The anti-surge line should also be checked to ensure that the control valve is fully open when the compressor is being started.

Before Starting the Compressor

Check that dependent systems are operational

• auxiliary lube oil pumps are available for use

Check that the compressor is purged of all air If the compressor is not completely purged of air, the gas/air mixture may burn or explode when the compressor is started.

We have to check that the compressor is not going to shut down because of a lack of gas, or because the main driver has run out of fuel, or for any other reason which is not directly related to the compressor itself.

Check that the suction line and compressor casing is free from liquids

Check that the discharge valve is in the correct position

Liquids are incompressible. If we try to compress them, the pressure increase maybe so high that the compressor is damaged.

Some centrifugal compressors are started up with the discharge valve closed and allowed to go through a re-cycle stage before they are put on line.

The high rotational speeds of centrifugal compressors also means that they develop high torque when starting. Trying to spin a casing full of liquid may stall the main driver during the start sequence.

Check that all relief valves are operational

• seal oil flows are within normal limits

Check that the lube-oil system is operating correctly

• auxiliary seal oil pumps are available for use

We should check that: • there is sufficient lube-oil in the reservoir

• main lube oil pumps are running and header tanks are full • lube-oil flows are within normal limits

Check that the seal oil system is operating correctly We should check that: • there is sufficient seal oil in the reservoir • any seal oil added to the system is of the correct type and grade • main seal oil pumps are running and header tanks are full

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Check that the lube-oil and seal oil cooling medium is available We should check that: • the lube-oil and seal oil coolers are operating correctly • the temperatures are being properly maintained Check that no current alarm or shutdown conditions exist In addition to the alarm and shutdown instrumentation (including inhibit alarms), centrifugal compressors are often fitted with various by-passes which allow them to start up. The most common bypasses are: • An anti-surge shutdown by-pass. Surge is a function of low flow, and the anti-surge shutdown operates when the flow through the compressor is low. Before start-up there is, of course, no flow. Therefore, the anti-surge shutdown must be by-passed during start-up, to prevent it shutting down the compressor due to the temporary, low flow conditions which exist at this time.

•

A high vibration shutdown by-pass. As the compressor starts up it may pass through two or more critical speeds which will cause high vibration. Unless the high vibration shutdown is by-passed, the compressor will be shut down as it passes through these critical speeds.

If you are involved in compressor operations you should become completely familiar with the equipment under your control. Your specific operating procedures should be followed, and safe working practices adopted at all times.

When the Compressor is Running •

Check that the pressures, levels, flows and temperatures are within operational limits. These checks must be made at regular intervals, at least once every two hours.

•

Check that the lube-oil and seal oil levels are maintained. If it becomes necessary to top up the systems with oil, the following points should be noted:

Always use the correct type of oil for each system. Always ‘top up’ the system with clean and dust free oil of identical grade. I have described a few checks that an operator of compression plant should carry out as part of his routine duties. The list is by no means exhaustive and is certainly not meant to be taken as an operating procedure.

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Summary of Section 4 In the final Section of this Unit on centrifugal compressors, we have concentrated on the operation of the system. The section was split into three parts:

•

In the first part we looked at compressor performance characteristics. You saw that the performance can be illustrated by a set of operating curves. These curves show the range of heads, flows, efficiencies and speeds within which a compressor can operate.



We studied a set of typical curves and used them to explain surge and stonewall. You saw how a compressor is protected against surge by an anti-surge system.

•

We then went on to consider some operating problems, alarms and shutdowns. We looked at some typical problems and saw how a compressor is protected against the damage that they can cause.

•

Finally we had a look at some of the operational checks which should be carried out before a compressor is started and when the machine is running.

Now that you have completed Section 4, you have come to the end of Unit 3 of the compression programme. I must emphasise once again that this unit is not meant to take the place of specific manufacturers guidelines or operating instructions. It is intended to give you a good basic grounding in the design, construction and operation of centrifugal compressors. Now, go back to the Training Targets and satisfy yourself that you have met those targets .

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Check Yourself - Answers

Check Yourself 3.1 a)

Compression ratio discharge pressure = suction pressure 90 psia = 3.6 = 25 psia

Check Yourself 3.2

The items marked with a tick are centrifugal compressor items. The others belong to a reciprocating compressor. Impeller

√

Diffuser

√

Piston Rod b)

Discharge pressure = Suction pressure x Compression ratio =

30 psia x 3.6

=

108 psia

Cylinder Shaft

√

Crank Casing

√

Blades

√

Cross Head Clearance Pocket

Check Yourself 3.3 1.

In a HORIZONTALLY split casing, the casing is made in two halves which are bolted together.

2.

At a rotating assembly’s CRITICAL speed it will reach a peak of vibration.

3.

In a closed impeller the blades are attached to both the HUB and the SHROUD

4.

The thrust force acting on the rotating assembly is taken up by two items. They are the BALANCING PISTON and the THRUST BEARING

5.

The DIFFUSERS guide the gas through the return passages to the eye of the next impeller.

6.

The most common internal seal is the LABYRINTH seal.

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Check Yourself 3.4

Check Yourself 3.5

Check Yourself 3.6 Impeller

Inboard element

(b)

FALSE The external seals are usually liquid film seals or other, equally efficient types.

1.

1.

a)

2.

Lube oil cooler YES

2.

Labyrinth teeth

(b)

TRUE

3.

Coupling

YES

3.

Impeller rim



(a)

Driver

YES

4.

Diffuser



(c)

5.

Thrust collar



(a)

b) c)

6.

Diaphragm



(c)

7.

De-gasser



(b)

8.

Journal bearing

(c)

9.

Balancing piston

(a)

10.

Inlet port

(c)



d) e)

FALSE 4. The oil which comes into contact with the gas is routed via the DE-GASSERS to the 5. reservoirs. 6. TRUE 7. TRUE 8.

NO - Rotating assembly

Balancing piston NO - Rotating assembly Diaphragm

NO - Non rotating assembly

Intercooler

YES

De-gasser

YES

9.

Reservoir

YES

10.

Thrust collar

NO - Rotating assembly

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Check Yourself 3.7

Check Yourself 3.8

a)

Using a sketch similar to that in Figure 34, your description should have been as follows :

Any three of the following :



-

destroy internal labyrinth seals



-

destroy bearings



-

stall main driver



-

shatter coupling and/or gear box



-

shatter impeller

b)

Inlet flow rate, and pressure difference between suction and discharge.

c)

The minimum speed at which the driver can be operated.

An anti-surge controller compares the inlet flow to the compressor with the pressure rise across the compressor. If the inlet flow rate is too low for a particular pressure rise the controller opens a valve in a line between compressor suction and discharge. This allows gas to flow from the discharge to the suction side of the compressor to keep the suction flow rate above that at which surge will occur.

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