oil pumping and metering.pdf

POLPetroleum Open Learning

OPITO

THE OIL & GAS ACADEMY

Oil Pumpingand Metering

Part of thePetroleum Processing Technology Series

Petroleum Open Learning

Designed, Produced and Published by OPITO Ltd., Petroleum Open Learning, Minerva House, Bruntland Road, Portlethen, Aberdeen AB12 4QL

Printed by Astute Print & Design, 44-46 Brechin Road, Forfar, Angus DD8 3JX www.astute.uk.com

OPITO 1993 (rev.2002) ISBN 1 872041 85 X

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Contents Page* TrainingTargets 4* Introduction 5

* Section1-CentrifugalPumps:TermsandConcepts 6 Liquids, Gases and Fluids Mass, Force and Weight Density and Secific Gravity Centrifugal Force Kinetic Energy and Pressure Energy Pressure Head Pressure Net Positive Suction Head (NPSH) Cavitation Flow v Differential Pressure

* Section2-ConstructionandOperationofCentrifugalPumps18 Impellers and impeller Speed Pump Casings Bearings Pump Configurations Centrifugal Pump Performance Curves A Centrifugal Pump Arrangement Minimum Flow System

VisualCues trainingtargetsfor you to

achieve by the end of the unit

testyourself questions to see how much you understand

checkyourself 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

OilPumpingandMetering(Part of the Petroleum Processing Technology Series)


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Contents(cont'd) Page* Section3-OilMeteringandSampling 33 Differential Pressure Metering Turbine Meters Metering Systems Meter Proving Sampling Systems

* Section4-PigLaunchingFacilities 42 Types Pig Launchers Pig Launching Problems Basic Rules for Pig Launching Safety Systems

* Section5-ATypicalOilPumpingandMeteringSystem 49 Booster Pumps Sampling System Metering System Oil Pipeling Pumps Pig Launching

* CheckYourself-Answers 65

VisualCues trainingtargetsfor you to

achieve by the end of the unit

testyourself questions to see how much you understand

checkyourself 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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TrainingTargets

When you have completed this unit on Oil Pumping and Metering, you will be able to:

Explain some of the technical terms and concepts which lie behind the operation of a centrifugal pump

List the component parts of a centrifugal pump.

Explain the operating principles of a centrifugal pump.

Describe the construction and operation of turbine and differential pressure meters.

Explain the function and operation of a typical meter run.

Describe the procedure for proving a meter

List the essential elements of an oil sampling system

Detail the main features of a pig launching system, and its method of operation

Describe a typical layout for the oil handling (or oil pumping and metering) section of a production facility

Tick the box when you have met each target


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Petroleum Open LearningOilandGasSeparationSystemsIntroductionOilPumpingandMeteringIntroduction


In this unit, we will be looking at the oil handling section of a production facility.

The equipment needed for this system will usually be situated between the final stage of separation (into oil, gas and water streams) and the point where crude oil leaves the production facility for transfer to a pipeline, oil tanker or terminal. The layout is illustrated in Figure1.

Figure 1: Oil Pumping, Sampling and Metering

The system is often referred to as the oilpumpingandmeteringsystem.

The unit is divided into five sections :

in Section1, we will look at some basic terms and concepts relating to centrifugal pumps.

Section2 concentrates on the construction and operation of centrifugal pumps.

Section 3 gives you an overview of the metering and sampling part of the system.

In Section4, pigs and pig launching facilities are described.

Finally, in Section 5, we will go through a typical oil pumping and metering system.

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Petroleum Open LearningPetroleum Open LearningOilPumpingandMeteringSection1-CentrifugalPumps:TermsandConcepts

TestYourself15 litres of water has a mass of 5 kg

5 litres of crude has a mass of 4.25 kg

5 litres of salt water brine has a mass of 5.5 kg

What are the specific gravities of gas oil and brine?

You will find the answers to Test Yourself 1 on page 65.

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mass of a certain volume of materialmass of an equal volume of reference substance

In this first section, we will look briefly at a number of concepts which relate to the operation of centrifugal pumps. I will also explain some of the terms often used when we are discussing how these concepts can be applied in practice.

Throughout this unit we will be concentrating on centrifugal pumps because these are the most common ones used in oil pumping and metering services.

Liquids,GasesandFluids

Both liquids and gases are called fluids because each has the ability to flow.

In this unit we will use the term fluid when describing something which can happen to a gas or a liquid. When we need to make a distinction, I will use the specific termliquid or gas.

Mass,ForceandWeight

The mass of an object is a measure of the quantity of matter present. This object may have various forces acting on it, the most important of which is likely to be the forceofgravity. You can easily demonstrate that there is a force acting on the object. Hold it out and release it - the force of gravity will pull it towards the earth.

Weight is a measure of this force acting on the object. Therefore, a one pound mass will have a force of one pound weight acting on it, due to gravity.

The two terms mass and weight cause a lot of confusion. Very often they are used as if they mean the same thing. In many cases, however, this is not all that important and I think that the brief explanation given above should be sufficient to guide you through the remainder of this unit without any undue problems.

DensityandSpecificGravity

The density of a substance is defined as the massperunitvolume of that substance. For the same material, density can be expressed in a variety of units. For example, the density of water is :

1 gram per cubic centimeter -1 gm/cm3

Specificgravity (s.g.) compares the mass of a certain volume of a material with the mass of an equal volume of a reference substance. In other words :

specific gravity (s.g.)

For solids and liquids, the reference material used is usually water. For gases, the reference is often to air.


It shows a spinning disk. If we let a drop of water fall onto the face of the disk, near to the centre spot, it will follow the type of path shown. This is because the drop is affected by two forces during its travel:

centrifugal force, which tends to throw the droplet outwards, causing it to speed up as it approaches the edge of the disk

friction, which will cause the disk to attempt to drag the droplet round with it as it rotates

The relative size of these two forces will determine the angle at which the droplet leaves the disk edge.

This angle is important, as you will see when we come to the section on Construction and Operation of Centrifugal Pumps (Section 2). The design features of the pump encourage a flow path for the liquid being pumped, which is very similar to the droplet trajectory in Figure 2.

This ensures that the pump imparts the maximum amount of energy to the liquid. In this case, energyofmotion, or kineticenergy is transferred.

Centrifugal Force

Have a look at Figure2.

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Kinetic Energy

We have seen that kinetic energy is energy of motion, or movement.

The amount of kinetic energy possessed by any moving object depends upon:

its mass (weight)

its velocity (speed)

In mathematical terms, kinetic energy (KE) can be calculated by using a formula:

KE = 1/2 mass x velocity2

If the mass is expressed in kilograms and the velocity in metres per second, the kinetic energy will be in joules.

To confirm your understanding of this relationship, try the following Test Yourself.

TestYourself2A small car has a mass of 1 000 kilograms, and is travelling at 180 kilometres per hour.

A large truck has a mass of 20 000 kilograms, and is travelling at 30 kilometres per hour.

Which one has the greater kinetic energy ?

You will find the answer to Test Yourself 2 on Page 65.

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Let us go into a little more detail on these pressure and velocity changes by considering six separate points in the process:

pointA : fluid is flowing along the pipe at a steady speed and (almost) constant pressure. You will remember that the kineticenergy of this fluid can be calculated by the equation:

KE = 1/2 mass x velocity2

The velocity of the fluid at point A is constant. In addition, the mass of fluid passing each point in the pipeline per unit of time (mass flow rate) is also constant. This means that the kinetic energy content of the fluid at that point is also constant.

pointB : the fluid starts to enter the restriction. The mass flow rate remains constant but, because the pipe diameter is smaller, the fluid velocity must increase.

Looking again at the kinetic energy equation, you will see that the kinetic energy of the fluid will start to increase at this point as the fluid speeds up.

Kinetic Energy and Pressure Energy

Figure 3 illustrates the flow of a fluid across a restriction, and how the fluid velocity and pressure vary during this process.

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Now let me introduce you to another principle of science - ConservationofEnergy.

This tells us that the totalenergycontentofasystemwillalwaysremainconstant

If the kinetic energy content of our system increases then, to compensate for this, some other form of energy possessed by the system must decrease. This other form of energy is pressure energy. Figure 3 shows that, as the velocity (kinetic energy) increases, the pressure (pressure energy) decreases.

pointC : this is a new steady state. The fluid has a higher velocity and a lower pressure but both of them are steady as the fluid passes across the restriction.

points D and E reverse the changes which occurred at pointsAandB.

It is worth noting that, across the process overall, a small reduction of pressure has occurred. Due to turbulence in the system, some pressure energy has been converted into heat energy. You will no doubt appreciate that, under conditions of high flow rates, high turbulence, or extended restrictions (say, a long pipeline run), pressure losses will be greater.

We will look into the effects of pressure loss and flow a little later on in this section.

Pressure

Pressure expresses the relationship between force (or weight) and area, as follows:

kilogram force (or weight) pressure = areaLike density, pressure can be measured in a variety of units. The most common are pounds per square inch (psi), or kilograms per square centimetre (kg/cm2). We normally use the SI term bar, as 1 bar is almost the same as 1 kg/cm2. (1.019 kg/cm2 to be exact).

Picture a metre cube of water:

This cubic metre of water weighs 1000 kg. In other words, due to the effects of gravity, it is applying a downward force of 1000 kg, spread over its base.

The pressure on the base of the cube is therefore:

1000 kg/m2

However, we have just seen that pressure is usually expressed in bar.

As you will see from Figure 4, the base of this water cube has an area of one square metre, or:

100 cm x 100 cm = 10,000 cm2)

So, on each square centimetre of the base a downward force of 1000 kg is applied. 1000 kg is applied.

The pressure on the base can, therefore, also be expressed as:

1000 = 0.1 kg/cm 2, or 0.1 bar. 10,000

10,000


HeadPressureThe term headpressure or head is often used in the oil and gas industry, especially when referring to pumps. It is the pressure developed by a head, or column height, of liquid.

In the paragraph entitled Pressure, we saw that the head pressure applied by 1 metre depth of water will be 0.1 bar. For 3 metres of water the head pressure would be 0.3 bar; for 30 metres, 3 bar, and so on.

Now let us combine what we know about specific gravity and head pressure. Try the following Test Yourself to combine these two factors together:

TestYourself3The specific gravity of crude oil is 0.85, and that for a particular salt water is 1.1.

What will be the head pressure developed by 3 metres of crude oil, and 4.5 metres of this brine?

The answers to this TestYourself are on page 65.

We have already shown that centrifugal force can impart kinetic energy to a substance as a result of a spinning action. We have also seen that kinetic energy can be converted into pressure energy.

Centrifugal pumps are dynamic pumps which, primarily, impart kinetic energy to the fluid being pumped. They do not create pressure directly. Pressure results from the liquid slowing down, and the kinetic energy converting to pressure energy.

Thepressuredevelopedwilldependonthedensityoffluidbeingpumped.

A centrifugal pump, working at a fixed flowrate, will generate the same height of head, but will generate a lower head pressure, when pumping crude oil, than when water is being pumped, because water is heavier than crude oil.

The different categories of head pressure referred to in pumping operations are shown in Figure5 on the next page.

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The suctionhead represents the head pressure present at the pump suction.

The discharge head represents the head pressure delivered by the pump.

The totalhead (which is the difference between suction and discharge heads) represents the additional pressure imparted to the liquid by the pump.

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NetPositiveSuctionHead(NPSH)I would like you to think about two common situations in which you have seen bubbles coming out of a liquid.

1. If you heat up a pan of water, two things happen:

long before the water boils, bubbles are seen rising through the liquid as dissolved air comes out of solution when the temperature begins to increase

at the boiling point, the liquid bubbles vigorously as the water is rapidly converted into steam

Water at sea level boils at 100C (212F). I am sure you will have heard, however, that the boiling point of water (or any other liquid) falls as you climb from sea level, so that it can be difficult to cook an egg properly on top of a high mountain. This is because atmospheric pressure falls the higher up we get.

2. If you open a bottle of fizzy lemonade, bubbles are seen rising through the liquid as dissolved gas comes out of solution when the pressure is released (reduced).

If these effects are observed in water and lemonade, it is reasonable to assume that they will happen in other liquids as well. So let us now visualise how these effects can influence the operation of a pump.

We already know that fluids can only flow from areas of high pressure to areas of low pressure. Suppose that the liquid being pumped enters an area of low pressure. Then:

if the liquid was near its boiling point, the pressure drop may cause the liquid to boil and thus release gas or vapour

if the liquid was near to the pressure at which dissolved gases are released, the pressure drop may cause these gases to come out of solution

In either case we can predict that, if the pressure is increased again, the released gases will go back into the liquid, either because boiling stops or the released gases re-dissolve.

When a centrifugal pump is running, a low pressure area is created at the suction. This encourages liquid further upstream to flow into the pump suction.

The accompanying drop in pressure may cause gas or vapour to be released for either of the reasons described above. It is important that we prevent this happening, for reasons that I will explain a little later under Cavitation.

We must therefore always have sufficient pressure at the pump suction to prevent gas or vapour release for whatever reason. The minimum pressure necessary to do this is called the netpositivesuctionhead(NPSH).

A further pressure reading which is relevant to the suction end of the pump is called the staticsuction line pressure. As the name implies, this is the measured pressure at the pump suction when pumping has stopped.

We now have three pressure values which relate to the pump suction:

a. the pressure at which gas or vapour is released

b. the static suction line pressure

c. the NPSH


TestYourself4As a check on whether you have understood what I have ben saying about pressures at the suction end of the pump, list these three pressure valves:

a. the pressure at which gas or vapour is released

b. the static suction line pressure

c. the NPSH

in order of decreasing pressure, and see if you can explain the reasons for your answers.

You will find the correct answers in CheckYourself4 on Page 66.

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A pump manufacturer will specify the NPSH and maximum operating temperature required for each pump to handle a given liquid effectively. The NPSH should be maintained over the entire range of the pump

To be safe, most pumps will be operated just above their NPSH. An adequate safety margin for most applications would be 1 metre, or 10%, head of water pressure above the NPSH specified by the manufacturer (whichever is the larger).

In general, the industry standard is to work in terms of head of water. This is because everyone knows the density of water and pumps can easily be tested to make sure that they produce the level of head specified.

CavitationWe have discussed at some length the importance of NPSH and other factors in preventing the release of gas or vapour bubbles in the suction of the pump. We will now look at why it is so important to prevent this.

If gas is released at this point in the system, it will give rise to an effect known as cavitation.

The formation of bubbles is, in itself, quite harmless. However, as the liquid containing these gas bubbles, or cavities, passes through the pump, the pressure will rise. Now we already know that, if gas is released from a liquid for the reasons I have described, an increase in pressure will drive this gas back into the liquid again.

As these tiny cavities created in the liquid collapse, the liquid tends to rush in from all angles to fill the cavity. The cavity is said to implode.

This inrushing liquid can transmit very large forces. When the bubbles are near a metallic surface, these forces are applied directly to the solid surface. When a pump is cavitating, this process is being repeated many thousands of times each second and the effect results in noise, vibration and eventual erosion of metal from the surfaces. In very severe cases, for example where the pump is handling liquids carrying small solid particles, the impeller can be eroded in a relatively short space of time.

An equally important factor is that severe cavitation can result in a failure of the pump to deliver flow at the expected head.

When pumping oil, the drop in head and efficiency is not quite so severe as with water because the liquid is composed of mixtures of different hydrocarbon compounds. The bubbles which appear will consist of lighter hydrocarbons such as methane or ethane. These can be more easily reabsorbed as the pressure is increased. When pumping water the bubbles are nearly always caused by the water boiling at a reduced pressure. In this situation the bubbles collapse violently and each implosion is of a high intensity.


FlowvDifferentialPressureTake another look at Figure 3. You will recall that it illustrates the conversion of pressure energy to kinetic energy, and the reverse, as a fluid passes through a restriction. Remember also that, because of turbulence, some pressure energy is converted to heat energy. This conversion is responsible for the pressure loss shown in Figure 3.

Any pipeline will contain a whole series of restrictions. These may be bends, changes in diameter, obstructions and rough internal surfaces, for example.

You will probably realise, therefore, that:

at low flow rates the turbulence caused by these restrictions may well be small, therefore minimising the pressure loss

at high flow rates the turbulence could be very high, as will be the pressure loss

We will now take a look at the relationship between flow and differentialpressure between two points in, say, a pipeline.

Have a look at Figure6.

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TestYourself5In the example we have just used, if the differential pressure fell from 70% to 40% of maximum, what would be the change in flow rate, expressed in litres per minute?

The answer can be found in CheckYourself5 on Page 66.

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This shows the relationship between flow and differential pressure, both expressed as a percentage of the maximum possible under those particular circumstances.

We can see, for example, that 50% of the maximum flow is equivalent to 25% of the maximum differential pressure.

Now let us suppose that, in our pipeline, we can generate up to 10 bar of pressure at the inlet and deliver up to 10 litres of-liquid per minute. Let us also suppose that, at the outlet, the liquid discharges into a pond at 0 bar.

We therefore have a differential pressure of 0-10 bar and a flow rate of 0-10 litres/min. Let us look at the conditions under different flows and pressures.

if we regulate the inlet flow to 1 litre/min (10% of maximum) we could expect very little turbulence. From Figure 6 we can estimate that the differential pressure will be 1 % of maximum, or 0.1 bar, at this flow rate.

if the flow is increased to 2 litres/min (20% of maximum), both turbulence and the pressure drop will increase. At a 20% flow rate, the differential pressure will rise to 4% of maximum, or 0.4 bar.

(You will have noticed that, when the flow rate doubled, the differential pressure increased by a factor of 4).

let us now increase the flow rate to 4 litres/min (40% of maximum). The differential pressure rises to 16% of maximum, or 1.6 bar. Again, as the flow rate doubles, from 2 to 4 litres/min, the differential pressure quadruples, from 0.4 to 1.6 bar.

This relationship between flow and differential pressure can be expressed as a mathematical equation :

F = DPxl0

where:

F = flow rate as a % of maximumDP = differential pressure as a % of maximum(DP means the square root of DP)

That ends our brief look at some of the key factors which affect the design and operation of centrifugal pumps. Before we go on to the next Section, however, try the following TestYourself.


SummaryofSection1

In this section, we have looked at some of the scientific terms and concepts which help us to understand the design and operation of centrifugal pumps.

You will remember, for example, that both liquids and gases are called fluids because they have the ability to flow. We saw how fluids flow from high energy areas to low energy areas.

We examined the relationship between mass, force and weight, and I tried to clear up some of the confusion which exists in the common use of these words.

Density and specificgravity were explained.

I illustrated centrifugalforce by asking you to visualise the movement of a water drop on a spinning disk.

This led us to a description of kinetic energy, and how kinetic energy and pressure energy can be interchanged. We introduced the concept of conservation of energy.

Netpositivesuctionhead(NPSH) was fully described, and we saw how important it was in relation to preventing cavitation.

We looked at differential pressure and flow. The relationship was expressed as a graph, and also as a mathematical equation.

You are now ready to take a look at the construction and operation of a centrifugal pump, and see how the terms and concepts covered in Section 1 can be applied to the design and performance of this type of pump.

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Petroleum Open LearningOilPumpingandMeteringSection2-ConstructionandOperationofCentrifugalPumpsThe centrifugalpump is the commonest form of pump in use today. It is relatively cheap, easy to maintain and is to be found almost everywhere when large flows are required.

We will first take a look at the basic configuration of a centrifugal pump and then at the component parts, to see what they do and how they work.

The type of pump illustrated in Figure7 is one of the simplest. It consists of:

a casing, which contains and supports the rest of the pump components. Access to the inside of the pump is via a vertical split at the back of the casing (not shown)

a suction flange, which directs the liquid entering the pump casing into the impeller

an impeller, which imparts kinetic energy to the liquid

a pumpshaft, connected through a coupling to a motor which drives the shaft and the attached impeller(s)

abearinghousing, which supports the shaft

a shaft seal, which prevents liquid escaping from the casing along the shaft

a discharge flange, which directs the liquid away from the pump

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Figure 8 shows a cross section through a single impeller pump, illustrating three more key items of equipment:

The wearrings, which act as seals between the high pressure discharge side and the low pressure suction side of the impeller

The wear rings are so called because they wear in preference to the impeller. They are sleeved on to the impeller, and may be replaced when worn

The balanceholes, which allow the packing to operate at suction pressure rather than discharge pressure. This reduces the differential pressure across the packer and impeller, and therefore reduces the thrust forces

The packing, which prevents liquid escaping from the casing

We will now examine some of these components in more detail.

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ImpellersWe have already seen that a spinning disk can impart kinetic energy to a drop of water on its surface. A centrifugal pump, which is a dynamic pump, does a similar job on the liquid it is pumping. The pump then converts this kinetic energy into pressure energy before the liquid leaves the outlet.

The elements of the pump which impart kinetic energy to the liquid are called impellers. We will now look at the three basic kinds of impeller and see how they differ from each other.

All impellers are fitted with curved vanes which spread out radially from the centre. The impellers are attached to the pump shaft and rotate with it.

Figure 9 shows the three most common types of impeller.

Your washing machine at home probably has a pump with an open impeller similar to the one shown at the top of the diagram. Open impellers are cheap to make but they are inefficient.

The one on your washing machine will be there to empty the machine. Washing machine pumps, however, have to cope with debris - buttons, fluff, coins and the like. An open impeller is ideal. It will handle most foreign objects and, if it is broken, it is cheap to replace.

The water pump on your car will probably be fitted with a semi-open impeller similar to the one shown in the centre of Figure 9. The semi-open impeller is a little more efficient and a little more expensive than the open impeller.

The car pump has to be reasonably efficient to provide engine cooling by circulating water around the engine and through the radiator. But, every bit of energy used in the water pump means that there is less available to propel the car itself. The semi-open impeller is therefore a compromise between efficiency and cost.

In the oil industry, the closedimpeller is the one most often used. This is shown at the bottom of Figure 9. It is more efficient than other types of impeller, but is also considerably more expensive.

This is because special techniques are needed to weld the vanes to the inside of the shroud which covers the impeller.

You should notice, in particular, the curve on each impeller vane, and compare this shape with the shape of the droplet trajectory in Figure 2. They are very similar.

Vanes are designed in this way to impart the maximum amount of kinetic energy to the liquid being pumped, and to ensure that this liquid leaves the impeller rim at a particular angle.

This angle will be matched by the shape of the volute, or angle of the diffusers, depending on the pump design. (I will talk about volute and diffuser casings shortly).


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ImpellerSpeedThe type of impeller selected will depend on the planned speed of rotation, and the type and size of pump.

As a general rule:

low pressure, high capacity pumps will have large diameter impellers with a low rotating speed

high pressure, high capacity pumps will have small diameter impellers with a high rotating speed

PumpCasingsWe already know that the velocity of the liquid increases as it passes across the impeller. We also know that, as the velocity decreases, the pressure will increase. Figure10 shows the two main types of casing which allow this to happen within the pump.

The upper diagram shows a volute casing. In this type of pump, the liquid leaves the tip of the impeller, and is thrown into a channel with an increasing area of cross-section. Here the liquid slows down and kinetic energy is converted into pressure energy.

The volute design ensures that it is aligned with the trajectory of the liquid leaving the impeller. This ensures efficient energy transfer and conversion.

The liquid is then guided towards the pump discharge flange.

The volute type of pump is the most common type in use.

The lower diagram shows a diffusercasing. In this type of pump, as the liquid leaves the tip of the impeller it moves through a set of angled vanes known as diffusers. Again, these are lined up with the direction of the pumped liquid as it leaves the impeller. The diffusers then guide the liquid into the outer section of casing where its velocity decreases and pressure increases before flowing to the discharge flange.

areasincrease

constantly


BearingsFigure 11 is an illustration of a simple bearing arrangement. The shaft is supported by two radial ball bearing races, which allow it to rotate with minimum friction.

Thrustforce is a force which is directed along the axis of the pump shaft. It arises because of the difference in pressure between the discharge and suction sides of the pump acting on the impeller. In Figure 11, the thrust force will be from right to left, (from high pressure to low pressure).

In this case, to counteract the thrust force, a ball bearing race (the thrust bearing) is mounted between two vertical plates. It allows the shaft to turn with a minimum of friction as it takes up this thrust force.

The slinger rings (also called flinger rings) are two slender rings, often of brass, which slide up and down the shaft as it rotates. The slinger rings dip into the lubricating oil and, as they turn, transfer oil onto the shaft. The oil then runs along the shaft and contacts the faces of the bearings. Centrifugal force throws the oil outwards along the bearing faces to lubricate and cool them.

The oil in this type of bearing is either topped up through an oilfillplug, as shown, or is automatically replenished via an oil bottle arrangement.

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SealsFigure 12 is an illustration of a typical packedseal. In this type of seal the packing consists of rings of asbestos rope which are impregnated with graphite. The rings are placed around the shaft and compressed into a packinggland by means of a glandfollower, the pressure on which can be adjusted by four bolts.

In some cases a lantern ring is fitted between sections of the asbestos packing so that any liquid which has leaked along the shaft can be removed.

The problem with this type of shaft seal is that small leaks almost always occur, whatever liquid is being pumped. These leaks are usually necessary in order to keep the packing lubricated and to prevent the shaft from overheating.

However, any leak would be dangerous when pumping oil or other hazardous liquid. In such cases, a mechanical seal would probably be used. A typical seal is illustrated in Figure13 on the next page.


The shaft enters the pump casing from the right hand side of the diagram and passes through a stationary seal. The stationary sealis fixed to the pump casing and does not rotate.

Attached to the shaft is a rotary (or rotating) seal. Leakage along the length of the shaft is prevented by O rings which seal the gap between shaft and rotating seal. The O rings turn together with the shaft and rotating seal.

The sealing faces of the rotating and stationary seals are usually of machined carbon or high grade stainless steel which are polished to a mirror finish. The two faces are held very closely together by a spring and by the pressure of the liquid in the pump.

A small amount of the liquid being pumped is often taken from the discharge of the pump, filtered, and then returned through the mechanical seal via the seal flush inlet. This liquid helps to keep the mechanical seal clean, cool and lubricated.

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PumpConfigurationsFigure 14 shows examples of how centrifugal pumps may be configured to increase flow, or to increase pressure.

In Figure 14a, a single pump is delivering 500 litres per minute with a total head of 3.5 bar: discharge head - suction head = total head.

Figure 14b shows that, to increase the flow, two pumps arranged in parallel are needed - that is, the pumps have a common suction and a common discharge.

In this case, we can :

run either pump on its own to produce a flow rate of 500 litres per minute and a total head of 3.5 bar, or,

run both pumps together to produce a flow rate of 1000 litres per minute and a total head of 3.5 bar.

In Figure 14c, we can increase the pressure by running two pumps in series. This means that the first pump discharges into the suction of the second pump. In this case:

both pumps must be run together

the combination of both pumps will produce a flow rate of 500 litres per minute and a total head of 7 bar.

In most instances where high pressures are required, it is easier to mount a number of impellers on a single shaft. These pumps are called multi-stage pumps. They give us high flow rates, and a gradual pressure rise over as many stages as required. Some main oil pipeline pumps may have more than eight impeller stages.

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Centrifugal Pump PerformanceCurvesEvery centrifugal pump is designed and manufactured for a specific purpose. This purpose is summarised in a pumpperformancecurve.

Figure15 shows a typical performance curve which gives us the following details about a specific pump:

on the left hand side of the curve, there are three vertical scales:

1. efficiency - from 0-100%. This compares the power the pump is using to the work it is achieving

2. power - from 0-24 kilowatts in this case. This indicates the amount of power the motor is consuming

3. totalhead - this indicates the pressure which the pump can achieve

on the top right hand side of the chart we can see the required NPSH (net positive suction head) in metresofliquid. (You will recall that NPSH was described in Section 1 on Page 13)

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the horizontal axis of the chart gives flow rate in cubic metres per hour.

in the body of the chart we find curves which show the relationship between:

1. NPSH and flowrate

2. efficiency and flowrate

3. power and flowrate

4. total head and flowrate

Take a few minutes to study Figure 15 and then try Test Yourself 6 and 7.

Test Yourself 6

When pumping 20 cubic metres per hour the pump will:

require a minimum of metres head of the liquid NPSH

develop metres total head of liquid

consume kilowatts of power

operate at efficiency

Test Yourself 7

When the flow rate increases to 40 cubic metres per hour this pump will:

require a minimum of metres head of the liquid NPSH

develop metres total head of liquid

consume kilowatts of power

operate at efficiency

The answers to these can be found in CheckYourself6and 7 on Pages 66 and 67.

27


Now, to summarise what we have covered so far in Section 2, try TestYourself8:

Check your answers in CheckYourself 8 on Page 67.

Test Yourself 8

Indicate with a tick, to which part or parts of the pump the following items belong.

Item Casing Impeller Bearing Seal

Shaft sleeve 'O' ring

Shroud

Lantern ring

Wear rings

Flush inlet

Vane

Slinger ring

Balance holes

Gland follower

Volute

Ball bearing race

Diffuser

28


ACentrifugalPumpArrangementBefore we look at a typical oil pumping system, let us think about those items of equipment which you aremost likely to come across.

Figure16 is an illustration of a typicalcentrifugal pumping arrangement.

29


30

The motor which drives the pump is called the main driver. In this case, the main driver is an electric motor, but for bigger pumps it may be a gas turbine or a diesel engine.

The motor has a set of localswitches for starting and stopping. In some cases a local ammeter is fitted to check whether the motor is running properly. In many instances, the pump motor may also be started from a remote location, such as a control room, either manually or via an automatic start system.

The motor shaft is linked to the pump shaft via a coupling, designed to transmit power from the motor to the pump, and to take care of any small shaft misalignments which may occur.

The flow of liquid into the pump is through a suction block valve, which can be used to isolate the pump from the upstream process if required. Occasionally, a strainer or filter (not shown) may be fitted to the suction line, downstream of the suction block valve, to prevent debris from entering the pump.

The pumpcasing is fitted with:

a casingvent valve, used to bleed off any gas or air in the pump before starting

a casingdrain valve, used to drain liquid from the pump after shutdown

The discharge of the pump is fitted with:

a dischargepressuregauge, which indicates the pressure produced by the pump

a dischargecheckvalve, which only allows flow in one direction, away from the pump. This valve, therefore, prevents liquid flowing back through the pump, back- spinning it and causing damage to the seals and bearings of both pump and motor

a discharge block valve, which can be used to isolate the pump from the downstream process, if required

MinimumFlowSystemAll centrifugal pumps require one other item of equipment for their protection.

If we look back at the performance curve in Figure 15 we can see that, when the pump is running at zero flow, it is still using about 4 kilowatts of power. We also know, from the performance curve, that the pump efficiency will have fallen to zero.

So, what has happened to the power we are using ?

The answer, of course, is that it is converted into heatenergy.

There would be great turbulence inside a pump with the impeller turning through liquid trapped within the pump. The temperature would rise, increasing the chances of cavitation.

In some instances, with large and powerful pumps, damage can then occur in a matter of seconds. In smaller machines it may take much longer - but damage will eventually occur.

To prevent this situation from happening, a minimum flow must be established and maintained through the pump at all times while running. This minimum flow level is specified by the pump manufacturer.

All centrifugal pumps which are at risk can be fitted with a minimumflowsystem. This ensures that, while the pump is running, there is sufficient liquid flow to ensure that no damage occurs.

In some instances, the minimum flow system consists of a simple orifice plate, sized for the correct flow. The plate is inserted into a line through which is re-cycled a fixed flow from pump discharge to pump suction at all times.

In other instances a flow measuring device is fitted into the suction of the pump. This device controls a flowcontrolvalve, inserted into a line which re-cycles a fixed amount of flow. If the flow falls below the pre-set minimum level, the flow control valve will open to restore flow rate to the minimum.


31

A simple and very common device is illustrated in Figure17. It is called a minimumflownon-returnvalve and serves the purpose of a check valve and a minimum flow valve.

Ih Figure 17a, there is no flow through the main part of the valve, but the two smaller valves are fully open to let liquid flow to the minimum flow system.

Figure 17b shows that there is some flow through the main part of the valve, but the two smaller valves are still partially open, allowing some liquid flow to the minimum flow system.

In Figure 17c, all flow is through the main part of the valve and the two smaller valves are fully closed. In this situation the pump is pumping at least a minimum flow through the main valve.


Summary of Section 2Ih this section, we have looked at the component parts and method of operation of a centrifugal pump.

These included:

pump casing (volute or diffuser)

impeller and impeller wear rings

pump suction and discharge

shaft bearing systems with radial and thrust bearings

shaft seals

We examined

how different types of pump casing played a part in converting kinetic energy into pressure energy

how we can change flow and/or pressure characteristics by changing pump configurations (parallel v. series)

In particular, we looked at

the construction and interpretation of a set of pump performance curves for a typical centrifugal pump and how they incorporate the concepts and ideas which we had previously encountered

a typical centrifugal pump arrangement with its inlet and outlet lines and associated equipment

Finally we looked at why centrifugal pumps are fitted with a minimum flow system, ensuring that they do not become damaged due to overheating and cavitation.

In the next section, we will take a look at a typical oilmeteringandsamplingsystem.

32

Petroleum Open LearningOilPumpingandMeteringSection3-OilMeteringandSampling

Test Yourself 9On an offshore installation there is a 1.0% error in the volume of crude oil being metered. The installation produces 60 000 barrels of oil per day.

If the price of crude oil is , say, US$25 per barrel, what is the market value of this discrepancy in the course of a year (assuming continuous production)?

You will find the answer in CheckYourself9on Page 68.

33

We have considered the basic design and operation of a centrifugal pump.

Now it is time to take a look at crudeoilmetering,meteringsystems, and samplingtechniques.

There are four main reasons for metering and sampling a flow of crude oil:

1. to measure the amount of hydrocarbons removed from the reservoir. This allows field production plans to be updated and revised.

2. to determine the amount of each component in a mixed oil stream. This is particularly important where the production from separate oil fields are mixed (perhaps as part of pipeline sharing agreements) prior to the point of sale.

3. to measure the product for tax purposes. This is called fiscal metering.

4. to ensure that no loss of product has occurred. In an offshore oilfield, the amount of metered offshore product, plus any losses or gains due to packingor unpacking of the pipeline (see below), is compared regularly with the amount of onshore metered product.

Multi-component liquids such as crude oil are slightly compressible. Increases or decreases in the overall pipeline pressure will produce small changes in the volume of oil contained within the pipeline. The terms packing and unpacking are used to describe these small changes in volume. If they are ignored, apparent losses or gains in the pipeline inventory can accumulate.

The sampling andmetering system is placed as late in the oil handling sequence as possible. There are a number of reasons for this:

it should be downstream of any boosterpump which is fitted. (On many installations, the crude oil passes through a booster pump to raise the pressure prior to entering the metering and sampling section. This ensures that no gas or vapour will break out of the liquid whilst it is being metered and sampled)

no further processing of the fluid occurs before export, and the fluid sampled and metered is representative of the fluid being exported

metering takes place downstream of water removal. At a water content higher than about 1%, serious discrepancies occur in meter accuracy which conflict with the objectives of metering and sampling

The process of metering and sampling is therefore given a very high priority. Meters themselves are checked regularly, using a permanently installedmeterprover. The meter prover itself is checked regularly to ensure that it, too, is accurate.

To emphasise this point, try TestYourself9 :


34

There are many methods used to measure fluid flow. It is worth noting, however, that most flow rates are arrived at indirectly by measuring some other property of the flowing fluid, and then relating the value of this property to flow rate by some form of calibration. This is true for the two most common devices used for metering produced oil:

the differentialpressuremeter the turbinemeter

For example, as you will see later on:

in the differential pressure meter, it is a pressuredifference which is measured directly

in the turbine meter, we measure the frequencyofelectricalpulsesIn this Unit, we will take a brief look at the differential pressure meter, and how it operates. We will then consider the turbine meter.

DifferentialPressureMetersDifferentialpressuremetering is one of the oldest methods of measuring flowrates. It is simple, accurate, reliable and relatively inexpensive. It will record volume flowrates (say, cubic meters per day), but mass flowrates (say, tonnes per day) can be calculated if the density of the oil is known.

The most common differential pressure device is one which uses a restriction, usually an orifice plate, in the pipeline. The pressure drop across this restriction is measured. This pressure differential can then be related to flowrate by the use of, for example, calibration tables or graphs. A large amount of calibration data has been published on this.

The orifice plate is popular because it has no moving parts and is very accurate if calibrated and maintained correctly.

In order to measure the pressure drop, there should be pressure tappings on either side of the orifice plate, as shown in Figure 18. These are usually located:

one pipe diameter upstream of the orifice plate and a half diameter downstream

or in the flanges which hold the orifice in the

orifice plate in the pipeline


35

The first method provides more accuracy, but the second method is most widely used.

in general, accurate metering can only be achieved when the orifice plate is designed, fabricated and installed with great care.

The most common type is the square-edge orifice plate, shown in Figure 18.

We must ensure that the flow entering the device is steady and free of eddies which would affect the accuracy of the meter. The orifice plate should, therefore, be placed at a point where temperature and pressure are constant. In addition, bends, valves and other fittings upstream of the orifice plate tend to disturb the flow pattern of the fluid approaching the plate. To avoid this, it is common practice to specify:

a minimum length of straight pipe both upstream and downstream of the orifice plate

or a flow straightening vane to be fitted up-

stream of the plate

A flow straightening vane is a length of pipe with a set of fins running along the inside. As the fluid flows along this stretch of pipe, the fins straighten the flow and prevent swirling. Flow straightening vanes are also used upstream of turbine meters.

Figure 19, demonstrates how the pressure changes as fluid passes through an orifice plate. The differential pressure is measured between points P1 and P2. Point P2 is positioned in line with the vena contracta - the point at which fluid velocity is at its highest, and pressure at its lowest.The differential pressure thus created will depend mainly upon:

type of fluid pipe diameter

orifice diameter

flow rate

inlet pressure

The differential pressure thus recorded may then be converted into a flowrate figure.


36

TurbineMetersTurbine meters are the most popular method of measuring produced oil. They are accurate, reliable and are easily proved and adjusted.

Turbine meters consist of a straight flow tube within which a turbine or fan is free to rotate. You can see this in Figure20. The flowing stream causes the turbine to rotate at a speed proportional to the flowrate. If the flow increases, the turbine will spin faster. If the flow decreases the turbine will rotate more slowly.

In most units, a magnetic pick-up system senses the rotation of the turbine rotor. As each blade passes the pick-up coil, an electric pulse is generated. Each pulse is counted and, as each pulse represents a known volume of liquid, the total flow of oil can be calculated. In some cases, two pick-up coils are installed, so that the two separate pulse counts may be compared with each other as an additional check.

One of the major advantages of a turbine meter is in its use for producing additional flow data. The electrical pulses generated can be fed into a computer system, which can then perform other, more complex, flow calculations. This additional information may be added to the final read-out.

It should always be remembered that the accuracy of a turbine meter depends almost entirely on the precision of the rotor and how consistently its speed of rotation can be related to flow. If the rotor becomes damaged, worn or dirty, then its capacity to measure flow accurately will suffer dramatically.


37

MeteringSystemsThe component parts of a typical turbine metering run are shown in Figure 21. These consist of:

A manually operated inletblockandbleedvalve, which allows the metering run to be positively isolated from the rest of the process upstream. The bleed facility allows the space between the two valve seals to be de-pressurised, proving that no liquid is passing across the valve.

A filter, to remove any particles which may damage the measuring element. The filter is fitted with a differential pressure switch (PDS), which gives an alarm if the pressure drop across the filter gets too high (due to filter blockage).

Flow straightening vanes, to remove turbulence and any tendency for the fluid to swirl.

A measuring element, in this case a turbine meter fitted with a pulse transmitter. The electrical pulses produced may be transmitted to the flow computer. (In the case of an orifice plate metering system, the differential pressure across the plate produces an electrical signal, which may also be sent to the flow computer.)


38

Therefore, in many meter runs, but not shown in Figure 21, you will find:

a thermometer, which measures the temperature of the stream being metered

a pressure transmitter

an on-line densitometer

MeterProvingYou saw, from Test Yourself 9, that small inaccuracies in measurement of oil can result in considerable revenue losses. In order to minimise any errors the meters are proved at regular intervals. The term proving is used in the oil industry to refer to the calibration of oil meters.

The procedure involves comparing the indicated (recorded) volume of oil passing through the meter with the actual (true) volume as measured by a very accurate device known as a prover. From this comparison a correction factor can be obtained which is then used to convert the observed flow readings to true values.

This correction factor is known as the meterfactor.

There are various types of meter prover, but the most common one is thepipeprover.

The basic principle on which a pipe prover works is as follows:

A slightly oversize, elasticsphere is installed in a special length of pipe. It is free to move within the pipe as it is pushed by oil flowing through. As it moves it forms a travelling seal against the inside of the pipe.

The prover is connected in series with the meter to be proved. So, the volume swept out by the sphere in a given time is identical to the volume passing through the meter.

Two detectors are installed in the special pipe near each end. These emit a signal when the sphere passes them, which is transmitted to the pulse counter of the meter. When the sphere reaches the first detector it starts the counter. When the sphere reaches the second detector it stops the counter.

The pulses, and therefore the volume, recorded by the meter should be the same as the true volume displaced by the sphere as it travels between the detectors. If it is not, the recorded volume and the true volume are compared, to arrive at the meter factor.

The meter factor then is

accurately calibrated volume of provervolume registered by meter

A flow control valve, which controls the flow of liquid through the metering run. When there are two or more metering runs, a central meteringcontroller will apportion flow between the different flow control valves to ensure that each meter run is operating within its limits.

A motor operated outlet block and bleedvalve (MOV), which allows the metering run to be positively isolated from the rest of the process downstream. This isolation is required when the meter run is out of service, or when it is being proved by the meter proving system.

A second, motor operated blockandbleedvalve (MOV), which is opened when the meter run is being proved. When this occurs the flow is diverted through the second MOV to the meter proving system.

In practice, the pressure, temperature and density of the oil may change while the flowrate is being measured. To compensate for these changes, readings of the temperature, pressure and density are taken. This information is then fed, together with data from the flow measurement device, into the flow computer. Corrected values for volume flow rate, mass flow rate, etc., can then be computed and recorded.


39

Pipe provers usually consist of a U-shaped or W-shaped length of pipe. Figure22 is an illustration of a bi-directionalU-shapedmeterproverloop.


40

A bi-directional U-shapedmeter prover loop, operates as follows :

The flow enters the meter prover through the meter under test.

In the position shown, the oil flow is holding the calibration sphere against the buffer. If the 4-way diverter valve is now turned through 90 degrees, the flow through the prover loop is reversed. This reversed flow picks up the sphere and carries it round the prover loop for the first pass. Two sphere detectors are mounted in the prover loop, and the internal pipe volume between these detectors is already known.

As the sphere passes sphere detector 'A', a signal to the flow computer records the flowmeter reading at that point.

When the sphere passes spheredetector'B', a new flowmeter reading is recorded.

The difference between these two meter readings, representing the metered volume of the prover loop, is now computed and stored.

The calibration sphere, at the end of the first pass, is now held against the other buffer.

The flow computer now turns the 4-way diverter valve through another 90 degrees to start the secondpass.

The second pass is now completed as above, but with the oil flow reversed.

The flow computer will then average the two metered volumes from the first and second passes and compare this average with the known volume. If the volume recorded by the meter under test is the same as the known volume then the meter has been proved.

If there is a discrepancy between the measured volume and the known volume, the flow computer will calculate a correction factor and then apply this to the meter under test. Another meter proving run will then take place.

When the flowmeter reading (including any correction factor) falls within 0.5% of the known volume, without adjustment, for at least five consecutive proving runs, it is classed as being accurate.

SamplingSystemsIt is not only important that the crude oil is metered accurately. It is equally important to gather information on the nature of the oil being pumped. The chemical and physical nature of the oil may change with time, as may the level of contaminants, such as water or solids, still present after the separation process.

A samplingsystem must therefore be installed to. determine the precise nature of the liquid being pumped.

Sampling systems have two main functions:

sampling for metering

sampling for analysis

Sampling for metering involves the use of an online density measuring system. This system continuously samples the fluid and passes the density results to the flow computer. The computer then combines values for density, pipeline pressure and temperature to calculate the mass flow.

Samplingforanalysisis carried out by a second system. At regular intervals, a pump extracts a small amount of the fluid being metered, and these small samples are stored in a sample jar or similar vessel. Periodically, this combined fluid sample is taken away to be analysed in detail.

An on-line basic sediment andwater (BS&W) system is also installed on most oil handling facilities. The BS&W analyzer ensures that the water and solids content of the crude does not exceed pre-set limits (typically less than 1%") without a warning being transmitted to the operator.

The automatic sampling systems described above are usually backed up by samples taken manually by the operator, as a check on the automatic systems.


41

TestYourself10

1. densitometer

2. flow straightening vanes

3. 4-way diverter valve

4. vena contracta

5. prover loop

6. BS&W analyser

7. booster pumps

8. pick-up coil

9. orifice plate

10. sphere detectors

11. turbine meter

12. block and bleed valves

The following pieces of equipment are used in the metering and sampling system I have described. Can you say briefly what their purpose is, and where in the system they are located ?

You will find the answers in CheckYourself10 on Page 69.

Before leaving this section, work through the followingTestYourself. It will help you to recall the topics we have covered:

SummaryofSection3During this section we have looked at:

the reasons why we need to meter the crude oil before it leaves the production facility

the different types of meter which may be used

a typical meter run and what equipment it contains

a bi-directional meter proving loop and how it works

a typical sampling system and the reasons for sampling the crude oil.

We will now take a look at the final stage in an oil productionfacility - the Pig Launcher.


42

OilPumpingandMeteringSection4-PigLaunchingFacilitiesIn previous sections, we have looked at the equipment used to pump the oil. I have described typical metering systems and how we prove that they are accurate. Sampling and analysis of the crude oil were touched upon.

In Figure 1 we saw that the last stage in a crude oil production facility is normally the pig launcher -the final item of equipment on the installation before the oil enters the main oil pipeline.

The oil which flows through the pipeline may have a small amount of residual water in it. There may be traces of sand, or wax may be deposited from the oil as it cools down. All of these materials may settle out and affect the efficiency of the pipeline.

Devices called pigs may then be pumped through the pipeline, from the pig launcher, to remove the water or sediments which have settled out from the oil.

Pigs should form a reasonably tight fit inside the pipeline, in order that

they perform their cleaning duties effectively

they are efficiently transported through the pipeline by the fluid flow

By the way, there are two main explanations given for the name "pig", both of which are equally unlikely !

The first is that the original pigs were made from stuffed pigskins, sent through water pipelines to clear them out

The second is that early pigs were made of wood, with metal bands around them to help withstand constant rubbing against the wall of the pipeline. As they travelled along the pipeline they "squealed like pigs" as the metal bands scraped along the pipe

I will leave you to choose which one you believe.

TypesofPigPigs come in a variety of shapes and sizes depending on the service which they are intended to perform.

Figure 23 on page 43 illustrates a few of the designs available. Their main uses are as follows:

the squeegeepig is often used for separating different liquids or gases when pipelines are being filled or emptied, or when the same pipeline is being used for different products.

It may also be used for lightweight cleaning duties and for de-watering gas pipelines

the brush pig is used for cleaning and de- waxing pipelines. (Scrapers may also be included in the design). Brush pigs in liquid service often incorporate a series of pipes which provide liquid channels through the pig centre. Some of the liquid behind the pig will pass through the pipes and, because of the angle at which these pipes are set, the pig rotates, thus improving the brushing effect. In addition, the jetting action this causes ahead of the pig stops a build up of debris at that point

the sphere is used mainly to de-water gas pipelines but it is occasionally used for very light cleaning work on oil pipelines

the foampig is most often used for the initial de-watering and cleaning of pipelines. Any welding rods, or other sharp objects which may have been left in the pipeline, embed themselves into the foam as the pig passes by

the foam brush pig is used in lightweight cleaning service, usually on gas pipelines


43

Figure 23: Pig Designs


44

A very specialised pig is shown in Figure24. This is the Kaliperpig or Linaiogpig.

As this type of pig travels along the pipeline, two wheels, positioned near the centre of the pig, press against the walls of the pipe and record how far the pig has travelled.

At the same time a series of fingers, mounted at the back of the pig, slide along the walls of the pipe and measure its diameter.

The information thus collected is recorded on a chart which is built into the pig. The chart can be analysed on arrival, to reveal variations in internal diameter (caused, perhaps, by dents or corrosion pitting) and precisely where these variations occur.

Pigs are becoming more sophisticated and, these days, are capable of measuring and recording a wide range of data related to the condition of the pipeline and contents.


45

PigLaunchersWe will how take a look atFigure25, which shows the basic layout of a piglauncher, and think about how it operates.


46

Under normal conditions, the crude oil supply bypasses the pig launcher and flows through valveX directly into the pipeline.

To load a pig into the pig launcher:

valveB and valveC should be closed the pig launcher must be de-pressurised and

drained of liquid

when these steps are completed, the pig launcher door - doorA- is opened and the pig placed inside the launcher

doorA is then closed the pig launcher is refilled with liquid and re-

pressurised using the pressurisingvalve the pig launch indicator is re-set to record

when the pig passes that point

valveBand valveCare then opened, and valveX slowly closed

the flow of oil is diverted through the pig

launcher and this flow forces the pig into the pipeline

as the pig passes the piglaunchindicator it activates a "flag" which tells the operator that the pig is in the pipeline

the operator can now open valveX, and close valvesBandC

PigLaunchingProblemsIt all looks pretty straightforward, so what can go wrong?

Well, some pigs are very reluctant to leave the pig launcher and it may take three or four attempts at loading, to get them far enough into the pig launcher for them to leave.

Again, pigs can break up as they traverse the pipeline. This may result in the non-arrival of a pig, and then damage to pigs which are sent down after it.

Pigs can stick in the pipeline. Some pig / pipeline combinations found onshore are so prone to sticking that the pig is fitted with a radio transmitter to assist in locating the sticking point. When a pig is stuck, the operator must decide whether to launch another pig in an attempt to shift the first one. If this doesn't work, you have two stuck pigs. Is it wise to try a third?

On occasion, foam pigs will leapfrog' each other inside the pipeline. Launched in the order 1,2,3, they arrive in the order 1,3,2.

Pigs may leave the launcher and enter the pipeline without triggering the 'pig launched' signal; or arrive at the other end of the pipeline without triggering the 'pig received' signal.

In addition, it should be remembered that the operation of pig launchers and pig receivers is amajorcauseofexplosions in theoilandgasindustry. You will understand, therefore, why the launching and recovery of pigs is an operation which must be treated with a great deal of respect.

BasicRulesforPigLaunchingAlways bear in mind the following basic rules:

stick closely to your own laid down proce dures and do not take any short-cuts

during pig launching and receiving operations, do not assume that any event has occurred or not occurred until you have checked and double-checked thoroughly

always make sure that you are launching the correct size of pig :

too narrow, and it may not travel

too wide, and it may stick, blocking the pipeline

too long, and it may jam on a bend, again blocking the pipeline

too short, and it may hang on a bend allowing the flow to bypass it


47

always ensure that the pig is properly positioned in the launcher so that it will leave cleanly when the flow is diverted

always remember to re-set the 'pig launched* device before you launch the pig, otherwise you cannot tell whether it has entered the pipeline or not

As I have already emphasised, opening and closing pig launchers is potentially dangerous and, because of this, most of the launching facilities are fitted with safety systems. These prevent the operator from opening the wrong valve or, worst of all, opening the fauncher door whilst the launcher is open to the pipeline.

SafetySystemsYou will note from Figure 25 that a number ofinterlocks have been labelled. I do not intend to go into any detail on these - this topic will be covered extensively by other Units in the PetroleumProcessingTechnologySeries.

As a simple illustration, however:

interlockA on the pig launcher door interlockB on valve B (inlet to the pig launcher) interlockC on valve C (outlet from the pig launcher) interlockD on the pig launcher low pressure switch

work together to ensure that the pig launcher door cannot be opened unless

valve B is closed valve C is closed the pig launcher pressure is low


48

TestYourself11SummaryofSection4In this section we have looked at:

the reasons why we need to pig a pipeline

the different types of pig which may be used

a typical pig launcher and how to launch a pig

and, very briefly, the need for safety systems

We will now look at a typical Oil Pumping and Metering System and see how it compares with what we have learned so far. Before that, however, try the followingTestYourself:

1. Why do pig launcher systems present an explosion hazard ?

2. What steps should always be taken before a pig launcher door is opened ?

The answers are given in CheckYourself11,which you will find on Page 70.


49

OilPumpingandMeteringSection5-ATypicalOilPumpingandMeteringSystemIn this section we will take a look at a typical oil pumpingandmeteringsystemand see how it relates to what we have covered previously in this Unit.

You may like to refer back to Figure 1, which shows the general layout.

A few assumptions have been made in the design illustrated:

the production operation is offshore and the main pipeline takes the oil to an onshore facility, where it is treated further

there is no crude oil buffer storage facility. Therefore, the separated crude oil is pumped directly frohi the second stage separator, through booster pumps and pipeline pumps into ah export pipeline system

the crude oil metering facility is located between the booster pumps and the pipeline pumps. As previously explained, this location ensures that there is a stable flow to the metering system and that the pressure is sufficiently high to prevent any gas bubbles forming

BoosterPumpsIf we look at Figure26, on the next page, we can see how the boosterpumpsystem works.The crude oil flows from the 2nd stage separator through an emergencyshutdownvalve,ESDV1. Valve ESDV 1 is common to the suction of all three pumps we are using here.

ESDV 1 will be closed by remote signals if an emergency occurs. Typical emergencies would be:

a very low oil level in the 2nd stage separator (part of a processshutdown because only the oil process would be closed down if this occurred)

a fire in the wellheads area (part of an emergencyshutdown, which would shut down all processes).

ESDV 1 also has an interlock (IL)which, if the valve is in the closed position will prevent any of the booster pumps from starting.

It should be noted that, after the pumps are running, the closure Of ESDV 1 will not shut them down via the emergency shutdown system. It only acts as an inhibit to prevent the pumps starting in certain circumstances. If ESDV 1 closes while the pumps are running, then the low-low pressure switch on the discharge of the booster pump (PSLL)would shut down the pumps

Downstream of ESDV 1, the line branches into three, which provide suction to each of the booster pumps. It is normal practice to specify that the piping configuration to the pumps is designed to distribute the oil flow evenly.

The first valve on the suction of boosterpumpAis HV1. HV 1 is a handoperatedvalve and it is also interlocked as an inhibit, to prevent the starting of booster pump A when it is closed.

Downstream of HV 1 and just upstream of the inlet to the pump is a T filter. This is usually a coarse screen, designed to prevent larger items of debris (gloves, helmets, spanners, etc.) from entering and damaging the pump.

The filter is fitted with a differential pressureswitch (PDS) which incorporates a highdifferentialalarm. This arrangement will give an alarm in the event of a high differential pressure caused by filter blockage. It should be noted that a low or zero reading here may be caused either by a clean filter or a ruptured filter!

The discharge of booster pump A is fitted with a pressure switch low (PSL) and a pressureswitch low-low (PSLL). PSL will give an alarm and PSLL will cause the pump to shut down in the event of low pressures.


50


51

After a shutdown, PSLL creates a potential problem. If the pressure at that stage is below the setting of PSLL, the pump cannot be re-started. A shutdown signal is still being sent from the Pressure Switch Low-Low. Something must be done to allow the pump to restart.

The problem is overcome by, automatically, bypassing PSLL for 30 seconds when the pump is started. This allows sufficient time to build up enough pressure to re-set the switch. If the increasing pressure does not re-set PSLL before the 30 seconds have elapsed, then the pump will shut down again. This system is called a time-pressure race, i.e., the pump is racing against time to generate sufficient pressure to re-set the switch.

The discharge of the pump is also fitted with a pressure switch high (PSH) and a pressureswitchhigh-high(PSHH). PSH will give ah alarm and PSHH will cause the pump to shut down in the event of high pressures, perhaps because of problems downstream.

The discharge of the booster pump is fitted with aminimum flow non-return valve (SV1), which we have already described in Section 2, Page 31 and Figure 17. To prevent the continuously re-cycled oil from becoming progressively hotter, it is routed all the way back to the 2nd stage separator via HV 2.

The discharge from pump A now passes through a hand-operated valve, before joining the flow from the other pumps. The combined flow then passes through level control valve LCV 2. This valve controls the oil level in the 2nd stage separator. The separator level controller will open this valve if the level rises, and close it if the level falls. We can see that, in the event of a failure of supply to the 2nd stage separator, the valve would close completely and the booster pumps would go on to minimum flow.

After passing across LCV 2 the oil flows to the sampling and metering systems.

You should note that the booster pump system is designed so that its discharge pressure is high enough to meet the required suction pressure at the main oil pipeline pumps, which we will look at later.


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SamplingSystemFigure 27 shows the layout of a typical sampling system.


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You will see that a side stream is removed from the inlet header to the metering system by one of two samplepumps,A and B.

This side stream is drawn through two continuous sampling devices (A and B) where a small sample is removed and stored.

As an illustration of the sampling routine:

sample device A may take a composite sample of five litres per day

sample device B may take a composite sample of 35 litres per week

one litre spot samples may be taken manu ally by the operator, as a back-up, at twelve hour intervals

After leaving the sample pumps, the sample stream flows to two densitometers (Aand B) and a basic sediment andwater (BS&W) analyzer before returning to the inlet header.

A densitometer is designed to measure the density of the sample stream fluid. It does this by comparing this fluid with a reference, whose density is known. The result is then passed automatically to the flow computer.

A BS&Wanalyzer checks for the basic sediment and water contained in the crude oil flow. Most pipeline operations have a maximum specification for BS&W which, typically, may be "not more than 1%". This means that no more than 1 % of the total volume pumped into the pipeline should be sediment and water.

If an increased BS&W level occurs for any length of time, the pipeline pigging programme is readjusted to increase the rate of pigging. This is required to prevent the sediments and water from blocking and corroding the pipeline.


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MeteringSystemIn the meteringsystem shown in Figure28,i have included just one meter run and a prover loop. The meter run, which we could designate run 'A', is from upstream of the inlet block valve (HV1) to downstream of the outlet block valve (MOV1). In a complete system there would be three or more parallel runs. I have indicated this in the drawing as additional runs 'B' and 'C'. A single prover loop is used and there are connections between each run and the prover, enabling it to be placed in series with any of the meters.


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When meter run 'A' is in service, the normal flow pattern would be through:

the inletblockvalve(HV1)

the filter (F)

the flow straightening vanes

the turbinemeter

the flow control valve (FCV1)

the outletblockvalve(MOV1)

and from there to the pipeline oil pumps.

When the meter in run 'A' is being proved, the flow would be through:

the inletblockvalve(HV1)

the filter (F)

the flow straightening vanes

the turbinemeter

the proverloopblockvalve(MOV2)

the 4-wayproverloopdivertervalve(MOV3)

the proverloop

the 4-wayproverloopdivertervalve(MOV3)

the prover loop flow control valve (FCV 2)

and from there to the pipeline oil pumps.

Note the flow computer in the drawing. You will remember from Section 3 that one of its jobs is to compare the volume indicated by the meter with the true volume of the loop to obtain a meterfactor. In addition, it ensures that there is equal flow between each of the meters being used. It does this by altering the settings of the appropriate flow control valves. If meter run TV were in normal service this would be FCV1. If meter W is being proved however, the flow would be controlled via FCV2. The flow reading from each meter is fed to the computer via a flow transmitter (Ft).

So, when the meter in run 'A' is being proved, the flow computer:

closes MOV1

opens MOV2

transfers control of flow from FCV1 to FCV2

allows flow to stabilise

operates MOV3 to start first proving run

operates MOV3 again, to reverse flow through prover and start second (and any further) proving runs

performs necessary calculations to obtain meterfactor

A few other points to note are:

1. interlocks are fitted to MOV1 and MOV2 to ensure that these valves are at the right setting (open or closed) before the meter proving starts

2. pressure relief valve PSV1 is located downstream of the filter and upstream of the flow straightening vanes. If HV1,MOV1 and MOV2 are all closed for any reason, the pressure inside the meter run may rise due to any temperature increase. PSV 1 is fitted to relieve this pressure

3. to ensure the accuracy of the prover loop, the sphere is always oversized by 1-2%. This ensures a tight fit between the surface of the sphere and the walls of the prover loop. The sphere is replaced on a regular basis, and it is normally the first item to be changed if the accuracy of the prover loop is suspect

4. An independent contract company is often used to prove the prover loop, say, on an annual basis


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OilPipelinePumpsIf you look at Figure29, you can probably see how the oilpipelinepumpingsystem works.


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It is rather similar to the booster pump layout, so we will concentrate only on the important differences:

When the minimum flow system is operating, crude oil is re-cycled from the discharge to the suction of the pump and is not routed back to a separator, as was the case in the booster pump layout. (The separators are upstream of the meters. Therefore, if the oil was re-cycled to the separators, it would pass through the meters twice, which, of course, would introduce errors into the flow measurements)

However, because the pipeline pumps are transferring a large amount of energy to the oil, this direct re-cycling would result in a rapid and substantial temperature rise. To prevent this from occurring, a re-cycle cooler is fitted to cool the crude before it is returned to the suction of the pipeline pumps. Offshore, the re-cyclecooler would often use seawater as a cooling medium (as shown in Figure 29) because it is cheap and plentiful

FluidCouplingPipeline pumps have a variable speed drive. The speed at which they operate is determined by the pipelinepressurecontroller (which we will look at later). If the line pressure is too low, then the controller increases the pump speed; if it is too high, the pump speed is decreased.

This speed variation may be achieved by a fluid coupling between an AC electric motor and the pump. Fluid couplings are also known as hydrauliccouplings.Acommon design is shown in Figure 30


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Figure 30 is a three dimensional cut-away drawing of the coupling assembly. You will see that the coupling comprises:

an inlet shaft, connected to the drive motor

an outlet shaft, connected to the main pump

The inlet shaft drives an oil circulating pump. The oil path is from the reservoir, via a cooler and small holding tank, into the circulating pump section. From the pump discharge, the oil flows to the scoopchamber

You should take particular note of the components labelled the runner and the impeller. They are both of similar design and look like a ring of cups attached to a wheel. The runner is a t the end of the inlet shaft, and the impeller at the beginning of the outlet shaft. Each turns independently of the other within the casing. The only connection between them is made by the circulating oil when the unit is in operation-hence the term fluid coupling.

The basis of operation is as follows:

the inlet shaft turns the runner, and drives the oil circulating pump. Note that the runner turns at 100% of the drive motor speed at all times

the cups on the runner pick up oil from the outer perimeter of the scoop chamber and throw it into the receiving cups of the impeller.

The runner is therefore acting as a pump

the oil striking the impeller cups turns the impeller, which is acting as a turbine

the impeller then turns the main pipeline pump

PumpSpeedControlThe amount of oil transferred between the runner and the impeller and, therefore, the main oil pump.

Figure31, on page 59, shows a series of cross sectional diagrams through a fluid coupling, which help us to explain this mechanism of speed control.


59


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The position of the scooptube will determine how much power is transmitted across the coupling.

In Figure 31 a the scoop tube is at maxi-mum extension, at a radius slightly greater than the outer boundary of the circulating oil. Therefore, all oil entering the scoop chamber is 'scooped' away by the open tip of the scoop tube and returned to the reservoir. The scoop chamber is virtually empty, and no oil remains for the runner to throw at the impeller. Power transmission is therefore nil, and the main pipeline pump is stationary.

At an intermediate extension of the scoop tube, (Figure 31b), a ring of oil can accu-mulate in the scoop chamber between the tip of the scoop tube and the outer bound-ary. This limited volume of oil is now avail-able for the runner to throw at the impeller. An intermediate level of power can now be transferred across the coupling to drive the main pump.

In Figure 31 c, the scoop tube is at mini-mum ra dius, the oil retained within the scoop chamber is at a maximum, and full power transfer is taking place.

PipelinePumpingPressureReturning to Figure29 again, the oil pipeline pump speed is controlled by the speedcontroller(SC) which takes its signal from the pipelinepressurecontroller.

If the pipeline pressure is too low, these controllers will speed up the oil pipeline pump by shortening the extension of the scoop tube.

If the pressure is too high, the controllers will slow down the main pump by increasing the radius of the scoop tube.

PressureTransmitterFinally, just upstream of the main outlet valve ESDV 2 is a pressure transmitter (PT) which sends a telemetry signal to the local control room, to the shore (in offshore locations), and to other oilfields sharing the same pipeline facility.

This safety feature is required to prevent over-pressuring the pipeline.


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PigLaunchingThe piglaunchingfacility is illustrated in Figure32.It is similar to the one I have described previously.

Normal flow through the system would be:

through ESDV2

through MOV1

through ESDV3, and then

to the pipeline

ESDV2 and ESDV3 are two emergency shutdown valves which are interlocked with the ESD system to ensure that the pipeline pumps cannot be operated when these valves are closed.

On an offshore installation, ESDV 3 may be situated on the sea bed. It is designed to ensure that ho oil can flow back to the installation in the event of platform malfunction. It is only operated in extreme emergencies such as a fire or large oil leak.


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Now,withreferencetoFigure32again,wecanlistthestepsinvolvedinlaunchingapig:

1. check that the pig is undamaged, the c

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Documents

centrifugal pump list

oil sampling system

differential pressure

sampling systems

types pig launchers

checkyourself answers

meter list

operation of turbine