Oil & Gas Separation Book 2.pdf

POL Petroleum Open Learning

Oil and Gas Separation Part of the Petroleum Processing Technology Series

OPITO

2

THE OIL & GAS ACADEMY

POL Petroleum Open Learning

Oil and Gas Separation Part of the Petroleum Processing Technology Series

OPITO

2

THE OIL & GAS ACADEMY

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

All rights reserved. No part of this publication may be reproduced, stored in a retrieval or information storage system, transmitted in any form or by any means, mechanical, photocopying, recording or otherwise without the prior permission in writing of the publishers.

Oil and Gas Separation Systems - Workbook 2


(Part of the Petroleum Processing Technology Series)

Contents

Page

*

Training Targets

2

*

Section 4 -Control of Separators

3

The Control Loop Separator Level Control Separator Pressure Control

*

Section 5 - Separator Safety Systems

Level Control and Systems Pressure Control and Safety Emergency Shut-down Valves

*

Section 6 - Operations of Separators

Routine Operation Checks Start-up Procedure Shut-down Procedure Blow-down Procedure

21

34

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

1


Training Targets When you have completed Workbook 2 of this unit you will be able to : • Explain the basic principals of process control • Describe the equipment used in separator level control • Describe a simple separator safety system • Explain the routine operational checks on a separator • Describe in simple terms a separator start up procedure

Tick the box when you have met each target.

2

Oil and Gas Separation Systems


Introduction Section 4 - Control of Separators In the last part of Section 3 of this programme you looked at the external features of separators. Two of the items you saw were pressure gauges and level sight glasses. These pieces of equipment are used to enable the operator to check pressure and liquid levels inside the vessels. Pressure and liquid level are features of the process which can vary. Each can increase or decrease with variations in separator throughput. However, in order to obtain optimum separation, the pressure and liquid level must be maintained at a constant value. So, apart from the equipment used to check the liquid level and pressure, separators have two major controls.

* Liquid Level Control * Pressure Control

In any continuous process such as oil and gas separation there are a number of factors which must be kept within certain limits. These are called the process variables. The four most common of these are.

* * * *

Liquid Level Pressure Temperature Fluid Flow

As I pointed out earlier, we are going to concentrate on Liquid Level and Pressure, but the basic method of achieving control applies to all four. It relies on having built into the system a suitable control loop.

Correcting Unit

In this section we will look at each of these control systems and see how they work. Please note that we will be covering the subject of control at a very basic level. Other programmes in this series will delve much more deeply into Instrumentation and Control. Before we proceed, however, let’s consider the fundamentals of process control.

The Control Loop There are 4 main elements in a typical control loop and these are :

* * * *

The The The The

Process Variable Measuring Unit Controller Correcting Unit

Figure 26 shows a simple block diagram of a control loop.

Process Variable

Measuring Unit

desired value measured value

correcting signal

Controller Figure 26 : The Control Loop

3


We can look at the four elements in turn.

The Process Variable This is that part of the process which has to be controlled within certain limits, i.e. Level, Pressure, etc. The actual value of the process variable which the operator wishes to maintain is called the desired value. We need not say any more about the process variable at this point.

The Measuring Unit This unit measures the actual value of the variable. It could be a Pressure Measuring Instrument, a Flow Measuring device and so on. The measuring unit obtains the measured value.

The controller may work using air (pneumatic operation), liquid (hydraulic operation) or electronics.

The Correcting Unit This part of the control loop is usually a valve. On receipt of the signals from the controller it opens or closes to alter the process variable. The measured value is then returned to the one indicated by the desired value. The following example should help you see how a simple control loop works. Look at Figure 27.

The Controller

Imagine in this simple example that water is entering the tank through the inlet line at the top of the tank. Joe has opened the valve on the outlet line so that exactly the same amount of water is leaving the tank as is entering. As long as this situation remains steady, the water level in the tank will stay constant. This level, as shown on the level indicator, is the measured value of the process variable. Also, because that is the level which Joe wishes to maintain in the tank, it is the desired value. There is no difference between the measured value and the desired value. Supposing that the flow of water entering the tank is somehow increased. What will happen to the water level? Of course, if nothing is done to the outlet valve, the level will start to rise.

It is the job of the controller to compare the measured value of the process variable with the desired value. If it senses a deviation between the two it then sends a correcting signal to the final element in the loop, the correcting unit. For instance, supposing you wanted to maintain the pressure in a separator at 250 psi, but the pressure had increased to 275 psi. The desired value is 250 psi and the measured value is 275 psi. There is obviously a deviation. A mechanism within the controller would sense this and instruct the unit to send an appropriate correcting signal.

In this figure the process variable is the level of water in the tank. The measuring unit is the level indicator. The correcting unit is the valve in the water outlet line. Finally, the controller in this case is the plant operator. Let’s call him Joe.

Figure 28 on the next page, shows this.

Figure 27

4


Figure 28 But Joe is keeping his eye on the level indicator and sees the change in level. He compares the new measured value with the desired value and notes that the level has increased. In order to reduce the level again the valve in the outlet line must be opened more. Joe does this manually and increases the outlet flow until the measured value of the liquid level once again matches the desired value. This is shown in Figure 29.

Figure 29 Joe, the controller, has maintained a constant level in the tank by : • Comparing the measured value of the level in the tank with the desired value. • Noting the difference between measured and desired values. • Sending an appropriate signal to the correcting element in the loop. (In this case an instruction to his hands to open the valve).

Of course having an operator like Joe standing by the tank all day long would be a waste of an operator’s time, and pretty boring for Joe. It would be much more sensible to have an instrument to do this simple control job. Let’s see how this basic principle of automatic control is applied to level and pressure control in a separator.

5


Separator Level Control

Test Yourself 7 Fill in the missing words from the following paragraph. That part of a continuous process which an operator wishes to control within certain limits is called the ................ ................, and its target value is called the ................ value. A measuring unit obtains a measured value from the process and feeds it to a .................................... whose job it is to compare the two values. If a deviation exists between the two values, the ........................ sends a correcting signal to the final element in the loop - a correcting unit which is usually a ................ ................ .

Level control in a separator is very similar to the level control system in the simple example I just gave. Before we look at the control loop used, let’s consider the reasons for maintaining the liquid level in a separator at a constant height. They may seem fairly obvious but I think it is worthwhile listing them here. Liquid Level Control is required : • To prevent liquids being carried out with the gas. (Known as carry-over.) • To prevent gas from leaving the separator through the liquid outlet. (Blow round.) • To help maintain the pressure on the vessel. (Fluctuating levels affect the pressure.) • In a 3 phase separator, to prevent oil from leaving through the water outlet or vice versa.

• To ensure optimum retention time.

Think first of all of an oil level control system on a 2 phase horizontal separator.

You will find the answers in Test Yourself 7 on page 42

Figure 30 shows the system in its most basic form.

6


The hardware associated with the system consists of: • The displacer mechanism - which is the measuring unit. • The level controller - in this case we are looking at a pneumatic controller.

• The level control valve - the correcting unit.

Let me describe each of these units and briefly explain how they work.

Displacer Mechanism This piece of equipment measures liquid level by a method which is based on the principle of Archimedes. The principle states that, if a body is immersed in a liquid, it will apparently lose weight equal to the amount of liquid it displaces. So, a cylindrical weight partially submerged in a liquid will have a certain apparent weight. (Less than its actual weight because of the buoyancy effect of the liquid.) If the liquid level rises or falls, then more or less of the cylinder will be submerged. Its apparent weight will therefore vary. The apparent weight can be measured to give an indication of the level of the liquid. Figure 31 on the next page, shows how the apparent weight varies with the liquid level.

Figure 30 : Oil Level Control System

7


The loss or gain of apparent weight has to be transmitted to the controller as a signal which is proportional to the increase or decrease in liquid level. This is done by means of a torque tube mechanism. Figure 32 shows the torque tube assembly.

Figure 31a

Figure 31b

Much of the cylinder submerged - buoyant effect means low apparent weight.

Less of the cylinder submerged - higher apparent weight.

8


A hollow rod called the torque tube (C) is connected via a torque tube plate (D), to the supported end of the float rod. The opposite end of the torque tube is welded to the outer flange of the assembly (indicated E). A small diameter shaft, the torque tube rod (F), fits inside the torque tube and is welded at the torque tube plate. This shaft protrudes through the outer flange where it is free to rotate. Look carefully at Figure 32 again and try to visualise what will happen to the assembly as the liquid level moves up or down. Imagine the level going down in the vessel. As it does so the apparent weight of the displacer will increase. The increased weight hanging on the float rod will give a twisting motion to the torque tube. (Remember that the torque tube is fixed at the flange end but only supported on the knife edge at the free end).

Figure 32 : Torque Tube Assembly The displacer cylinder may be contained in a chamber which is connected to the separator but mounted outside it. The liquid level in the chamber is obviously the same as the level in the separator. Having the displacer placed outside the separator like this means that it is unaffected by any turbulence in the vessel.

Because the torque tube rod is welded to the torque tube, it will rotate as the tube is twisted. The rotation of this rod at its free end will be in proportion to the increase or decrease in liquid level. The rotation is transmitted via linkage to the next element in the control loop, the controller.

The displacer is suspended from one end of a float rod, marked (A) on the figure. The other end of the float rod is supported on a knife edged bearing (B).

9


Level Controller We have seen already that the job of a controller is to compare 2 signals, the measured value signal and the desired value signal. If a deviation exists between the two, the controller then has to send a correcting signal to a control valve.

In order to perform its job the controller has 4 separate, but interconnected, units. They are :

• The Differential Mechanism

• The Flapper/Nozzle Assembly

• The Feedback Unit

• The Pilot Relay

CONTROLLER measured value Desired value

DIFFERENTIAL MECHANISM

There are various types of controller, but in this section we are going to look at the basic principle of operation for one type of pneumatic controller.

to recorder

20 SUPPLY AIR

FLAPPER/NOZZLE UNIT

FEED BACK UNIT

PILOT VALVE

TO CORRECTING ELEMENT

Figure 33 shows the relationship of the four units in the form of a block diagram

Figure 33

10


One end of the desired value link acts as a pivot point for the differential arm. The position of the pivot point can be altered by means of an adjusting knob at the other end of the link (not shown).

We can see how each of the units works, and how it interacts with the others. Differential Mechanism

The measured value link is also connected to the differential arm. It is connected at the other end to the measuring unit, or element (not shown).

It is the job of the differential mechanism to compare the desired and measured values of the process variable. If a deviation exists between the two, the unit feeds this information to the next unit in the controller, the flapper/nozzle assembly.

From the centre of the differential arm a deviation link transmits any motion of the arm to the flapper/ nozzle assembly.

There are two types of differential mechanism which can measure the deviation, and these are known as :

• The Motion Balance Mechanism

• The Force Balance Mechanism

Look again at Figure 34. You can see that, providing there is no movement of the M.V. link relative to the D.V. link, there will be no movement of the deviation link. However, if the measuring element causes the M.V. link to move, there will be a movement of the deviation link.

Motion balance mechanisms use two mechanical linkages to compare the measured and desired values (abbreviated as M.V. and D.V.).

Force balance mechanisms use pressure applied to bellows to compare desired and measured values. Figure 35 on the next page, shows this -again, in a very simple form.

Figure 34 shows this in simple form

Figure 34 : A Motion Balance Mechanism

11


Supposing the measuring element output gave an increased pressure signal. The M.V. bellows would expand against the D.V. bellows. This movement would cause the free end of the force bar to move. The movement could then be fed to the flapper/nozzle assembly as a deviation. Force balance is the mechanism most commonly used in Pneumatic Controllers. I have mentioned several times the flapper/nozzle unit. Let’s look now at this piece of equipment. Flapper/Nozzle Assembly The flapper/nozzle unit consists of three items, the flapper, the nozzle and the restrictor. Figure 36 is a simplified drawing of the unit.

Figure 35 : Force Balance Mechanism The output from the measuring element is fed as a pressure to the measured value bellows. The movement of the bellows is opposed by a second set of bellows, the desired value bellows. These are pressurised by a signal which relates to the desired value of the process variable. Sandwiched between the two sets of bellows is one end of a bar known as the force bar. The bar is pivoted using a fulcrum and the other end of the bar is free to move. I’m sure that you can visualise what happens when a deviation between desired value and measured value occurs.

12


It is the job of the assembly to send a correcting pressure signal from the controller to the final element in the control loop, the control valve. Here is a basic explanation of how that is achieved. An air supply is fed to the line upstream of the restrictor, typically at a pressure of 1.3 bar. This air can pass through the restrictor to the nozzle outlet and also the variable back pressure outlet. If the flapper is positioned away from the nozzle it is possible for the air to pass through the restrictor and out through the nozzle. Because the diameter of the restrictor is small compared to that of the nozzle, there will be very little pressure build-up in the space between the restrictor and the nozzle. This means that there will be no build-up of pressure in the back pressure line. However, if the flapper is moved towards the nozzle, the area of nozzle through which the air can pass is reduced. This means that the nozzle back pressure will increase. If the nozzle is completely covered then the back pressure would build up to the supply pressure of 1.3 bar. Figure 37 shows a graph of the back pressure obtained from a flapper/nozzle system against the position of the flapper relative to the nozzle.

Figure 37

You can see from the graph, that the section of the curve between points 1 and 2 is almost a straight line. The nozzle back pressure varies from 0.2 bar at point 1, to 1.0 bar at point 2. Between the 2 points, the back pressure obtained will be proportional to the distance that the flapper is away from the nozzle. Pneumatic instruments are usually designed to operate over a standard pressure range. This range must lie on the straight line portion of the graph which you saw in Figure 37.

With a supply pressure of 1.3 bar, the operating range chosen is from 0.2 to 1 bar. So, nearly all pneumatic instruments will record, transmit and control within this 0.2 to 1 bar range. You can probably imagine now, how the controller is able to send a correcting signal to the correcting unit. If the movement from the differential unit is linked to the flapper, a varying back pressure signal will be obtained from the nozzle. This signal will be proportional to the deviation between measured and desired values of the process variable.

13


nozzle

In an actual controller, the movement of the flapper is extremely small. The total movement required to change the signal from 0.2 to 1 bar is only a little more than 0.1 mm. Such a small movement means that a direct linkage from the differential unit to the flapper is impracticable. The deviation signal must be adjusted to compensate for this. This brings us to the next unit in the controller, the feedback unit.

Feedback Unit Feedback simply means feeding back the output signal of the controller to the input of the same unit. This allows relatively large movements of the differential unit to cause very small movements of the flapper. We will see how this is done shortly. Feedback is also used to introduce more complex control actions to the loop. These control actions are beyond the scope of this programme. However, you will come across them if you follow other programmes covering Instrumentation and Control, in the Petroleum Processing Technology Series. We can see now how a simple feedback unit works. The mechanism is shown in Figure 38.

Figure 38 : Feedback Unit The pivot point of the flapper is attached to the movable end of a set of bellows. The movement of these bellows is opposed by a spring.

The increased bellows pressure will move the pivot point of the flapper against the spring until the spring and bellows forces are balanced.

The nozzle back pressure is fed to these bellows in addition to being the controller output signal to the correcting unit.

As this happens, the movement of the pivot point will lift the flapper away from the nozzle.

When a deviation occurs, the flapper moves towards the nozzle and causes an increase in back pressure.

This sequence of events, movement of the flapper towards then away from the nozzle, will continue until a steady state is reached.

14


control signal

When this state is reached, the flapper will have moved very slightly nearer to the nozzle. Just enough to increase the output to the correcting unit.

Pilot Relay The final unit in our pneumatic controller is the pilot relay. This is a device which is connected to the nozzle back pressure line.

diaphragm

Its function is to act as a signal amplifier.

actuator spring

The controller output has to operate an actuator on the correcting element which is usually a control valve. This actuator, as you will see, requires a relatively large volume of air. Because of the restriction in the supply line to the nozzle, only a limited volume of air can be supplied to the actuator. The pilot relay can boost this air supply, for proper operation of the valve.

valve stem

I think that this explanation of what the pilot relay does is sufficient at this stage.

The Level Control Valve The function of a control valve is to throttle, or regulate, the rate of flow of a fluid.

valve plug

valve body

You will remember from our simple example earlier, that Joe had to open and close the valve in the outlet line from the tank. He was operating a level control valve manually. In an automatic control system, however, the control valve is operated by the signal from the controller. In the case of a pneumatic controller the signal is air pressure, which varies between 0.2 and 1 bar.

valve seat

There are many different types-of control valve in use. Figure 39 is a simplified drawing of a typical valve which could be used in a level control application.

Figure 39 : Level Control Valve

15


The valve is called a diaphragm motor valve and consists of the following items.

• diaphragm

• actuator spring

• valve stem

• valve body

• valve plug

• valve seat

The actuator spring is attempting to hold the valve in the open position by pushing up the diaphragm and lifting the valve stem. This is called a normally open valve.

If there is a complete loss of control pressure, the actuator spring will cause the valve to move into the fully open position. This type of valve is sometimes called a fail open valve. It is possible to change the action of the valve or the controller so that a loss of control signal would cause the valve to close. In this case the valve would be referred to as a fail closed valve.

Test Yourself 8 will help you draw together all these aspects of control loop operations.

You will notice that the valve illustrated in Figure 39 has two valve plugs and seats. It is known as a double ported valve. Although single ported valves are sometimes used, the double valve is preferred for level and pressure control duties on separators. Take another look at Figure 39 and satisfy yourself that you understand how the control valve works, and how it fits into the complete level control loop.

The control signal is applied to the top of the diaphragm. Increasing pressure of the control signal overcomes the resistance of the actuator spring and gradually closes the valve. The valve is arranged so that a signal pressure of 3 psi will just start to close the valve. With a signal pressure of 15 psi, the valve will have moved to its fully closed position.

16


We have just been looking at separator level control. You will remember that the other major separator control is that of pressure. Let's finish off this section by having a look at separator pressure control.

Test Yourself 8 The following items from a level control loop are part of the measuring unit, the level controller or the level control valve. Indicate by a tick in the box provided, to which part of the system each belongs.

Measuring Unit

Level Controller

pilot relay

differential mechanism

torque tube rod

actuator spring

valve plug

flapper nozzle assembly

diaphragm

feedback unit

float rod

valve stem

Level Control Valve

Separator Pressure Control As with level control, the basis of a pressure control loop consists of : • a process variable (in this case, in the separator)

the pressure

• a measuring unit (some form of pressure measuring device) • a controller (again, a pneumatic controller in this example)

• a correcting unit (once more a control valve)

The controller and control valve work in the same way as the units used in level control. The measuring unit, however, is obviously going to be different.

You will find the answer to Test Yourself 8 on page 43

17


Think for a moment about what the measuring unit has to do. It must transmit the measured valve to the controller as a signal. In level control this is done by the torque mechanism which transmits rotation of the tube via a linkage. Somehow the pressure measuring unit must perform a similar function. We have already looked at a pressure measuring instrument in Section 3, the bourdon tube type pressure gauge. This mechanism could be used to transmit a measured valve signal to the controller.

Activity Think about the way in which a pressure gauge works. How could the measured pressure be transmitted to the flapper of a controller ?

18


In a most simple way, the transmission could be as is shown in Figure 40.

As the valve opens, more gas is allowed to leave the separator and the pressure is reduced. Obviously, if the pressure in the separator goes down, the movement of the bourdon tube pushes the flapper towards the nozzle. This will cause the output signal pressure to rise, causing the control valve to close. If less gas now leaves the separator, the separator pressure will then increase.

Test Yourself 9 In the pressure control loop I have just described, is the control valve in the fail open or fail closed mode?

Figure 40 : A Pressure Control System You can see that the free end of the bourdon tube is connected via a linkage to the flapper/nozzle assembly of the controller. In the set up shown, if the pressure in the separator rises, the bourdon tube tries to straighten and the movement pushes the flapper away from the nozzle. This causes the output signal from the controller to fall which in turn causes the control valve to open.


19


Summary of Section 4 Before going on to summarise Section 4, I should emphasise once again that the subject of process measurement and control is very complex. We have only scraped the surface of the subject in this section but you should now know how a simple control system works. We started the section by considering the four most common process variables : • Liquid Level • Pressure • Temperature and

• Fluid Flow

Of these, we concentrated on the control of liquid level and pressure. You saw that a suitable control loop is required to achieve control, and such a loop has four main elements :

• • • •

The The The The

process variable measuring element controller correcting unit

With the aid of a simple example involving an operator called Joe, we saw how the elements of a control loop work together to maintain the liquid level in a tank at a constant value. From there we moved on to consider a separator level control system. We saw that the hardware associated with a typical system consists of:

• The displacer mechanism • The level controller • The level control valve

We looked at each of these elements of the system in some detail and saw how they are constructed and how they work. We paid particular attention to the controller with its four separate, but interconnected, units which are known as :

• • • •

The The The The

After a detailed look at the principle of operation of a level control loop we finished off the section by working through the basics of separator pressure control. Here you saw that the basis of a pressure control loop is the same as that for level control. However, the measuring element is obviously a different unit to the measuring element of a level control system. We used a bourdon tube type pressure measuring element in the example we considered, but you should remember that many other types of measuring instruments are available. In the next Section we will be looking at additional equipment and instrumentation which may be fitted to a separator to ensure safe operation of the vessel or train of vessels.

differential mechanism flapper/nozzle assembly feedback unit pilot relay

20



Section 5 - Separator Safety Systems In the last section we looked at control of the two main process variables in a separator, i.e. Level and Pressure. These two control systems usually operate with relatively few problems. However there is always the possibility that, for some reason, they fail to maintain control. This may happen, for example, because of instrument malfunction in the control loop. If this should occur a potentially hazardous situation will arise. Think for a moment about the possible consequences of losing either level or pressure control in a separator.

Activity Jot down in the space below your ideas of what might result from the following separator malfunctions.

• oil level goes too high

• oil level goes too low

• pressure continues to increase

• pressure continues to decrease

• water level goes too high

• water level goes too low

21


Here are a few ideas of mine. How do they compare with what you have written down? • If the oil level goes too high, a situation will be reached where oil gets carried over with the gas, causing problems downstream

We have already looked at similar equipment in Section 1. There, you saw that flow lines can be fitted with pressure switches to warn the operator of high or low flow line pressures. You also saw that these pressure switches are tied into the Emergency Shutdown (ESD) system.

• Should the oil level go too low, there is a danger of gas leaving the separator through the oil outlet

Let us now consider a typical 3 phase separator, together with its protective devices.

• If the pressure increases too much, there is a risk of exceeding the safe working pressure of the separator

Level Control and Safety

• In the situation where the pressure falls too much, there will be insufficient pressure to push the liquids from the separator • If the water level rises above the weir, water will contaminate the oil leaving the vessel • Should the water level go too low, oil will flow from the separator through the water outlet

Let’s start by having a look at the oil level in a separator. We can see what would be the sequence of events if the oil level started to rise, and continued to rise. First of all you will remember from the previous section that an increase in level will cause the level control valve to open. Take a look at Figure 41 which shows the oil accumulation and outlet side of a 3 phase horizontal separator.

LCV 01 Figure 41

In this section, we are going to look at the equipment designed to prevent such situations arising in a typical separation system. The section will concentrate on three variables (oil level, water level, and pressure) and the degrees of protection afforded by this equipment.

22


You will see that this is a simple illustration of a level control loop.

to rise or fall, then the separator must be protected automatically.

LC 01 is the level controller and LCV 01 is the level control valve. So in this case an increase in oil level causes LC 01 to open LCV 01.

This is done by having a second degree of protection using level switches connected to ESD valves. These switches are connected to the separator independently of the level controller.

However, the level might continue to rise (due to equipment malfunction, etc.), and start to approach a hazardous situation. In order that the operator can be warned of the situation, alarm signals are generated by the controller.

Figure 42 shows this.

HIGH and LEVEL SWITCH LOW- LOW. If the level reaches the setting of either of these switches, a signal is sent to the ESD system which automatically isolates the vessel and makes it safe, by activating the appropriate ESD valves. We will look at the location and operation of ESD valves shortly. It is quite common for all the instrumentation relating to separator level control and safety, to be located externally to the vessel. In such a case, the instrumentation can be mounted on pipework sometimes referred to as an instrument bridle.

If you look again at Figure 41, you will see LAH 01 connected to the controller LC 01. The letters stand for LEVEL ALARM HIGH

Figure 43 on page 24, shows a typical set up incorporating an instrument bridle.

If the level should reach the setting of LAH 01 an audio/ visual alarm would be generated. This is usually a noise (rapid high pitched beeper) and a flashing light which would indicate the alarm condition at a location which is normally manned, e.g. a central control room. The alarm would alert the operator, who could then try to rectify the situation before the actual hazardous situation is reached. If a falling level is the problem, you can imagine that a similar alarm is generated by LAL 01. If the situation is not rectified and the level continues

Figure 42 You will notice that the switches are designated LSHH and LSLL These stand for LEVEL SWITCH HIGH-

23


You can see that the bridle is connected via valves to the top and bottom of the separator. Take a look at Figure 43 and identify the bridle which incorporates the level controller, the level alarms, the level switches and the sight glasses. You will remember from Section 3 that the sight glasses give a visual indication of the actual level inside the separator. Note that there are two sight glasses which overlap each other. The normal operating level would lie within the overlap, enabling the level to be checked through both sight glasses. We have just looked at the oil side of the separator. In a 3 phase vessel, the water level must also be controlled. Of course, in this section of the vessel, the water is covered by a layer of oil. So there is an interface between the water and oil. It is this interface which is measured by the controller. Apart from that, the control and safety of the water end of the separator works in a similar manner to the oil end.

Figure 43 : Instrument Bridle

24


Pressure Control and Safety

Test Yourself 10 Make a simple sketch of the water outlet end of a separator. Your sketch should show an instrument bridle with the relevant instrumentation.


You saw in the introduction to this section that an increase or decrease in pressure in the separator is also potentially hazardous. Let's look now at this problem. I am sure that by now you will have realised that there are several degrees of pressure protection on a separator. Look at Figure 44. This shows a simple pressure control loop.

Figure 44

25


PC 01 (the pressure controller) activates PCV 01 (the pressure control valve).

This operating pressure is well below the maximum which the separator is capable of holding.

In addition PC 01 generates alarm signals (PAH 01 and PAL 01) if the pressure goes too high or too low.

However, the vessel could be ruptured if the pressure went high enough.

Separate pressure switches PSHH ( Pressure switch high-high) 01 and PSLL (Pressure switch low-low) 01 are connected to the ESD system. The switches are shown in Figure 45.

Because of this, a further level of protection against excessive pressure is fitted to separators, in common with other pressure vessels. These are called Pressure Safety Valves (PSVs).

Pressure Safety Valves To prevent the separator system from becoming overpressured, every separator is fitted with Pressure Safety Valves (PSVs). (These valves may also be called Pressure Relief Valves, or PRVs). The set pressures of the PSVs are determined by the maximum operating pressure of the separator. The sizing of the PSVs is determined by the maximum amount of gas which may be required to be vented.

PSVs are special valves fitted to the top, or gas sectionof the separator. At a pre-determined pressure, which is higher than the set pressure of PSHH 01, the valves will open and gas from the separator is vented. Usually the vented gas goes to a flare. Let us take a more detailed look at pressure safety valves.

Figure 45 If either of these switches are activated, the separator is made safe by isolating it via ESD valves. Separators are designed to operate at a certain pressure.

26


Figure 46 show a typical PSV arrangement. There are two PSVs fitted to the separator - PSV ‘A’ and PSV ‘B'.

The PSVs are normally situated on the top of the vessel upstream of any demister pad. This is to ensure that, should the demister pad become blocked, then the PSVs will still function properly. Separators are fitted with two PSVs, either one of which will cope with the full pressure relief requirements of the vessel.

The usual method of operation is that one PSV is ‘on-line’ whilst the other PSV is isolated and on ‘stand-by’.

PSV A

The isolating valves are interlocked so that: *

both PSVs may be on line,

*

PSV ‘A’ may be on line, or,

*

PSV ‘B’ may be on line.

The interlock system ensures that it is never possible for both PSVs to be isolated from the separator at the same time. From the shading of the valves you can see that, in the illustration, PSV ‘A’ is on-line and PSV ‘B” is on standby.

Figure 46 27


Pressure Safety Valves can vary in design and construction but the most common type is one which relies on a spring to hold a valve closed. Figure 47 is an illustration of this type of valve.

When the pressure beneath the valve seat reaches a pre-set value, the valve lifts against the spring tension and allows gas to escape. Figure 47 shows the valve in the relief position.

In circumstances where the separator may be handling corrosive gases such as carbon dioxide (CO2) or hydrogen sulphide (H2S), then a rupture disc may be fitted between the separator and the PSV.

Rupture Discs A rupture disc is a disc of malleable metal which is designed to burst at a pre-set pressure. Figure 48 is an illustration of a rupture disc and how it is installed. The rupture disc is fitted to protect the seats of the PSV from corrosion and is normally set to fail at a pressure just below the set point of the PSV. To assist the disc to spread out as it fails, a disc cutter is normally positioned just above the rupture disc.

Figure 47

This is shown in Figure 48 on the next page.

28


Activity rupture disc

A pressure switch is often fitted in the space above the rupture disc and below the PSV. It is indicated as P.S. in Figure 48. If the switch is activated it sends an alarm to the operator in the control room. Can you work out why the pressure switch is fitted?

disc cutter rupture disc

Figure 48 : A Rupture Disc Installation

29


The set pressure of the switch is below the lifting pressure of the PSV. If the rupture disc bursts or leaks, then the pressure switch will be activated. The pressure switch tells the operator that the rupture disc is no longer operable and requires changing.

Emergency Shutdown Valves We have said that the High-High and Low-Low switches will be tied into the ESD system to stop liquids or gases from entering or leaving the separator. This is achieved by the use of Emergency Shutdown Valves (ESDVs). ESDVs are valves which are operated as part of the ESD system. They are normally air or hydraulically actuated valves. The valves are spring loaded to fail to their ‘safe’ position in the event of an air or hydraulic failure. The ESDVs ensure that the process is isolated in a safe condition in the event of an Emergency Shutdown. Figure 49 shows a separator with its ESDVs.

LCV-02 ESDV3

ESDV2 LCV-01 Figure 49

30


The ESDV’s in Figure 49 are situated as follows : • ESDV 1

-

on the main fluid inlet line

• ESDV 2

-

on the oil outlet line

• ESDV 3

-

on the water outlet line

• ESDV 4

-

on the gas outlet line

Also indicated in the drawing are the four main controllers associated with a 3 phase separator.

Activity Look at Figure 49 and locate the four different controllers. Write down what each controller does and what it uses to achieve the control required. YES. There are FOUR controllers, its not a printing error!

31


The controllers are as follows : • Level Controller LC-01 controls the oil level on the downstream side of the weir by opening and closing LCV-01.

Before we finish this section, let us look, once again, at the overall sequence of events which could occur if control of one of the process variables is lost. Use Figure 50 to follow the sequence.

• Level Controller LC-02 controls the oil/water interface level on the upstream side of the weir by opening and closing LCV-02. • Pressure Controller PC-01 controls the pressure in the separator by opening and closing PCV-01. • The overall level control for the left hand side of the separator is achieved by a weir. The weir is the simplest type of level control ever invented. You will see from the drawing that there is one further ESDV - ESDV 5. This is situated on the top of the separator and is known as a 'blow-down' or 'depressurising' valve. If the separator has been isolated via the ESD system, it could still remain fully pressurised, and it may be necessary to depressurise the vessel as an extra safety precaution. If so, this is done through the blow-down valve. If ESDV 5 operates, it will vent all the gas from the separator to the flare, thus dropping the pressure.

32


Let us assume that the problem is a rising oil level. The following will happen :

• LC-01 will open LCV-01

Summary of Section 5

If the level continues to rise

• LAH-01 will generate an alarm

If the level still rises • LSHH-01 will send a signal to the ESD system which will then close ESDV 1 When ESDV 1 closes, no more reservoir fluids will enter the separator.

In this section on separator safety systems we have covered a very complex subject in a rather simplistic way. Once again I must emphasise that the situations I described are not taken from any particular process or separation system. Each process is unique. If you are involved with process operations, you should make sure that you are completely familiar with the equipment on your own particular installation.

Remember, safety is your concern.

Of course, the sequence I have just described is just one small part of the whole ESD system. Other things may happen. Depending on the hazardous situation which has been detected, other ESD valves may open or close.

At the start of this section we considered the consequences of losing either level or pressure control in a separator. You saw that loss of control could result in a potentially hazardous situation.

For instance, in addition to closing ESDV 1, the system may also close ESDVs 2, 3 and 4. This would completely isolate the vessel. The vessel may then be depressurised by opening ESDV 5 and venting the gas to a flare system.

• generate alarms

• cause an emergency shutdown

A complete ESD system is very complex. In essence however it can be described as a system of sensors, actuators and valves which are capable of automatically shutting down a process or part of a process. This renders the plant safe in the event of a hazardous situation arising.

We looked first at the problem of level control. You saw that an increasing or decreasing level can :

Moving to pressure control, you saw that alarms and shutdowns are also initiated by the pressure going too high or too low. You also saw that additional pressure safety features such as Pressure Safety Valves and Rupture Discs may be fitted to a separator. We finished the section by having a brief look at ESD Systems and valves, and you saw an example of a simple sequence of events which could occur on loss of level control. Now that you have completed Section 5, you can move on to the final section in this unit, where we will look at separator operations.

33



Section 6 - Operation of Separators In this, the last section of the programme, we are going to look at some routine separator operations. You must remember however that each system is unique and will have its own operating procedures. Here we will consider general operations of a hypothetical separator train.

Routine Operational Checks The operator of a separation system will often have other systems under his control. This is because, normally, separators are smooth running items of equipment with very few operational upsets. The operator will usually check the following, on a regular basis, during the course of a shift: •

Levels : All sight glasses and level controllers. The operator should ensure that the sight glasses are easily readable and that the levels which they indicate correspond to the levels indicated by the level controller. This may involve draining the sight glass and blowing it clear with gas. Figure 51 illustrates the type of valve found at the top and bottom of most sight glasses. You have seen this type of valve before. It is similar to that shown in Figure 25 on Page 35 in Workbook 1. You will remember that we referred to it as a ball check valve.

Figure 51 : Ball Check Valve

34


The valve shown in Figure 51, however, is capable of shutting off any of the three outlets. It seals by the movement of a free floating ball. Its operating position is indicated in the drawing, with the valve stem fully retracted. If the sight glass breaks, whilst the valve is in this position, the flow of oil and gas, as they escape, will cause the ball to move and seal off the leaking glass. *

Pressures : All pressure gauges and pressure controllers. The operator should ensure that all the vessels are working at the desired pressure and the readings on the pressure gauges correspond with the readings being given by the pressure controllers. If differences in these readings are discovered, the cause should be investigated and the fault remedied.

*

Leakages : All vessels, interconnecting pipework and instruments. The operator should ensure that there are no escapes of liquids or gases from any of this equipment.

We will now take a look at some of the problems which can occur during normal operations.

Emulsions A common operating problem is that caused by the water and oil forming an emulsion. This is a mixture of two immiscible liquids where one of the liquids is dispersed throughout the other in the form of very small droplets. In the oilfield, the dispersed liquid is usually the water. An emulsion may be classed as ‘tight’ or loose’. Milk is a tight emulsion. It is a mixture of butter fats and water and it cannot be easily broken. Salad dressing is a loose emulsion. It is a mixture of oil and vinegar. When you shake the bottle an emulsion forms and the small globules of oil and vinegar can be seen with the naked eye. If you let the bottle stand for a few minutes the emulsion will break down and the oil will begin to float on the top of the vinegar. If emulsions are found in a separation process they may be tight or loose. The type will depend, for example, on the nature of the oil being produced and the amount of water present. One of the functions of a separator is, of course, to remove the water from the oil. The presence of an emulsion could obviously make this more difficult. In fact, in extreme cases, water removal from an emulsion may have to be done in a special treatment plant.

However, in some cases, the emulsion can be treated in the separator itself. This involves the injection of a chemical into the well fluids. This chemical, which is called a demulsifier, helps to break down the emulsion and allows the separator to do its job.

Foaming Another problem which may be found in separators is that of foaming. This is caused when the oil fails to release the gas quickly enough as it passes through the vessel, and a layer of oily bubbles forms on top of the liquid surface. The level control displacer on the oil side of the weir is designed to operate in a liquid. It cannot float in foam. When the float sinks in the foam it indicates a false, low level to the level controller and the oil outlet valve will close. This can result in the carry-over of liquids with the gas stream and a possible shutdown of the gas facilities downstream. To stop this happening, anti-foam agents are often injected into the inlet stream to prevent foaming.

35


Slugging A third, but less common, problem in separators is that of slugging. Slugging occurs when, for some reason or another, there is an intermittent, rather than a constant, flow of well fluids into the separator. In some instances the flow may cease altogether for a few seconds and then a slug will arrive. This intermittent flow can cause rapid fluctuations in separator levels and pressures. The controllers react to these changes by rapidly opening and closing their respective valves in an attempt to bring the situation under control. In severe cases the control system may become unstable resulting in a shutdown. These are the three most common problems associated with the operation of separators. We will now have a look at a simple start-up procedure and a simple shut-down procedure for our separator train.

Start-up Procedure Before a start-up procedure is initiated, a number of checks have to be made to ensure that the separation train is ready. We will assume that the separators are empty, but are in a condition to receive hydrocarbons. We will first check that: • all the valves, in the inlet manifold upstream and the pipework downstream, are in the correct open/closed position for start-up. • well fluids will be available when we require them. •

all the sight glass bridles, sight glasses, level controllers, pressure gauges, pressure controllers etc. on the separator are on-line and able to function properly.

As you work through the next few paragraphs, refer to Figure 52 on the next page to remind yourself of the valve numbering. Let us consider the status of the vessel before we introduce the well fluids.

As the vessel is empty and depressurised, the following switches will have been activated : • LSLL-01. This will have closed ESDV 1, ESDV 2, ESDV 3 and ESDV 4 through the ESD system

• LSLL-02. This will have closed ESDV 3

• PSLL-01. This will have closed ESDV 1 and ESDV 4 The fact that ESDV 1 is closed means that we are not able to get fluids into the separator through this route. We will need to inhibit the output signal from LSLL01 before we can open ESDV 1. We will then have to keep it inhibited until we have established a level of oil, over the weir, higher than LSLL-01. When this occurs LSLL-01 will automatically re-set and will cease to have an effect on ESDV 1 and the other ESDVs. LSLL-02 only closed ESDV 3 so, although it will be activated, we will not require to inhibit it in order to open ESDV1. PSLL-01 will also be activated. This switch also will prevent us from opening ESDV 1. However, this is not a problem. If you refer to Figure 52, you will see that there is a small by-pass line around ESDV 1.

36


37


So, having completed our checks, and with LSLL-01 by-passed, we are ready to start-up our separator. We first of all open the small by-pass around ESDV 1. This will allow a small flow of well fluids into the separator. The pressure will begin to rise and the liquid level at the back of the weir will also rise. When the pressure passes the set-point of PSLL-01, ESDV 1 will be opened. (Remember that LSLL-01 is still inhibited.) ESDV 4 will also be opened and, as the pressure in the separator rises to the desired value, PC01 will take over control. As the pressure in the separator is increasing, so will the liquid level upstream of the weir. By using the sight glasses we can keep a check on the level of the well fluids as they fill the space at the upstream side of the weir. The oil will now start to spill over the weir into the oil end of the separator. We can then observe the level of the oil building on the downstream side of the weir.

When the oil level reaches the set point of LSLL01, we can re-activate it. ESDV 1 is now fully under the control of the ESD system. ESDV 2 will also be opened and LC-01 will take over control of the oil level.

Shut-down Procedure

When ESDV 1 is open, and the system is operating automatically, we must always remember to close the by-pass valve. If we don’t do this, and ESDV 1 activates, there will still be flow into the separator via the bypass, thus defeating the objective of the ESD system.

If none of the valves leak, the pressure and levels will be maintained at that point.

When the water level reaches the set-point of LSLL-02, ESDV 3 will be opened. With the water level at the desired value, LC-02 will maintain control.

In order to maintain levels and pressures, and thus facilitate easy start-up, the ESD valves are closed before these levels and pressures fall too low.

The ESD system on the separator is now fully commissioned and will operate if we have a problem.

If the separator is shut down in this condition, then it can be brought back on stream much more quickly.

If we slowly reduce the flow of fluids into the separator, levels and pressure will fall and the controllers will close the relevant control valves.

Unfortunately this seldom happens. Control valves often do leak and levels and pressure will continue to fall and, eventually, will activate the ESD system.

At this stage we have flow of fluids into the separator and all controllers on-line in automatic control. When we are satisfied that everything is functioning normally, we can begin to increase the flow of fluids into the separator up to the operating rate.

38


Blow-down Procedure One procedure we have not mentioned is blowdown. As we have already explained, ESDV 5, which is shown in Figure 52, is a blow-down valve. It is fitted so that we may depressurise the separator in a rapid, but controlled, manner. The control logic of a blow-down valve on a separator is often designed so that it will not open if any of the other ESDV’s on the vessel are open. The blow-down valves may be activated :

One other feature on Figure 52 should be mentioned. The restriction orifice (RO), downstream of ESDV 5, is a flat plate with a precision hole bored through its centre. The RO serves the following functions : • it reduces the pressure drop across ESDV 5, thus reducing wear on the valve • it can be accurately sized so that the flare system is not overloaded by too much gas Now that you have completed this section, try the following Test Yourself.

• automatically, by the ESD system, in the event of a hazardous situation arising. • by a manual signal from the Main Control Room, or a local depressuring panel. The blow-down valve would not normally be used as a depressuring valve for maintenance or during normal operations.

39


Test Yourself 11 The following statements refer to : a b c

operational checks, separator problems, or separator start - up procedures

Indicate to which of the three areas, a, b or c, each statement belongs. 1 2 3 4 5 6 7 8 9 10

Inhibit output signal from the level switch low low. Increase flow of fluids into the separator. Ensure level in the sight glass is readable. Inject demulsifying chemical. Bypass the ESDV on the separator inlet Ensure that the vessel pressure is at normal operating value. Inspect the connection between vessel and instrument bridle for leakage. Liquid carry over occurs because of foaming. Observe the level building on the upstream side of the weir. Check that the inlet manifold valves are open.


40


Summary of Section 6 In Section 6, the final section in this unit on Oil and Gas Separation Systems, we have had a brief look at the operation of separators. We started by considering the routine operational checks that an operator may have to make on a regular basis during the course of his shift or tour of duty. You saw that the operational variables of level and pressure are constantly monitored. You also noted that the operator keeps a close look out for possible leakages which could give rise to potentially hazardous situations. Although separator operations are usually trouble free, there are some problems which may be encountered. You saw that potential problems included :

• emulsions

• foaming

• slugging

Finally we looked at three basic procedures, including start-up, shut-down and blowdown. We went through a step-by-step procedure to be followed when starting a separation system. The procedure was a hypothetical one, based on the separators described in previous sections of the unit. We then looked at shut-down and blowdown in more general terms.

Before you leave this unit and move on to another unit in the Petroleum Processing Technology Series, I must make some final comments regarding the operation of process plant: • The Unit that you have just completed relates to separation in general. It is not meant to describe any particular plant or process •

If you are involved in the operation of processing facilities, you should remember that each plant is different. You must be completely familiar with the specific plant and equipment under your control

• Laid down procedures and operational guide-lines must be followed, and safe working practices adopted at all times

41


Check Yourself 7 Fill in the missing words from the following paragraph. That part of a continuous process which an operator wishes to control within certain limits is called the PROCESS VARIABLE, and its target value is called the DESIRED value. A measuring unit obtains a measured value from the process and feeds it to a CONTROLLER whose job it is to compare the two values. If a deviation exists between the two values, the CONTROLLER sends a correcting signal to the final element in the loop � a correcting unit which is usually a CONTROL VALVE.

42


Check Yourself 8

Check Yourself 9

The following items from a level control loop are part of the measuring unit, the level controller or the level control valve. Indicate by a tick in the box provided, to which part of the system each belongs.

Measuring Unit

Level Controller

3

differential mechanism

3

pilot relay torque tube rod

Level Control Valve

3

actuator spring

3

valve plug

3

flapper nozzle assembly

3

diaphragm feedback unit float rod

If signal pressure is required to close the valve a loss of signal pressure causes the valve to open. It is therefore in a fail open mode.

3

3

3

valve stem

3

43


Check Yourself 10

Check Yourself 11 Indicate, to which of the three areas, a, b or c, each statement belongs.

Figure 53 Figure 53 shows the water end of a 3 phase separator. An instrument bridle is connected to the separator with the top connection in the oil and the bottom connection in the water. The instrumentation on the bridle is identical to the instrumentation on the bridle at the oil end of the separator.

This instrumentation consists of: level controller incorporating level alarms, high-high and lowlow level switches and, of course, one or more sight glasses. (Only one is shown in Figure 53 for simplicity).

1

Inhibit output signal from the level switch low low.

(c)

2

Increase flow of fluids into the separator,

(c)

3

Ensure level in the sight glass is readable,

(a)

4

Inject demulsifying chemical.

(b)

5

By-pass the ESDV on the separator inlet,

(c)

6

Ensure that the vessel pressure is at normal operating value.

(a)

7

Inspect the connection between vessel and instrument bridle for leakage.

(a)

8

Liquid carry over occurs because of foaming.

(b)

9

Observe the level building on the upstream side of the weir.

(c)

10 Check that the inlet manifold valves are open.

(c)

44

Oil & Gas Separation Book 2.pdf

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