Controlling Centrifugal Pumps

CONTROLLING CENTRIFUGAL PUMPS

© Walter Driedger, P. Eng., 2000 May 20.

walter(at)driedger(dot)ca

First published in

Hydrocarbon Processing

, July 1995.

This

Adobe®

file is available for download.

INTRODUCTION. The centrifugal pump is

one of the simplest pieces of equipment

from the controls and instrumentation point

of view. It is a two port device with a well

defined characteristic. Its purpose is to

provide the necessary pressure to move

liquid at the desired rate from point A to

point B of the process. Figure 1-1 shows a

'generic' process with a centrifugal pump

connected to deliver liquid from A to B.

Figure 1-2 shows the characteristic curve

of an actual pump (a single stage vertical

turbine pump) together with the

characteristic curve of the process, known

as the system curve. The intersection of

the two curves defines the operating point

of both pump and process. It would be fortunate indeed if this operating point

is the one actually specified for the process. It is impossible for one operating

point to meet all desired operating conditions since the operating point is, by

definition, exactly one of an infinity of possible operating points. In fact the

entire point of controlling the pump is to modify its characteristic so that its

actual operating point is the one that is required at every instance in time.

P

= Differential pressure, or

head, at the operating point of

the pump and also of the

process.

Q

= Flow rate, at operating

point, of the pump and also of

the process.

P

= Maximum differential

pressure across the pump (at

shutoff).

Q

= Maximum discharge flow

of the pump.

P

= Static (Minimum) differential pressure between points B and A of the

process.

The minimum static differential pressure of the process is frequently zero, as

in a closed, circulating system. If the pump is in parallel with other pumps that

are maintaining the system pressure, then P

is greater than zero. It is clear

from the outset that if P

is greater than P

, no amount of process control

can force the two curves to intersect. The pump is simply inadequate. How is

process control like cutting off a rope? You can always cut off more, but you

can't cut off less.

Assuming the pump is more than adequate for the process requirements at

the moment, what is the best way to trim it back to the desired operating point,

P

, Q

? There are three possible locations to place a valve: At the discharge,

at the suction, and as a recycle valve. Each will be discussed in turn.

DISCHARGE THROTTLING. Since the pump exists to serve the

requirements of the process, and one of the primary purposes of

instrumentation is to adapt the equipment to the process, let us consider the

pump from the point of view of the process. It can be viewed as a constant

pressure device with an internal restriction. It is the restriction that gives it the

"curve". It seems natural to put a valve on the discharge to further restrict the

pump. This has the effect of rotating the curve of the pump/valve system

clockwise around P

, as can be seen in Figure 1-3.

At this point I must warn the

reader that we are about to

encounter a paradigm shift. (!)

The combination of pump and

valve will be presented as a

"black box" with a single

characteristic curve which I shall

term the "modified" pump curve.

The more traditional way of

looking at the situation is from the

point of view of the pump. It sees

the process system curve as

having rotated counter clockwise

around P

. Figure 1-3 shows that

the flow, Q

, is the same for both

cases. The difference between the two pressures is the Delta P across the

valve. Since the purpose of the pump is to serve the process requirements,

and the purpose of the valve is to adapt the pump to the process, it makes

sense to consider the valve to be part of the pump system and to use the

modified pump curve rather than the modified system curve in our discussion.

In any case it can be seen that a discharge valve can be used to achieve any

operating point on the system curve so long as the point is below the pump

curve.

SUCTION THROTTLING. The second possibility for control using valves is to

place the valve in the pump suction line. This would have an identical effect

on the characteristic curve, but the method has a fatal flaw – cavitation.

Cavitation is a phenomenon that occurs when the pressure of a liquid is

reduced below its vapour pressure and brought back up above the vapour

pressure again. Bubbles of vapour form in the liquid and then collapse upon

arriving at the higher pressure region. The collapse occurs at sonic speed

ejecting minute jets of extremely high velocity liquid. Wherever these jets

impinge on a solid surface extreme erosion occurs. Over time even the

hardest materials will be destroyed. Therefore it is of utmost importance that

this pressure reduction never occurs. It is prevented by having sufficient

pressure available at the pump suction so that the pressure drops that occur

as the liquid is drawn into the eye of the impeller are at all times above the

vapour pressure of the liquid at its current temperature.

An explanation of the term Net Positive Suction Head (NPSH) is in order. This

is the pressure of the liquid at the pump suction in terms of feet or meters of

liquid head above the vapour pressure of the liquid. The actual NPSH under

operating conditions is called NPSHA and the minimum required by the pump

to prevent cavitation is called NPSHR. Clearly NPSHA must be greater than

NPSHR to avoid cavitation. It is safe to leave a margin of about one meter.

These peculiar definitions are very reasonable in terms of the pumps actual

characteristic but they cause some problems to the controls engineer. It

means that the gauge pressure equivalent of a given NPSHA is proportional

to the density of the liquid and is also affected by its temperature. The vapour

pressure can rise dramatically as the temperature rises. This means that the

NPSHA can fall without a noticeable change in pressure.

Anything that would reduce the net positive pressure at the pump inlet below

the NPSHR must be absolutely avoided. Thus suction throttling is never used

to control pump flow.

RECYCLE CONTROL. The third

remaining possibility for pump

control with valves is to bleed

some of the discharge flow back

to the pump suction or to some

other point on the supply side.

Once again we can view the

result as a modified system

curve or as a modified pump

characteristic. Figure 1-4 shows

both. Each curve is a rotation of

the original: The modified

system curve as a clockwise

rotation around P

. Note the

little "tail" at the left of the

modified system curve. This represents the flow through the recycle valve

before the discharge check valve opens to the process. The modified pump

curve has a counter clockwise rotation around the hypothetical intersection of

the pump curve with the flow axis.

This family of curves shows several problems with recycle control. Firstly, the

pump is not rated to discharge more than the flow rate at the end of the curve.

It is possible, of course, to run the pump with a wide open discharge,

minimum



P, but it is unhealthy for this particular pump to run at such a high

rate. Excessive flow may cause cavitation damage. (Excess flow cavitation is

not caused by NPSH problems but by high velocity within the internal

passages of the pump.) This restriction means that the minimum discharge

pressure may not be lower than the one corresponding to the maximum flow.

In other words, the modified pump curve cannot reach all points on the system

curve.

Secondly, although many pumps are capable of operating near zero

discharge pressure, the very flat pressure vs. flow curve for much of the lower

range for most pumps means a change of flow has very little effect on the

discharge pressure. Thus it would take a very large amount of flow to produce

a small drop in pressure. In control terms this means that control would be

very 'sloppy'. Discharge throttling on the other hand, allows the pump to

develop the head that 'suits' it. The unwanted pressure is dropped across the

valve. (Note that the curve for this particular pump rises rather steeply. It will

be more easily controlled than

most.)

Thirdly, this method is often

inefficient. Figure 1-5 shows a

system curve, a pump

characteristic, a discharge

modified characteristic, and a

recycle modified characteristic.

Above these is a pump power

requirement curve. In the case

of discharge control, the pump

is adapted to the process by

dropping its discharge

pressure. If one follows the

flow line vertically to the actual

pump curve and then beyond

to the power requirement curve

one arrives at its power

requirement. In the case of

recycle control, the pump is

adapted by reducing the

discharge flow. Following the

pressure line to the right to the actual pump curve and then upwards to the

power requirement curve one arrives at the power requirement for recycle

control. Note that the power requirement curve tends to slope upward as flow

increases. Therefore recycle control consumes more pump horsepower than

discharge throttling when both achieve the same operating point. This is not

always so. If the power requirement curve were flat, there would be no

difference. Notice on the curve that there is a slight drop in horsepower near

the right hand end. If circumstances were such that the operating point

corresponded to a downward sloping power curve, recycle control would be

more efficient. This is rare.

SPEED CONTROL. There is, of

course, one other means of

adapting a pump to the changing

demands of the process: Speed

control. The virtue of this method

is that it reduces the energy

input to the system instead of

dumping the excess. Figure 1-6

shows a system curve

superimposed on a family of

curves for a variable speed

pump. The curves reach all parts

of the system curve below the

full speed curve. Therefore this

is an effective means of control.

Note, however, that these curves have one feature in common with recycle

control: At the far left end of the system curve the pump curve and the system

curve are almost parallel. (The particular pump chosen for this example has a

rather steeply rising curve near shutoff. Most are considerably flatter.) In

mathematical terms this means that the intersection is poorly defined. In

practical terms this means that it is difficult to maintain a precise operating

point and that control is 'loose' at high turndown.

In practice, variable speed drives for centrifugal pumps are still relatively

uncommon. For small pumps the power savings are not significant and for

large pumps the associated electronics become very expensive. Also, they do

not have the high reliability of valves. Variable speed steam turbine drives are

quite common in the larger horsepower ranges. Electric variable speed drives

are used in certain specialized applications such as pumps that are

embedded inside a high pressure vessel. In such cases there are no

alternatives.

RIDING ON THE CURVE. Last but not least: No control at all! The fact is that

the majority of pumps in the world run with no control at all. The exact flows

and pressures are not critical and the pump has been reasonably well

selected. The discharge pressure will rise to partially compensate for

increased back pressure. It falls as the back pressure decreases so that the

flow does not increase as much as it otherwise might. The pump is allowed to

"ride on its curve". When this situation is acceptable, leave well enough alone

and don't try to fix what ain't broke. (Be careful though, the machine may still

require minimum flow and other protections as detailed in the section on

Machine Protection.)

MEASUREMENT. The appropriate measurement for the controller depends

on the demands of the process. Flow control is a frequent requirement. Two

rules guide the location of the flow measurement: Make sure that side streams

are included or not, as required, by the measurement and make the

measurement at the highest convenient pressure. The latter requirement is to

avoid any possibility of flashing or cavitation within the measuring device. In

general the best place to measure flow from a centrifugal pump is between

the recycle Tee and the discharge throttling valve. The exception is when the

discharge is at an extremely high pressure and the suction has adequate

NPSHA. In that case a suction measurement may be best.

Level control of a vessel is one of the most common requirements

. The

vessel may be either upstream or downstream. It is quite possible to connect

the Level Controller directly to the discharge valve. Frequently, however, the

vessel serves to buffer a downstream process from upstream flow variations.

In that case it is not desirable for level control to be precise. Perfect level

control implies that the flow out is exactly equal to flow in at all times. Often it

is desired that the downstream flow remain as uniform as possible while

keeping the level within bounds. In simple terms, it is desired that the flow out

is the average of the flow in. The vessel absorbs the instantaneous

differences. This simple requirement is more difficult to accomplish than it may

seem and deserves a discussion entirely of its own. A simple arrangement

that is often satisfactory and is widely used is to have the Level Controller

cascade to a Flow Controller on the pump discharge. The flow loop keeps the

discharge 'constant' while the Level Controller gradually raises or lowers the

setpoint as the level in the vessel rises or falls.

Another common requirement is to control the pressure of either upstream or

downstream equipment. The tap for the pressure transmitter should be

connected at the point where it is desired to control the pressure. Note that a

pressure tap between the pump and a discharge throttling valve is probably

meaningless. A careful look at many pump curves will show that the

characteristic near shutoff is quite flat and may even slope downward.

Pressure control cannot be accomplished when the pressure curve is flat. If

the slope is the 'wrong' way, control will work backwards and drive the valve

away from the set point. In this case the minimum flow should be set so that

the pump cannot operate in the positive slope region of the curve. (It is, of

course, possible to reverse the action of the controller so that it can operate to

the left of the peak. But in that case, what will happen if the operating point

moves to the right? It is extremely difficult to design control systems that can

operate continuously along a characteristic curve that has a local minimum or

a maximum in it.)

There is a second, more serious, problem with pressure control. Centrifugal

pumps are essentially constant head machines. The discharge pressure for a

given pump rotating at a fixed speed is proportional to the density of the liquid.

This means that if the liquid has a constant density, the discharge pressure is

constant. The "curve" of the pump curve is produced by losses and other

affects caused by flow. Unless there is a flow through the system, there is only

one pressure and that is the shutoff pressure. If it is desired to control the

pressure of a vessel being charged by a pump, it is best to pressure control a

valve at the outlet of the vessel and let the pump ride on its curve. If the

vessel must be dead ended, only recycle flow at the pump can control

pressure to a setpoint.

ON / OFF CONTROL. On/off

control is used in many

situations where the object is

simply to move a liquid from

point A to point B and the exact

pressure or flow rate is

unimportant. A typical example

is the sump pump. The simplest

arrangement employs a level

switch with a very broad

deadband. This is used

together with a Hand/Off/Auto

switch to turn the pump on and off. The schematic is shown in Figure 1-7. The

LSHL contact opens when the level is below its setpoints. "M" represents the

motor contactor which energizes the motor whenever the contactor is

energized. "M" also represents the auxiliary contact that is closed whenever

the contactor is energized.

If it is important that the level

never goes beyond the upper or

lower setpoints, the Start/Stop

arrangement is preferred. It is

illustrated in Figure 1-8. The

process sensing switch has a

separate output for the upper

setpoint (On) and the lower

setpoint (Off). (Two switches

may be required.) The manual

switch consists of a Start and a

Stop button or a combined Start/Run/Stop selector with a spring return to

centre. The operator may start or stop the pump whenever the level is

between the two setpoints. He cannot stop it when the level exceeds the high

setpoint unless he locks it out. He cannot start the pump below the low

setpoint. A variation of the circuit places the left connection of the start button

to the left of the low level switch. With this arrangement it is possible to drain a

vessel below the low set point by holding the start button on. The pump will

stop as soon as the button is released.

With both of these arrangements, there must be sufficient deadband between

the high and low setpoints to make certain that the pump does not cycle on

and off too rapidly. Excessive wear of both the motor and its starter will result

if this occurs. Rapid cycling is a sign of an over-sized pump.

MACHINE PROTECTION. Once the process requirements have been met,

the attention of the process control engineer turns to protecting the

equipment. Centrifugal pumps are fairly undemanding. In general they have

only two requirements: that the NPSHR is met at all times and that a certain

minimum flow is maintained. To meet the first requirement is generally a

piping design problem. In cases of doubt, a low pressure shutdown switch

may be added to the suction line. A second look at the explanations of NPSH,

above, shows that determining the setpoint of the switch is not necessarily a

simple matter if there is any possibility of the liquid density changing. Things

get even more complicated if the vapour pressure is very sensitive to

temperature. A rise in temperature that causes the liquid to boil will cause the

net positive pressure to fall to zero even though there is an increase in actual

pressure. LPG and LNG pumps are notorious for NPSHA problems.

Fortunately most pumps can tolerate brief periods of cavitation without

noticeable damage.

When a pump is taking suction from a vessel, a low level shutdown switch is

essential. The switch, or transmitter, must be separate from any level control

devices.

To meet the second requirement,

minimum flow, is somewhat more

difficult. A centrifugal pump adds

energy to the liquid that the

moving liquid carries away. If flow

is blocked, the temperature within

the pump will rise steadily until

the liquid boils (net positive

pressure is now zero). Damage

to the pump is quite likely. For

this reason some form of

minimum flow is almost always

included on larger machines. The

simplest arrangement is a fixed

restriction orifice on a line leading

back to the supply side of the

pump. The preferred destination of the recycle flow is back to the vessel from

which it came. This allows the heat to dissipate before it is recycled back into

the machine. Restriction orifices have two drawbacks: They waste energy

when the process demand is sufficiently high to meet all minimum flow

requirements and also they limit the maximum pump output.

A more efficient method of recycle control requires that the discharge flow of

the pump itself is measured, and that a valve in the recycle line is opened

when the process does not draw the required minimum flow. The most

straightforward way to accomplish this is shown in Figure 1-9. Note that the

recycle line tees off upstream of the control valve. It is precisely when the

control valve is closed that the recycle is needed. There is a small problem

with controlling the minimum flow in this way: The measurement orifice in the

discharge consumes energy and also slightly reduces pump capacity. A

second problem is that the actual signal being measured is the



P across the

orifice plate. Since flow varies as the square root of



P, a minimum flow of

40% of maximum flow implies a controller whose set point is only 16% of the

measurement range. A typical instrument accuracy is 1%. Therefore an error

of 7% of the setpoint can be expected. Fortunately the minimum flow need not

be held very accurately. Recycle control is sometimes accomplished using a

local pneumatic controller mounted directly on the valve. Note: Alwaysuse a

fail-open valve.

Various schemes have been devised to infer the required valve setting from

the net discharge flow measurement. These require the flow downstream of

the recycle Tee to be subtracted from the required minimum flow. The recycle

valve is then opened in proportion to the difference, if it is positive. To do this

accurately one must know the valve and actuator characteristics. There is no

feedback to confirm that the correct flow is occurring. Since the flow is usually

above the minimum flow, the valve is usually closed. This will cause the

controller to wind up and be slow in responding when a low flow condition

suddenly arises. Fortunately pumps can tolerate short periods of low flow so

this is not a problem.

One method of minimum flow control that is occasionally proposed is to put a

flow control loop on the recycle line with the set point equal to the minimum

flow. This solution is worse than a fixed restriction. When discharge flow is

high, the discharge pressure falls. Flow through a fixed orifice will reduce

somewhat. A flow control loop will open the valve further to maintain constant

flow precisely when it is not needed. At this point the operator will be tempted

to manually close the valve. Then, when a discharge blockage occurs, there

will be no minimum flow at all!

There are a number of devices available, called Automatic Recirculation

Valves, or ARC valves, that combine the functions of net discharge

measurement, recycle control, recycle valve and discharge check valve all in

one device. These devices can be very effective but they suffer from one

drawback: lack of flexibility. In cases where the pump and process

characteristics are well known, they can be an ideal solution. Pipelines, for

example, have many identical pumps operating under steady conditions.

Once the correct components are known, application is routine. It must be

kept in mind, however, that both the process and pump data provided to the

controls engineer for a new facility are often tentative. ARC valves have very

little margin for error when the reality turns out differently from the theory. One

particular problem that can occur with the older style ARC valves that operate

in an open/close mode, and even with some that modulate, is instability. It

occurs as follows:



The discharge valve begins to close due to a reduced process

demand.



The ARC valve senses the reduced flow and opens the recycle

valve.



The pump discharge pressure drops.



The discharge Flow Controller senses that it is being starved of

flow and opens the discharge valve.



The ARC valve sense the increased flow and closes.



The pump pressure rises.



The discharge valve closes.



The cycle repeats itself.

Note that ARC valves are not positioned by conventional actuators. They are

positioned by the process liquid itself and are capable of very rapid action.

Instability results in violent slamming of the recycle valve, scaring the

operators and severely damaging the reputation of the controls engineer. Very

little can be done at this stage other than to remove the ARC valve and to

attempt to modify its characteristic by changing the spring or boring out the

recycle ports. The latter spoils the hardened seats required in high pressure

drop applications and leakage is inevitable. Boiler feed pumps seem to be

especially prone to these problems. Note that ARC valves are quite expensive

and often cost more that a complete flow control loop. They are, however,

extremely effective, and simple, under the right circumstances. Their use often

simplifies the piping arrangement and essentially eliminates routine valve

maintenance.

ARC valves are best bought as part of the pump package. In this way the

responsibility for ensuring that they match the pump rests with the party that is

most familiar with it.

The pump curves used in this article represent an actual pump but are by no

means typical of all pumps. Multistage pumps, in particular, may have little

quirks in the curves that can complicate controls. If the characteristic curve

droops as it approaches the zero flow axis, (the shutoff pressure is less than

their peak pressure) the minimum flow setting must be well to the right of the

peak or severe instability can result. Boiler feed pumps discharge into a

compressible volume. If they have a reverse slope near shutoff, they may

experience surge much like a centrifugal compressor does. Note that API STD

610

, the American Petroleum Institute standard for centrifugal pumps,

explicitly bans a drooping characteristic.

SEAL FLUSHING and COOLING. Pumps in certain special services require

flushing and/or cooling fluids to be injected into the seals. The details are

provided in API STD 610

, Appendix D. In general the instrumentation is

rather simple, consisting of rotameters, pressure gauges and thermometers.

In certain hazardous services, sealing becomes a more complex issue. If the

danger of a seal leak is sufficiently serious, specialized leak detection may be

required. One simple method is the installation of a pressure switch, or better

yet, a transmitter, between the tandem seals. This can then be connected to

the plant alarm system.

SAFETY. Centrifugal pumps are not generally hazardous pieces of

equipment. However, there is one special safety consideration whenever a

pump is drawing volatile hydrocarbons or other flammable liquids from a

vessel with significant capacity. (API RP-750

, defines this as five tons.)

Volatile liquids have a low viscosity and seal leaks are not uncommon. The

leaked liquid often catches fire and it is absolutely essential that the pump be

shut down to prevent feeding the flames further. In such situations it is

desirable to have a remotely controlled block valve between the pump suction

and the source vessel. This valve and its actuator should be fire safe. Since

closing the valve can cause low flow damage to the pump, it must have a limit

switch to shut down the pump whenever the valve is not fully open. It should

also have both opened and closed status indication in the control room so the

operator can be fully confident that the valve is open when the pump is

running and that the valve is closed when a hazardous situation exists. If the

block valve has an electric actuator, it is a good idea to have an alarm on the

main panel to indicate if there is a power failure, if the local switch is not in the

'Remote' position, or if there is any other reason the valve might not work

when called upon to do so. In extremely critical processes, one may wish to

interlock the pump so that it cannot start unless the valve is in working

condition.

Any indoor pump in flammable service should have adequate fire detection in

the building. Ultraviolet detectors are preferred because they are sensitive to

flame. They are extremely fast acting since they do not depend on heat

buildup or the generation of smoke. (There is an exception to this rule: If the

flammable material produces a lot of smoke, it may obscure the vision of the

UV detector. In such a case one might be advised to install both smoke and

UV detectors.) A certain amount of care must be taken when UV detectors are

installed. They are sensitive to sunlight and to welders. The sensitivity to

welders is probably a good thing since it forces all welding to be co-ordinated

with the control room. The sensitivity to sunlight means that they must be

positioned so that they are unlikely to 'see' the sun. The usual position is high

up under the eaves of the building in diagonally opposite corners. This is not

always fool proof. The author is aware of one case where a pipeline

in the parking lot. The welder was directly in line with a gap around a pipe that

went through the building wall. "Smart" combined UV/IR detectors are

becoming available that are able discriminate between sunlight, welding arcs

and fire. This type is also suitable for outdoor use.

Fusible link sprinkler systems are extremely reliable and can contribute greatly

in cooling down a fire that is too hot to approach. Their drawback is that they

only become active once considerable heat has been developed. In critical

applications they are best used together with a faster detection system.

It may be worthwhile installing flammable vapour detectors near the base of

large pumps if leakage is a possibility.

Never overlook the placement of check valves. This is a safety related issue

that should not be left to other disciplines as check valves are an integral part

of the functioning of many control schemes. It is generally self-evident that

parallel pumps need check valves on each individual discharge. This check is

also needed downstream of the control valve on single pumps. When the

pump is not running, the discharge valve will most probably go wide open. A

reverse flow could have some peculiar effects on the upstream process. A

check valve is also required downstream of the recycle valve if a fire safe

valve is necessary. Any time the fire safe valve isolates the pump from the

supply vessel, the recycle valve will open wide. ARC valves should be

checked to make sure all necessary check functions are included.

ACCESSORY INSTRUMENTS. Centrifugal pumps require few accessory

instruments. Since the purpose of the pump is to develop pressure, it is a

good idea to have a pressure gauge on the discharge. If the application

requires a low suction pressure interlock, a pressure gauge should also be

provided at the suction. It would be nice to have a local flow indicator but they

are invariably expensive and inconvenient to install so they are rarely used. A

thermometer on the suction may serve to warn of cavitation if the vapour

pressure is temperature sensitive.

PARALLEL PUMP INSTALLATIONS. Centrifugal pumps are frequently

operated in parallel. Their smooth operating curve allows this to be done

without complication. If it is intended that the pumps are usually operated

individually and not simultaneously, it is sufficient to have a common

discharge throttling valve and suction block fire safety valve. However it is

essential that each have its own recycle arrangements. Do not be swayed by

the argument that the two pumps will never be run simultaneously. The most

obvious reason for simultaneous operation is to switch from one to the other

so that maintenance can be done without shutting down the process. In this

case the pump that is being started will be operating against a blocked

discharge check valve and is in no position to make use of a common recycle

valve. Remember that the throttling valve is there to serve the process but the

recycle is there to protect the machine. You don't share seat belts do you?

Parallel variable speed pumps obviously have individual controls. The most

effective arrangement is to provide constant flow controls to the majority of the

pumps. The setpoints should be at the peak efficiency for each individual

pump. The remaining pump should have its controller set to handle the

swings. Actually this an example of the complex subject of Supply and

Demand Control and deserves a discussion of its own. Note that is

meaningless to have two pumps each on pressure control pumping into the

same header. They will not share the load.

SERIES PUMP INSTALLATIONS. Sometimes centrifugal pumps are

operated in series. The usual situation is when a multistage pump has an

NPSHR greater than what is available. In such a case, a single-stage pump

with a low NPSHR is used as a booster. This is common with boiler feed

pumps especially if the pump is drawing hot water whose vapour pressure is

already elevated.

Process demand control is applied to the high pressure pump. The booster

pump should be on discharge pressure control. The author was involved in

one situation where oil field injection water was drawn from a cistern

connected directly to a river. In this case the booster pumps were pressure

controlled by recycle back to the cistern. This allowed the recycle water to

keep the water in the cistern agitated, preventing an accumulation of silt.

It is not unusual for a group of booster pumps in parallel to supply a group of

high pressure pumps in parallel. In such cases care must be taken to ensure

that the various operating combinations are matched in capacity.

Every individual pump in a series installation must have its own minimum flow

arrangement.

SUMMARY. Figure 1-10 shows a complete set of instrumentation for a typical

centrifugal pump application. The drawing illustrates a pump drawing volatile

hydrocarbons from a large surge vessel. The following features are

illustrated:



A level / flow cascade loop on the pump discharge to provide

process control.



A check valve on the discharge downstream of the control valve

to prevent reverse flow when the pump is shut down.



A fire safe motor operated valve (MOV) in case of seal leakage

and fires.



An interlock from the MOV to stop the pump if the valve is not

fully opened.



A low level interlock from the vessel to stop the pump if the

vessel loses its liquid seal.



A pressure gauge on the suction to indicate adequate NPSHA.



A thermometer on the suction to indicate potentially high vapour

pressure.



A minimum flow recycle loop back to the vessel.



A check valve on the recycle line to prevent reverse flow when

the pump is shut down, especially when the fire valve is closed.



A pressure gauge on the pump discharge to indicate that the

pump is working.

REFERENCES

1. Driedger, W. C., "Controlling Vessels and Tanks"; Hydrocarbon

http://www.driedger.ca/ce6_v&t/CE6_V&T.html

2. API STD 610, Centrifugal Pumps for General Refinery Service.

http://www.cssinfo.com/apigate.html

3. API RP 750, Management of Process Hazards.

http://www.cssinfo.com/apigate.html

CONTROLLING POSITIVE DISPLACEMENT PUMPS

© Walter Driedger, P. Eng., 2000 May 20. walter(at)driedger(dot)ca

First published in Hydrocarbon Processing , May 1996.

This Adobe® file is available for download.

INTRODUCTION. The positive displacement pump is in some ways an even simpler device to control than the centrifugal pump discussed previously1. It has the same function, namely to provide the pressure necessary to move a liquid at the desired rate from point A to point B of the process. Figure 2-1 shows a 'generic' process with a positive

displacement pump (in this case a gear pump) connected to deliver liquid from A to B.

There is a great variety of positive displacement pumps. They are divided into two broad categories: Rotary and reciprocating. From the controls point of view, however, they are all similar. Their

characteristic curve is so simple that it is rarely drawn. It is essentially a straight vertical line, as shown in Figure 2-2. (For some reason PD pump curves are usually shown with the pressure and flow

axis exchanged. I will not follow that convention in this article.) All are constant flow machines whose pressure rises to whatever value is necessary to put out the flow

appropriate to the pump speed. If the discharge is blocked, the pressure will rise until something yields -- preferably a relief valve. Close examination of the curve shows a slight counter clockwise rotation. This is due to internal leakage.

For positive displacement pumps the major cause of leakage is the small amount of reverse flow that occurs before a check valve closes and possibly past the check valve after it is closed. Leakage past the piston is negligible. Diaphragm operated PD pumps have no cylinder to leak past. Rotating PD pumps, such as gear pumps or progressing cavity pumps have internal clearances which permit a small reverse flow, called "slip" or "blowby". There is another reason why the curve may rotate to slightly lower flows at higher

discharge pressures: The driver may slow down as the load increases. None of these have a significant affect in curving the slope of the characteristic enough that this slope can be used for control. For most practical purposes the slope is vertical. The system curve of the process is also shown on Figure 2-2. Its intersection with the pump characteristic defines the operating point.

As always, the process controls engineer has the responsibility of matching the capacity of a specific piece of equipment to the demands of the process at every instant in time. Rarely does the actual system curve fall exactly on the one used for design and

selection. As with any two port device, there are three locations in which a control valve can be placed: On the discharge, on

the suction, and as a recycle valve. DISCHARGE

THROTTLING. Discharge throttling does not work! Looking at the

process from the point of view of the pump, discharge throttling rotates the system curve counter clockwise so that the modified system curve intersects the pump curve higher up. The additional pressure is dropped through the valve so that the

pressure and flow to the process is (almost) exactly the same as before. The "almost" is due the small

increase in internal leakage that results in an equally small reduction in flow. An increased wear rate and a shortening of the life of the machine are the only results of this approach. If the pump is seen from the point of view of the process so that the valve is considered part of the pump, the same result is obtained. To obtain a modified pump characteristic curve, the pump curve must be rotated clockwise around the intersection with the pressure axis. The problem is that this hypothetical intersection is far off the top of the operating range. It is the point where the pressure is so high that 100% internal leakage occurs. The machine would self-destruct from excess pressure if one were stubborn enough to attempt to find this point. The rotation of the curve can still be performed on paper and it amounts to a slight shift to the left. Shown in Figure 2-3, it is virtually identical to the unmodified curve. To cut a long story short, you can't control a PD pump with discharge throttling.

SUCTION THROTTLING. Suction throttling has the same effect on the characteristic curve as discharge throttling and doesn't work either. PD pumps have a Net Positive Suction Head Required (NPSHR) just as

centrifugal pumps do. In fact their requirements are even more stringent. Therefore restrictions and pressure drops in the suction lines must be similarly avoided.

RECYCLE CONTROL. This leaves recycle control as the only means of using a valve to control a PD pump. The valve is installed in a line teeing off from the discharge and leading back to the source of the liquid, possibly a surge tank. It must be fail open , of course. Figure 2-5 shows its effects on the characteristic curves. Viewing the process from the point of view of the pump, its effect is to rotate the system curve clockwise around its intersection with the pressure axis. Note that the little "tail" at the bottom left of the modified system curve is due to the flow through the

recycle valve before the discharge check valve has opened. The flow through the pump is essentially as before but the pressure to the process has been reduced. Process flow will, of course, also be reduced by the amount flowing through the recycle line. Viewing the pump from the process gives a different perspective on the same phenomenon. This time it is the pump curve that is rotated counter clockwise around its

intersection with the flow axis. This modified pump curve gives the effect of greatly increased internal leakage. From the point of view of the process, this is exactly what is happening. Note that I have not used the same operating points in Figure 2-3 as I did in Figure 2-5. It is simply impossible to show any significant reduction in flow on a curve representing the effects of discharge throttling.

Recycle control is an efficient method of control for PD pumps. Since the flow rate is essentially constant, the power requirement is roughly proportional to discharge pressure. Since the effect of recycle is to drop the discharge pressure, it results in significant reductions in power requirement. Nevertheless there is still wasted power in proportion to discharge pressure times recycle flow.

Recycle valves experience rather severe service if the pressure drop is high. Cavitation will destroy them if they are not appropriately selected. Two approaches exist to deal with this problem: The first solution is to drop the pressure in many small stages through the use of many twists and turns in the valve trim. The second is to tolerate the resulting cavitation by shooting the liquid as a jet through a small hole in the middle of a disk. The jet then blasts directly into the discharge piping. The line diameter is often increased immediately downstream of the valve and the wall thickness is also increased. In this way the jet cavitates down the middle of the pipe. It makes a terrific racket.

In either case it may be necessary to put a fixed restriction downstream of the valve. It should be sized so that the ratio of the high to intermediate pressure is the same as the ratio of intermediate to low pressure. Keep in mind that the restriction will reduce the rangeability of the valve by making it act like a quick opening valve. This is because the restriction becomes the dominant factor in the line once the valve is about half way open. From that point on, the valve has little control.

Recycle lines for PD pumps should be run back to the suction vessel. This allows any entrained bubbles to escape. If they do not, they can build up to the point where pump capacity is impaired. It may even vapour lock.

SPEED CONTROL. Speed control is an obvious method of controlling the flow rate of PD pumps since flow is essentially proportional to speed. Pressure can also be

controlled by sliding up and down the system curve. Any point on the system curve can, in theory, be reached. Most drivers,

however, have low speed limits which limit the turndown of the system.

Variable speed electric motors are somewhat modified versions of normal motors. They require special provision for cooling and lubrication at low speed. In addition, they require specialized electronic power

supplies called "invertors". These units provide power of the appropriate frequency and voltage. They are, unfortunately, still quite expensive and do not have the reliability of control valves. There is another reason why large variable speed electric drives are seldom used with reciprocating pumps. The large inertia of the system means that speed changes cannot be made quickly. If it is possible for a valve in the process side to close suddenly, a variable speed electric cannot reduce speed fast enough to prevent a severe pressure rise. A recycle valve will be required to protect the pump, as detailed below in the section on machine protection. A more simple type of electronic control is frequently used for small chemical injection pumps.

OTHER MEANS OF CONTROL. The great variety of types of PD pumps results in a variety of specialized means of flow control. A pneumatic actuator may be used to vary the geometry of the crank arrangement of a reciprocating pump so that each cycle displaces a greater or lesser amount of cylinder volume. Direct acting diaphragm pumps driven by compressed air or some other gas can be controlled by regulating the gas supply. There is also a technique known as "lost motion" whereby the crank

arrangement first compresses a spring or volume pocket before it begins to work on the piston or diaphragm. These specialized methods are usually integral parts of the

equipment and the controls engineer simply connects a pneumatic or milliamp signal to the appropriate input port. None of these methods changes the essentially constant flow nature of the pump curve. (The flow is still "constant" but at a different value.)

The efficiency of hydraulic or eddy current couplings is about the same as that of recycle control. This is because the torque on both sides of the coupling is proportional to  P. The power lost in the coupling will be proportional to torque times the reduction in speed. In other words, all unused power is being dumped. If the pressure does drop with a reduction in net discharge flow, then there will be a power savings. A valve is a cheaper way of accomplishing the same thing.

"Stroke Counting" is a method used when fixed amounts of liquid must be injected at specific intervals such as in batch processes. An electronic device is used to count the number of revolutions of a PD pump. After a sufficient number has been counted, the pump is shut off. When this method is used for pH control, the correct number of strokes can be calculated from a titration curve.

MEASUREMENT. The most common application for PD pumps is in high-pressure service. The flow rates vary from extremely small to moderately large. Pressure control is very common. Since the control valve tees off the discharge header, it is not

significant where the sensing transmitter is placed. Keep in mind that the discharge will be pulsating. The pulsations may be relatively small for a rotary pump or they may be extremely large for a simplex (single cylinder) reciprocating pump. The degree of pulsation also depends on the effectiveness of the hydraulic pulsation dampeners that are often supplied with the pumps. If pressure or flow control is critical, the control systems engineer should encourage the biggest economical discharge dampeners. Small pulsation dampeners, called snubbers, should be installed on all instrumentation such as pressure gauges, switches and transmitters. This will extend their life as well as

improve the signal. Many transmitters have built-in adjustable electronic damping. These should be adjusted so that the time constant is approximately twice the period of the expected pulses at the lowest speed. The phenomenon known as "aliasing" makes digital control systems such as a distributed control system (DCS) especially sensitive to pulsations. Aliasing can be best explained with the help of a diagram as shown in Figure 2-7. The rippling curve shows the actual flow rate of the discharge as it varies with time. The Xs show the points at which the DCS samples the measurement. The DCS gets the totally misleading impression that the system flow is slowly rising even if the average is quite constant. The usual reading the DCS gets is one of totally random fluctuations. Analog damping, either hydraulic or electronic, is absolutely essential for digital control. It prevents aliasing by filtering out high frequency components before they are sampled.

Flow control measurements have similar problems to pressure measurements. An additional problem arises in the case of an orifice plate or similar head type measuring system. Since the  P varies with the square of flow rate and it is the  P that is averaged, the resulting signal is not the average of the flow rate. Rather it is the square root of the average of the square of the flow rate. (Electrical engineers recognize this as the RMS -- root mean square.) As long as the shape of the pressure signal, over time, does not change, flow will be

proportional to, but not equal to, root  P. The more cylinders in the pump, the smoother the waveform will be and the closer the measured to the actual reading. Discharge pulsation dampeners also help considerably. The measured flow on "ideal" (undamped, pure sinusoidal flow waveform) simplex and duplex pumps is 11% higher than the actual flow. An "ideal" triplex pump yields a measurement that is 1% high.

Flow measurements on the discharge of high pressure pumps should be avoided. This may not be possible if the pump has a recycle loop that returns, as it should, to the suction vessel. In that case remember that the flow sensor will experience not only high pressure but also a high level of pulsation. Turbine meters are easily damaged. I am told that coriolis-type mass flow meters do well in this service.

Certain classes of reciprocating pumps, known as metering pumps, have a very precise volume of liquid delivered with each stroke. The RPM of the pump can be used as an accurate flow measurement. However, individual calibration is required if this accuracy is to be realized. Note that even small amounts of entrained air or other bubbles can cause serious errors. Metering pumps are commonly applied for chemical injection. There is a simple way to calibrate them if extreme accuracy under varying conditions

isn't an issue. A large glass cylinder is teed into the suction piping. If a valve between the cylinder and the supply is closed, the time it takes the pump to draw down the level by a fixed volume can be used to calculate a flow rate. The cylinder also serves as a level gauge to a supply tank. In some applications the fact that the pump is capable of developing high pressure isn't even an issue. It may be metering directly into an open tank or a low pressure line. In such cases the pump may need a back pressure valve on the discharge to ensure that the check valves seat properly. This item is usually

supplied by the vendor as part of the pump package.

PD pumps are not generally used for level control in the process industries. The great variety of types of PD pumps invariably provides exceptions to every generalization. The direct acting, pneumatically powered diaphragm pump is one of these exceptions. It is ideal for sumps containing sludges. The pump can be controlled by an entirely

pneumatic control system thus eliminating all electrical connections. This has the added advantage of being absolutely safe in hazardous locations.

MACHINE PROTECTION. The greatest danger to positive displacement pumps is overpressure. The rigid, unyielding nature of the pump characteristic means that

overpressure is certain if the discharge is blocked. Many smaller (non API) pumps1, 2, 3, such as the gear pumps used to supply lube oil for larger equipment, have integral relief valves to release pressure from the discharge back to the suction. In the majority of cases, an external relief valve must be supplied by the user. It must be connected as closely to the pump discharge as possible and must not have any means of blocking either its inlet or its outlet. It should discharge back to the pump supply. If, for any reason, the discharge is blocked and the relief valve is not capable of relieving, the pressure will rise very rapidly until something busts. It may be connecting rods, the check valves or even the cylinder head. Don't count on the motor stalling because events unfold very rapidly and the inertia of the system is sufficient to cause major damage. The most likely point of failure is the

bolting on the discharge flanges.

Direct acting pumps, such as those driven by compressed air, may not need a discharge relief if it can be shown the maximum pressure of the driving fluid is incapable of causing excess pressure.

It is often advisable to install a high discharge pressure shutdown switch or transmitter in addition to the relief valve.

Good engineering practice dictates that operating controls be provided to avoid shutdowns or relief valve operation for normal operating situations. If it is possible for the pump discharge to be blocked under normal operating conditions, a pressure

control loop must be provided on the discharge. This consists of a pressure transmitter, a controller and a recycle valve. If there is already a flow control loop on the discharge, a pressure override controller must be added. A common arrangement is shown in Figure 2-8. A deviation alarm on the pressure controller provides the pre-alarm for the high pressure shutdown. Whenever the pressure is above the setpoint of the controller, the alarm is on. This has the advantage of having only one setpoint for the two

functions. Since the valve is fail open and the lower of the two signals drives the valve to the safe state, a low selector is chosen to pass the correct signal to the valve. Once again it must be stressed that overpressure conditions can arise extremely quickly. All components of the system must be selected with speed in mind. DCS controls with a scan rate slower than ½ second may be too slow. In any case, the valve may be too slow. Despite your best efforts it may be impossible to limit the pressure rise. In such cases it may be necessary to eliminate the high-pressure shutdown and to accept occasional relief valve action.

The suction side of the pump may also require protection. A relief valve is required unless all suction piping is rated for the full discharge pressure. Liquids, especially water, are quite incompressible. Even the smallest reverse leakage through a check valve can raise the pressure of a blocked suction sufficiently to rupture the line. This can happen even after the pump has been shut down! The discharge dampener will contain liquid at full pressure unless it has been relieved. The line rupture may occur minutes or even days after the pump has been shut down and isolated, depending on the relative sizes of the discharge and suction dampeners and the leakage rate. (Been there, seen it.)

A low-pressure shutdown switch or transmitter is required on the suction side of larger pumps. The NPSHR of reciprocating pumps is further complicated by what is termed the "acceleration head". (See the previous article in this series, Controlling Centrifugal Pumps 1, page 7, for a more detailed discussion of NPSHR and NPSHA. Note that there is one difference between NPSH for centrifugal and PD pumps: For a PD pump NPSH is specified in pressure units instead of elevation. This is because the operation of PD pump is not dependent on liquid density. ) When the piston of a simplex pump begins its intake stroke, the liquid in the suction line is essentially stationary. The entire line contents must be accelerated rapidly to its maximum velocity, approximately three times the average velocity. There are two reasons for this three to one ratio: Firstly, the liquid isn't moving at all for half the cycle. Secondly, even when it is moving the velocity starts at zero and builds up to a maximum at mid stroke before reducing to zero again at the end of the stroke. The "suction" required to draw the liquid into the cylinder reduces the pressure sufficiently that air or vapour bubbles may develop. When these collapse during the discharge stroke, if not sooner, cavitation occurs. If the bubbles do not collapse, as in the case of air dissolved in water, serious hammering can occur in the cylinder. The air may accumulate to the point that the pump becomes vapour locked. Remember that air can compress into the internal clearances of the cylinder and then expand again on the intake stroke without ever being forced out of the discharge check valve. The low suction pressure shutdown device should be accompanied by some sort

of pre-alarm. Acceleration head problems are greatly reduced for multi-cylinder pumps. Suction dampeners also contribute to making the flow rate more even.

Minor mechanical failure in PD pumps can cause significant vibration and subsequent serious damage to the entire machine. For this reason it is the rule to include a vibration switch on larger equipment. This switch need not be the extremely sensitive,

multichannel system used on high-speed machinery. We are not monitoring the gradual deterioration of delicate bearings. What we are looking for is an abrupt event of

considerable magnitude. Even the simplest switch will suffice. The usual type of switch is termed a "seismic" switch. It works by having a small weight held in place by a magnet against the force of a spring. A "bump" dislodges the weight from the magnet and allows it to open the shutdown contact. The usual means of "calibration" is a light whack with a hammer. A pre-alarm is not possible.

Larger PD pumps may have special lubricating requirements for the cylinders. The oil is supplied by small reciprocating injectors (miniature PD pumps) drawing from a small reservoir. The reservoir needs a low-level alarm which should also inhibit startup. A shutdown may not be necessary since damage from low oil level is not immediate. The reservoir is supplied from a larger lube oil tank through an integral float valve. The tank requires a low and a high level alarm. These can be provided by a single transmitter. Variable speed pumps, especially those driven by engines, may require an overspeed trip. This should come from a separate sensor from the governor since it may be a governor failure that has caused the overspeed. A simple method is a small bolt mounted in a hole in the rim of the flywheel and held in place by a spring. Centrifugal force causes the bolt to project from the rim and trip a limit switch mounted on the frame.

SAFETY. There are no inherent dangers associated with PD pumps other than

extremely high pressure or leakage of toxic or hazardous materials. Actually diaphragm pumps are especially suitable for toxic service since they have no rotating or sliding seals. The possibility of leakage or even rupture and a subsequent fire must be

considered whenever flammable materials are being handled. Fire detection methods similar to those discussed in Controlling Centrifugal Pumps 4, page 10, may be necessary.

It is possible that a diaphragm may rupture during service. If the liquid is particularly hazardous, a double diaphragm may be used. In that case a tap will be provided by the manufacturer to install a pressure sensor for alarm or shutdown.

A fire safe block valve is needed on the suction whenever flammable liquids are being drawn from a reservoir with significant capacity5. Its interlocking must be handled slightly differently from that associated with a centrifugal pump. It is not advisable to slam shut the suction valve even if the pump is stopped simultaneously. Full vacuum may be induced during the rundown. If this causes air to be drawn into the piping an

extremely hazardous situation is created. It is best to use a time delay circuit so that the suction valve is not closed until several seconds after the pump has been tripped.

It may also be desirable to have a fire safe block valve on the discharge. Since most PD pumps are in high-pressure service, there may be the potential of pressurized fluid forcing its way backward past the discharge check valve into a fire. Automatic closure should also be interlocked to occur at least several seconds after the pump has been turned off.

ACCESSORY INSTRUMENTS. Any instrument used to control the process or to provide some safety or machine protection function should, if possible, have a simple local device to verify its operation. In the case of PD pumps that means pressure

gauges at both the suction and the discharge. Pressurized pulsation dampeners require pressure gauges to ensure that they are properly charged. Large reciprocating pumps have oil filled crankcases. A gauge glass (by vendor) and a thermometer should be provided.

The cylinder lube reservoir requires a sight glass. This is supplied by the vendor on API pumps1, 2, 3 . The tank needs a level gauge glass whose span is broad enough to cover both alarm settings.

If the machine is equipped with cooling water jackets, there should be a thermometer on the outlet of every jacket. A single thermometer on the supply is a good idea. High outlet temperatures may not mean the pump is overheating!

The variety of PD pumps implies a variety of special requirements. Be sure to discuss these with the pump vendor to make certain that nothing "obvious" has been

overlooked.

PARALLEL PUMP INSTALLATIONS. PD pumps are quite suitable for parallel operation. Since the discharge pressure of each pump rises as necessary, all pumps will discharge into the common header. A common recycle valve is sufficient for flow or pressure control.

Starting up a pump that is discharging into a header that is already pressurized by other pumps may overload its driver. To prevent this it is necessary to have an individual recycle valve on each pump. This may be a slow acting ball valve. Starting the pump then becomes a simple timed sequence in which the valve is first opened, then the pump is started, and finally the valve is closed again. The pump should also be shut down in the same sequence. Remember that the ball valve will be opening against the full discharge head and may need a large actuator. In water service it is extremely important that the appropriate water resistant grease is used.

If variable speed pumps are used, the majority should be placed on fixed speed. One pump is then selected for process control to take the swings in demand.

SERIES PUMP INSTALLATIONS. PD pumps are not generally installed in series. Since series pumps must both discharge an identical flow and both are discharging a "constant" flow, it is extremely unlikely that the two can be matched without complex controls. It is common, however, to have one or more parallel centrifugal pumps

servings as boosters to one or more parallel PD pumps. The centrifugal pumps serve to provide the NPSH that the PD pumps require. The PD pumps in turn can provide a very high discharge pressure.

The centrifugal boosters should have sufficient flow capacity to supply the pulsating requirements to the PD pumps. This means the full peak flow, not the average. If they need controls they should be on pressure control by way of a recycle valve since there should be no interference in the suction to the PD pumps.

A warning: It may happen that the PD pump has a very low discharge pressure for some reason -- perhaps the piping has been removed for maintenance. It is then possible for the booster pump to push liquid through the various check valves and out the discharge without the PD pump being turned on at all. In fact, the flow may be even greater than if the PD pump were running!

SUMMARY. Figure 2-9 shows a typical arrangement for a positive displacement pump application. The following features are illustrated:

- Centrifugal booster pump with recycle pressure control and a minimum flow restriction orifice. - Low suction pressure shutdown with alarm. - Pressure gauge on the suction. - High vibration shutdown and alarm on the crankcase. - Thermometer and a sight glass in the crankcase.

- Discharge pressure controller with an alarm. The controller works through a recycle valve.

- Discharge pressure relief valve.

- High discharge pressure shutdown with an alarm. - Discharge pressure gauge.

A thermal relief valve must be around any isolation valve on the PD pump suction so that internal leakage does not over-pressure the piping.

REFERENCES

1. API STD-674, Positive Displacement Pumps -- Reciprocating. http://www.cssinfo.com/apigate.html

2. API STD-675, Positive Displacement Pumps -- Controlled Volume. http://www.cssinfo.com/apigate.html

3. API STD-676, Positive Displacement Pumps -- Rotary. http://www.cssinfo.com/apigate.html

4. Driedger, W. C., "Controlling Centrifugal Pumps"; Hydrocarbon Processing, July 1995.

http://www.driedger.ca

5. API RP 750, Management of Process Hazards. http://www.cssinfo.com/apigate.html

CONTROLLING SHELL AND TUBE EXCHANGERS

First published in Hydrocarbon Processing , March 1998.

This Adobe® file is available for download.

INTRODUCTION. Shell and tube heat exchangers are among the more confusing pieces of equipment for the process control engineer. The principle of operation is simple enough: Two fluids of different temperatures are brought into close contact but are prevented from mixing by a physical barrier. The temperature of the two fluids will tend to equalize. By arranging counter-current flow it is possible for the temperature at the outlet of each fluid to approach the temperature at the inlet of the other. The heat contents are simply exchanged from one fluid to the other and vice versa. No energy is added or removed.

Since the heat demands of the process are not constant, and the heat content of the two fluids is not constant either, the heat exchanger must be designed for the worst case and must be controlled to make it operate at the particular rate required by the process at every moment in time. The heat exchanger itself is not constant. Its

characteristic changes with time. The most common change is a reduction in the heat transfer rate due to fouling of the surfaces. Exchangers are initially oversized to allow for the fouling which gradually builds up during use until the exchanger is no longer capable of performing its duty. Once it has been cleaned it is again oversized.

WHERE DO WE MEASURE? At the fundamental level, there is only one variable that can be controlled -- the amount of heat being exchanged. In practical situations it is not possible to measure heat flux. It is always the temperature of one fluid or the other which is being measured and controlled. It is not possible to control both since the heat added from one is taken from the other. Therefore the first consideration is to specify the place at which the temperature is to be kept constant. This is usually within a piece of equipment somewhere downstream of the outlet of one of the fluids. Assuming there is not much temperature change along the piping, the measurement may be anywhere between the outlet itself and the point of interest, perhaps at the base of a distillation tower. In cases where the measurement is being made downstream of a bypass valve, the further downstream, the better the mixing will be, and the more representative the measurement. On the other hand, too far down-stream may result in process dead time that can make control difficult. In cases where the "other" fluid is the one being

manipulated, it is often quite sufficient to make the measurement directly downstream of the outlet nozzle of the exchanger.

WHICH STREAM DO WE MANIPULATE? The second consideration is which stream to manipulate. The complications arise from the fact that exchangers have four ports and involve two different fluids, either of which may change phase. The former feature alone allows eight different valve arrangements. Figure 3-1 allows the reader to figure them all out. The diagram assumes that it is the fluid on the shell side whose