32 Control Valves

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CHAPTER32Control Valves

Iam an anticommunist. That was not always so. But working in theformer Soviet Socialist Republic of Lithuania altered my views

about communism. Figure 32.1, drawn to scale, illustrates thereason for my revised political orientation. Really! Thats the way thepiping and control valve looked in the field. It happens because of

bad design practices:

1. The mechanical engineer specifies a pump so that it producesexcessive head and flow, so that he cannot be accused ofindustrial sabotage by undersizing the pump.

2. The piping engineer oversizes process lines to avoid being

branded an enemy of the people.

3. The Soviet instrument engineer must then install a smallcontrol valve that will consume all the excess pressure withoutoperating too far in the closed position.

Control valves only work properly within a certain range ofpositions. When a control valve is mostly closed (perhaps 5 to 10 percentopen), opening it a little bit more will increase flow a lot. When a controlvalve is mostly open (perhaps 80 to 90 percent), opening it a lot more

will hardly increase the flow at all. To summarize, a control valve canonly respond to a change in position in a linear fashion, between 20 to80 percent open. To avoid the control valve operating in the nonlinear,mostly closed position, the Soviet instrument engineer installed a smallcontrol valve that has a giant P at 50 percent open.

If a control valve is operating too far closed, I will partially closethe upstream gate valve to force the control valve to operate in the 20to 80 percent range. This is a bad practice. The high velocity passing

between the gate and the seat will erode the valve seat. Then, when

the valve is shut, it will leak through and fail in its function as anisolation valve.

A normal installation will have the control valve one line sizesmaller than the process piping. For example:

Line size = 10 in, control valve = 8 in


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Source: A Working Guide to Process Equipment


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398 A W o r k i n g G u i d e t o P r o c e s s E q u i p m e n t



Thus, simply changing the trim to a maximum size in an existingcontrol valve may not eliminate the control valve as an hydraulicrestriction. The term trim refers to the internals inside the controlvalve. If the control valve is smaller than the line size, reducing thecontrol valve pressure drop to a minimum will be more complicatedthan just changing the trim. The piping reducers shown in Fig. 32.1,on either side of the control valve, will also have to be resized. Thiswill be a big job if these piping reducers are welded rather thanflanged. I made this mistake on a process revamp project in

Lithuania. I specified that the maximize size trim on a control valvebe installed. But I forgot to specify that a full line size control valvealso be installed.

Many engineering design organizations adhere to the followingpractice in sizing control valves and pumps. The control valveoperating at normal design conditions will consume 50 percent of thefrictional losses through the system. For example, for a pumparoundcircuit on a distillation tower:

Frictional loss through heat exchangers = 60 psi Head increase of flowing fluid equivalent to 20 psi (about

50 ft)

Pressure drop through control valve = 60 psi

Total differential pressure developed by the pump = 140 psi

While this practice may result in good process control, it alsocauses energy waste, due to parasitic pressure loss in the controlvalve. Its a conservative way to design pumping systems. But forme, its just communism. Its an abdication of our responsibility todesign process equipment that benefits the long-term energyefficiency of the plant.

6"

Control valve

1"

FIGURE 32.1 Over-sized pump & piping cause under-sized control valve.



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C h a p t e r 3 2 : C o n t r o l V a l v e s 399

32.1 Pumps and Control ValvesThe relationship Ill now describe between a control valve and a

pump applies to a constant speed motor driven centrifugal pump.This covers more than 90 percent of the systems we encounter.For variable speed pumps, such as steam turbinedriven pumps,

control valves should not be used. The facility shown in Fig. 32.2 is acontrol scheme that is used not infrequently at older plants. Its a finepiece of technology, which has been lost with time. No control valveis used. The turbine speed is altered to directly maintain the vessellevel. With the increasing use of variable speed alternating currentmotors, elimination of parasitic control valves should become more

common.For the ordinary fixed speed motor centrifugal pump, I suggestthe following rules when selecting a control valve:

The valve should operate between 40 and 60 percent open atdesign conditions.

If I want to allow for future expansion, Ill oversize the valveand use an internal trim of reduced size.

L

Vessel

Steam

TurbinePump

FIGURE 32.2 Variable speed pump eliminates control valve.



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The normal pressure drop across the control valve shouldbe sufficient for the pump to operate (see Fig. 32.3) on the

flat part of the pumps performance curve. These curvesare published in reference manuals by the pump vendors.The flat portion of the curve in Fig. 32.3 has been labeledgood.

The control valve should not be sized for the steep portion of thecurve. I have labeled this part of the curve bad in Fig. 32.3.

The idea is for the centrifugal pump and control valve to work asa team to control flows. The pump and valve have to be selected

together if the valve is to control the process with a minimum ofwasted pumping power.

32.2 Operating on the Bad Part of the CurveI last saw this problem at a 100,000 BSD flashed crude charge pump inCorpus Christi, Texas. It looked like the pump was cavitating. Thepump discharge pressure was erratically low. The flow was alsoswinging. The control valve position was erratic. I checked the motordriver amps, which were also erratic. All observations indicatedcavitation.

As I did not have reference to the pump curve in the field, I nowproceeded as follows: I switched the control valve from automaticflow control (FRC) to manual. The flow became steady, as did thedischarge pressure and motor amperage. Now I understood theproblem was not cavitation, but a control problem.

PowerHead

Flow

Bad

Good

FIGURE 32.3 Centrifugal pump curve showing region of good controlability.



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As I manually closed the valve, the amperage on the motor drivervent up. This meant that the pump discharge pressure was increasingfaster than the flow was decreasing. Check the flow versus power

curve in Fig. 32.3. The pump was operating on the steep or badportion of its performance curve.

Normally, motor amps increase as a downstream control valve isopened. If you observe the opposite, then the control valve operationwill be erratic, because you are attempting to control the flow too far outon the pumps curve. This is another reason why the centrifugal pumpand the control valve need to be designed to work together as a team.This is exactly what the communist engineers in Lithuania failed to do.

32.3 Control Valve PositionThe control valve position shown on the control screen is not theactual control valve position. The valve position displayed in thecontrol center is the required valve position. It is quite wrong toassume the valve position shown in the control room represents thereal valve position in the field.

Liz used to make the same mistake. She would tell me to uproota tree stump, then assume it was done. She is smarter now. She

inspects the job in the field. Liz suspects Ill cover the stump withearth and pretend that I extracted it from our garden. Its the same inthe plant. To know the real control valve position you must go intothe field and inspect the valve.

Control valves have a tendency to stick. Even though the valve isshown to be moving on the panel screen, it may not be moving in thefield. This is easy to check. The stem of the valve should be shiny and

bright. This indicates that the stem has been traveling up and downrecently. If the stem is covered with dirt, ask the panel operator tostroke the valve. This means he will fully open the valve and thenfully close the valve. The valve stem has a position indicator. You canobserve in the field if the valve opens and closes to its full extent.

Some control valves move in fits and starts. They stick and thenjerk open suddenly. When I consulted in a refinery in Aruba, the fuelgas pressure was extremely erratic. High fuel gas pressure causedexcess gas to be flared. Low pressure caused propane to be vaporizedinto the fuel gas header. Sometimes the plant would be flaring fuelgas and making up with propane simultaneously!

I worked on this problem for six years. Control experts devisedadvanced, real-time, computer control programs. Meetings were heldto resolve the issue. Finally, I observed that the control valve on theliquid propane flow to the vaporizer would stick in a closed positionand suddenly jerk open to 60 percent, where it would stick. It took sixyears for me to identify and two hours to correct the cause of theunstable fuel gas pressure. Of course, this erratic valve position wasnot seen on the control center panel.



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32.4 Valve Position DialsFigure 32.4 shows a control valve installation in the field. Next to the

control valve there will be a small cabinet with three dials displayed:

Supply: This is the supply air pressure. It should not change.If there is no instrument air pressure, the control valve willnot work.

Signal: Sometimes labeled Input. This is a signal from eitherthe control center (if control is remote) or from anotherinstrument (if control is local).

Output: This is the instruction to the control valve as to its

required position.

The Signal and Output dials should have the same reading. Letsassume that this is an air-to-close valve. That means that more airpressure closes the valve. Typically, the supply air pressure is 30 psig.If the air output pressure to the control valve is 3 psig, the valveshould be 100 percent open. If the air output pressure to the controlvalve is 15 psig, the valve would be shut. We say the valve operates

between 3 to 15 psig air pressure. If the output air pressure was

9 psig, the valve should be 50 percent open. But if the valve is stuck,it may be in any position. It will not open regardless of air pressure tothe diaphragm. Or the diaphragm itself may have malfunctioned.

Instrument air

Panel

Supply

Signal Output

Diaphragm

FIGURE 32.4 Air-to-close control valve.



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32.5 Air-to-Open ValvesMany valves are air-to-open valves. The process engineer decides

whether a control valve should be air-to-close or air-to-open, basedon the fail-safe position of the valve if the instrument air pressure islost. For example:

A back pressure control valve on a wet gas absorber shouldfail in an open position on loss of instrument air pressure.

A fuel gas regulator for control valve to a furnace should failin the closed position on loss of instrument air pressure.

For many (but not all) control valves, you can tell if air pressure issupposed to open or close the valve. If the tubing to the diaphragm ison the top, as shown in Fig. 32.4, then the air pressure is usually (butnot always) going to close the valve. When in doubt, ask the paneloperator to move the valve. Touch the stem with your gloved fingerand see what happens to the air pressure of the output dial.

32.6 Saving Energy in Existing Hydraulic Systems

The pumping power lost across a control valve is approximately:

KWH =

(GPM) (s.g.)2200

where KWH = power in kilowatts converted to heat in the controlvalve

= pressure drop, psi

GPM=

(U.S.) gallons per minute s.g. = specific gravity

Installation only of a larger control valve, or larger valve trim,will save zero energy regardless of the result of the above calculation.In addition to modification of the control valve, the upstreamcentrifugal pump must also be modified. Assuming that the pumpis driven by a fixed speed AC motor, the size of the pumps impellerhas to be reduced. The smaller impeller plus the larger control valvewill result in:

% KWH

(DN)

(DO)

3

3= 100 100

where % KWH = percent of power saved on the motor DN = Diameter of new impeller DO = Diameter of old impeller



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The pump vendor publishes a family of curves for different sizeimpellers. Using the rules described in this chapter, you can select anenergy efficient combination of a new pump impeller and a new or

modified control valve.The size of the motor is not insignificant. The power required to

run a motor depends on what it is driving, not on the horsepowerrating of the motor itself.

Even on a brand new unit, you should not be surprised if 30 to50 percent of the amperage on a driver can be saved. Again, forturbine-driven pumps the goal is not to optimize the size of thecontrol valve but to totally eliminate the control valve.

32.7 Control Valve BypassesThe hydraulic capacity of a system is sometimes limited by pumpcapacity and sometimes limited by the sizes of the control valve. Todiscriminate between these two problems:

1. With the control valve wide-open, open the control valvebypass.

2. If the flow increases by a lot, the control valve is too small.

3. If the flow increases very little (less than 5 percent), theproblem is the pump.

Operating with a control valve bypass open is not permitted bymany operating companies. In theory, I agree that control valve

bypasses should be kept closed. In practice, I frequently have openedbypasses. For example, we have a fired heater that is limited by theunit charge pump. Opening the bypasses around the heater passcontrol valves seems reasonable and safe. But suppose there is a leak

in a heater tube. The panel operator closes the pass control valveremotely to stop the fire, but the flow continues through the bypassvalve. The reader can imagine how I became so smart on thissubject.

Running for a long period of time with a control valve bypassopen in hydrocarbon service should be considered a change inoperation requiring a HAZOP review. For me, as a field troubleshooter,Ill always close all control valve bypasses I opened during myinvestigations before I leave the plant in the evening.

32.8 Plugged Control ValvesWhen pumping dirty water through a small ported control valve (1 inor less), the valve may easily plug. I can back-flush the valve to blowthe dirt out (refer to Fig. 32.5):



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1. Open the control valve D 100 percent.

2. Close valve B.

3. Open valve C.

4. Open drain valve A. (Perhaps routing the drain via a hose tothe flare or enclosed sewer might be safer.)

Water will now flow backwards through the control valve.Especially since there will be little or no back pressure at valve A, I

have found this to be a highly effective method to clear small pluggedcontrol valves.

D

C

A

Drain

BDirtywater

FIGURE 32.5 Clearing a plugged control valve.



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