Control Valves in Process Plant
Embed Size (px)
Transcript of Control Valves in Process Plant
ROBERT KERN, Hoffmann - La Roche Inc.*
In recent years, a second group of control valves hasreceived wide .acceptance. In these types, the actuatorrotates a butterfly flap, plug or disk around its axis(Fig. 3). Size for size, these valves usually have highercapacities and less flow resistance than the contoured-plug valves. Generally, control valves with rotating axesare suitable fora wide range of flow-control applications.
Characteristics of Valve Plugs
In the following brief discussions, only the generalfeatures of each control valve are given. For completedetails about a specific control valve, consult the manu-facturers' literature.' .
One major group of control valves resembles the globevalve (Fig. 1). In place of a handwheel, an actuatormoves the valve stem and plug, thereby opening andclosing the valve. The usual act-ilator is an air-operateddevice whose housing contains a diaphragm that sepa-rates it into two compartments. The diaphragm (andattached valve stem) is balanced in its position by aspring on one side and air pressure on the other. In flowcontrol, the air pressure changes in response to a signalresulting from the measurement of the differential pres-sure across an orifice or other flow-sensing element.
The single-ported control valve (Fig. 1) finds use wheretight shutoff is required in addition to flow control. Thedouble-ported control valve (Fig. 1) has two seat ringswith two plugs on a common stem. This is a highercapacity valve than the single-seated one of the same size.With hard seat rings and high temperatures, the double-seated valve cannot shut off tightly. The valve accessories,shown in Fig. 2, ~l1ow for various operating functionsand conditions. To meet your author, see Chern. Eng., Dec. 23, 1974, p. 66.
Control valves are the basic .regulatory devices in anyprocess operation handling fluid streams. Hence, we mustbe thoroughly familiar with the different types of these'valves and their flow characteristics. This enables us tomeet process conditions, and to ensure proper installationin the fluid system.
Major Types of Control Valves
For proper performance in any piping 'system, he e are the design relations, sizingformulas and installation procedures for seleCting and using control valves for fluids.
CE Refresher _
CHEMICAL ENGINEERING/APRIL 14,1975
CE REFRESHER /
CONTROL valves handle many types of process fluids. and are actuated by air In response to a process signal-Fig. 1
Single-Seat (Equal-percentagecontoured plug, fails closed)
Double-Seat (Equal-percentageported plug, fails open)'
AlternativeActuator and Plugs
ROTARY actuatormoves flap, plugor disk-Fig. 3
Without air pressure in the pneumatic actuator, thevalve can be in clos~dor open position. These alternative
available; or where pressure drop across the control valvevaries greatly. '
The modified parabolic-flow characteristic falls be-tween the linear and equal-percentage characteristics.This type of plug (usually V-port) finds use where themajor part of the system pressure drop is available forcontrol.
Actuators (also called operators or valve positioners)lift the valve stem and plug above its seat, or move theplug in the seat cylinder. Butterfly or ball-type controlvalves have the actuators side~mounted because the actu-ator stem rotates the hIve axle. Plug characteristics canbe influenced by the linkage between actuator stem andvalve axle.
The valve housing ,and the operator's yoke are separatepieces. Hence, after a valve is installed, the operator canbe rotated around the valve stem or valve axle, relativeto the valve body. This enables a convenient position tobe chosen for the actuator, in order ,to provide accessto operating points on the valve.
Hydraulic, mechanical and piston operators are alsoavailable.
86 APRIL 14, 1975/CHEMICAL ENGINEERING
Bellows Bonnet(Seal between valve and
packing box in toxic service)Extension Bonnet
(For cryogenic temperatures)
Lubricator forvalve-stempacking box
Finned Bonnet(For temperatures higher than 4000 F) .
Handwheels(For manual operation at
startup or air failure)
C" is a capacity index indicating the flow of 60F watcrin gpm, which will pass through the completcly openvalve under a pressurc dilTerence of I psi between thcinlet and outlet Oanges. Obviously, if S = I and6P =I- psi, then C" =Q.
Capacity indexes for the butterfly valve are also givenat two throttling positions of the flap, in addition to thefully open position. .
Valve flow coefficient, C", depends o.n the internaldimensions of the valve and the smoothness of surfaces.Tests made by manufacturers (using water or ai~ at pre-determined pressure difference) establish C" values.Manufacturers give the following definition:
Cv = Q( VSiylJ
mentation and equipment engineers when d~ciding onfail-safe' positions for control valves so as to assureorderly. shutdown procedures~
Capacity Coefficients of Valves
limit Stop(Restricts stem movement!
CHEMICAL ENGINEERING/APRIL 14,1975
positions are accomplished by reversing the seat ring andplug, or by reversing the location of the actuator spring.from below to above the diaphragm (Fig. 1).
One concern of the designer is to select valves that willfail-safe in the event of instrument-air failure. In princi-ple, a control valve fails safe if temperature and pressureof the process system do not increase after the controlvalve becomes inactive.
For example, fuel-oil control valves to heater burnersshould fail closed. At the same time, feed to heater tubes(in most cases) should fail open to avoid overheating thefurnace tubes. The feed-control valve to fractionatingcolumns usually fails clos.ed. Steam supply to reboilerfails closcd. Rcflux-drum vapor outlet and reflux pump-discharge valves fail open. Control valves in minimum- .flow bypass lines at centrifugal-pump dischargc lines,compressor bypass lines, and reciprocating-machine by-pass lines fail open.
Reactors are protetted under controlled conditions,and usually the feed-control valve fails closed. Generally,a designer of flow systems should consult process, instru-
ACCESSORIES extend usefulness of control valves by providing for extreme and unusual conditions-Fig. 2
Pneumatic Positioner(Or transmitter) .
CE REFRESHER I
and Pe is the critical pressure, psia.For simplicity: tl.P. == PI - Pv ' provided that Pv