Your best bet in control valves

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					                                                                                                      Valves | automation basics



Your best bet in control valves
By Hans Bauman




C
        ontrol valves may be the most          A valve operating strictly in the on-off          vary the output signal sufficiently to make
        important, but sometimes the           mode (such as the hydronic solenoid valve         the actuator and the valve move to a new
        most neglected, part of a control      in your home heating system) could be re-         position. The dead time, TDv, is the time it
loop. The reason is usually the instrument     placed by a simple ball valve operated by a       takes for the pneumatic actuator to change
engineer’s unfamiliarity with the facets,      pneumatic cylinder, a type we usually refer       the pressure in order to move to a differ-
terminologies, and areas of engineering        to as an automated valve.                         ent travel position. It is most commonly
disciplines, such as fluid mechanics, met-         The distinction between automated and          related to the dead band of the actuator/
allurgy, noise control, and piping and ves-    control valves is the ability of the latter to    valve combination, or, in case a positioner
sel design that can be involved depending      modulate or assume an infinite number of           is used, the dead band of the valve divided
on the severity of service conditions.         throttling travel positions during normal         by the open loop gain of the positioner
   Any control loop usually consists of a      control service. Physically, there are three      plus the positioner’s dead band (dead band
sensor of the process condition, a trans-      basic components of a control valve:              keeps the valve from responding instantly
mitter, and a controller that compares         n	 The valve body subassembly. This is the        when the signal changes, which, in turn,
the process variable the transmitter re-          working part and, in itself, a pressure        causes dead time). The valve itself should
ceives with the set point, (the desired           vessel.                                        never have a dead band of more than 5%
process condition). The controller, in         n	 The actuator. This is the device that po-      of span, that is, less than 0.6 psi for a 3-to-
turn, sends a corrective signal to the final       sitions the throttling element inside the      15 psi signal span or 0.8 mA for a 4-20
control element, the last part of the loop        valve body.                                    mA signal. The positioner/valve combina-
and the muscle of the process control          n	 Accessories. These are positioners,            tion should have no more than 0.5% of
system. While the sensors of the process          I/P transducers, limit switches, hand-         signal span. Ignoring process dynamics, a
variables are the eyes, and the control-          wheels, air sets, position sensors, sole-      positioner may, therefore, improve matters
ler the brain, the final control element           noid valves, and travel stops.                 by an order of magnitude.
represents the hands of the control loop.         There is no such thing as the ideal con-          In order not to box yourself in price-
This makes it the most important, alas         trol valve, but we can attempt to develop         wise, try not to specify what valve type
sometimes the least understood, part of        a workable compromise. The closest thing          to quote, but rather what features do
an automatic control syst
				
DOCUMENT INFO
Description: The closest thing to an ideal valve should have a constant gain throughout the flow range, such as a linear installed flow characteristic, no dead time with packing tightened, and a time constant that is different from that of the process by at least a factor of three. Besides the obvious, such as good quality workmanship, correct selection of materials, and noise emission, you should pay special attention to two areas: * Low dead band of the actuator/valve combination (with tight packing) * Tight shutoff, in cases of single-seated globe valves and some rotary valves (if required) The prime concern of an operator of a process control loop is to have a stable loop.
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