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from the ENGG department...
Control valve
Continuous process industries, DCS questions. topic
Posted by Sumit on 6 February, 2006 - 11:08 pm
Can i use a Control valve without positioner?? if yes, how?? and will there be any effect on the operation of control valve and can i get precise control with this?
The control vale is for Steam line & How water line.


Posted by Johnny Crab on 9 February, 2006 - 12:05 am
Sumit,

Read this:

http://www.isa.org/InTechTemplate.cfm?Section=Article_Index&tem plate=/ContentManagement/ContentDisplay.cfm&ContentID=43794

and think about your question.

Make sure you are clear on the parts "Actuator" and "Positioner". Also, refer to Flowserve/Valtek or Fisher or ISA spec sheet forms for existing valves to reinforce your concept(s) of the parts. The vast majority of control valves here use current-to-pressure(I/P) converters.

We also have a handful of skid packages that were supplied with locally-controlled valves which SEEM to have no positioner but are tubed to a nearby level controller sometimes this one:
http://www.documentation.emersonprocess.com/groups/public/documents /bulletins/d103194x012h.html

JC


Posted by Bud Buyer on 9 February, 2006 - 12:21 am
A valve positioner does several things for precision valve operation:
- uses valve stem position feedback to precisely position the valve stem according to the control signal
- uses an air supply pressure greater than the control signal pressure to ensure enough energy can be delivered to the actuator so that the actuator can overcome line pressure and friction to position the stem where it should be.

In answer to your question, yes, a typical air operated modulating control valve can operate without a positioner, where the 3-15 psi control signal operates against a 3-15 pound spring.

But you lose the advantage of a positioner, so the stem can stick and there it sits, in the wrong position, and there's nothing to correct it.

And you have to make sure that the 3-15 pound spring can close against whatever your maximum line pressure is. That's a normal check in any case, but valve assemblies with positioners can be equipped with much stronger springs because there is a positioner operating with a greater air supply. Without a positioner, you're limited to a spring that matches your control signal range.

I have used valves without positioners on low pressure cold and hot water lines when budgets wouldn't allow otherwise. But the control loop would not have been considered "critical".

Bud


Posted by Shailesh C Patel on 9 February, 2006 - 12:48 pm
Hi sumit,

In general, control valves shall be furnished with valve positioners for the following applications:
a) All temperature control valves
b) Split range applications where two or more control valves are operated by a single controller.
c) Valves with high pressure drop (5 kg/cm2 for single seated, 10 kg/cm2 for double seated).
d) Valves handling viscous or flashing fluids or slurries.
e) Valves having extension bonnets and cooling fins.
f) Butterfly, V-Ball valves, rotating plug and other special valves.
g) Valves having other than maximum 1.0 kg/cm2 diaphragm actuators pressure.
h) Valves for liquid level control and other wide proportional band control.
i) Valves with high unbalanced forces.
j) Valves 6 inches body size and larger
k) 3-Way valves
l) Compressor anti-surge valves.
m) Valves with extra deep packing.
n) For averaging liquids level control service.
in other case you should not use positioners,

Regards,
Shailesh C Patel


Posted by Jonas Berge on 11 July, 2006 - 1:11 am
Really? Why not use positioners in all valve applications?

Jonas Berge
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Posted by Dick Caro on 10 February, 2006 - 12:06 am
Yes, you CAN use a control valve without a positioner since the overall feedback from the control loop will correct any residual valve positioning problems - eventually. The problem is loop dynamics. Positioners provide a form of cascade secondary position control that makes the primary loop perform faster and with less cycling. For steam and hot water service, you probably do not need tight control loop performance, so avoidance of the expense of a positioner is probably a reasonable compromise.

Dick Caro
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