Control Valves and Pressure Drop

D

Thread Starter

Diwakar.S

Could anyone help me to know how a specific value or constant pressure drop could be maintained in a control valve for obtaining its inherent characteristics when it is seldom possible to maintain a constant pressure drop across a valve under working conditions?
 
W
Diwaker,

It's not apparent what kind of valve you're asking about, but in general, you could solve this problem by using a flow control valve, or by using a flow controller in conjunction with an on-off valve. It's hard to answer such a general question with a specific cure-all solution, as there are so many different types of valves and media; accuracy requirements would also dictate how this problem is solved. But here is an example - for a similar requirment, I've used analog proportional flow control valves set to a
specific value and then used as a digital (on-off) device in discrete control systems, both with hydraulics and pneumatics. I don't know if this is something one encounters a need for in process control systems, but I imagine it could be done in a similar fashion.

Regards,

Willy Smith
Numatics, Inc.
 
PUT a DP transmitter across the valve, and put a PID controller or a PID block to control the DP to a constant value by opening or closing the valve.
Anand
 
J

Johan Bengtsson

Anand's answer is the correct answer to your question, this reply is just to point out that you don't want a constant pressure drop under working conditions, that is why you have a controllable valve in the first place.

In normal working condidtions you would probably want a constant flow (or a flow depending on something else) you make this reality by measuring the flow and adjusting how much the valve should be open. By doing this you alter
the pressure drop across the valve and thereby maintain a constant flow (or change it according to your wishes).

/Johan Bengtsson

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P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
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B

Bruce Durdle

ON THE OTHER HAND ...

One approach that is used with for example large feedwater flows for boiler applications is to use a variable speed turbine-operated pump as the
primary control mechanism. To get quick response, a conventional feedwater valve is also used. The feedwater valve is adjusted to control the flow,
and the pump speed adjusted to maintain a specified pressure drop across the valve - as low as possible consistent with effective control and plant efficiency.

You can never say "Never" when talking about process control applications ...

Bruce
 
J

Johan Bengtsson

I know, I usually try to avoid that. I should have put in the word "normally" somewhere, forgot to do that. Still you excpect the pressure drop to change (that is still why it was put there in the first place) but you want to control the averge pressure drop by some other means.

Thank's for the example.

/Johan Bengtsson

----------------------------------------
P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]pol.se
Internet: http://www.pol.se/
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The inherent flow characteristics of a control valve will normally supplied by the valve manufacturer for a constant pressure drop of 1 PSI
when using water at 60 F and at the factory test bench. This is the only way the manufacturers can
specify their vave flow characteristics. However for detail dynamic response of the control valve under user applications (fluid comp.temp. press drop ) installed flow characteristic of the control valve shall be determined for fast
response or critical process. There are dynamic simulation packages like pipe net software which are of quite help.
Regards,
M.Sadra
National Iranian Gas co
 
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