4 replies to this topic

[nikhilbha]

Need assistance on selection of design Cv for pressure control valve based on calculated Cv

[katmar]

The rule of thumb was always to aim to select a valve that would have the calculated Cv when about 70% open. Obviously valves go up in step sizes so you cannot get it to be exactly 70%. Of course it will also depend on your process variability.  The design Cv is calculated on 1 point.  If you know the process will often deviate from that design point then you should include that information in your decision.

 

If the full-open Cv of the valve is too small to cope with process extremes how dangerous does the deviation become?

 

In the "old days" it was often found that if the valve was too big, and therefore operated towards the closed end of its range, then the control system would "hunt" and not reach a steady state.  With modern valve positioners and self-tuning controllers this is much less of a problem and I have seen valves run rock steady at 10% open.

 

Use the rule of thumb as a starting point and take the process circumstances into account to come to a reasoned selection.  You may find that the valve supplier will be able to give relevant advice as well.

[Ajay S. Satpute]

Nikhilbha,

 

Please provide available process data to help you with numbers.

 

Regards.

 

Ajay

[breizh]

Hi ,

Probably good to read !

 Hope this helps.

Breizh

[JMW]

There is also a program from Fisher that may be useful:

http://www2.emersonp...edownloads.aspx