Re: Re: control valves

From: <Jack>
Date: Sun Nov 04 2007 - 08:40:00 EST

Steve.....

thanks for the description of control valve authority. What would you say about similar requirements regarding three way control valves in a circuit? Should there be similar head loss (or authority) sizing? Any pitfalls to sizing a three way valve in a circuit?

Jack

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Jack Osborn, P.E.
Mechanical Engineer / Project Manager
Lee & Ro, Inc
925 937 4050 ext 402

Hi there
For accurate control of simple systems it is necessary for the control valve to have a fairly high head loss across it compared to the entire system pressure loss. The ratio of valve loss to total system (including valve) loss is called authority, and for good tight control it should be around 0.5 (1:2) or better. A system with all the head loss across the valve will have an authority of 1. An authority of 0.5 means 50% of the head is lost across the valve, and the remaining 50% is lost across the rest of the system. So for control we want the valve to have a high head loss which means its trim will be small, and the rest of the system to have a low head loss which means big pipes.
So thats the reason why you see smaller that line-size control valves, apart from cost of course.

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[Non-text portions of this message have been removed] Received on Sun Nov 04 08:40:00 2007