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Control Valve And Pipe Size


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#1 linda_pro

linda_pro

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Posted 01 March 2011 - 12:56 AM

Dear all,
I would like to ask you about control valve and pipe size.
I have a control valve for Sea water service, installed on 3 inch pipe, calc Cv is 8. it reduces pressure from 174 barg to 6.9 barg for flushing service.

when i check with Fisher, they said that 3 inch valve size, high pressure trim, 3 stages must be used. The attached file is preliminary sizing and their catalog (the 4 inch inlet/outlet line size are not correct, it should be 3 inch but Fisher confirmed that the results is not change if inlet and outlet line are 3 inch).

It means control valve and line size are the same. I've never seen this case in my previous project and then i would like to ask it can be happend in engineering design.

Any feedback is appreciated.
Many thanks

Linda

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#2 katmar

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Posted 01 March 2011 - 05:10 AM

There is no reason why the control valve has to be smaller than the line size. It mostly works out that the valve is smaller, but it doesn't have to be. Your situation is a bit unusual in that to get a Cv of 8 you would normally be looking at a 1/2" or 3/4" valve and the only reason I can see why Fisher have selected such a big valve for you is because of the extremely high pressure drop and they need that body size to fit in the 3 stage trim.

#3 linda_pro

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Posted 01 March 2011 - 08:54 AM

Thanks for your feeback, Katmar.
I've just never seen before then i am a bit worry :D
You are right. The high pressure drop may be the reason of this situation. thanks very much.

#4 joesteam

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Posted 02 March 2011 - 08:09 AM

Linda_pro is correct, the CAV III trim is designed to prevent cavitation by putting the fluid through a number of smaller pressure drops rather than just one large one. So the valve with this trim has a very low Cv compared to the body size. You could actually get the same valve without the CAV III trim that would have a much larger Cv - and you would use a smaller body size - but it would not last long in your application.




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