Valve Cv rating?

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uez2find Participant Join Date: Dec 2009 Posts: 4	Valve Cv rating? 12/29/2009 10:20 AM
	What is meant by the Cv rating of a valve? I seen it noted on valve data as Cv 2% or egual % or something to that line. I'm just a lowly instrumentation tech and I feel that I should have a handle on this term. Thanks.
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lyn Guru Join Date: Oct 2008 Posts: 42355 Good Answers: 1693	#1 Re: Valve Cv rating? 12/29/2009 11:27 AM
	OMEGA ENGINEERING - Technical Principles of Valves
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phoenix911 Guru Join Date: Aug 2007 Posts: 23647 Good Answers: 420	#2 Re: Valve Cv rating? 12/29/2009 11:34 AM
	Cv Rating is the Rate of flow depends on the pressure drop across the valve So Cv is the valve coefficient of flow and represents the flow of water in GPM with a 1 PSI pressure drop across the valve. Throttling characteristics can also shown with the valve at different openings Here's the Formula to determined Cv at different pressure drops Actual Flow (GPM ) = Cv √ (DP-sq) Cv = Coefficient of Flow D P = Pressure Change Sg = Specific Gravity The actual Cv is usually determined from Empirical Information Example: For example, a valve has a Cv rating of 2.5. This means when 2.5 GPM of water passes through the valve, the valve will cause a 1 pound pressure drop, and it will use up one pound of energy head. Another way of expressing this: one pound of pressure drop is equal to 2.31' of head loss. So if a valve has a Cv rating of 2.5 and you pump 2.5 GPM through the valve, the valve itself will create 2.3' of head loss. here's some usefull links http://www.omega.com/TechRef/techprinc.html http://www.airoil.com/cv.html http://www.pipingdesign.com/controlvalves.html Hope ths answers your question. p911 __________________ “ When people get what they want, they are often surprised when they get what they deserve " - James Wood
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Anonymous Poster	#3 In reply to #2 Re: Valve Cv rating? 12/30/2009 4:37 AM
	realy great hats of you
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nick name Guru Join Date: Mar 2007 Location: City of Light Posts: 3943 Good Answers: 183	#4 In reply to #2 Re: Valve Cv rating? 12/30/2009 8:58 AM
	"Actual Flow (GPM ) = Cv √ (DP-sq)" Equation has an error of operation sign and an other as symbol: Following equation is correct Q = Cv (DP/Sg)^0.5 Those equations present the difficulty to go from a unit system to an other. Today fluid simulation soft give a quite good Cv values even before the valve has been build and tested.
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phoenix911 Guru Join Date: Aug 2007 Posts: 23647 Good Answers: 420	#5 In reply to #4 Re: Valve Cv rating? 12/30/2009 11:04 AM
	Yes it is imperial units. here's the conversion. CFD simulation software can give a pretty good simulation, But not everyone can have that. But it did answer the question of Cv. The equation Cv*√(DP-sg) is, in fact derived from the Bernoulli Equation. Regarding it's accuracy, this is for incompressible, inviscid flow. here is for others. I believe it original came from trying to develop a linear answer/formula for the problem because equation assumes a square-root relationship between pressure drop and flow. And to be more difficult.....or precise if you order the whitepapers from ANSI/ISA S75.01 this should be the best and more accurate. Thanks for bring this up about the equation nick name, p911 __________________ “ When people get what they want, they are often surprised when they get what they deserve " - James Wood
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phoenix911 Guru Join Date: Aug 2007 Posts: 23647 Good Answers: 420	#6 Re: Valve Cv rating? 01/01/2010 3:31 PM
	By the way welcome to CR4. I was just looking through CR4 and this is some good advice, before posting a question. Search CR4 because this question may have been asked already. p911 __________________ “ When people get what they want, they are often surprised when they get what they deserve " - James Wood
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