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Member

Join Date: Apr 2009
Posts: 6

Valves and Flow Coefficients

05/08/2009 4:01 AM

Does anyone know how to calculate a Cv value or a butterfly valve. I can find plenty of examples of how to calculate the Cv that you require a valve to be for your process but what if you design a butterfly valve and want to calculate/predict the Cv value.

Which is what I'm trying to do any help would be appreciated.

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Pathfinder Tags: Cv flow valve
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Power-User

Join Date: Mar 2009
Location: United Kingdom
Posts: 308
Good Answers: 28
#1

Re: Valves and Flow Coefficients

05/08/2009 9:46 AM

You could intially take a simplistic approach and use an Orifice - Cv calculator.

http://www.spiraxsarco.com/resources/calculators/orifice/orifice-diameter-to-flow-coefficient.asp


OR

Multiply the effective area S calculated in mm2 by a constant to produce a supposedly equivalent Cv.
The most common conversion is S=18.45Cv.

Neither is a perfect solution by any means ...but perhaps a start point.

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Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
Posts: 4331
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#2

Re: Valves and Flow Coefficients

05/08/2009 4:20 PM

Why would you need to? Information from manufacturers is readily available, and over the full range of opening, 0 to 90°. This information comes from actual tests on the valves, not calculation. I assume valve design is on mechanical and construction considerations, not Cv (maybe one eye kept on streamlining the disc if low pressure drop is wanted). Cv is then what's measured on test.

Cheers.......Codey

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Participant

Join Date: Feb 2009
Location: Johannesburg South Africa in a quiet Garden Suburb of Edenvale
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#3

Re: Valves and Flow Coefficients

05/09/2009 12:38 AM

Hi John:

The comment from Woody is relevant as a first step calculation ie measure (calculate) the flow area of your Butterfly valve and use the Spirax Sarco calculator to obtain a CV Value. Because a Butterfly valve does not look at all like an orifice as the disc provides obstruction and turbulence the calculator can only be used as a first step calculation to give you an idea of what the Cv could look like.

The only way to get an accurate fix on Cv is to have it measured on a flow rig.

I hope this helps

Best of luck with you new design - I would like to have a look at it when complete

Peter

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Member

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Posts: 6
#4

Re: Valves and Flow Coefficients

05/09/2009 8:12 AM

Thanks for the replies. It's been a great help in that it confirms what I was beginning to realise that it can't be accurately calculated but measured on a test rig.

Thanks again.

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Anonymous Poster
#5

Re: Valves and Flow Coefficients

05/10/2009 3:53 AM

Hi! If you know the Cv of the required valve, ask the manufacturer to give you his catalogue with Cv values listed with sizes. Choose the one you need. That's it. Anyhow, even for other valves as well, we actually don't calculate Cv of valve but take vendor value. initial guide of 11 X (port)2 is only a guestimate figure. cheers//Goyal

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Anonymous Poster (1); Codemaster (1); JohnSuton (1); Peter Telle (1); woodygb (1)

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