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Anonymous Poster

how to calcualate the flow rate base on valve opening% for equalpercent valve

09/06/2009 6:56 AM

Dear all,

Is there any body know how to calculate the flow rate pass through the equal percentage control valve base on valve opening%.

Thank so much,

Nany

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#1

Re: how to calcualate the flow rate base on valve opening% for equalpercent valve

09/06/2009 9:24 AM

Besides its not that much accurate, you can have a good approach by using the valve discharge coefficient.

Contact your valve manufacturer and ask him or download from its web site the valve cv curve X % opening. Based on this value, and correcting for your fluid density, pressure and temperature, you can get to a good value.

But - again - if you're talking about process control, accurate measurement, you should go for a flow measurement device.

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#2

Re: how to calcualate the flow rate base on valve opening% for equalpercent valve

09/07/2009 6:30 PM

Most Control Valve manufacturers make available in their internet sites charts showing Cv x % of valve opening.

There is a specific chart for every valve body type and flow characteristic curve.

If you have the valve opening (stem position) you can find out the corresponding Cv in these charts.

If you are dealing with liquids, flow (Q) = Cv √(dP/SG), where dP is the differential pressure and SG the specific gravity.

Do remember that Cv available in the manufacturers charts refer to the inherent flow characteristics, calculated considering dP constant.

The resulting flow calculation using the above method is expected to be way beyond +/- 10% error margin. Useful if you do not have any flow measurement at all.

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#3

Re: how to calculate the flow rate base on valve opening% for equal percent valve

09/08/2009 5:55 PM

Hi Nany

The smallest passage, flow, in any system controls the total flow.

If the load had an equivalent Cv of 4 and the metering valve was a Cv of 8 when fully open it would have very small affect on the system flow or load until the metering valve were reduced to below 50% (a Cv of less than 4) of the load.

If you are seeing some unusual results this may be the cause.

Tom

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#4

Re: how to calcualate the flow rate base on valve opening% for equalpercent valve

09/09/2009 7:17 AM

Where;

A1 = Pipe cross section area

A2 = the cross section area for the gap on the valve open

such that A2= A1*(1- valve opening %)

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#5

Re: how to calcualate the flow rate base on valve opening% for equalpercent valve

03/14/2024 12:12 PM

Yes. The valve manufacturer will, and their local friendly valves distributor will too. Telephone?

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