December 10, 2008

Equal Percentage Valves - Opening Vs Flow

In case of equal percentage valves, the flow varies according to the following definition.

'Any % change in the opening from its current existing value changes the flow by same percentage of its current value at current opening.'

Means if current opening is say 10% & u change it to 15% than the change in opening from its current value (of 10%) is 50%, so the flow will also increase by 50% of its existing value at 10% opening. This means if flow was say 20% of the total range (Or capacity of the valve) at 10% opening it will become 30% (50% higher compared to 20%) of the total range.

So how to calculate it????

Now you know the definition so you can generate an equation which gives you all the values where 0% opening or lift is 0% flow while 100% lift is 100% flow.

So the curve for these valves look like this.

Long back I generated an equation from a general curve for an equal percentage valve. Its fixed for all kinds of valves from any manufacturer generally. There may be minor variations in case of specially designed valves otherwise it is same.

The equation is.

% Flow is = .06 + 0.49 * X -.019 * X^2 +.0005 * X^3 - 0.0000502 * X^4 + 0.0000000242 * X^5

Where X is % Lift or opening , X = 60 for 60% opening & not 0.6

Hope it is useful for all.

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omkranthi1 said...


There is a zero missing in the 5 th term isn't?

gate valves said...

i appreciate it for sharing the info. great post. great blog.

Anonymous said...

its wrong i can prove it

Anonymous said...

something wrong somewhere... I try and put it in Excel but produce some bizzare result, e.g. 60% lift produce %flow = -562,714!

Anyone tried it out?

profmaster said...

Did you use 0.6 for 60% or 60 in the equation.

vishal said...

Hey,let my valve opening initially is 0% ie closed,now the command goes to open,say it is 1%,then what will be the %flow???????

Engineer 55 said...

I also ran this calculation in Excel and got the same result (at 60%) as "Anonymous": -562.714 Back to the google searching....

Engineer 55 said...

I added the extra zero in the fifth term as omkranthi1 recommends (-0.00000502*X^4) and the result comes out within +/- 1% at 0 and 100% stem positions. Looks like a simple mistake with the post. Thanks for the equal percentage characteristic.

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Irfan Qadir said...


and the result is OK

Anonymous said...

for all equal-percentage control valves, the general equation is:

% max flow (or % max Cv) = 100/T*exp(H*ln(T))

where T = valve rangeability or turndown (typically 50 for globe type)
H = % valve open/lift (75% would be inputted as 0.75)

so if you are using a globe valve, the above equation reduces to:

% max flow = 2*exp(H*ln(50))

if you plot this against the equation given in the post, they produce similar results.

R. Kannan said...

I Really appreciate, this has been defined very clearly by simple example

Anonymous said...

Either, I do not understand your explanation of the Equal Percentage curve, or I do not agree. Let me give you an example: The valve is fully open, thus valve lift is 100% and valve flow is 100%, right? Then I reduce the valve lift to 90%, changing it by -10%. Then according to your explanation (as I understand it), the flow should be 100%-10% = 90% of maximum flow. However this is not right, and it is fairly obvious to see from the curve you are using, where the flow at 90% valve lift is approximately 70% of maximum flow.

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