Control Valve

I believe the 0.96 and 0.28 are specific numbers for a certain situation and fluid.

I found this site which might explain what you need to know.

Are you working with steam?

Try googling a bit more. I googeled "control valve Sizing Calculation"

can anyone give me the derivation from which critical pressure ration factor arrived?

Yes! See document below page 13.

couldnt find the document.. plzzz help me out

i couldnt find the doc. plzzz help me out

And here another one. Had the search still open!

Why is the area of a circle always ∏ • r² ..? it just is....

It's a numerical constant....

Its just a constant normally found by experiment.

The values 0.96, 0.28 come from Emerson and are applicable for all liquids

If like Emerson you wish to carry out 1000's of experiments with different liquids, pressure drop and valves; tabulate these resultsand then formulate equations with which people can size valves. Then allow that work to be peer reviewed (because every engineering company for which I have worked uses it) then please feel free to offer an alternative valve sizing equation.

I would agree that the numbers seem a bit odd and random but as noted above the ratio of a circles circumference to its diameter is a somewhat odd number.

As to what the number represents I have never been clear in liquid service. In gas service the critical pressure ratio is the point at which the flow through the valve goes sonic and at which there is no increase in flow if you reduce the outlet pressure but the concept isnt as clear cut in liquids.