Valve CV Units -Query

[CHEMSTRONG]

In Process Dynamic Simulation using hysys , we need to enter valve Cv as per datasheet..We generally use Valve sizing chart to do this.. In this Universal Gas sizig Cv is given as per line size.. But for particular stream and valve combnation, i may inlet and outlet pressures , flows in diff units than Cv unit .. Then is it apt to use Cv in Us units when my H & MB for that stream - units are different... Suppose I may have inlet pressure in different units and also outlet pressure in diff units.. Also density in diff units other than Cv specified in valve datasheet , then sometimes Cv in US units.. so how can I apply Cv in US gallons to flow in line as per H & MB in m3.. Need some detailed inut on this...Is it just the relation factor.. and it does not matter what are inlet and outlet flows as per H & MB.. Request help from all members now..

 

[katmar]

In the USA Cv is defined in terms of USgpm for flow, SG (a dimensionless ratio) for density and psi for pressure drop. The units of Cv (in the US) are USgpm/(psi[sup]0.5[/sup]).

The is also a British definition of Cv, which is very rarely used anymore, and is similar to the US definition but uses Imperial gallons per minute in place of the USgpm. The SG and psi units are the same as for the American Cv.

In Europe the valve flow coefficient is called Kv. This is based on m[sup]3[/sup]/hour for flow and the pressure drop is in kgf/cm[sup]2[/sup].

All of the valve coefficients are gradually being superceded by the "Area Coefficient" Av which is defined in SI units - i.e. m[sup]3[/sup]/s, Pascal and kg/m[sup]3[/sup].

Just to make things a bit interesting and more confusing the resistance of a valve can also be expressed in terms of K, the Resistance Coefficient.

I have put all this into a paper which is available at

http://www.katmarsoftware.com/articles/pipe-fitting-pressure-drop.htm

There are plenty of places on the web that will give you the conversion factors between all of these.



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