I will be using a needle valve for my practical setup of project work. It is based on pulse tube cryocooler. I need to calculate the coefficient of discharge for needle valve for report purpose. Needle valve is of Swagelok make and is of M series with flow coefficient as function of number of turns open. I want to know, how can I use this flow coefficient chart to calculate discharge coefficient for same needle valve?
For any type of orifice,
Coefficient of discharge (Cd) = Coeff. of flow/velocity (Cv) x Coeff of contraction (Cc).
where
Cd = actual discharge/theoretical discharge
Cc = Area of vena contracta/ area of orifice
Cv = actual velocity at vena contracta/theoretical velocity
If you know coeff. of flow/velocity (Cv) , you will be able to calculate coeff of discharge (Cd), once Cc is known. Obviously Cc changes depending on the shape of the jet at vena contract however a value between 0.61 - 0.64 is usually accepted.
Hope it helps.
Paresh, is the fluid changing the state during flow through the swagelock? If yes, the the speed of sound in the gas phase is limiting the flow through the valve seat as far a I know.
Volker
0.61 to 0.64 is ok for sharp edged orifices, some needle valves might show coefficients out of this range
Hi Gian,
That's correct. These coefficients depend on the design of the valve and manufacturer's handbook/catalog is the best source to get the values. However, as far as I know, manufacturer only provides flow coefficient (Cv) value because that governs the sizing/capacity of the valve.
If you are using a Swagelok valve, i would have a look to a swagelok catalogue where you can find the definition, may be it is what you are looking for
http://www.swagelok.com/downloads/webcatalogs/EN/MS-06-84.PDF
In industrial use the flow coefficient Cv or Kv is the dimensional number that gives you the volumetric rate of water under a deltaP unit deltaP. Its value can spread from very low 0.0005 to very high 250.I guess its name comes from "volumetric Coefficient". It is the american way of thinking: you don't need to know the phisics behimd it, you just need a number and an easy rule.
Very llittle to do with the "Velocity Coefficient", Cv, just the same name, used by scietists, that is the ratio to a Bernoulli velocity, that can range from 0 to 1 and is adimensional.
I am looking for information regarding helium flow which is oscillating in nature, as it is used in cryocooler. The helium gas does not change the phase. It remains in gas state only. What I want is, governing mass flow rate equation for needle valve and value of coefficient which is contained by the equation.
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