4 replies to this topic

[ChemEng01]

Should we use cp/cv or cp/cp-R for control valve sizing equations? 

 

I'm calculating a gas breakthrough through a control valve and calculating Fk = k / 1.4. 

 

Since the ideal cp/cv is used for PSV sizing I presume I should use the ideal Cp/Cp-R for calculating the gas breakthrough? 

 

API states that the ideal k value should be used, however I cannot find a similar quote in any control valve sizing literature. 

 

Appreciate any help

 

Thanks

 

 

[breizh]

Hi ,

Go through this document and follow the nomenclature ..

k=cp/cv

 

Good luck

 

Breizh

[ankur2061]

ChemEng01,

 

Refer this excellent post on discussion related to Cp/Cv (isentropic exponent) value to be used for PSV sizing.

 

http://www.cheresour...522-psv-sizing/

 

Regards,

Ankur.

[Mahdi1980]

ChemEng01,

 

If I understand your question, you wanna know if Cp/Cv is required to size a control valve.

The answer is that it is not required for sizing a control valve through which a gas is flowing.

For sizing a control valve, flow rate and pressure drop to be specified and the Specific gravity of the gas to be determined.

The S.G. of gas depends on pressure, temperature and composition.

Cv = Q/(Y. (dp/S.G.)^0.5)

Cv = sizing parameter of Valve

Y = opening ( in design the valve is full open so Y=1)

dp = pressure drop

 

So, Cp/Cv is not the main parameter.

 

Mahdi

[ChemEng01]

Thanks for the input guys,

 

I suppose it's up to me which one to use then. I normally just go for the ideal cp/cp-R as its more conservative. Will have to check how much difference I get using the real k instead next time. 

 

Mahadi 1980. 

 

I was using the cp/cv in the control valve sizing for the ratio of specific heats factor Fk. (Fk = k / 1.4). It just makes a difference when calculating the pressure drop limit i.e. choked flow as dP/P1 = a maximum of xT * Fk (where XT is the pressure drop ratio factor supplied by the vendor, and dependent on the geometry of the valve).  

 

I'm using the ISO / IEC / current Massonelian / current FISHER equations. The equation you supplied is the equation for liquid flow through a valve. 

Edited by ChemEng01, 13 May 2016 - 01:47 AM.