Cv

[simar187]

I previously posted a thread, “Cv and acceptable leakage of air,”

http://www.eng-tips.com/viewthread.cfm?qid=205910&page=1.

In that thread, MikeHalloran, mentions that you can correlate Cv to air flow, but that he didn’t know how to do it off the top of his head. I was hoping either he, or someone else might be able to tell me how exactly to do that or where I might be able to find some useful information regarding the correlation. He also, posted a link to another thread,

http://www.eng-tips.com/viewthread.cfm?qid=124819&page=6,

in which anegri, mentions that Cv is “is not derived from any equation in particular, but empirically defined (and measured).” He goes on to further state with respect to Cv that:
“So, to be precise, it is not applicable to discuss about the accuracy of the definition; one should consider instead the accuracy of the measurements (or of the predictive algorithms) adopted to determine the Cv values, and then the accuracy of the flow rate calculations in which Cv values are used.”
These predictive algorithms are of interest to me and again, I was hoping either he, or someone else might be able to tell me to elaborate on these algorithms such as where they could be found, or where I might be able to find some useful information regarding them. Your help would be greatly appreciated.

 

[MikeHalloran]

Did you try the equation that JimCasey gave in his response?
Note that it gives a Cv with units different from those for water Cv.

I know this waterCv/airCv correlation has been discussed in detail on the site, I just can't remember where.



Mike Halloran
Pembroke Pines, FL, USA

 

[proinwv2]

Have you looked at ISA S75.01 and S75.02? I believe that these will explain this in detail with equations.

I am on the road at this time, so I can't look at them for you.



Paul

http://www.ostand.com

 

[israelkk]

Try this site of the Lee Company it is a great company and great info.

http://www.theleeco.com/EFSWEB2.NSF/Engr!OpenView

On this page you will find the correlation between CV and equivalent hole/orifice diameter and/or Lohms (the Lee company fluid resistance coefficient system for flow of fluids and gases).

If you know the CV or flow for water you can calculate the Lohms and then from the Lohms to calculate the gas flow or CV.

 

[vzeos]

simar187,

There seems to be some confusion here. Maybe I’m the one who is confused. If I understand your question correctly, you say that you have a butterfly style valve with a Cv of 32042 and you want to find the air equivalent to the Cv value so that you can compare the valve leakage/blow-by to the air equivalent of the Cv value.

As has been mentioned by others, Cv, the liquid sizing coefficient, is the valve capacity in terms gpm of water flowing @ 1 psid. You can calculate the valve capacity in terms of SCFH by using the Gas Sizing Equation explained on the attached webpage (

http://www.fisherregulators.com/technical/sizingcalculations/#SizingforGasorSteamService)

in the Sizing for Gas or Steam Service section. Note that in the Gas Sizing Equation, the Cv value is the liquid sizing coefficient of your particular valve. You would solve the Gas Sizing Equation for Q_SCFH at dP = 1 psid and this would represent a gas valve capacity comparable to the water valve capacity. Pay strict attention to the equation limitations.

Regarding the Derivation of Cv, the following link describes how the valve equation is derived:

http://www.fisherregulators.com/tec...s_of_Valve_Sizing_for_Liquids_d350408x012.pdf

 

[Ashereng]

The Cv required of a valve is determined through calculations.

The Cv of the valve is a measured number, usually provided by the manufacturer of the valve. There is no "method" which "calculate" the Cv of a particular valve that is used, that I am aware of. I belive that most Cv values are empirically determined. I have not heard of the "predictive algorithms" mentioned above. Then again, I have not worked on the valve manufacturer side of things so they may indeed exist. And if they do, I would still think that they are valve specific.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[simar187]

Thanks for all your help. I was able to arrive at a solution. Once again, thanks!