correlation between gases and liquids Cv

[vahid.z]

As I know, Cv is the capacity of a valve to pass water in GPM with 1psi pressure drop. so the Cv of a valve doesn't depend on the medium and fluid type (gas or liquid).
regarding this, can I use water to measure Cv of a Valve that will work to pass Air?
In other word, if I measure Cv of a valve by using water and Air, the results will be the same? (I think Cv is the same and Flow is different which can be calculated by using Cv and other parameters like comprehensibility according to the related formula. Am I correct?)

 

[Mous1747]

Cv must be determined by testing with water
definition of Cv : numerically equal to the number of US gallons of water at 60 F THAT WILL FLOW THROUGH THE VALVE IN ONE MINUTE WHEN THE pressure drop across the valve is 1 psi

 

[Danlap]

Hi,

It is slightly more complicated than your summary.
Very well explained in Emerson handbook

http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf

about "Sizing valves for fluids" and "sizing valves for compressible liquid", also the different approach to calculate mass flow and volumetric flow.

Two phases medium, molecular weight, critical pressure, and other constanta should be taken into account.
Cv value of the same valve is remain the same, but your expectation might not in regards with required/calculated flow output.

Suggest to use the formula in the handbook and re-check the result back and forth. And ask peer(s) to challenge your calculation.

Good luck,
regards,
MR

https://nosuchvalve.com

All valves will last for years, except the ones that were poorly manufactured; are still wrongly operated and or were wrongly selected

http://www.eng-tips.com/faqs.cfm?&rat1=2&fid=376

 

[fwcatr]

vahid.z

If I am understanding your question correctly, it is possible to conduct a Cv test using water and using air. The air test can be tricky though as you need to have very low pressure drop ratios. There are some cases where the water Cv and the air Cv can be close, but there are also cases where they can not be close. If you have a copy of ANSI/ISA-75.02.01, review the sections for testing and the sections for calculating the coefficient C. I would recommend using the Cv from the water test as I have found that to be more applicable.

 

[vahid.z]

Thank you for the answers,
I have read ANSI/ISA-75.02.01 standard, but I didn't find a clear answer.
In fact, I am manufacturing valves (I am new at this) and I must report the Cv of the valves. the valves are for air flow. it is very difficult for me to do the Cv test with air so, I decided to measure Cv by using water. Can I do this?

Best regards
Vahid

 

[MortenA]

Take a look at the info Danlap pointed to (the Fisher control valve handbook) and check out chapter 5 from around page 110 and forward you will find the basic equations etc for who Cv is used with regards to compressible and in compressible fluids.

Best regards, Morten

 

[vahid.z]

Dear Morten
I have read the handbook, like ANSI/ISA-75.02.01 standard, the handbook presents different formulations for compressible and in compressible fluids.

however, all the valve manufacturers give a unique Cv (at 100% travel) for a valve, regardless of the fluid type. It means Cv of a valves doesn't related to the fluid type.
So, can I measure the Cv using water??

I think the formulation are mostly used to calculate Flow rate for different fluids.

I am in a condition that have to make a decision about the test fluid.

best regards
vahid

 

[fwcatr]

Vahid.z

Yes, you can do a Cv test using water even if your valve will be installed in an air application. Have you completed a compressible fluid sizing?

 

[vahid.z]

fwcatr

No I have not done any sizing.
I will only report the Cv. The procedure and formula to do Cv measurement using water is so easy.
which kind of sizing I should do?

 

[fwcatr]

vahid.z

Go through the sizing exercise for compressible fluid

Can use the previously stated reference by Danlap, the control valve handbook

http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf

 

[vahid.z]

fwcatr
thank you for your helps,

Why should I do sizing?
taskmaster asks me only Cv value and measuring procedure.
sizing means determination of the flow rate in a valve (with known Cv) for a specific fluid under a given condition (pressure, temperature and so on).
best regards

 

[fwcatr]

vahid.z

Sizing exercise would determine the suitability of the chosen valve with the stated process conditions, to ensure valve is not undersized or oversized.

 

[Danlap]

Hi vahid.z

Align with fwcatr that you should do the sizing.
In your case, knowing Cv only is like driving with early version GoogleMap. It will only tell distance and approximate time from point 1 to point 2.
And confirm it with sizing, is like driving with Waze or similar app. It is already interpolate the traffic, stop light, etc. So it will tell you the more accurate speed (your fluid flow rate).

On ideal condition, both app will show the exact same result if only there is no traffic, stop light and using the same vehicle. Your vehicle is for sure different, now your using water but it will be implemented for air. At whatever pressure and temperature you have in mind.

Kind regards,
MR

https://nosuchvalve.com

All valves will last for years, except the ones that were poorly manufactured; are still wrongly operated and or were wrongly selected

http://www.eng-tips.com/faqs.cfm?&rat1=2&fid=376