Unit conversion Cv to Kv 1

[Luuk21]

Hi,

I'm having some trouble converting a Cv (imperial) formula to a formula that can be used for Kv (metric).

It concerns a Cv calculation for a gas flow trough a control valve. The formula is:

Cv = [Qg * (SpGrg * T)^0.5] / [1360 * ([P1 - P2]*P2)]
This is for sub critical flow.

Cv = capacity coefficient for fully open control valve
SpGrg = specific gravity gas compared to Mw air
Qg = gas florate ft^3/h at 14.7 psia and 60 deg F
T = flowing temperature, deg R
P1/P2 = in/out pressure in psia

I used the following conversions;
Kv = 0.862 Cv
Qg = 0.0283168466 m3/h
T = 5/9 K
P = 0.0689475729 bar

I would expect a clean outcome, however, the Pressure factors do not cancel out. Substituting R for P-X does not help.

I used WolframAlpha for conversion - what am I missing here? See attachment for overview

https://www.wolframalpha.com/input?...(P*14.5037738)-(R*14.5037738))*R*14.5037738))

 

[RVAmeche]

Wolfram Alpha is generally good at math but I don't know I'd trust it to have industry equations or know industry terms/variables.

There's numerous Cv to Kv converters out there:

https://spartan-pd.com/resources/cv-kv-calculator/

 

[Luuk21]

There's numerous Cv to Kv converters out there:

https://spartan-pd.com/resources/cv-kv-calculator/

Hi,

Thanks, this converter only is applicable for liquids as far as I can tell. Still usefull though.

I need a converter for Gas (crit/non-crit) and Steam (crit/non-crit), there are some Gas equations to be found, but no Steam correlations.

But it should be simple math, a converter does usually not show what it's doing, my company doens't accept just a number without a calculation.

 

[fel3]

Here is something I put together about five years ago. See if this helps. It's USCS to USCS, not USCS to SI, but you should be able to modify the calculation to suit.

============
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[RVAmeche]

If your company needs a calculation I have my doubts they'd blindly trust Wolfram Alpha results.

 

[Luuk21]

If your company needs a calculation I have my doubts they'd blindly trust Wolfram Alpha results.

If the input formula gives the same result, I can prove it's correct.

Solved the issue - the trick was to see P - R as one term (pressure drop).