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Partial Volume of Chemical Vapor

Partial Volume of Chemical Vapor

Partial Volume of Chemical Vapor

(OP)
Hello friends, I am reviewing a relief valve pressure setting calculation from a consultant and I have a question about partial volume. The equation used is

Vchem = Pvt/Pt

Where
Vchem is the partial volume of the chemical vapor as a ratio
Pvt is the saturated vapor pressure and
Pt is listed as the total pressure, i.e. chemical & air vapor + atmospheric

The chemical vapor pressure (Pvt)is given as 2.48 psia
The chemical + air pressure (Pt) is known based on tank measurement and is 2.5 psig, or, 17.2 psia

It seems that the result should be

Vchem = (2.48 psia)/(17.2 psia) = 0.144 or 14.4%

But the result given is 0.078 or 7.8%

It seems to me that they have doubled up on the atmospheric pressure

Vchem = (2.48 psia) / (17.2 psia + 14.7 psia) = 0.078 or 7.8%

I would appreciate feedback on whether it is correct to add atmospheric to the absolute pressure.

Thanks in advance.



RE: Partial Volume of Chemical Vapor

Possibly, they're confusing 17.2 psia with 17.2 psig. Confirm with the ones doing the calculation.

RE: Partial Volume of Chemical Vapor

(OP)
JStephen,

Thanks for the reply. I looked at some of their other work which uses the same form and had been vetted by a 3rd party reviewer. They had indeed doubled up.

I used to count sand. Now I don't count at all.

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