Partial Volume of Chemical Vapor

[SandCounter]

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

V_chem = P_vt/P_t

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

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

It seems that the result should be

V_chem = (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

V_chem = (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.

 

[JStephen]

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

 

[SandCounter]

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.