Equation to calculate the back pressure for a PSV discharged to atmosphere
Equation to calculate the back pressure for a PSV discharged to atmosphere
(OP)
This is a natural gas PSV. Though the discharge is to atmosphere, there will be little increase in back pressure on PSV. I remember there is an equation to calculate this. Can anyone please give me a hint?
Thank you!!
Thank you!!





RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
The way it was explained to me was "back pressure" is simply the pressure loss exhibited by the piping system downstream of the PSV.
That might be simplistic, but it helped me.
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
I used the D-W equation. For 2 m pipe, 1884 scfm and I got the pressure drop to be 3.1 KPa.
I also used Panhandle A equation, and the pressur drop 0.02 KPa.
What might be wrong in my 2 calcuations?
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
Is that 2 m length or diameter? Which ever it is, we need the other one too. Fittings??? Composition of the NG or MW? Come to think of it, why not attach your calcs so we don't have to run the numbers, just look it over for mistakes.
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
Thanks again for your quick reply. It is a 3 in NPS venting pipe with 2 meter long
I found a error in my calculation before. Now I got three different results:
(1) using D-W, read friction factor from Moody Chart (0.015), I got 3809 Pa pressure drop
(2) using D-W, calculate friction factor from Haaland equation(0.004), I got 990 Pa pressure drop
(3) using Panhandle A Equation, I got a 1864 Pa pressure drop.
I did a google search and found Panhandle A Equation is mainly for large pipe diameter and 5-11 million Re numbers. So I guess it is not suitable for my case.
Then my quesiton is which would be proper way to get the friciton factor? Which result should I trust btw Case 1 and 2?
I am trying to figure out how to upload a file...
Thanks!!
Jasmine
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
Thanks a lot!
Yes, after I change 3.6 to 1.8, (2) gives a consistent result. Thanks for the good and quick advice!
Jasmine
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
I did get that wrong equation from Wikipedia. It shows the correct equation now.
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
I used API 521, and the got the Mach number 0.466. the pressure drop is almost same, around 3.8 Kpa.
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere
Jasmine, you do good work.
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Equation to calculate the back pressure for a PSV discharged to atmosphere