Equation to calculate the back pressure for a PSV discharged to atmosphere 1

[Jasminezxm2]

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!!

 

[Latexman]

Darcy–Weisbach equation.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[PEDARRIN2]

The D-W is excellent, but I was always confused by the term "back pressure" and how the equation solved for it.

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.

 

[Jasminezxm2]

Thanks for the reply.
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?

 

[Latexman]

Who do you think we are, the Amazing Kreskin?

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.

 

[Jasminezxm2]

Hi Latexman,

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

 

[Jasminezxm2]

There it is. The excel file

 

[Latexman]

My handy dandy spreadsheet gave me about 4000 Pa. Your Haaland equation seems to be wrong. Check some journal articles. Don't believe what you see on the internet. Your 3.6 constant should be a 1.8 according to my references. That fixes it.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[Jasminezxm2]

Letexman,

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

 

[Latexman]

Sure. Btw, there's a button for that.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[Latexman]

Where did you get that incorrect value? It needs to be fixed.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[Latexman]

I fixed it on Wikipedia.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[Jasminezxm2]

Good Job, Latexman.
I did get that wrong equation from Wikipedia. It shows the correct equation now.

 

[CMA010]

Using API STD 521 (§ 7.3.1.3.3) yields a built-up back pressure of ± 0.17 bar and 0.6 Mach at the outlet.

 

[LittleInch]

This is all great stuff, but in reality do you need to calculate this? What is the set pressure and does a fraction of a bar make any real difference? For 2 m of pipe exiting to atmosphere, I would have assumed 1 bar max and worked accordingly unless the set pressure was really low.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Jasminezxm2]

Thanks, CMA
I used API 521, and the got the Mach number 0.466. the pressure drop is almost same, around 3.8 Kpa.

 

[Latexman]

LI, yes, several parameters must be verified and recorded for a PSV. One of those parameters is the outlet tailpipe pressure drop as a % of set pressure depending on the type of PSV. When I'm sizing, checking, or auditing, the outlet pressure drop must be rigorously calculated at rated capacity and documented in the PSV record for the worst case scenario for each phase relieved (vapor, liquid, and two-phase).

Jasmine, you do good work.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[maddocks]

Be careful using Panhandle for anything other than long single phase gas lines and even then, we often correct for sctual conditions or if we can reference back to real pressures. It can be horrifically wrong sometimes.