8 replies to this topic

[black friday]

Hi all,
I'm looking for some assistance regarding a calculation.
The gas flow rate across a PCV for a PSV sizing calc is what I'm having problems with. A high pressure vessel vents to a low pressure vessel through a PCV (details in spreadsheet). I've calculated 313Nm3/hr as the volumetric flow rate. However I've found some design documents, which while not kind enough to specify how they've reached this figure, indicate that it is to high. I should be getting an answer below 250Nm3/hr apparently.
If anyone might take a look at the attached spreadsheet and see if I have correctly calculated the flow rate across a Cv=4.4 Cg=156 valve from 550kPaa to 220kPaa for pure N2 service that would be greatly appreciated.
Thank you.

Attached Files

•  CHE PCV Calc.xlsx   10.83KB   48 downloads

[breizh]

Scenario : N2 valve full open

My calculation with different formulae leads to :
355 Nm3/h
445 kg/h

Cv= Q/279 *(d*T/ (p2-p1)/(p2+p1))^0.5
M= 355 * Cv *((p2^2-P1^2)/T)^0.5

Q: Nm3/h
M : kg/h
d : Mw N2/Mw air :28/29
T:K
Delta P= P2-P1 with P bars abs



Note : similar formula to yours in Carl Branan , rules of thumb for chemical engineers
.
Gc= Q /( (520/(G*T))^.5 *P1*[sin(3417/C1*(DP/P1)^0.5)]
deg

Q :SCFH ;T deg R ;P PSIA , P1 valve inlet pressure

Hope this helps


Breizh

Edited by breizh, 21 February 2012 - 02:47 AM.

[black friday]

Thanks for the feedback Breizh. The second formula you mention is the same formula I used with degrees rather than radians.

[black friday]

Actually would you mind giving a reference for the equation you used Breizh?

[breizh]

Hi,

All the equations are coming from API 520 or API 521 , which are the references used to issue the document in French I have with me ( Year 1990) .

Hope this helps.
Breizh

[katmar]

With gas flow, if you hold the upstream pressure on a valve constant and gradually decrease the downstream pressure you will eventually get to a situation where decreasing the downstream pressure does not result in any increase in the flow rate through the valve. At this point the valve is said to be "choked".

If you do not have any details of the valve, or if you are doing a rough estimate, then it is usually assumed that the choke occurs when the pressure drop is 50% of the absolute upstream pressure. However, with a well designed globe valve the pressure drop can exceed 50% before the valve chokes. The valve specification sheets usually term this x_T . In your case it seems to be specified as C₁.

If you recalculate your valve, but limit your pressure drop to 50% of the upstream pressure you will get around 227 Nm³/h for the flow. But it seems that you have a good quality valve that is designed not to choke at 50%.

[ihg]

From the data given, I think you have a DN20 or DN25 Fisher 95H regulator. Using equations is not going to give you good answers with a PCV, as the valve opening depends on too many variables.
See http://www.documenta...d100117x012.pdf
for the recommended method.

[S.AHMAD]

1. For k=1.4 (specific heat heat ratio), the chocked pressure is 0.528 times the upstream pressure. Based on upstream pressure of 550 kPaa then the chocked pressure is about 290 kPaa. Since your downstream pressure of 220 kPaa is lower than the chocked pressure then your system is most probably a chocked flow system.
2. So, the right approach is to use adiabatic equation to determine/verify the chocked flow conditions
3. It is an iterative calculation.
4. I am unable to explain here, but you may refer to any textbook on compressible fluid mechanic. One of the good book is Albright's Chemical Engineers Handbook.

Edited by S.AHMAD, 22 February 2012 - 07:42 PM.

[breizh]

Black friday ,

Take a look at this calculator ( gas control valve sizing) and the theory associated .

http://www.pipeflowc...egulator-sizing


Hope this helps

Breizh

Edited by breizh, 22 February 2012 - 10:11 PM.