17 replies to this topic

[Psingh]

Dear all,

 

Please tell me the best method / industrially accepted method to select governing case for PSV sizing :

 

1. Choose maximum volumetric flowrate(M3/hr) at given releving conditions from all applicable cases and size my PSV for this maximum load.
2. Calculate orifice area for all applicable cases and select the maximum area and there by the governing case for PSV. 

Best regards

Pramukh

[breizh]

Hi Pramukh ,
Consider the document attached to support your query .

Breizh

[Zauberberg]

You need to read API 520 and 521. Some older revisions can be found online.

[fallah]

pramukh,

 

The second method is acceptable to specify governing scenario as sizing case of PSV; but the cases under investigation should be credible rather than to be just applicable...

[Zauberberg]

If you type "flare design manual" on Google, you'd be surprised by the number of available references.

[Linda1978]

Hi,

 

You can use chapter 5 of GPSA for a preliminary sizing.

There are some simple sizing methods in design procedures (you may find some old ones online)

you can refer to Flare Gas System Pocket Handbook (I don't know if the name is exactly this)

 

Based on my experience (and recommendation in above references) I recommend studying all cases and selecting the highest size.

[latexman]

I was taught to check/design the inlet and outlet pipe and fittings for the selected size/PSV using the controlling liquid case, the controlling vapor case, AND the controlling two-phase case.

[fallah]

Hi,

 

You can use chapter 5 of GPSA for a preliminary sizing.

There are some simple sizing methods in design procedures (you may find some old ones online)

you can refer to Flare Gas System Pocket Handbook (I don't know if the name is exactly this)

 

Based on my experience (and recommendation in above references) I recommend studying all cases and selecting the highest size.

 

Actually the OP wants to know about the best method to select the governing case for PSV sizing not PSV sizing procedure itself...

 

Indeed, highest orifice area among the ones for various credible scenarios will dictate the governing scenario not highest size...

[Psingh]

Thanks every one for your quick help!!

 

@ Fallah, you nailed the point !!

 

While sizing PSV orifice for single PSV mounted on my system,

 

I was thinking that except fire case all cases would have 10 % pressure accumulation and I can find the max flow rate and select out the governing case but when I bring in the Fire case the scenario starts getting complicated.

Since the Delta P across the orifice would change in fire case vis a vis other cases and I can't use the max load of all relevant cases.

 

Being more specific i was thinking of single phase relief, I wonder how complicated it will turn up in multi-phase relief and what would happen in multiple PSV's.

 

Please help me in understanding further....

 

@ Breizh: thanks for sharing the PDF file but it could not answer my question specifically.

 

B.R.

Edited by pramukh, 07 April 2015 - 01:49 PM.

[fallah]

 

@ Fallah, you nailed the point !!

 

While sizing PSV orifice for single PSV mounted on my system,

 

I was thinking that except fire case all cases would have 10 % pressure accumulation and I can find the max flow rate and select out the governing case but when I bring in the Fire case the scenario starts getting complicated.

Since the Delta P across the orifice would change in fire case vis a vis other cases and I can't use the max load of all relevant cases.

 

Being more specific i was thinking of single phase relief, I wonder how complicated it will turn up in multi-phase relief and what would happen in multiple PSV's.

 

Please help me in understanding further....

 

 

pramukh,

 

In governing case selection for a PSV you should determine the required orifice area for each credible scenario; then no matter every scenario is fire case or non fire case and also no matter the relief is single phase or multi phase. Of course the relieving fluid should be evaluated at relieving pressure and temperature which might be single or multi phase and it's obvious that orifice area determination for multi phase relieving fluid in more complicated. For pressure accumulation in fire case the relieving pressure is to be considered as 1.21 times of the set pressure in relevant orifice area determination. Please let's know if still you need anymore clarification...

Edited by fallah, 07 April 2015 - 02:13 PM.

[fallah]

Linda1978,

 

Would you please explain about the reason of your negative feedback on my post no. 8?

Edited by fallah, 08 April 2015 - 01:58 AM.

[Psingh]

@ Fallah, 

 

Thanks for clarifications!!!

[AymanAyadi]

pramukh,

 

As already said by Fallah, you should calculate required PSVs Orifice area for every credible scenario (blocked outlet case 10% pressure accumulation, fire case 21%....) choose the critical one (higher area) and than choose the standard effective orifice areas.

Regards,

[Linda1978]

Sorry Mr. Fallah,

 

Just a mistake.

[mosayef]

dear all,

 

I need to design PSV for natural gas processing plant, but i have confusion to decide the basis of my PSV sizing calculation.

 

as we know, the Pressure from gas reservoir will decrease over time . at the beginning of production, the pressure at gas processing plant will be around 500 psi, but at the end of plant life, the pressure will decrease to 150 psi. do i have to size the PSV based on 500 psi or 150 psi ?

 

as far as i know, the calculated area of PSV is inversely proportional to upstream relieving pressure. so if i design my PSV at the lowest pressure possible, it will give the higher calculated area. but PSV is designed for overpressure protection, isn't it ? so not the lowest pressure possible i guess, but it will give smaller calculated PSV area if I design at 500 psi.

 

what should i do ? i need your help. thanks

 

 

sorry if it is not appropriate to be posted in here

Edited by mosayef, 09 May 2015 - 08:56 PM.

[fallah]

mosayef,

 

How about the PSV relieving rate 1) at the beginning of production, and 2) at the end of plant life? It's not expected to be constant with such pressure change of gas reservoir...

[Linda1978]

I think there is a pressure control system on plant inlet line and PSVs are located downstream of this system, so the reservoir pressure is not the controlling case (pressure will be regulated).

But the production rate shall be considered for PSV sizing. I recommend using the start of run (or the highest production rate) as the governing case. However, it is beneficial to study lower possible flow rate to select proper size and number of PSVs, more accurately.

 

Good LUck

[mosayef]

thanks all  

 

the pressure in the inlet of gas plant eventually will also decrease over time,  the production rate at the beginning is predicted around (70 MMSCFD , P = 500 psia, Case A), and at the end of life is around 20 (MMSCFD, P=150 psi, case B )

 

so basically i have to compare these two and i should pick the highest calculated area are between them in every scenario ? am i wrong ?

let say if the case B will give the higher calculated area of PSV, i have a doubt if the set pressure of the PSV shall be set at the lower pressure. or is it just for area calculation ? but in the reality i can adjust the set pressure at 500 psi ?

 

this is my first time i design the PSV

Edited by mosayef, 10 May 2015 - 05:22 PM.