Hi,
i am involving to sizing psv for a manifold of gas, set pressure of psv must be 320 bar and I need to reach pressure of 2 bar gage at the inlet of flare line,as my calculations have shown size of orifice will be almost 6" with 318 bar differential pressure.
I want to know if any psv as i described above is practical?

your answer will be really appreciated.

Looking at the way you present your facts, i guess you are pretty fresh in PSV related...

First, grab API RP 520 Part 1, study the section related to vapor relief under critical flow and subcritical flow...for clear understanding of PSV and hydraulic.

You should not say "I need to reach pressure of 2 bar gage at the inlet of flare line...". Conventionally the PSV downstream pressure will be back pressure created by the flow from PSV downstream to the flare tip. It is flow, pipe surface roughness and line size dependent.

If you have read above recommended standard on particular section, you will aware that the PSV should be under CRITICAL flow with 110% of 320 upstream and downstream less than 52%-55% of 110% x 320 barg.

If you have size you valve base on driving force of 320 barg - 2 barg, then you probably have made a mistake. Again, please read the section as recommended above.

Hi,

thank you JoeWong for your answer, but probably i could not present my problem well.
the back calculation from flare tip to the downstream of mentioned psv (including roughness, diameter of dicharge line ) showed that back pressure must be 2 bar gage with respect to the flow that shoud be relieved,with regard to psv condition the calculated size of psv is 6".

so i just want to know if this psv size under such condition is practical or not, I think i can find it in API 526 but this differential pressure caused by psv and resulting temperature of -14 C at downstream of psv is somewhat unusual to me.

please let me know if it is usual, and if it is, should i inject methanol again in discharge line to prevent hydrate formation despite of injecting of methanol at the well head?

mazdak,





PSV with inlet pressure of 110% x 320 barg relieving gas, i am almost certain that it is under CRITICAL (Choked) flow.

Under critical (choked) flow, maximum driven force across PSV nozzle is (PSV upstream pressure - Critical pressure) as sonic velocity occur in "vena contracta". You may read more related "vena contracta" in "Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve (click here)".

To understand why maximum driven force is (PSV upstream pressure - Critical pressure), this post "A refresh to Process Engineer on few phenomenons in restriction orifice (click here)" may give you some idea.

You mentioned that you have used upstream pressure of 320 barg and downstream pressure of 2 barg (built up back pressure) and used the driving force of 318 bar to calculate PSV "orifice". There are 2 main problems here.

i) If your vessel designed to ASME Section VIII Div I, the upstream pressure shall be 110% of 320 barg for non-fire contingency and 121% for fire contigency.

ii) driving force should not be (320 - 2). It should be (110% x 320 - critical pressure)




JT effect causing significant low temperature downstream of PSV is NORMAL.

But with pressure difference of 318 bar, you only get -14 degC downstream seem a little on high side. Anyway, constant enthalpy flash from 110% x 320 to calculated back pressure will advise you this temperature.

Hydrate formation issue has been discussed in earlier post "Tracing Of Flare Network (click here) ".

I think (and also has been discussed in this forum) from 110%*320 bar to 52-56%*110%*320 bar there is isentropic process and between 52-56%*110%*320 bar and 2 bar isenthalpic one through which major temperature drop occurs(jt effect).
Also, thought "vena contracta" occurs in RO and CV , not in PSV if considered as a nozzle.
Regards

Could you please specify maximum allowable pressure in flare line.

Dear all,

discharge line is carbon steel,ANSI 150 pressure class,. and back pressure is 2 bar i must add some device to drope the pressure from critical pressure to 2 bar at the inlet of discharge line, could please tell me how can i find more about that.

thank you for your answers.

Reminder

Hope this help you...
Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve

Thanks for your advise,but you are refered to your last paragraph in above post regarding procedure of calculating downstream temprature. You mentioned "flash from 110%*320 to calculated pressure",but i think the upstream pressure for flash should be critical pressure (around 55%*110%*320).
Regards

Ah...yes. You are right. You are amusing.

I am keep on viewing the PSV as "orifice". Thus, flash from upstream pressure to calculated back pressure.

If we view it as perfect nozzle, then should be isentropic flash from upstream pressure to critical pressure and follow by isenthalpic flash from critical pressure to backpressure.

Thanks for highlight.