9 replies to this topic

[Jiten_process]

Dear all

I have a question. I tried search through old post but i could not get the post exactly related to my question.

What is the overpressure(accumulation) to be considered incase of multiple relieving scenario applicable for one PSV.

As per API 10% accumulation (overpressure) is for single PSV non fire case and 21% for single PSV fire case. But what if one equipment has PSV applicable to fire case, blocked outlet and other credible cases also.

I have one exchanger in my case, there are two relieving scenario applicable to tube side of an exchanger. One is firecase and second one liquid thermal expansion due to blocked in heat input. Now calculated required relieving rate for fire case is smaller, 100kg/hr then the required relieving rate incase of liquid expansion case (280kg/hr).

for above case, when i make PSV datasheet, what % overpressur should i mentione 10% considering the sizing scenario is liquid expansion or 21% since fire case is also applicable though not the sizing scenario.

Please reply.

I understand that you need mentioned 10%, because your controlling emergency scenario is not the fire case. Normally you will find more than 1 scenario and then you need chose the worst case scenario.

[Lowflo]

The answer depends on which scenario is controlling. In your case , fire is the controlling scenario, so you size the PSV for 21% accumulation.

BTW, the controlling scenario is the one that requires the largest orifice - not the greatest mass flow. Do the math and you'll find it takes a much larger orifice to pass 100 kg/hr vapor, as compared to 280 kg/hr liquid.

[Jiten_process]

Thank you all

yes lowflow you are correct...in my case governing is the fire case coz orifice size for liquid relief is less eventhough the mass flowrate is higher due to large density difference between vapor and liquid.

However my question was general as it popped up in my mind when i was doing the calulation. what i understand from your reply is that accumulation is to be defined based on controlling scenario...so let say in multiple cases if controlling scenario is fire, i will mention 21% overpressure which then eventually will be applicable to other scenarios also despite of being nonfire cases. If so, then in a way it is deviation to API, isnt it? API does not allow accumulation more than 10% for single PSV for non fire case.

Is that so that accumulation is governed by the controlling scenario not the lowest one (which is 10%) incase of multiple scenarios.

What others have to say on this ?

[S.AHMAD]

Jiten
1. You are confuse and misunderstood the API overpressure.
2. The 10% and 21% allowable overpressure is for sizing the PSV. The set pressure is the design pressure. Not the design pressure + overpressure.
3. What it means is that, during fire case the system pressure is allowed up to 1.21 times the set pressure.
4. Once you have selected the fire case as the worst case scenario, the accumulated pressure for thermal expansion will never go beyond 10% of the set pressure since the orifice is bigger than the relieve requirement for the thermal expansion.
5. You can determine the accumulated pressure for the thermal expansion case using the thermal expansion flowrate and the orifice area for the fire case.
6. Alternatively, you can determine the rated capacity for thermal expansion at 10% overpressure which is going to be greater than 200kg/h (over capacity for thermal case).
7. In the data sheet, you should include (as notes) the relieving rate for other valid scenarios. This is useful for future reference.

Edited by S.AHMAD, 09 March 2012 - 03:14 AM.

[fallah]

jiten,

You should consider 21% overpressure for sizing a PSV when:

1) Fire case is only applicable scenario
2) Fire case is governing case among multiple applicable scenarios

Fallah

[Jiten_process]

@fallah

okay..point no. 2) of ur reply says : consider 21% overpressure incase fire case is governing in multiple applicable scenarios.

In above case, eventually for other scenarios except fire case, over pressure will also become 21%. Is it right ? since overpressure cannot be different for one PSV for multiple scenarios.

that means relieving pressure in all scenarios will be 21% overpressure than set pressure right ?

Plz confirm or correct my understanding...

[fallah]

@fallah

okay..point no. 2) of ur reply says : consider 21% overpressure incase fire case is governing in multiple applicable scenarios.

In above case, eventually for other scenarios except fire case, over pressure will also become 21%. Is it right ? since overpressure cannot be different for one PSV for multiple scenarios.

that means relieving pressure in all scenarios will be 21% overpressure than set pressure right ?

Plz confirm or correct my understanding...

jiten,

For other scenarios except fire case, over pressure will never be more than 10% since the PSV orifice size has already been greatest among those of other scenarios calculated with 10% oover pressure in PSV sizing stage.
Overpressure can be different in one PSV for multiple scenarios (due to different relief loads with fixed orifice area) and their relevant relieving pressures as well.

Fallah

Edited by fallah, 13 March 2012 - 06:49 AM.

[S.AHMAD]

Probably comment of post #6 is not very clear to you? Once the worst case scenario is selected for sizing, the other credible scenarios are automatically covered and the PSV orifice size for other scenarios is bigger than their relieving rate requirement.

Edited by S.AHMAD, 13 March 2012 - 07:51 PM.

[Jiten_process]

Okay I got it now.

Thanks a lot to everyone for their time.