12 replies to this topic

[Ghasem.Bashiri]

Dear Process Experts
Assume a vessel with operating pressure of 15 Barg and Design Pressure of 28 Barg.
(Design Pressure is based on upstream pump shut-off pressure).
As first point: Is there any limitation to gap between operating and design pressure? In my vessel Design Pressure is more than 70% higher than operating pressure.
Refer to API 520 definition, I used a PSV with set pressure of 28 Barg on this vessel.
But Refer to ASME and also API 520 I can set PSV on MAWP which is higher thad design pressure.
Which one is correct if MAWP and design pressure is available.
Finally MAWP is MAWP as maximum allowable, But why code permit us to have pressure accumulation on vessel (21%). In this case vessel pressure will be higher than MAWP and some error or damage will be happened.
Please advise.

[sheiko]

Which one is correct if MAWP and design pressure is available.

Both are correct but I would choose the highest (ie normally the MAWP), as it may lead (depending on the difference between MAWP and design pressure) to a smaller (thus cheaper) safety relief valve.

Finally MAWP is MAWP as maximum allowable, But why code permit us to have pressure accumulation on vessel (21%). In this case vessel pressure will be higher than MAWP and some error or damage will be happened.

Read this: http://www.eng-tips....d=239600&page=9

Edited by sheiko, 09 July 2011 - 07:08 AM.

[Ghasem.Bashiri]

Dear Sir
I checked all forum in cheresources, ASME Code and API 521 before writing my point.
Therefore I ask my point again and sure that it is some misunderstanding from my side:

I checked Design Pressure and MAWP definition in API 520.
Therefore I assume that vessel can not withstand pressure higher than MAWP.
But I dont know why ASPE permit 21% over to MAWP in fire case.
And why it is differ with 10% in conventional scenario.

Ghasem

[fallah]

Dear Sir
I checked all forum in cheresources, ASME Code and API 521 before writing my point.
Therefore I ask my point again and sure that it is some misunderstanding from my side:

I checked Design Pressure and MAWP definition in API 520.
Therefore I assume that vessel can not withstand pressure higher than MAWP.
Can withstand in the short time of the overpressure relief same as hydrotest duration with 1.3 times of design pressure.
But I dont know why ASPE permit 21% over to MAWP in fire case.
PSV in most fire cases not to be able to protect vessel because the vessel would usually fail before PSV opening.Indeed,it will lead to smaller PSV size.
And why it is differ with 10% in conventional scenario.
Based on my previous answers it is more than that of conventional scenario and 21% came from multiplying 1.1*1.1=1.21 means accumulation in fire case is 10% over with respect to other cases.
Ghasem

[Ghasem.Bashiri]

You mean that vessel should be subjected to Hydtostatic Test Pressure as 1.3 MAWP?
And based on this test we can sure that vessel can withstand 21% overpressure during fire case?
But Because MAWP is not available during process design we assume design pressure as set pressure of PSV.
And therefore 21% of desgin pressure can be lower than 21% MAWP.

Assume I have MAWP and Design Pressure: Which one is better to set for PSV.
I know with higher set pressure PSV will be smaller.

And I think we use fire case PSV spare in warehouse becasue we assume vessel will be failed during fire case and we want to save spare PSV, is it ok?
Actually refer to item that I searched I think Fire PSV is only for plant protection and not vessel protection.
Please add some note to this forum becuase it is common mistake for most process engineer that I know.
Ghasem

[fallah]

You mean that vessel should be subjected to Hydtostatic Test Pressure as 1.3 MAWP?
And based on this test,and also based on the fact that allowable stresses are adequately taken lower than yield stress,we can sure that vessel can withstand 21% overpressure during fire case?
But Because MAWP is not available during process design we assume design pressure as set pressure of PSV.
And therefore 21% of desgin pressure can be lower than 21% MAWP.
Yes to all.
Assume I have MAWP and Design Pressure: Which one is better to set for PSV.
I know with higher set pressure PSV will be smaller.
Correct.
And I think we use fire case PSV spare in warehouse becasue we assume vessel will be failed during fire case and we want to save spare PSV, is it ok?
OK.
Actually refer to item that I searched I think Fire PSV is only for plant protection and not vessel protection.
No clear/correct statement.PSV in fire case mostly installed in order to buy a bit time for operator intervention.
Please add some note to this forum becuase it is common mistake for most process engineer that I know.
Please don't hesitate to ask for further clarification.
Ghasem

[Ghasem.Bashiri]

Which Pressure should be used to determine fluid properties and PSV sizing in fire case.
MAWP or Design Pressure?
Can we say that when we use design pressure for PSV in flare network and as design pressure<=MAWP always then we have some safe margin for equipment design?
GB

[GS81Process]

Which Pressure should be used to determine fluid properties and PSV sizing in fire case.
MAWP or Design Pressure?
Can we say that when we use design pressure for PSV in flare network and as design pressure<=MAWP always then we have some safe margin for equipment design?
GB

The fluid properties should be evaluated at 121% of MAWP, assuming that the PSV set pressure is at the MAWP. Since it is a fire case it is likely that you would only require the latent heat of vapourization at the relieving condition.

Edited by GS81Process, 12 July 2011 - 03:34 PM.

[Ghasem.Bashiri]

MAWP or Design Pressure?
Before detail design of equipment MAWP is not available for such calculation.
I want to know if I use Design Pressure as common practice, is it over design in compare to using MAWP for PSV sizing or not?

[fallah]

MAWP or Design Pressure?
Whichever is selected to be as set pressure!
Before detail design of equipment MAWP is not available for such calculation.
I want to know if I use Design Pressure as common practice, is it over design in compare to using MAWP for PSV sizing or not?
Yes,it could be considered as overdesign (in orifice area point of view),provided that there would be a reasonable difference between MAWP and Design Pressure and also not to consider more overpressure than 10% (up to 10%*MAWP) due to mentioned difference.

Edited by fallah, 13 July 2011 - 06:48 AM.

[GS81Process]

MAWP or Design Pressure?
Before detail design of equipment MAWP is not available for such calculation.
I want to know if I use Design Pressure as common practice, is it over design in compare to using MAWP for PSV sizing or not?

Design pressure is fine. Even if the calculated orifice size is slightly larger because design pressure<MAWP, this is not typically a concern because you will have to select the next largest orifice letter size anyway.

[aquileslima]

What is the real pressure of vessel's failure ? 1x, 2x, 3x the MAWP? Can we relate these values?

[GS81Process]

Vessel failure typically occurs at 3 to 5 times the MAWP, but vessel deformation may occur at as low as twice the MAWP. Depends on what code is being used.

Edited by GS81Process, 12 December 2011 - 04:35 PM.