Allowable overpressure - Overpressure factor
Allowable overpressure - Overpressure factor
(OP)
Hello,
Recently, I have been doing work on PSV datasheet that have to be filled up by "Allowable over pressure (%)" and "Over pressure Factor". But it gets me really confusing to differentiate between these two parameters. As far as I know, these two parameters are quite same!
If someone could enlighten me about this I would be very grateful.
Many Thanks,
shiplu48
Recently, I have been doing work on PSV datasheet that have to be filled up by "Allowable over pressure (%)" and "Over pressure Factor". But it gets me really confusing to differentiate between these two parameters. As far as I know, these two parameters are quite same!
If someone could enlighten me about this I would be very grateful.
Many Thanks,
shiplu48





RE: Allowable overpressure - Overpressure factor
It would have helped if the entire datasheet had been attached, so we could get a flavor of the content and context. But I'll take a stab at it.
Relief device technology must follow some strict definitions of terms as set forth by the applicable Code. Unfortunately, people, even Engineers, get sloppy.
A datasheet should have instructions or an accompanying procedure. I recommend you track that down, if possible, to see if you can learn what was meant.
For the remainder of my reply, I refer to the Code I know the best, ASME and API. If you are working with different Codes, you will have to check that out.
To me, even the term "Allowable over pressure (%)" is mostly meaningless. I think "allowable accumulation" or "maximum allowable accumulation" was meant. ASME and API refers to "Allowable Accumulations" and not to "Allowable overpressures". Below are excerpts from API 520 Part I.
3.1
accumulation
The pressure increase over the maximum allowable working pressure of the vessel, expressed in pressure units or as a percentage of maximum allowable working pressure (MAWP) or design pressure. Maximum allowable accumulations are established by applicable codes for emergency operating and fire contingencies.
3.35
overpressure
The pressure increase over the set pressure of the relieving device. Overpressure is expressed in pressure units or as a percentage of set pressure. Overpressure is the same as accumulation only when the relieving device is set to open at the maximum allowable working pressure of the vessel.
Accumulation is always relative to the pressure vessel's MAWP, and overpressure is always relative to a relief device's set pressure.
For the second term, "Over pressure Factor", I think someone added the word "Factor" where it is not needed. They got sloppy and strayed from strict adherence to the ASME and API definitions. I think they meant just "overpressure", but I'm guessing. Try to find the datasheet instructions or procedure.
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Allowable overpressure - Overpressure factor
For your convenience, I have attached a portion of datasheet. I cannot upload whole as it is a big file.
Regards
shiplu48
RE: Allowable overpressure - Overpressure factor
Here it is.
RE: Allowable overpressure - Overpressure factor
Keep the entry for Allowable Overpressure and delete the one for Overpressure Factor. Overpressure Factor isn't a term that's used in common parlance.
Other problems that I noticed:
RE: Allowable overpressure - Overpressure factor
Thanks for your information. I am also feeling the same way.
RE: Allowable overpressure - Overpressure factor
Max and min in this case would typically refer to at least one operating and / or instrumented control loop failure, and is limited by some physical passive means or instrumented safeguard.
RE: Allowable overpressure - Overpressure factor
OPF < 1.3
1.3 < OPF < 2.0
2.0 < OPF < 3.0
OPF > 3.0
The overpressure factor is used in risk assessment exercises to address potential consequences of failure of the relief valve, which would obviously depend on the overpressure factor. OPF below 1.3 would likely cause no consequences, OPF higher than 1.3 but below 2.0 would probably result in gasket leak, OPF between 2.0 and 3.0 would result in major flange release and/or small bore piping rupture, whereas OPF above 3.0 would lead to vessel rupture.
Of course, these are somewhat arbitrary ranges and caution should be exercised when they are applied to any particular vessel.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Allowable overpressure - Overpressure factor
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Allowable overpressure - Overpressure factor
Dejan IVANOVIC
Process Engineer, MSChE