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Allowable overpressure - Overpressure factor

Allowable overpressure - Overpressure factor


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,


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.

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.

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,

To a ChE, the glass is always full - 1/2 air and 1/2 water.

RE: Allowable overpressure - Overpressure factor

Dear Latexman, thanks for your kind information.

For your convenience, I have attached a portion of datasheet. I cannot upload whole as it is a big file.


RE: Allowable overpressure - Overpressure factor

This is a very poorly written datasheet. In the section of the form that you show, there are numerous entries that invite confusion and misinterpretation. That's the opposite of what you want a spec sheet to accomplish. You should revise this datasheet so the entries are unambiguous and clearly understood by the vendor. Or better yet, just use the API datasheet form (see App D in API 520 Pt. I).

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:
  • Phase - The listed units are "barg" ??
  • Backpressure - This form has two entries, one for "constant" and the other for "variable". The form should report the "built-up" and "superimposed" backpressure. The vendor will be confused by your form. They'll think that constant and variable backpressure are both referring to superimposed backpressure, and they'll search the form for the built-up backpressure and not find it.
  • % Required Blowdown - The user doesn't specify a specific blowdown value. The actual blowdown will vary within a relatively broad range.
  • Body Type - ?? Don't know what was intended by this. Perhaps this is intended to communicate whether the PSV is an API 526 valve or some other type. If so, revise the entry to API 526 Type Valve - Yes or No
  • Body Size and Bonnet Size - Neither of these are specified. Instead, the inlet and outlet sizes are specified.

RE: Allowable overpressure - Overpressure factor

Dear don1980,

Thanks for your information. I am also feeling the same way.

RE: Allowable overpressure - Overpressure factor

One parameter that sometimes gets overlooked is the max and min operating pressures. Your d/s only asks for "operating pressure". Same goes for operating temp - should be expanded as min to max operating temperature.
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

Overpressure factor may sometimes refer to the ratio of pressures between the source of pressurization and the subject vessel's design pressure, and it is usually categorized in 4 groups/levels:

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.

Process Engineer, MSChE

RE: Allowable overpressure - Overpressure factor

Thanks Dejan, I can see the logic of those ranges! When I googled the term I saw it was a common term in the Structural Engineering field.

Good luck,

To a ChE, the glass is always full - 1/2 air and 1/2 water.

RE: Allowable overpressure - Overpressure factor

Hi Latex, we also use these (or slightly modified, depending on client's standards/preferences) in LOPA assessments. It helps in visualizing different levels of consequences at various ovepressure factors or, putting it the other way round, it shows that considering equal consequence for different levels of overpressure would lead to ultraconservative designs in cases when OPF is below 1.3 and below 2.0.

Process Engineer, MSChE

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