7 replies to this topic

[pankajthool]

Hi all,

My question is that whether the allowable overpressure 10%/16%/21% per code should be based on PSV set pressure or MAWP of the equipment. If PSV is set at the MAWP, then there is no problem. But what if PSV is set below the MAWP of the vessel?

[fallah]

pankajthool,

Allowable overpressure values of 10%/16%/21% for a PSV per code should be based on MAWP of the relevant vessel...

Overpressure is the pressure increase (or percent of set pressure) over the SP of the PSV. Obviously, if PSV set pressure to be below MAWP, relevant allowable overpressure could be higher than 10%/16%/21% of the PSV set pressure...

Fallah

Edited by fallah, 07 June 2012 - 09:49 AM.

[pankajthool]

Fallah,

Thanks. This is useful.

[sheiko]

% expressed relatively to MAWP (or design pressure) is called accumulation, not overpressure.
On the other hand, overpressure is relative to set pressure.

Edited by sheiko, 22 June 2012 - 06:16 PM.

[fallah]

sheiko,

It is very clear when in response to OP query i stated that "Allowable overpressure values of 10%/16%/21% for a PSV per code should be based on MAWP of the relevant vessel..." i meant MAWP as PSV set pressure!

Indeed, then i clearly stated "Overpressure is the pressure increase (or percent of set pressure) over the SP of the PSV".

On the other hand, overpressure is the same as accumulation when PSV set pressure is equal to MAWP...

Fallah

Edited by fallah, 23 June 2012 - 01:35 AM.

[latexman]

But what if PSV is set below the MAWP of the vessel?

Then you have to be very, very careful with the nomenclature so there are no misunderstandings. Overpressure is pressure in excess of the relief device set pressure. Accumulation is pressure in excess of the vessel's MAWP.

Then, by definition, the allowable accumulation of 10%/16%/21% per code is based on MAWP of the equipment, and overpressure is whatever it calculates to be based on the sizing pressure and set pressure.

When the set pressure = MAWP, and in practice this is very common, we tend to get sloppy with the nomenclature. I know I do.

[pankajthool]

Thanks,

It means that we should always consider vessel MAWP first for relief valve sizing when PSV set pressure is equal to or less than vessel MAWP. Am I correct?

However, if PSV is provided to protect the downstreams, like say Vessel "A" has MAWP of 100 psig , PSV on it has set pressure of 90 psig ,and downstream vessel "B" has MAWP of 90 psig then I guess we should consider PSV set pressure plus 10%/16%/21% overpressure and not vessel MAWP. Please correct me if I mis-understood something.

Edited by pankajthool, 24 June 2012 - 11:31 AM.

[latexman]

PSV set pressure plus 10%/16%/21% overpressure does not exceed allowable accumulations on Vessel "A". This complies with Code, as it always will when set pressure < MAWP and the 10%/16%/21% accumulation rules are applied to overpressures. The math guarantees it.