6 replies to this topic

[rohollah]

Dear all

As per API 520 maximum allowable over pressure is

110% set pressure for single valve

116% set pressure for multiple valves

and 121% for fire case.

but we have a relief valve which has over pressure 25%

how can justify that?

best regards
rohollah

Attached Files

•  PSV-20551 A1.pdf   76.73KB   103 downloads

[latexman]

Mandatory requirements (i.e. legal requirements) in Pipe and Vessel Codes refer to accumulation. Accumulation - Vessel pressure above the MAWP, expressed as a percentage of MAWP (100[sizing pressure - MAWP]/MAWP). What is the accumulation in your case?

Overpressure is used to size PSVs, and to my knowledge is not a legal requirement. Overpressure is the pressure difference between sizing (relieving) and set pressures,expressed as a percentage of set pressure (100 [sizing pressure - set pressure]/set pressure).

Accumulation and overpressure are only equal when setpressure = MAWP.

[fallah]

Dear all

As per API 520 maximum allowable over pressure is

110% set pressure for single valve

116% set pressure for multiple valves

and 121% for fire case.

but we have a relief valve which has over pressure 25%

how can justify that?

best regards
rohollah

Dear Rohollah,

It was used for sizing of PSV's in liquid services, before the ASME code incorporated requirements for capacity certification of such valves.

Please see the section of 3.9 of API 520 Part I Ed. 2000 for more explanations.

Fallah

[Lowflo]

There are a number of possible explanations.

Was this valve purchased before ~1985? I didn't see a date on the attachment. In 1985, ASME code was updated to include liquid valves. Prior to that, liquid valves required 25 -30% overpressure to become fully open. Today's liquid trim valves only require 10% overpressure.

Is the set pressure below the MAWP? If so, then 25% overpressure might be OK. Remember, there's no limit on valve overpressure. The 10%, 16% and 21% limitations apply exclusively to accumulation.

Another possible explanation is that the set pressure is less than 30 psig (~ 2 barg). ASME allows a 3 psi accumulation for those cases in which the calculated value is less than 3 psi. that occurs when the set pressure is very low (less than 30 psi).

[fallah]

Remember, there's no limit on valve overpressure. The 10%, 16% and 21% limitations apply exclusively to accumulation.

Lowflo,

Valve overpressure is limited and to be such that "valve overpressure+valve set pressure" not to be exceeded than Maximum Accumulated Pressure (1.1* MAWP, 1.16*MAWP and 1.21*MAWP).

Fallah

[rohollah]

Dear Fallah

Thanks a lot for your good explanation.
Is there any relation between accumulated pressure and life time of the vessel?
with regard to corrosion allowance is it correct to say ,accumulated pressure decreases due to corrosion?

best regards
Rohollah

[fallah]

Is there any relation between accumulated pressure and life time of the vessel?
with regard to corrosion allowance is it correct to say ,accumulated pressure decreases due to corrosion?

Dear Rohollah,

Maximum accumulated pressure would be decreased with respect to the time during life time of the vessel.

Indeed, accumulated pressure decreases due to corrosion occurance.

Fallah