Boiler PSV Overpressure

Question

Boiler PSV Overpressure

aikmeam (Chemical)

(OP)

2 Jul 03 10:30

Can anyone advise me on the allowable overpressure for PSVs installed on Boilers (say operating at 54 barg), and which reference is that taken from? I recall a 3% figure at the back of my head but not quite sure of the reference.

davefitz (Mechanical) · Answer 1 · 2003-07-03 08:43:19

Per ASME I, for a drum-type boiler, the safety valves shall be set and designed to prevent the boiler presure from exceeding 6% over MAWP. This rule, initially set in 1914, still stands.

The first valve must be set to initailly lift at or below MAWP, and the remaining valves can be set to lift at a maximum of 3% over MAWP, and the "accumulation" may not exceed an additional 3%, ensuring not more than 6% overpressure.

JAlton (Mechanical) · Answer 2 · 2003-07-05 10:56:42

Davefitz is correct.  Here are the paragraph references.

ASME Section I, Paragraph PG-67.2  defines the 6% Maximum Boiler Overpressure requirement.

Para. PG-67.3,  One or more Safety Valves on the boiler proper shall be set at or below MAWP.  If additional valves are used the highest pressure setting shall not exceed MAWP by more than 3%.

Para. PG-72.1,  Safety Valves shall be designed ... to attain full lift at a pressure no greater than 3% above their set pressure.

J. Alton Cox
President
DeLuca Test Equipment
www.delucatest.com

aikmeam (Chemical) · Answer 3 · 2003-07-07 07:51:18

Thanks. So, simply means that in my case:

PSV1 set at 54 barg with accumulated overpressure not exceeding 55.62 barg.

PSV2 set at 55.62 barg with accumulated overpressure not exceeding 57.29 barg.

Is that correct?

kyu (Chemical) · Answer 4 · 2003-07-18 17:45:00

I don't think that's quite correct, although no one else said anything, so if I'm wrong, I hope someone will correct me!

I believe that the first valve (the one set at 54 barg) can have 6% overpressure, so that its maximum relieving pressure is the same as the second valve (57.2 barg).

Not that this will make much difference to the calculations....

JAlton (Mechanical) · Answer 5 · 2003-07-21 09:02:36

Your numbers are correct. PSV # 1 is "at or below MAWP" and its overpressure is 3%. PSV #2 does not exceed 3% > MAWP and its overpressure is 3% of set. Therefore the total overpressure does not exceed 6% of MAWP. Your PSVs are in compliance with ASME Section I requirements. This is based on Nameplate Stamping Information. Another question for you. Do you have actual Test Results for these PSVs? The Set Pressure Tolerance at the specified Set Pressures (approx 800 psi) should be +/- 10 PSI or < 1 BAR. Suggest you confirm this if possible. Also, do you have blowdown results? That is reseat information?

J. Alton Cox
President
DeLuca Test Equipment
www.delucatest.com

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Boiler PSV Overpressure