1 reply to this topic

[kasri]

Dear all,
For PSV we set one pressure (MAWP or Design pressure) , after for relieving pressure we set 10% ( conventional) and 21% ( fire case) .for example a vessel has MAWP=100 psig if my PSV set pressure also 100 psig .then relieving pressure is 121 psig ( fire case).
Please clear me if a vessel's MAWP is only 100 psig that means it can withstand up to 100 psig, then our relieving pressure is higher than MAWP(121 psig) means how the vessel can withstand.
Please clear me whether my understanding is wrong.
And also clear me why 3% limit on pressure loss in inlet line is there any relation with relieving pressure to set higher than set pressure.Normally PSV open at set presssure and it can go up to reliving pressure means the system can meet pressure above MAWP.
Why the syste
Regards,
Kasri

[paulhorth]

Kasri
Your understanding is correct. The vessel design codes allow the relieving pressure to exceed the MAWP by the specified amount. This is on the basis that a relieving condition is a rare event which is of short duration. The vessel can withstand this pressure because it has been hydrotested to either 1.3 or 1.5 x the design pressure. In contrast to this, in principle the vessel could sit at its design pressure for an indefinite time, so this is considered as the maximum WORKING pressure.

The old steam railway engines had their relief valves blowing all the time,so the relieving presssure was a working condition. I don't know if the boiler design codes for this service allowed the relieving pressure to be higher than the design pressure.

Paul