ASME I PSV Reseat Pressure & Max Operating Pressure
ASME I PSV Reseat Pressure & Max Operating Pressure
(OP)
Hi all,
In ASME I 2004 PG-72.1, it's written that "After blowing down, all valves set at pressures of 375 psi (2.7 MPa) or greater shall close at a pressure not lower than 96% of their set pressure, except that all drum valves installed on a single boiler may be set to reseat at a pressure not lower than 96% of the set pressure of the lowest set drum valve."
However, in ASME I 2007 of the same paragraph, the reseat pressure of 96% below set pressure requirement is not there anymore.
1. What are the possible reasons that this requirement is removed?
2. What is the minimum difference (good engineering practice or code requirement?) between maximum operating pressure and PSV reseat pressure for drum?
Thanks in advance for your help!
In ASME I 2004 PG-72.1, it's written that "After blowing down, all valves set at pressures of 375 psi (2.7 MPa) or greater shall close at a pressure not lower than 96% of their set pressure, except that all drum valves installed on a single boiler may be set to reseat at a pressure not lower than 96% of the set pressure of the lowest set drum valve."
However, in ASME I 2007 of the same paragraph, the reseat pressure of 96% below set pressure requirement is not there anymore.
1. What are the possible reasons that this requirement is removed?
2. What is the minimum difference (good engineering practice or code requirement?) between maximum operating pressure and PSV reseat pressure for drum?
Thanks in advance for your help!





RE: ASME I PSV Reseat Pressure & Max Operating Pressure
If it is assumed to be too large, it would affect the selection of the design pressure of the main steam line and HP bypass station.
Other associated issues are (a) for multiple boilers on a header system ( as with modern multiple HRSG's feeding one STG) the pressure drop from boiler outlet stop valve to the STG may vary between each HRSG, so the one with the highest main steam line DP would have its releif valve set pressure and reseat pressure govern max load operations and (b) automatic runback of boiler heat input based on not overpressuring the HP main steam line may also be affected by the reseat set pressure.
RE: ASME I PSV Reseat Pressure & Max Operating Pressure
Anyone has any idea for question 1, i.e why is the requirement of max reseat pressure removed from ASME I?
Thanks.
RE: ASME I PSV Reseat Pressure & Max Operating Pressure
RE: ASME I PSV Reseat Pressure & Max Operating Pressure
Concerning the first question.
Good news, ASME I safety valves have always been a clear problem to test on site, especially the blowdown. This was due to the allowance 7%full open / -4% reseat to compare withe ASME VIII or french code (NF32100) requirements which are +10% / -7%. From a boiler manufacturer point of view, they were also more costly.
As for a good practise, I would say that we generally consider an operating pressure around 90% of the SVs set pressure, which is the pressure required by API 527 for the leak test. This is only true for spring operated SVs, but ASME I do not allow piloted ones, if I remember well.