ASME I PSV Reseat Pressure & Max Operating Pressure 2

[ashaari]

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!

 

[davefitz]

I am not certain of the minimum difference - it should be provided by the relief valve mfr, as it relates to the mechanical tolerances and the effect of component warm-up following initial lift. If the minimum difference is set too low, there will be chattering and subsequent seat damage , followed by leakage .

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.

 

[ashaari]

Thanks davefitz for the reply. I'll check with my relief valve mfr then.

Anyone has any idea for question 1, i.e why is the requirement of max reseat pressure removed from ASME I?

Thanks.

 

[Duwe6]

Because, for the boiler, it doesn't matter. The PSV is there to prevent overpressure events. No *BOOM* allowed. Reseat pressure and leakage are operational issues, not a design issue. I agree that they both are major concerns, but they belong in NBIC and your operations SOP, not in the Boiler Design Code.

 

[GBENARD]

Hi,

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.