Epoche,
I would suggest that the most critical case of "thermal relief" would be in the case of a fire, would it not?
That does make sense. Normally when I think of a fire case, I think of vapor generation. So, with my PSV in question, it is probable that a fire case was examined and it was determined that no vapor generation would occur and the only cause of concern would be hydraulic expansion. In this scenario, it does make sense that a 21% overpressure is being used.
For a bit more clarification, if the external source of heat was not a fire, but maybe something such as heat tracing, would this be a 10% or a 21% allowable overpressure scenario?
I believe I am about to answer my own question: I just re-read ASME Sec VIII, and it specifically states for the pressure not to rise above 21% of the MAWP when a pressure vessel “can be exposed to a fire or other unexpected sources of external heat”. And a hydraulic expansion scenario happens with an external source of heat; meaning 21% overpressure?
I appreciated your help, I believe I just needed a quick refresher on the definition of 21% overpressure.
There is a lot more confusion than what you state.
A thermal relief valve only relieves LIQUID. It is not designed to relieve vapor or gas. If you place the thermal relief device at the TOP of a tank, you must ensure that the tank will ALWAYS BE 100% LIQUID-FULL when it is loaded. The thermal relief device is normally placed in a liquid flowing pipe for that reason. This device is not designed to handle the Pool-Fire Case in a vessel because it will be subjected to increasing vapor generation within the vessel as the liquid level is being depleted. This device only relieves unduly generated HYDRAULIC liquid pressure - usually from thermal increases external to the piping (or vessel). These thermal increases are usually due to sun radiation or exposure to surrounding equipment - not to a pool-fire case.
As I mentioned above, I typically think of a fire causing some sort of vapor generation, so when I saw the 21% allowable overpressure, I instantly assumed fire and was wondering why this valve would be a thermal one. With the feedback I've received and the extra reading I did, I believe it was wrong for me to assume this was to involved fire, when it could possibly be any type of external heat (21% allowable overpressure doesn't necessarily = fire).
To comment on some of your concerns that you mentioned, the vessels with these PSVs will be 100% liquid filled. Because the actual source of external heat is not made known on these datasheets, I will look into what would happen if a fire were to happen and the time-frame needed for it to happen.
Thank you for your help!