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Relief devices for fire case - acceptable overpressure

Relief devices for fire case - acceptable overpressure

Relief devices for fire case - acceptable overpressure

(OP)

There is a slight change in the wording of S. VIII D.1 UG-125(c)(2), that rules the overpressure of safety devices for vessels exposed to fire:

Up to 2005, this paragraph mentions the use of "supplemental" devices for the "additional" hazard  imposed by the fire, in which case such "supplemental" devices can operate with an overpressure of 21%.

The wording was changed with Addenda 2006, and mentions the use of "supplemental" devices only for cases in which the devices used to satisfy UG-125(c) [overpressure of 10%, general case] and UG-125(c)(1) [multiple valves, up to 16% overpressure) have insufficient capacity.

Questions:

1. Is the current wording meaning that there "has to be" another relieving scenario that is not fire related, and that this scenario requires devices to operate with 10% overpressure?

2. If the only relieving scenario foreseen is fire, is it correct under the current code to install only one relieving device with 21% overpressure?

3. If the answer to the second question is yes, why would there be paragraph UG-125 (c)(3), that includes several restriction, yet allows only 20% of overpressure? Seems reasonable that if the fire case in (c)(2) allows an overpressure of 21%, relief devices could be design for 20% instead without additional restrictions.

4. Is the wording changed with Addenda 2006 a cosmetic change or there has been a real change in the requirements?  

From previous discussions, it has been my understanding that before 2006, the code required a relief device with 10% overpressure,and the paragraph (c)(3) was there to provide an exception by which a single valve could be protecting a vessel for fire case only, with 21% overpressure.

I'd appreciate any input on this topic.


 

RE: Relief devices for fire case - acceptable overpressure

RE: para. (c)(3): I think ASME is providing an exception to liquified compressed gas storage to the effect that you can have overpressures greater than 10% (or 16%) for non-fire scenarios; i.e. up to 20%.

RE: Relief devices for fire case - acceptable overpressure

garfio,

Consider reposting in the "Safety Relief Valve engineering (PSV)" forum.

-MJC

 

   

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