garfio
Mechanical
- Jul 17, 2005
- 86
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.
