Relief device overpressure under ASME code

[garfio]

(I posted these questions in the ASME forum but was suggested to try here.)

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 20% overpressure for specific situations.

I'd appreciate any input on this topic.

 

[CJKruger]

garfio, I couldn't lay my hands on this addenda. But here are my comments based on your description of the change:

1) No, I don't think so. There are many references in the code where they refer to fire as the only relieving case.

2) Yes, I thinks so. The other cases has a zero load, which is insufficient for the fire case (and thus require use of the "supplemental" valve). Provided you do the set pressures per UG-134b.

3) Sorry, I have not use 125.c.3 often, as it applies to ambient vessels without permanent supply piping.

4) I think it is cosmetic, but will watch this thread to see what others think.

 

[garfio]

CJKruger or any other contributor:

Could you give me some examples as were in the Code there are references to fire been the only relieving case?

It seems to me that paragraph 125.c.3 would not be required if we could just install a single valve for fire scenario set at 100% MAWP and with 20% overpressure based on 125.c.2.

Why would there be an "exception" to a rule (125.c.3), with more restrictions (compress gases, ambient temperature, no permanent connection, etc), if we could do the same (set at MAWP and 20% overpressure) based on 125.c.2?

The only reason to have 125.c.3 then, would be to allow this situation, meaning that 125.c.2 requires a valve with overpressure of 10% for other scenarios (which "needs" to exist).

Any ideas about the correct interpretation?