I’m looking at an evaluation of PSV’s on heat exchangers and have come across a statement “ Three exchangers fail the 2/3’s rule” What is the 2/3’s rule?
It has to do with overpressuring the low pressure side of the exchanger in the case of tube rupture. API RP520/521 says that it is not necessary to protect the low pressure system (=exchanger + piping + whatever is attached to it) for tube rupture if the test pressure is at least equal to the design pressure of the high pressure side. Since the test pressure used to be 1.5 times the design pressure, the design pressure of the low pressure side should be at least 2/3 (= 1 / 1.5) of the design pressure of the high pressure side. Recently, the test pressure has become 1.3 times the design pressure, and the design pressure of the low pressure side should be 10/13 of the design pressure of the high pressure side, to avoid sizing the low pressure side relief valve for tube rupture.
Summarizing: check the test pressure of the low pressure system against the design pressure of the high pressure side. If it is equal or greater, no tube rupture to be taken into account.
Check also:
thread798-42871
thread135-23361
thread135-41046
thread124-23358
It may be 1.3 or 1.5 depending on when and to what stress table it was designed. That needs to be determined is accessing what relief requirements are for a heat exchanger.
