(2) jte (Mechanical)
5 Nov 08 11:34
A good question. There are also several changes to the ratings from the 1996 ed to the 2003 ed. For example, at 200 deg F the rating of a CL-600 flange in material group 1.1 was raised from 1350 psi to 1360 psi while at 300 deg F the rating dropped from 1315 psi to 1310 psi.
The direct answer to your question is in B16.5 2003 (and at least as far back as the 1977 ed) in the last sentence of 1.3:
Where such components were installed in accordance with the pressure-temperature ratings of an earlier edition of this Standard, those ratings are applicable except as may be governed by the applicable code or regulation.
Another reference is Interpretation 2-38:
Subject: Use of Earlier Editions of B16.5
Question: According to B16.5-1988, can flanges manufactured in accordance with an earlier edition of B16.5 be rated at the higher pressure-temperature ratings in the current edition?
Reply: No. Se para. 1.3
Now, have I seen flange rating "cherry picked" by edition. Yes. Is it hazardous? Maybe. I guess the bottom line is that if the joint seals - no problem! But make sure you are in a ethically and legally defensible posture (e.g. you verified that the material spec didn't change, the manufacturing requirements didn't change...) if it leaks and causes a problem.
When referring to the Code, you have to recognize that the committee does not have infinite volunteer resources (material spec's change, B16.5 requirements change...), and is really not in a position to retroactively change flange ratings. You really wouldn't want to be told that your line at 300 deg F now has to be derated since the flange rating changed, would you? So, the same applies to increases in ratings.