PipingEquipment-
Regarding CA and rerates: Yes, the CA can be modified up or down as required and supported by calc’s. For example, I may have a vessel with an MAWP of 100 psi and a CA of 0.125”. If my Process friends tell me that they need the MAWP increased to 110 psi, and my UT’s indicate that we’ve seen no more than 0.050” corrosion over the last 20 years of operation, I may consider dropping the future CA to a smaller value, perhaps 0.070” if that’s what it takes to “make it work.” On the flip side, I might be informed that Inspection is concerned with excessive corrosion, and Process tells me that they don’t have a problem resetting the relief valves to 70 psi. Now I can rerate the vessel to a lower MAWP to buy a bit more CA. I will say that I generally don’t like this approach as most rerate calc’s I see don’t consider structural loadings, and I’m always concerned that even if pressure governed the initial design, the engineer working the rerate may not recognize that perhaps something else (such as structural stability) begins to govern if the CA is increased.
I get the feeling that the discussion between your QC manager and yourself might be a result of confusion in when the start point is for determining the corrosion allowance. Say the vessel was fabricated from 1” plate. It has been in service for some years and has corroded by 0.125”. The original CA at the time of fabrication was 0.25”. Now we do a rerate (say for 516-70 material and a temperature increase from 200°F to 300°F). What is the CA? Well, I’d suggest you take a step back and recognize that it is not so much the CA which is important as the required thickness of the shell at retirement. Do you retire the vessel when it hits 0.75” wall thickness or do you retire the vessel when it has corroded by 0.25”? Do you care if the 0.25” corrosion occurred as 0.125” before the rerate and 0.125” after? No. Now, changing the CA to reflect the 0.125” corrosion which has taken place becomes confusing: Do you run your rerate calc’s with a “new nominal” thickness of 0.875” and a “new CA” of 0.125”? This can only lead to confusion unless all of the original documents and drawings are tossed out and new ones made to replace them. Of course you can’t do that: The weight of the vessel for wind and seismic calc’s must be determined on the conservative basis of new thickness, as the corrosion is not likely to be exactly uniform everywhere. Bottom line: Run the numbers as though the vessel was fresh from the fab shop and never seen service. Determine the max CA, verify that the existing wall thickness is above the minimum required, and let the user’s qualified inspection group determine remaining life.
As to the UG-125 question… Seems to me that UG-125(1) is clear that it is not the fabricator’s responsibility to install the PRD. UG-125(2) backs up TD2K’s comment about who has the “big picture” in order to set relief requirements. Finally, UG-125(3) seems to make it clear that the Manufacturer need not supply relief devices. Not quite sure where the confusion is coming from…
jt