Three different sets of Temp & Pressure on a Vessel's nameplate. 1

[schaali1]

I’m trying to calculate Mean Allowable Wall Thickness (MAWT) of a Flare knock out drum (Design code of construction is ASME VII Division 1 – W82). Peculiarly there are following three sets of design pressure and temperature values stamped on the Vessel’s name plate as well as mentioned on its GA drawing.

Case I - 7.4 barg @ -79/90 C
Case II – 5.6 barg @ -79/199 C
Case III – 3.6 barg @ -79/224 C

The problem is if I take the worst case scenario meaning max pressure and temperature value of all three cases i.e. 7.4 barg @ 224 C, the required MAWT comes out to be more than the current nominal wall thickness of the vessel which essentially means it was not fit for service from the day 1. But when I calculate MAWT based on above individual cases separately, the required MAWT in all three cases is less than the nominal wall thickness of the vessel. My questions are

Does ASME VIII code allow that a Vessel’s nameplate can have more than one set of temperature and pressure stamped?

Assuming worst case scenario, am I correct here in calculating MAWT based on max: pressure and temperature of all three cases?

Regards,
Schaali

 

[Jmoore1]

Vessels can be have more than 1 operating service, but should only be designed to the absolute worst case scenario. Im guessing that the flare can get a combination of the 3 or 1 case since it is a flare knockout drum...therefore it is impossible to have both 7.4 barg and 224 C but it is somewhere in between...the vessel should have been stamped for one design of the worst case combination between the 3 possible services...if I am assuming the reason for the different cases correctly.

 

[trottiey]

Vessels can be designed and stamped for more than one pressure and temperature combination, see UG-116 footnote 37. Your vessel calculations need not combine pressures and temperatures from different cases; the nameplate does not permit these to be coincident.

Each individual element must be designed for the worst case coincident pressure and temperature per UG-21. This would be one of the three cases you listed, but it might be a different case for each element. In practice, if you're using a cmputer, you just run all your calculations three times.