PetroStreet

Dear Experts,

What is the basis for different thickness limits for PWHT of pressure vessel and piping on the same material & design conditions? For example, why the requirement of PWHT begins from 19mm in case of CS piping and 38mm in case of CS pressure vessel?

There are differences in PWHT requirements and hardness checks when comparing ASME BPVC Sec. VIII and ASME B31.3. One difference is what you highlighted for thickness qualified for PWHT of P.No. 1 material. Other than this, you would also be finding other differences like heat treatment temperature range etc.

Now the actual reason can be best known to the ASME committees who laid ASME B31.3 and ASME BPVC Sec. VIII requirements.

What comes to my mind is that piping caters not only longitudinal and circumferential stresses but also the torsional stresses due to the piping layout consisting of certain piping length, branches, elbows and piping support. I mean that piping has to travel a distance and to follow a layout which also causes torsional stresses. In order to deal with these stresses also, weld joints must be free of any residual stresses and must be in acceptable hardness range so that they must be capable of bearing additional torsional stresses. That becomes the reason of process piping code containing some strict requirements regarding PWHT requirements, temperatures and hardness checks for piping in all the P.Nos. unlike ASME BPVC Sec. VIII.

Actually for carbon steels P.No. 1 material, almost all ASME codes have different requirements of PWHT. ASME B31.1 and ASME B31.3 for power piping and for process piping respectively requires a P. No. 1 material to be post weld heat treated when the thickness goes above 20 mm whereas ASME BPVC Sec. VIII exempts PWHT up to 38 mm. Also for ASME Sec. I (power boilers), PWHT requirements starts from 20 mm thickness.

Carbon steel has low hardenability and some exemptions in ASME Sec. I, ASME B31.3 and ASME B31.3 must be included like 38 mm criteria for ASME Sec. VIII. Pre-heating requirements may be increased, carbon content limitation may be imposed but relaxation in thickness limit must be provided.

Anyway till we do not receive any update from ASME Committee, we have to just follow all the relevant requirements of code in order to have 100% compliance. Although one code case 174 for ASME B31.1 has extended the thickness limitations up to 38 mm but with increased requirements.

I agree with mechcolor. ASME does set different thickness limitations in almost every piping and BPVC code. I did read a paper on this somewhere also which discussed and highlighted the same things as you highlighted.
ASME Committees must consider waiving off PWHT up to 38 mm for P.No.1 material against piping codes ASME B31.1 and B31.3 also like in ASME BPVC and yes they may impose some other checks.
And as mechcolor said, when meeting the code is just to meet everything it says, no shortcuts and yes no arguments also.