Weld Buildup on a Corroded Propane Vessel Without PWHT
A vertical ASME Section VIII Division 1 pressure vessel stores propane at ambient temperature. The shell is SA-516 Gr. 70, nominal 0.750 in., MAWP 250 psig, and the vessel is registered with the National Board. The original NB-2 data report shows the vessel was post-weld heat treated (PWHT) and radiographed in accordance with the original code. An internal inspection finds a localized corroded area on the shell with a remaining thickness of 0.430 in. over an area about 6 in. by 10 in.; the minimum required thickness for the current MAWP, calculated by the owner's engineer, is 0.560 in. An R-stamp holder proposes to restore the area by weld buildup. The holder says the area is small, so no PWHT is needed, and proposes only visual inspection after welding. The plant wants the vessel back in service within 10 days.
1. May the vessel remain in service at 250 psig while the repair is planned?
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No. The measured 0.430 in. is below the 0.560 in. minimum required for 250 psig, so the shell cannot be shown to meet the current MAWP. The small area does not automatically exempt it. The owner may ask for a fitness-for-service assessment, but that is not a substitute for code compliance unless the jurisdiction and authorized inspector accept it; the inspector should not permit operation at 250 psig on the basis of a verbal plan. The practical options are: reduce the MAWP to a level supported by the remaining thickness and re-mark/register as required; repair the area; or replace the shell section. Until one is completed, the vessel should be restricted or taken out of service. The 10-day schedule does not change the thickness deficiency.
2. Evaluate the R-stamp holder's weld buildup proposal: is visual inspection after welding sufficient, and is PWHT required?
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No, visual inspection alone is not sufficient. Weld buildup is a pressure-retaining repair and must be made under a qualified WPS/PQR with welder qualification, surface preparation, preheat and interpass controls, and NDE appropriate to the repair. Because the original vessel was PWHT and radiographed, the repair procedure must address whether PWHT is required; weld buildup on a restraint-restricted shell commonly requires PWHT unless the code provides an exemption, and the holder's 'small area' argument is not a code exemption. The inspector should require the repair plan to state the PWHT basis, verify furnace/thermocouple records if PWHT is performed, and require surface NDE at minimum plus volumetric examination if the procedure or code calls for it. A hydrostatic test may also be required. The R-1 report should record the procedure, NDE, PWHT, and final thickness.
3. What should the inspector do if the repair cannot restore 0.560 in.?
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Then the vessel cannot retain 250 psig. The inspector should require the owner to reduce the MAWP and have the nameplate and data report revised as required by the jurisdiction and NBIC, or replace the vessel. The inspector should not approve a return to 250 psig based on a weld that leaves thickness below minimum. Document the final thickness and the reduced MAWP; verify the relief valve set pressure is adjusted to the new MAWP. The R-1 and any registration update must reflect the change. If the owner wants to retain 250 psig, the repair must restore the required thickness and be supported by the design and NDE records; otherwise the inspector should withhold acceptance.