PWHT? 3

[SBlowers]

A heat treated vertical pressure vessel needs replacement of top dished head. After welding the new head, does the vessel require to be heat treated fully or a partial heat treatment of top half would be sufficient? What are the general conditions to adhere for stress relief?

Thanks, SBlowers

 

[vesselfab]

was the original vessel pwht

how thick is the weld joint to shell?

was the pwht for service and is the vessel still in the same service?

 

[stanweld]

Per NBIC only the head to shell weld need be post weld heat treated. Alternatives to PWHT could also be perforemed if analysis for service (stress corrosion cracking, cyclic service, non-lethal service, etc.) permits and AI and Jurisdiction and Owner accepts.

 

[vesselfab]

stanweld....openings in head??

 

[pHreaC]

PWHT is needed only once per weld over 38mm (CS, or 32mm if not preheated during welding).
If it has undergone PWHT already I would think only the new weld would be needing PWHT.

Same thing when making a vessel in two parts, pwht-ing them.
Then welding a closure seam and local pwht on the closure seam only.

grtz

 

[metengr]

Let’s be clear about this - if weld repairs are performed to the NBIC, the repair welds only would require PWHT based on original code of construction PWHT requirements for fabrication welds. The NBIC does not provide requirements for code of construction PWHT. The NBIC defaults back to the original code of construction for repair welds unless PWHT for technical reasons cannot be performed because of harm to the vessel itself. In this case, if the original code of construction required PWHT because of material composition and thickness, the repair welds would require PWHT.

The same also holds true for NDT, unless other options are evaluated by the Inspector and, if necessary, from the Jurisdiction.

 

[stanweld]

vesselfab,
If openings are in the new head, the head and nozzles can be independently post weld heat treated prior to welding the head to the shell. After making the head to shell weld it is then subject to PWHT.