Postweld Heat Treatment
Note PWHT is a very important factor that can have a significant effect on the integrity and reliability of the vessel. Omitted or improperly performed postweld heat treatments can lead to failure of the vessel.
The pressure vessel specifications recommend attaching sufficient thermocouples to the vessel during postweld heat treatment to assure that the minimum temperature for postweld heat treatment required by the ASME Code or the specifications has been obtained, and the temperature should not vary more than 50°F over the entire vessel. Temperature variations during heating and cooling should not vary more than 150°F over the entire vessel, to prevent damage by the development of thermal stresses.
Postweld heat treatment can be performed locally for individual welds, when a completed vessel is too large to fit inside of the heat treating furnace. However, it is very important to make certain that the temperature gradients resulting from the local heat treatment do not develop thermal stresses that damage the vessel. A pressure vessel engineer should review the fabricator’s procedure for local heat treatment of the vessel.
It is important that no welding is performed on pressure containing components of a vessel after it is postweld heat treated. Occasionally, by oversight, some external attachments to the vessel shell must be welded to the shell after the postweld heat treatment has been performed. If this occurs, a second PWHT is required.
The Company inspector should maintain sufficient surveillance to assure that the postweld heat treatment is properly performed in accordance with the ASME Code and any additional requirements in the pressure vessel specification, and that no welding is performed after the vessel is heat treated.
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