Requirement of PWHT of pressure vessel depends on various factors like material of construction, Thickness & other design criterion. Usually, pressure vessels with plate thickness 1.5 inch or over are required to undergo PWHT.
Partial or full PWHT is performed of the vessel, It depends on the size vessel and capacity of the furnace.
Types of PWHT:
Following types of method are used for PWHT.
- Furnace PWHT: PWHT of vessel is done in the furnace with single firing. This method is most preferable method since all the parameters (heating, Soaking, cooling) can be controlled effectively. Oil firing & electrically heated furnaces are used in this method. In case PWHT of complete vessel cannot be done in one go due to capacity constraint of furnace, PWHT can be done part by part.
- Internal firing Method: Second method is to cover the vessel with insulation and to provide heating burners on different locations. Usually, oil firing burners are used in this method along with additional electrical heating system.
3. Localized PWHT: In this method PWHT of welds is done using electrical resistance heating.