PWHT Requirements in Pressure Vessel Inspections
What are PWHT requirements in pressure vessel inspections?
Bob's Reply: I prefer to answer to this question in two separate questions as below:
1. When and where PWHT is mandatory by code requirement?
2. How PWHT must be done when it is mandatory?
Pressure Vessel Inspections-Answer to question 1:
If we focus only in ASME Code Section 8 Div 1, there are lots of conditions which cause the PWHT to be mandatory. Some of them are:
a) Based the requirement of UW-2(a) when our service is lethal and our material is carbon steel or low alloy steel.
b) Based the requirement of UW-2(c) when our pressure vessel is unfired steam boiler and our material is carbon steel or low alloy steel.
c) Based the requirement of UW-2(d) when our pressure vessel is subjected to direct firing and our material is carbon steel or low alloy steel.
d) Based the requirement of UCS-56 when our nominal thickness exceeds a specific value, for instance 38 mm for P No.1 material or 0 mm for P No.3 material.
UCS-56 also gives min. PWHT temperature and min. holding time (soaking period) based on P-Nos. and thickness respectively.
e) Similarly to the UCS-56, the same apply for other martial groups i.e. UNF-56, UHA-32, UHT-56 and etc.
f) Based the requirement of UCS-68(b) when our MDMT (Minimum Design Metal Temperature) is below -55 Degree F.
Pressure Vessel Inspections-Answer to question 2:
When one of above requirement applies to us, then we must do PHWT based the requirement of UW-40 which detail to us the PHWT procedure.
Some important requirements in this clause are:
a) For Furnace PWHT, Loading Temperature shall not exceed 800°F, heating rate 400 deg F/hr/inch of thickness, cooling rate 500°F /hr/inch of thickness.
Still air cooling permitted below 800°F. During soaking period, temp difference between hottest and coldest part shall not exceed 150°F.
b) Minimum overlap for PWHT in multiple heats is 5 ft.