Fion Zhang/ Charlie Chong

General Reading

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When and where PWHT is mandatory by code requirement?

a) UW-2(a) when our service is lethal and our material is carbon steel or low alloy steel.

b) UW-2(c) when our pressure vessel is unfired steam boiler and our material is carbon steel or low alloy steel.

c) UW-2(d) when our pressure vessel is subjected to direct firing and our material is carbon steel or low alloy steel.

d) 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.

e) UCS-56, the same apply for other martial groups i.e. UNF-56, UHA-32, UHT-56 and etc.

f) UCS-68(b) when our MDMT (Minimum Design Metal Temperature) is below -55 Degree F

1. UW-2(a), when service is lethal and material is carbon steel or low alloy steel.2. UW-2(c), when pressure vessel is unfired steam boiler and material is carbon steel or

low alloy steel.3. UW-2(d), when pressure vessel is subjected to direct firing and material is carbon

steel or low alloy steel.4. UCS-56, when nominal thickness exceeds a specific value, for instance 38 mm for P

No.1 material or 0 mm for P No.3 material. 5. UCS-56, the same apply for other martial groups i.e. UNF-56, UHA-32, UHT-56 and

etc.6. UCS-68(b), when MDMT (Minimum Design Metal Temperature) is below -55

Degree F.

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How PWHT must be done when it is mandatory?

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When one of above requirement applies to us, then we must do PWHT based the requirement of UW-40 / UCS-56, which detailed to us the PWHT procedure. Some important requirements in this clause are;

1. For Furnace PWHT; a) Loading Temperature shall not exceed 800ºF, b) Heating rate 800ºF /hr /inch of thickness,c) During heating period the temperature variation shall be smaller than 250ºF

within 15ft interval length.d) Above 800ºF, cooling rate 500ºF /hr /inch of thickness. e) Still air cooling permitted below 800ºF. f) During soaking period, temperature difference between hottest and coldest part

shall not exceed 150ºF. g) Minimum overlap for PWHT in multiple heats is 5 ft.

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