PWHT

A1) The welding itself, and stopping people getting hurt by it.

Following is an extract from ASME Sec 8 Div 1 regarding PWHT:

UW-40 PROCEDURES FOR POSTWELD HEAT TREATMENT

(a) The operation of postweld heat treatment shall be performed in accordance with the requirements given in the applicable Part in Subsection C using one of the following procedures. In the procedures that follow, the soak band is defined as the volume of metal required to meet or exceed the minimum PWHT temperatures listed in Table UCS-56. As a minimum, the soak band shall contain the weld, heat affected zone, and a portion of base metal adjacent to the weld being heat treated. The minimum width of this volume is the widest width of weld plus 1t or 2 in., whichever is less, on each side or end of the weld. The term t is the nominal thickness as defined in (f) below.

Apart form above, you can check the PWHT temperature and time requirements from UCS 56/UHA 32 (as applicable).

Other general requirements are as per UCS 56 d:

(1) The temperature of the furnace shall not exceed 800°F (427°C) at the time the vessel or part is placed in it.

(2) Above 800°F (427°C), the rate3 of heating shall be not more than 400°F/hr (200°C/ hr) divided by the maximum metal thickness of the shell or head plate in inches, but in no case more than 400°F/hr (222°C/hr). During the heating period there shall not be a greater variation in temperature throughout the portion of the vessel being heated than 250°F (139°C) within any 15 ft (4.6 m) interval of length.

(3) The vessel or vessel part shall be held at or above the temperature specified in Table UCS-56 or Table UCS-56.1 for the period of time specified in the Tables. During the holding period, there shall not be a greater difference than 150°F (83°C) between the highest and lowest temperature throughout the portion of the vessel being heated, except where the range is further limited in Table UCS-56.

(4) During the heating and holding periods, the furnace atmosphere shall be so controlled as to avoid excessive oxidation of the surface of the vessel. The furnace shall be of such design as to prevent direct impingement of the flame on the vessel.

(5) Above 800°F (427°C), cooling shall be done in a closed furnace or cooling chamber at a rate3not greater than 500°F/hr divided by the maximum metal thickness of the shell or head plate in inches, but in no case more than 500°F/hr (278°C). From 800°F (427°C) the vessel may be cooled in still air.

Wha is settlement regarding to hydrotest?

> This is specifically observed during hydrotest of mounded vessels. During hydrotest, enormous weight of water is transferred to the compact sand on which the vessel is supported and the sand may sag/get more compacted. The settelment is observed to get general idea regerding stability of the mounded vessels after hydrotest.