ELEMENT OF POST WELD HEAT TREATMENT
Today in this post we know about the element of post weld heat treatment or we can say treatment of weld after welding or it is called stress reliving.
Thermocouple –material of thermocouple to be used shall be the nickel-chromium / nickel –aluminium type. It should be calibrated; in addition recalibration shall be done if there is justified reason to believe that they are in error.
Where the thermocouples are being joined by capacitance discharge method, these are to be attached with two shorts as shown in fig.
The distance between two wire shall be minimum 6 mm. Insulation must be remove from end of wire shall not be more than 15 mm. Care shall be taken to connect positive of thermocouple with positive side of compensating cable similarly negative side of thermocouple must be connected with negative side of compensating cable.
Thermal insulation – All thermocouple attached shall be adequately insulated to avoid temperature misreading caused by effect of radiation. Thermal insulation shall be achieved employing:
Ceramic fibre, as form of mat, minimum density 96 kg/m³
Knitted silica mat, minimum density 200 kg/m³
Compensating Cables – thermocouple extension cable shall be copper & constant compensating cable with connection through plug and socket as:
Copper lead (+ve) to nickel chromium (-ve)
Constant lead (-ve) to nickel aluminium (+ve)
Heating element- PWHT work under this procedure shall be perform by induction heating using nichrome wire covered with ceramic beads/heating pad / flexible ceramic pads as per parameter specified in applicable design code and approved WPS.
Location and no of thermocouple – following guideline shall be followed as minimum requirement of number and location of the thermocouple.
One thermocouple for pipe diameter up to 6 inch NPS.
Two thermocouple at 180° for pipe diameter above 6 inch to 12 inch NPS.
Three thermocouple at 120° for pipe diameter above 12 inch NPS.
Thermocouple shall be fixed within the area of heating band.
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