Dear Friends,
Could you please tell me , in Preheating process for Hot tap as per Shell DEP 30.10.60.30-Gen. (WELDING ON PRESSURISED PIPES (AMENDMENTS/SUPPLEMENTS TO ANSI/API RP 1107)) December 1998, Clause 4.8 (b) and (c) PREHEATING AND INTERPASS TEMPERATURE)
why we are increasing the temp. to 250 °C , then decreasing to 150 °C before starting the welding and why the time taken for 250°C to cool to 150 °C shall be recorded
4.8 PREHEATING AND INTERPASS TEMPERATURE
(b) Longitudinal groove welds:
Prior to the commencement of welding, both seams shall be heated simultaneously to 250 °C over a minimum band width of twice the thickness of the fitting either side of the weld preparation. After welding has commenced, the preheat maintenance temperature shall be 150 °C.
(c) Circumferential fillet welds
With the fitting in position on the live pipeline, a minimum length of 300 mm of the weld zone shall be preheated to 250 °C. The temperature of the run pipe within this zone shall be monitored and the time taken for the zone to cool to 150 °C shall be recorded. This recorded time shall define the time limits between the cessation and the re-commencement of preheat during welding for the deposition of the first two layers of the weld metal including the buttering runs on the run pipe. Thereafter, the temperature shall be monitored so that welding does not occur when the weld zone is at a temperature of less than 150 °C.
Best Regards,
Mohamed El-Sayed
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