Indian Welding Journal

Mahendra Pal ¹, Mayank Banjare ¹

Affiliations
1 Indian Oil Corporation Limited, Guwahati Refinery, PO-Noonmati, Assam, Guwahati-781 020, IN

Abstract

In a Process unit there are several streams that are exchanging heat to optimize the unit operation. In the HGU at IOCL, Guwahati refinery repetitive failures were observed in a 2" 0, SS 304 pipe at this hot and cold mixing point. The investigation revealed that the two streams were handling fluid at a temperature of 40°C and 160°C, the difference being 120°C. These huge temperature differences lead to thermal gradient across the wall thickness of the pipe and also along the length of the pipe surface in the flow direction. The inner surface of the pipe seeing a higher temperature than the outer surface and therefore more expansion at inner surface. Due to this thermal fatigue phenomenon and hindered expansion severe stresses were observed at the inner surface of pipe leading to crack initiation and further propagation across the wall thickness. As a temporary measure the joint was replaced with identical pipe with higher thickness (schedule), and as a permanent solution it was suggested to replace the mixing point with an injection "Quill" design to avert the huge thermal gradient.

Keywords

Thermal Gradient, Thermal Fatigue, Quill, Process Mixing Point.

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Mayank Banjare ¹, Vimal Kumar ¹, B. K. Muduli ¹

Affiliations
1 Indian Oil Corporation Limited, Panipat Naphtha Cracker, Panipat-132140, IN

Abstract

During the planned shutdown in one of the Indian Oil, Petrochemical complex, Linear low density polyethylene plant fine longitudinal cracks were observed in the fillet weld of the heavy thickness sieve plate to bracing plate weld joint of a low pressure separator vessel. The crack was observed to be propagated (80-90% of total weld length) on the both side of fillet weld (toe region) of bracing and sieve plate weld joint in bracing. The ischolar_main cause of the cracks was suspected to be improper heat treatment in shop resulting in cracking while in service for six months. Since the sieve plate and bracing are made of heavy thickness chrome-molybdenum quench- tempered steel also known as high strength steel, in-situ repair and in-situ post weld heat treatment is difficult. Repair of eight no. of joints with total length 12 meters, took much time. This paper discusses the various problems encountered during the repair of cracks. A thorough understanding of the technicalities involved in in-situ repair and heat treatment involved of low alloy high strength heavy thickness steel will help in proper planning and in reducing the repair time.

Keywords

Quench-Tempered Steel, Low Pressure Separator, Sieve Plate, High Strength Steel.

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