Root Cause Analysis of Failure in Hot and Cold Mixing Point in Hydrogen Generation Unit (HGU) due to Thermal Fatigue Phenonmenon

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.