Failure Analysis Case History No. 005

Industry: Chemical Process
Specimen Description: U-bend exchanger tube
Material: 316 stainless steel
Environment: Shell-side - Hot Waste fuel
Tube side - Cooling water
316 stainless steel u-bend exchanger tubes in a waste fuel heeat exchanger, resulting in a retube of the exchanger. The tube-side cooling water had been blocked in for an unknown time with hot (420F), shellside fuel oil still on the shell-side prior to the tube failures.
Time in Service: 5 years
Laboratory failure analysis findings showed that the 316 stainless steel exchanger tubes suffered chloride stress corrosion cracking, in straight sections as well as in the U-bend of exchanger tubes. Microhardness measurements showed that the hardness of the U-bend was comparable to that in the straight section (HV 158) and suggested that the U-bend had been stress relieved after cold forming. It was believed that the root cause for failure was blocking in the cooling water, allowing the hot oil to heat the water to temperatures which initiated chloride stress corrosion cracking in the approximately 800 ppm chloride containing cooling water


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