During a nonroutine maintenance operation at the Tesoro petroleum refinery in Anacortes, WA, on April 2, 2010, a nearly 40-year-old heat exchanger located in a naphtha hydrotreater unit catastrophically failed. An explosion and intense fire killed seven employees who were working in the immediate vicinity. The U.S. Chemical Safety and Hazard Investigation Board (CSB) attributed the catastrophic failure to damage caused by high-temperature hydrogen attack (HTHA).
HTHA can occur in carbon steel equipment exposed to hydrogen at temperatures over 400°F (205°C). Higher alloys, such as C-1/2Mo steel and Cr-Mo steel, typically become susceptible to HTHA at higher temperatures and hydrogen pressures. The failure risk increases as temperatures and hydrogen pressures increase.
When the temperature increases above 400°F, molecular hydrogen dissociates to form atomic hydrogen on the surface of the steel. Atomic hydrogen diffuses into the metal and reacts with free carbon and metal carbides to form methane gas. Methane, which cannot diffuse out of the steel, collects and forms internal voids and fissures, and causes the internal pressure to increase. The voids and fissures grow and finally crack to relieve the internal pressure, weakening the steel. High stress (applied or residual) can accelerate HTHA.
Organizations like the Materials Technology Institute (MTI) and the American...
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