A facility experienced a major fire in its Resid Hydrotreater Unit (RHU) that caused millions of dollars in property damage. One employee sustained a minor injury during the emergency unit shutdown and there were no fatalities.

The RHU incident investigation determined that an 8-inch diameter carbon steel elbow inadvertently installed in a high-pressure, high-temperature hydrogen line ruptured after operating for only 3 months. The escaping hydrogen gas from the ruptured elbow quickly ignited.

This incident occurred after a maintenance contractor accidentally replaced an alloy steel elbow with a carbon steel elbow during a scheduled heat exchanger overhaul. The alloy steel elbow was resistant to high-temperature hydrogen attack (HTHA), but the carbon steel elbow was not. Metallurgical analyses of the failed elbow concluded that the HTHA severely weakened the carbon steel elbow.

Incident Date
Jul 28, 2005
  • Piping/Fittings/Valves
  • Piping
  • Heating Equipment
  • Heat Exchanger
Damage and Injuries
Probable Cause
When Incident Discovered
Lessons Learned

In the future, refining, petrochemical, and chemical industries need to review material verification programs to ensure that the maintenance procedures include sufficient controls and positive material identification (PMI) testing to prevent improper material substitutions in hazardous process systems.

Human Factors Based Design
Designers should consider the entire process system life cycle, including planned maintenance, to avoid piping configurations that allow critical alloy piping components to be interchanged with non-compatible piping components.

Positive Material Verification Programs
In-situ alloy steel material verification using x-ray fluorescence or non-destructive material testing is an accurate, inexpensive, and fast PMI test method. Facility owners, operators, and maintenance contractors should ensure that the verification program requires PMI testing, such as specified in API Recommended Practice 578, or other suitable verification process, for all critical-service alloy steel piping components that are removed and reinstalled during maintenance.

At a minimum, piping components and their respective locations should be tagged or marked before removal, and the correct installed location should be verified after reinstallation.