A prefeed heater in a hydrotreater unit experienced a fire while being prepared for shutdown during a turnaround. The prefeed hydrotreater heater was a mixed operation heater that typically operated in mixed mode with hydrogen and hydrocarbon bitumen, but at the time of the incident was operating in hydrogen only mode in order to hot hydrogen strip the hydrotreater.
The heater had been in operation since 2011 and had a design fluid temperature of 425 °C (797 °F), a tube metal temperature of 570 °C (1058 °F) and design pressure of 16,800 kPag (2437 Psig). The normal operating pressure was 15,000 kPag (2176 Psig). The radiant and convection tubes were fabricated from Type 316Nb stainless steel (UNS31640). There was a tube rupture in the radiant section that caused the fire and the subsequent failure investigation established that the temperature in the radiant section at the rupture location had reached in excess of 850 °C (1562 °F) for over a 15-minute period prior to rupturing. The following paper outlines the failure analysis and root causes which caused the incident, while detailing the recovery and construction of the heater, including the fitness-for-service methods and inspections conducted. The heater was partially rebuilt with the convection section tube material being recovered and the radiant section being rebuilt in a different metallurgy.