Explosion on the Hydrotreatment Unit of a Refinery
The release and fire occurred during what appeared to be a normal operation in a reactor section of a Residue Hydrotreater Unit, when an eight-inch pipe connected to a heat exchanger failed at the flange. Recycled gas, primarily hydrogen, was suddenly released when the pipe broke away. The hydrogen, under high pressure and temperature, was ignited, sending a large jet fire shooting an estimated 75 feet westward from the flange. Damage was localized along that fire path. Initial information indicated the hydrogen was pressurized at approximately 3,000 pounds per square inch at a temperature of over 500 degrees Fahrenheit. Heat from the fire deformed piping and bent structural beams.
Event Date
July 28, 2005
Record Quality Indicator
Region / Country
Event Initiating System
Classification of the Physical Effects
Nature of the Consequences
Cause Comments
The immediate cause was the failure of a carbon steel elbow, severely weakened by high temperature hydrogen attack and ruptured after operating for only 3 months. According to the CSB bulletin (see references), the investigation determined that the 8-inch diameter carbon steel elbow had been installed inadvertently in a high-pressure, high-temperature hydrogen line during a maintenance. The contractor accidentally switched a carbon steel elbow with an alloy steel elbow during a scheduled heat exchanger overhaul in February 2005. The alloy steel elbow was resistant to high temperature hydrogen attack (HTHA) but the carbon steel elbow was not. The maintenance contractor was unaware of the material differences in the elbows and the plant operator did not require the contractor to implement any special precautions to preventinadvertently switching the elbows or any post-reassembly testing to confirm the alloy elbows were reinstalled in the correct locations. These findings hint at a root cause related to organization, specifically in maintenance control and contractor management
Facility Information
Application Type
Application
Specific Application Supply Chain Stage
Components Involved
Reactor, pipe, heat exchanger, flange
Storage/Process Medium
Design Pressure
200
Location Type
Location description
Industrial Area
Operational Condition
Pre-event Summary
DESCRIPTION OF THE UNITResidual material from the crude oil processing unit is processed in the Residual Hydro-treatment Unit (RHU) to remove nitrogen, sulfur, and metals. Hydrogen is pressurized to about 3000 psi (ca. 200 bar), and then preheated in the RHU heat exchangers to about 600 F (315 C). The preheated hydrogen passes first through a furnace to increase the temperature, and it is then injected into the reactor feedstock. Hydrogen combines with nitrogen compounds and sulfur within the feedstock in the presence of the catalyst inside the RHU reactors to form hydrogen sulfide and ammonia. Light hydrocarbon, such as gasoline, is then processed in downstream refinery units.
Environmental Damage
0
Property Loss (onsite)
N
Property Loss (offsite)
N
Lessons Learned
Lessons Learned
The accidental switch between the two elbows of different steel composition was possible because:(1) it was visually impossible to distinguish the two elbow with compositions, which had exactly the same dimensions.(2) the maintenance contractor was unaware of the differences, and precautions were not in place to avoid switching.Therefore, the CSB report recommended the following options to avoid recurrence: (A) Piping systems could be designed such that incompatible components cannot be interchanged;(B) All elbows could be made of the same steel resisting high temperature hydrogen attack; (C) The material verification procedure, already in sue for incoming new components, could be adopted also for critical piping component PMI equipment maintenance, (the test is simple to perform and quickly differentiates between carbon steel and alloy steel).
Event Nature
Emergency Action
Unknown
Release Type
Release Substance
Detonation
No
Deflagration
No
High Pressure Explosion
No
High Voltage Explosion
No
Flame Type
Source Category
References
References
CSB (US Chemical Safety and Hazard Investigation Board) Safety Bulleting No. 2005-04-B, October 12, 2006, dedicated t othe BP accident of July 28, 2005.
https://www.csb.gov/file.aspx?DocumentId=5612
(accessed December 2020)
CSB (US Chemical Safety and Hazard Investigation Board) News Release, July 30, 2005
https://www.csb.gov/csb-investigators-continue-assessment-of-july-28-ex…
(accessed December 2020)