Explosion on the hydro-treatment 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 psi (approximately 210 bar) ) at a temperature of over 500 F (260 C). Heat from the fire deformed piping and bent structural beams.

The INITIATING 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 prevent inadvertently switching the elbows or any post-reassembly testing to confirm the alloy elbows were reinstalled in the correct locations.

These findings suggest at a ROOT CAUSE related to organisation, specifically in maintenance control and contractor management.

Reactor, pipe, heat exchanger, flange

DESCRIPTION OF THE UNIT
Residual material from the crude oil processing unit is processed in the Residual Hydro-treatment Unit (RHU) to remove nitrogen, sulphur, 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 sulphur within the feedstock in the presence of the catalyst inside the RHU reactors to form hydrogen sulphide and ammonia. Light hydrocarbon, such as gasoline, is then processed in downstream refinery units.

CSB Chairman said that "Although this latest explosion and fire fortunately caused no injuries, such accidents are not an acceptable part of normal operations. The accident occurred at an entirely different part of the Texas City facility, but we are interested in knowing whether the two accidents (ref. major accident 23/03/05) share any common root causes such as lack of an effective mechanical integrity program. Such a program would make sure the equipment is safe and serviceable."

This incident involved the installation of a hydrogen pipe elbow made of the wrong steel. The accidental switch between the two elbows of different steel compositions happened durign maintenance and was performed by a contractor, was possible because:
(1) it was visually impossible to distinguish the two elbow, 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 use 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).