The incident occurred in the catalytic hydrotreatment plant of an oil refinery. The plant, which began operations in September 1997, has a capacity of 1650 tons/day of light fuel oil and 1450 tons/day of heavy fuel oil. The plant was designed to desulfurize the light and heavy fuel oil fractions produced in the refinery by treating them with high-pressure hydrogen over a catalyst to remove sulfur (producing hydrogen sulfide as a byproduct). The plant has two heating/reaction/fractionating sections to treat the two fuel oil fractions, but a single gas purification and compression section for the recycled hydrogen gas.
The heavy fuel oil reactor feedstock from the vacuum distillation plant is sent to the heavy fuel oil treatment section through three pipelines equipped with flow control systems. Once the entrained water is eliminated, the feedstock is pre-heated and then transferred to the reaction section at a pressure of 60 bar. Before the feedstock enters the reactor, it is mixed with the hot recycled hydrogen and heated to optimal temperature for the catalytic reaction (on average 360°C) in the feedstock/hot oil heat exchanger. The reactor effluent, consisting of desulfurized fuel oil and a gas mixture, is cooled down and sent to a liquid-gas separator. The gas, which is mainly of hydrogen, is washed and purified to remove any hydrogen sulfide before being recycled. The desulfurized heavy fuel oil feeds a pre-stripping column, and the overhead stream from this column is sent to the stripping column in the light fuel oil section. The pre-stripped heavy fuel oil fraction is heated and sent to the main fractionating column, which separates the following products: non-condensable gas, virgin naphtha, light desulfurized fuel oil, and heavy desulfurized fuel oil. The light fuel oil is sent from the main fractionating column to a stripper, where any entrained heavier hydrocarbons and water are eliminated. The heavy fuel oil is cooled down and transferred to storage.
The incident occurred in the heavy fuel oil purification section, which includes the reactor, the feedstock /hot oil heat exchange train, the hydrogen injection (quench) piping, and the reactor control instrumentation. A jet fire affected the 3-inch hydrogen quench line, which ruptured after a six-minute exposure, resulting in ignition of the hydrogen. The shift personnel who were present in the plant at the time of the rupture testified that a strong hissing sound was heard for a few seconds, followed with the ignition of the released product, but there had not been any noticeable pressure changes (e.g., pressure waves). The shift foreman, who was standing on the main access ramp to the atmospheric distillation plant, testified that he saw the jet fire ignite from the top and propagate down (from approximately 18 meters above ground down to approximately 14 meters in the area between the heat exchangers and the reactor). The fire took a cylindrical form, affecting the above-mentioned equipment under the diathermic oil preheater. Approximately 30 minutes after the fire started, an 8-inch fuel pipe in the diathermic oil system ruptured and caused a subsequent ignition of the fuel oil product.
Immediately the shift foreman activated the onsite emergency plan and informed the gate guard to alert the fire brigade, which was done at 21:40. The onsite emergency response team during that shift consisted of 6 people: the shift manager in charge of emergency management, the gate guard responsible for external communications, and four firefighters from the plants that were involved in the incident. The team, assisted by other shift personnel (known as the operative team), participated from the beginning of the event in implementing the emergency response. The operative team consisted of 8 workers, including shift foremen, control room operators, and plant operators. Approximately three minutes after the fire started, the fire brigade from the nearest town (with 6 fire fighters) arrived onsite, followed by other nearby fire brigades (with a total of 50 firefighters). The fire was kept under control and evolved without noticeable changes until the valves were closed and the fuel was consumed, according to the facility's emergency response plan. The fire was extinguished at 1:20 (about 3 hours and 40 minutes after it started) and the state of emergency was called off by the fire brigade at 1:45.
- Heating Equipment
- Heat Exchanger
Based on the results of the company investigation and analysis of an amateur video, the company determined that the incident could have been caused by the failure of one of the following plant components:
· pipes leading to the reactor pressure gauges
· the recycled quench gas pipe at the bottom of the reactor
· the diathermic oil pipe (hot oil) entering or exiting the heat exchanger
· the heat exchanger flanged joints and connection lines.
The company determined that a release from the hot oil circuit could not have triggered the fire, based on the evidence from the pressure data in the circuit, which showed that the failure occurred 30 minutes after the fire started. The video confirms the pipe rupture 30 minutes after the fire began. For the same reason, a release from the hydrogen pipes is not considered likely, as the records demonstrate that the hydrogen pipe failed seven minutes after the fire began. When the heat exchanger flanged joints were dismantled, it was seen that the joint gaskets were not damaged. Thus, the company considers the failure of a pipe from the reactor pressure measurement gauges to be the most likely cause of the accident (although there is no conclusive evidence to identify the specific failure that caused the pipe to rupture). This assumption is supported by the following facts:
· This pipe is located in the area corresponding to the epicenter of the fire.
· The area corresponds to the area visually identified by the witnesses.
· The product release (hydrogen and fuel oil) from one of these pipes can cause a 6-meter long jet flame, as occurred.
· The product supposedly released would have had a high enough temperature and pressure to self-ignite or ignite against a plant hot spot (e.g., the hot oil circuit).
· The damages recorded were caused by overheating (flame exposition) and were not caused by overpressure or explosion. The pressure measurement records confirm no significant pressure changes at the beginning of the event.
The company decided to rebuild the hydrotreatment plant, in compliance with regulations, and to introduce the following process design changes:
· complete separation of the light fuel oil section and the heavy fuel oil section to avoid the possibility of "domino effects"
· lowering the maximum height of the heat exchanger installations from 25 meters to 15 meters to facilitate fire-extinguishing operations
· redesign of the piping system to minimize adjacencies
· relocation of the valves on the hydrogen quench line to enable depressurization
· reduction of the number of measurement gauges
· insertion of valves in a safe area for depressurizing the hot oil circuit.