| Case Name |
Leakage and fire from a flange with a special shape at the reactor outlet at a gas oil medium-pressure hydrocracker |
| Pictograph |
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| Date |
October 27, 2002 |
| Place |
Ichihara, Chiba, Japan |
| Location |
Refinery |
| Overview |
A fire occurred on changing operating conditions of a medium-pressure hydrocracker for gas oil at a refinery. Process fluid leaked from the flange with a spacer at the reactor outlet after the operation temperature of the reactor was lowered, and a fire occurred. There was large fire damage to piping in the area. The spacer shrank as temperature fell but the bolt did not shrink, and the tightening torque of the flange decreased. Parallelism of the flange was not sufficient, and use of the spacer seemed to be a true cause of the accident. |
| Incident |
At a refinery, sudden leakage and fire occurred during usual operation at a medium-pressure hydrocracker that desulfurized and decomposed heavy gas oil and vacuum heavy gas oil. Gas of high-temperature and high-pressure leaked through a flange with a special shape located at the reactor outlet, and a fire occurred. |
| Processing |
Manufacture |
| Individual Process |
Reaction |
| Chemical Reaction |
Hydro-desulphurization |
| Substance |
Hydrogen, Fig2 |
| Gas oil |
| Type of Accident |
Fire |
| Sequence |
On October 27th, 2002, the feed rate was decreased and the reactor operating temperature was lowered to 320 °C from 370 °C.
About 22:30, high-temperature and high-pressure gas leaked from a spacer flange at the reactor outlet, and ignited. First, a small fire occurred from a small leakage. Due to the flames, the flange bolt expanded thermally. In addition, the quantity of leaked oil increased. Hydrogen piping, steam piping, and fuel oil piping on the rack were also damaged, and the fire spread as the fuel oil blew out.
05:03 of October 28th, The fire was confirmed to be extinguished.
(history of equipment)
In 1962. The equipment was constructed as an indirect heavy oil desulphurization unit, and operation started.
In 1983. An additional reactor was constructed, and the unit was remodeled into a hydrodesulphurization and decomposition unit for heavy gas oil.
On October 27th, 2002. A fire occurred. |
| Cause |
Operating temperature was lowering on the day prior to the fire, the spacer of the flange with the spacer contracted thermally. However, the bolt of the flange did not shrink thermally to the same extent as the temperature drop. Therefore, tightening torque of the spacer at both ends of the gasket became insufficient, and gas leaked. Although there is no description of the ignition source, it could be static electricity caused due to the spouting material, or gas temperature.
The flange with a spacer is a special flange that has a flange space enlarged by inserting a piping component between the flanges. The gasket is put at both ends of the spacer, and two gaskets and the spacer are tightened together. Tightening becomes difficult, and the inside spacer approximates to the process temperature because of direct contact with the process fluid. However, as the tightening bolts are located away from the spacer external surface, the thermal expansion is affected less by the process temperature change.
Strict attention is required for the temperature change because the bolt that is longer than the usual bolt has to be used. |
| Response |
The plant was shut down urgently, and depressurizing and nitrogen substitution for each device were carried out. Other plants at the refinery were shut down. The private fire brigade was called. The public fire brigade turned out. Thirty employees took refuge in the explosion-resistant control room. |
| Countermeasures |
Facilities countermeasures:
1. Removal of the unnecessary spacer and flange.
2. Gradient correction of the flange plane.
3. Material change of the bolt.
4. Inspections of the operations with large changes of operating conditions and strengthening inspection contents.
Improving reliability of facilities and operators' education:
1. Facilities management for age degradation is executed.
2. Making use of outside information.
3. Thoroughly strengthening hazard management and management of changes. |
| Knowledge Comment |
1. At a plant after prolonged operation, there might be concern about age degradation at unexpected places. Changes of shapes, etc. should be concerned even if it is merely piping.
2. The flange with a spacer is constructed from three parts with a long bolt. Although tightening management is also important, it should be considered that the tendency of following a temperature change is smaller than a conventional flange. |
| Background |
1. The gradient at both flanges seems to have already been produced in the turnaround shutdown in 2001. There is an 8-mm difference between flange gaps when the spacer was removed after the accident, and this occurred due to strain of the piping. In short, the flange, which should be parallel at the plane, was not already parallel. This was forcefully tightened with the bolt. Therefore, the unbalance of tightening torque which resulted from a temperature fall appeared more strongly.
2. Why was the flange with a spacer, and moreover, a 8-cm long spacer was used? From a report of the company where the fire occurred, there was a need to use it for operation and maintenance reasons. Concretely, it is estimated that the spacer was used at the piping disconnection when the reactor opened. If this is correct, it seems better to insert one short pipe instead of the spacer, although one coupler of the flange is increased. It is considered that the reason was not technical but cost.
Considering above, two basic causes are speculated; first, the problem of managing an old plant and second, the idea of design. |
| Incidental Discussion |
When the author was an incoming employee, a flange in which the plane was not parallel from the strain from the piping, was pulled with chain blocks, and was tightened forcefully with bolts. If such work were carried out even now, there might be concerns about a similar accident as in this report, or insufficient strength of a related flange. What is the reality? |
| Reason for Adding to DB |
Example of fire caused due to insufficient maintenance of a special shape flange |
| Scenario |
| Primary Scenario
|
Poor Value Perception, Poor Safety Awareness, Inadequate Risk Recognition, Carelessness, Insufficient Precaution, Inadequate Handling, Insufficient Analysis or Research, Insufficient Prior Research, No Care of Flange with a Spacer, Planning and Design, Poor Planning, Poor Design, Usage, Maintenance/Repair, Distortion of the Flange, Malfunction, Poor Hardware, Uneven Stretch, Secondary Damage, External Damage, Leakage/Fire, Loss to Organization, Economic Loss, Direct Manetary Damage 86 million yen
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| Sources |
Kyuu Miyano. Outline of the medium-pressure hydrocracker accident and countermeasures. Industry and protection. Vol.19. No.36. pp.12-13 (2003)
The company's investigation committee. Flange plane gradient, tightening pressure drop. Industry and protection. Vol.18. No. 43. pp.2-3 (2002).
Fire and Disaster Management Agency. Outline of accidents of dangerous materials 2002. pp.26 (2003)
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| Physical Damage |
Hydrogen piping, high-pressure steam piping, fuel oil piping ruptured. |
| Financial Cost |
¥ 86 million (Fire and Disaster Management Agency) |
| Multimedia Files |
Fig2.Chemical formula
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| Field |
Chemicals and Plants
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| Author |
KOBAYASHI, Mitsuo (Office K)
TAMURA, Masamitsu (Center for Risk Management and Safety Sciences, Yokohama National University)
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