Case Ditails

Scenario
Case Name Leakage and explosion at a heat exchanger having a special structure caused due to insufficient maintenance of a vacuum gas oil hydrodesulfurization unit
Pictograph
Date October 16, 1992
Place Sodegaura, Chiba, Japan
Location Refinery
Overview On October 16th, 1992, an explosion and a fire occurred on starting a vacuum gas oil hydrodesulfurization unit after changing a catalyst at the refinery. While the operating condition was being adjusted, a leak of hydrogen and a vacuum gas oil mist was generated from the feed-effluent heat exchanger at the reactor section. A loud explosion and a fire followed. A channel cover and lock bolts of the heat exchanger came off, and flew about 120 m away, severely damaging buildings of the a neighboring factory. The refinery shut down the whole fuel oil refining plant immediately. A tank of the neighboring factory also caught fire due to the influence of the explosion. The cause of the accident was that internal parts of the heat exchanger of a very large and special structure called the bleach rock type had deteriorated and hardened due to inadequate long-term maintenance.
Incident An accident occurred at startup after finishing catalyst exchange of a vacuum gas oil hydrodesulfurization unit in a refinery. After restarting the unit, the operating conditions were adjusted to almost usual levels, and hot bolting of high-temperature equipment and piping was being done. At that time, an explosion and a fire occurred at a heat exchanger E-2801. 17 persons including hot bolting workers and supervisors near the heat exchanger were killed or injured. Moreover, tanks and buildings of the adjacent factory were damaged by a blast and scattered debris from this accident, and some of leaking lubricant oil was ignited.
The operation conditions of the heat exchanger were in the design limit at the time: the pressure was about 8 MPaG at the tube side and about 7 MaPG at the shell side, the temperature was 111-199 °C at the tube side, 262-187 °C at the shell side.
Processing Manufacture
Individual Process Maintenance
Process Flow Fig2.Vacuum gas oil hydrodesulfurizing unit flow sheet
Chemical Reaction Other (hydro-desulfurization)
Substance Hydrogen, Fig3
Gas oil
Type of Accident Leakage, explosion, fire
Sequence On October 1st, 1992; the unit was shut down, and exchange of a desulphurization catalyst started.
On October 14th; restarting of the unit began. A series of startup tasks such as hot oil circulation at 300 °C, etc. started.
At 13:00 on October 16th; the operating conditions were almost reached at usual conditions, and operation shifted to a commercial run.
At about 15:15; hot bolting of the heat exchanger E-2801B (1.5 m in outer diameter, 9 m length, and 41.5 tons weight) started.
Between 15:47-48; leak of smoke (a hydrogen and vacuum gas oil mist) was found at a detection hole at the upper part of the heat exchanger. Operators waited at the windward side near the heat exchanger.
About 15:52; channel covers and a lock ring, etc. came off, flying about 130 m, striking buildings and damaging them severely. Leaked vacuum gas oil and a hydrogen mist exploded and burned.
16:16; public firefighters turned out and urgent de-pressurization of high pressure gas release was performed.
18:35; the fire was extinguished.
The unit started operation in 1976. Inspection and repair of the heat
exchanger were conducted in 1979, 1981, 1982, 1988, and 1991.
According to these records the accident occurred 16 years after start-up.
Cause 1. The cause of gas leakage; The heat exchanger was very large and operated at high pressure and temperature pf 7-8M PaG and 350 °C. It has a special structure called the bleach rock type. A gasket retainer, which presses the gasket and prevents leaks between inside and outside of the heat exchanger, was deformed by repeated rising and lowering of temperature, and its diameter decreased. Therefore, the gasket retainer reached the gasket groove inside corner of the heat exchanger shell. Hydrogen gas began leaking from the part, and it was considered that the leakage rate increased with a rise of temperature and pressure. At turnaround maintenance in the previous year, 1991, grinder repair was carried out at the groove inside corner and the gasket retainer inside corner, therefore, the gasket retainer was easily deformed.
2. The cause of dispersion of a lock ring, etc.
(1) The set bolts of the flange hardened because of repeated warming and lowering of the temperature. Therefore, the set bolts could not perform their original purpose of absorbing stress generated by thermal expansion. For this
reason, the stress on the channel cover increased. As a result, repeated
raising and lowering of the temperature diminished the lock ring, and the bite of the screw tip decreased.
(2) An internal pressure rise due to gas leakage caused the channel barrel
to swell and be deformed. In addition, the bite of the screw decreased.
(3) Due to the effects of (1) and (2), the tips of the screw threads collapsed,
because increased stress on the screw outside was applied.
(4) For the above reasons, the number of bites of the screw thread
decreased, and led to dispersion.
Countermeasures Clarification of exchange manuals of parts and so on during an overhaul.
Strengthening of inspection management.
Knowledge Comment This accident shows how hazardous it is if maintenance work is executed without a sufficient understanding of the roles and the structures of parts.
Education of workers of subcontractors is also important because maintenance work is often done by a subcontractor's workers.
Background The heat exchanger had a special structure, and the construction crew did not fully understand the structure and purposes of parts when maintenance was being done. Although there was an inspection error, it can be considered an accident caused by maintenance following inadequate investigation.
Reason for Adding to DB Example of explosion caused due to insufficient maintenance of a special type heat exchanger
Scenario
Primary Scenario Poor Value Perception, Poor Safety Awareness, Poor Recognition of Importance about Structure and Parts, Carelessness, Insufficient Understanding, Insufficient Recognition of Risk, Usage, Maintenance/Repair, Repair, Oversight of Secure Deterioration, Failure, Large-Scale Damage, Rapture, Secondary Damage, External Damage, Explosion, Bodily Harm, Death, 10 peson died, Bodily Harm, Injury, 7 person injured, Loss to Organization, Economic Loss, Direct Monetary Damage 2900 million yen
Sources High Pressure Gas Safety Inst. of Japan, Accident investigation report of The F petroleum Co., Ltd. S refinery. (1993)
High Pressure Gas Safety Inst. of Japan High pressure gas protection overview. pp.173-180, 202-203 (1993).
Chiba Labour Standards Bureau, Report of F petroleum explosion investigation committee. 1993 November (1993).
Assoc. for Safety Techniques of Dangerous Materials Cases of accidents with dangerous materials. Dangerous material accident seminar. pp.102-104 (1996).
Number of Deaths 10
Number of Injuries 7
Physical Damage At S refinery of F petroleum, the unit in which the accident occurred was severely damaged by the explosion and fire. Some of the fluid in the unit spilled and burned. Many buildings in the refinery were damaged by a hot blast.
At S lubricating oil factory of N petroleum Co. Ltd., many facilities and machines were damaged by a hot blast and scattered debris
Financial Cost F petroleum ¥2,480 million, N petroleum ¥380 million (Report of the explosion investigation committee)
Consequences An explosion sound was heard at the point 8 km away from the site, and the shock of the explosion was felt even at the city hall about 6 km away, and its windows were damaged by the blast.
Multimedia Files Fig3.Chemical formula
Field Chemicals and Plants
Author KOSEKI, Hirosi (National Research Institute of Fire and Disaster)
TAMURA, Masamitsu (Center for Risk Management and Safety Sciences, Yokohama National University)
Scenario