| Case Name |
Explosion caused due to generation of a combustible gas-air mixture at a naphthalene oxidation reaction plant |
| Pictograph |
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| Date |
March 10, 1998 |
| Place |
Kawasaki, Kanagawa, Japan |
| Location |
Chemical factory |
| Overview |
On March 10th, 1989, cavitation occurred at naphthalene feed pumps from a naphthalene storage tank to a mixing chamber in an oxidation reaction plant. The cavitation of one pump continued. The naphthalene feed was interrupted. The air inlet valve at the discharge side of the oxidation air blower was closed more tightly because the reaction temperature began to drop. The operation became unstable, and a fire occurred in the mixing chamber due to a static electric spark in series B pump. Then the rupture disk operated. At this time, the naphthalene concentration in the mixing chamber rose as the inlet air flow decreased. |
| Incident |
At a naphthalene oxidation plant, two pumps for feeding raw materials, which supply naphthalene to the mixing chambers of A and B series from the naphthalene storage tank, caused cavitation simultaneously. Therefore, naphthalene feed was interrupted. Gas purging was carried out from the discharge vent valves of both pumps. Series A pump was soon normalized, but cavitation continued at series B pump. Afterwards, the air inlet valve for oxidation was closed more tightly because the reaction temperature of series B began to drop. Anti surging equipment of the air blower was actuated, operation became unstable, there was an explosion in the mixing chamber of series B, and the rupture disk operated. |
| Processing |
Manufacture |
| Individual Process |
Reaction |
| Process Flow |
Fig2.Unit process flow
|
| Chemical Reaction |
Oxidation |
| Substance |
Naphthalene, Fig3 |
| Type of Accident |
Explosion |
| Sequence |
On March 9th, 1998, 12:01. The plant started up.
On March 10th, 03:50. Simultaneously, cavitation occurred at two naphthalene feed pumps of series A and B. Only series A pump returned to normal operation when gas purging was carried out from the discharge vent valve of both pumps. Naphthalene feed to the mixing chamber of series B failed, and the reaction temperature began to drop.
04:00. The air inlet valve was closed more tightly, and the air flow rate to the mixing chamber was reduced. Anti-surging equipment of the air blower was actuated, and operation of the mixing chamber became unstable.
04:09. The air feed rate dropped, and the naphthalene concentration in the mixing chamber exceeded the lower explosion limit. Ignition and explosion occurred due to static electric sparks in the mixing chamber, and the rupture disk operated. The supply of naphthalene was stopped. |
| Cause |
1. A combustible gas-air mixture was generated by the action of the anti-surging equipment of the oxidation air blower. The naphthalene concentration in the mixing chamber exceeded 51 g/Nm3, which is the lower explosion limit.
2. Ignition sources: Steam condensate was entrapped in the naphthalene feed system, and static electricity was generated in the mixing chamber by atomization of the condensate.
During turn around, steam condensate moved into suction piping of B series naphthalene feed pump through a seat leak of a valve. After start-up, the condensate was evaporated by the high temperature of steam which was used for steam tracing. This generated steam flowed into the two feed pumps, and the cavitation occurred. |
| Response |
The naphthalene feed was stopped after the explosion. |
| Countermeasures |
The operation standard must be revised so that the flow has to be returned to the recirculation system to prevent naphthalene from flowing into the mixing chamber when naphthalene feed pumps cause cavitation.
Utility piping connected directly to the process fluid piping should be isolated from process piping. |
| Knowledge Comment |
A dangerous condition might be caused due to making an adjustment during unstable operation of a plant. A safe response procedure must be prepared beforehand. |
| Background |
The main cause was startup of the plant with steam condensate in the naphthalene feed pump and the suction piping. Engineers and managers must understand that the presence of condensate can cause unstable operation.
It is considered that the operators panicked as they could not find the cause of the pump failure. It is important to carry out education and training on pump failures.
Steam piping was connected directly to the process piping with only one valve. Therefore, it was natural that steam or steam condensate (high pressure) moved into the process piping at atmospheric pressure during a turnaround shutdown. The main reasons seemed to be process design and operation management. |
| Incidental Discussion |
Some problems might be caused as water enters a high-temperature unit by evaporation after start-up. |
| Reason for Adding to DB |
Example of explosion caused due to pump cavitation |
| Scenario |
| Primary Scenario
|
Poor Value Perception, Poor Safety Awareness, Inadequate Risk Recognition, Carelessness, Insufficient Precaution, Inadequate Handling, Ignorance, Insufficient Knowledge, Poor Experience, Planning and Design, Poor Planning, Poor Purge Planning, Regular Operation, Nonobservance of Procedure, Malfunction, Poor Hardware, Cavitation, Secondary Damage, External Damage, Explosion, Bodily Harm, Injury, Loss to Organization, Economic Loss, Direct Monetary Damage 5 million yen
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| Sources |
Kawasaki City Fire fighting station Prevention division. Fumio Itakura. Case of fire caused by static electricity. Dangerous material accident case seminar. pp.21-36(1999).
Kawasaki City Fire fighting station. Prevention division. Peace section. Naphthalene oxidation reaction facility rupture phenomenon. Material of the Kawasaki City complex safety countermeasures committee. (1998)
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| Number of Injuries |
1 |
| Physical Damage |
A rupture disk broke. Deformation of an earth screen and a backfire-preventing wire mesh. |
| Financial Cost |
¥ 5 million. (Material of the Kawasaki City complex safety countermeasures committee) |
| Multimedia Files |
Fig3.Chemical formula
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| Field |
Chemicals and Plants
|
| Author |
DOBASHI, Ritsu (School of Engineering, The University of Tokyo)
TAMURA, Masamitsu (Center for Risk Management and Safety Sciences, Yokohama National University)
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