Case Ditails

Scenario
Case Name Partial leakage of hydrochloric acid gas from an absorber due to an earthquake
Pictograph
Date December 17, 1987
Place Sodegaura, Chiba, Japan
Location Chemical factory
Overview About 11:35 on December 17th, 1987, a strong earthquake occurred during usual operation of a vinyl chloride monomer plant. The substation was stopped by a malfunction of an over-current relay due to the earthquake during emergency shutdown operation. Although the emergency power generator started, fluctuations of cooling water caused by the earthquake were incorrectly interpreted as a fall of a liquid level. The emergency power generator was stopped immediately by an interlock that mistook an abnormality with cooling water. Following the total power failure, the alkali circulation pump of the absorber stopped, and the gas was discharged.
Incident A strong earthquake occurred. A vinyl chloride monomer plant was shut down immediately. Some hydrochloric acid gas within a reaction section was led to the absorber. In parallel with an emergency shutdown, plant patrols checked for a gas leak, a liquid leak, and a bad smell. No abnormalities were found. However, information on a bad smell was reported from the local government and the fire department, and two persons of an adjoining company were also treated by an intravenous drip. It was found that a small amount of hydrochloric acid gas leaked out from the absorber as a result of an investigation.
Processing Manufacture
Individual Process Reaction
Chemical Reaction Halogenation
Substance Hydrochloric acid, Fig2
Type of Accident Leakage, Environmental Pollution
Sequence 11:08 on December 17th, 1987. Following a strong earthquake, an emergency shutdown of the plant started. As a power failure occurred due to a malfunction of an over-current relay in the substation at almost the same moment, an emergency power generator started. However, the emergency power generator also stopped. As an interlock was operated by an emergency shutdown, some hydrochloric acid gas in the reaction section was led to the absorber.
11:14. The operators started the emergency power generator again. In parallel with an emergency shutdown, a plant patrol carefully checked a gas leak, a liquid leak, and a bad smell. No abnormalities were found.
About 11:35. The fire department and the patrol vehicle of the local government asked about the bad smell. Although the actual state was investigated, no unusual, bad smell was detected. No leak was found by subsequent patrol.
From time, range, wind, and the kind of a bad smell, an emergency shutdown/restart of the emergency power generator, etc., it was assumed that about 6 kg of hydrochloric acid gas had been discharged from the top of the absorber for about two minutes.
Cause A power failure occurred due to a malfunction of an over-current relay of a substation during an earthquake. The emergency power generator started for backup. However, the level sensor of the cooling water tank mistook the fluctuations of cooling water in the tank caused by the earthquake for a fall down in the water level. Therefore, the emergency power generator was stopped by an interlock. Five to six minutes were required to manually restart the generator and start a circulation pump. When the circulation pump for a sodium hydroxide liquid stops, the effectiveness of the absorber is lost after four to five minutes. It is assumed that hydrochloric acid gas was discharged from the absorber for about two minutes until the pump restarted.
Response The vinyl chloride monomer plant was shut down immediately. Plant patrol was made. The emergency power generator was started again.
Countermeasures Operation of the circulation pump of the absorber was changed to remote starting at the control room.
A level indicator was installed in the cooling water tank of the emergency power generator.
The low level interlock of the cooling water tank was replaced with an alarm.
The manual was restudied and thorough education was provided according to the changes above-mentioned.
The communication system in the factory should be restudied to prepare the whole factory for an abnormal state.
Knowledge Comment If facilities to be used in an emergency stop, damage spreads. Daily management of maintenance, daily checks, and keeping functions at the time of a natural disaster are important.
As earthquakes happen frequently in Japan, instrumentation that does not mistake sloshing for a low liquid level is required.
Background The over-current relay malfunctioned during an earthquake. This might have been a problem of daily maintenance. Sloshing was incorrectly interpreted as a surface fall, and this might be a design problem.
Incidental Discussion Although the accident seems to have occurred due to an earthquake, the accident was actually caused by a malfunction of the over-current relay, and having mistaken sloshing for a low liquid level. Preventing misunderstanding of sloshing is a nationwide theme.
It is written in the "cause" column that the absorption capacity is effective for four to five minutes even if a circulation pump stops.
This was cited from information sources. However, as the absorber is a packed column, when a pump stops, functions are maintained at most several 10 seconds. This is because there is very little liquid at the packed part, and the liquid drops to the bottom immediately.
Reason for Adding to DB Example of leakage caused due to mistaking surface fluctuations for level lowering
Scenario
Primary Scenario Insufficient Analysis or Research, Insufficient Practice, Lack of Imagination, Poor Value Perception, Poor Safety Awareness, Inadequate Risk Recognition, Planning and Design, Poor Planning, Poor Design, Malfunction, Poor Software, Instrument Mis-operated, Bad Event, Electrical Failure, Private Power Generator Stop, Secondary Damage, External Damage, Leakage
Sources High Pressure Gas Safety Inst. of Japan, Vinyl chloride monomer manufacturing plant, Transient emission of hydrochloric acid gas from an absorber during earthquake. Accident examples of Petroleum refinery and Petrochemical units, pp.175-182 (1995).
Number of Injuries 2
Consequences About 11:35 on December 17th, 1987, based on a report made via a hot line from a local fire department, and a report of a bad smell from a local government patrol vehicle, there was judged to have been a discharge of hydrochloric acid gas.
Multimedia Files Fig2.Chemical formula
Field Chemicals and Plants
Author ITAGAKI, Haruhiko (Japan National Institute of Occupational Safety and Health)
TAMURA, Masamitsu (Center for Risk Management and Safety Sciences, Yokohama National University)
Scenario