Case Ditails

Scenario
Case Name Explosion in the piping caused due to a back flow of ammonia in the amination of 5-chloro-1,2,3-thiadiazole
Pictograph
Date February 13, 1980
Place Sakaide, Kagawa, Japan
Location Chemical factory
Overview To manufacture 5AT (5-amino-1, 2, 3-thiadiazole), which is a crude law material for medical supplies, 5CT and liquid ammonia were supplied into a reactor through a mixing chamber. A manual and an automatic operation were repeated because the temperature of the reactor was not stable. The supply of 5CT was stopped when a pressure abnormality was detected, and preparations for cutting off the liquid ammonia were made. The ammonia supply pressure dropped suddenly, and an explosion occurred immediately. It was considered that the ammonia flowed backward through the 5CT piping, led to the abnormal reaction. Multiple malfunctions of facilities and operation caused the accident.
Incident 5AAT, which is a raw material of medical supplies, can be obtained by acetylation of 5AT, which was obtained by amination of 5CT with liquid ammonia. The stainless steel high-pressure piping for charging 5CT into the reactor at the 5AT manufacturing plant exploded. The accident occurred in the first operation of a newly established plant.
5AAT ; 5-acetylamino-1,2,3-thiadiazole, 5CT ; 5-chloro-1,2,3-thiadiazole, 5AT ; the 5-amino-1,2,3-thiadiazole
Processing Manufacture
Individual Process Reaction
Process Flow Fig2.AT reaction plant figure
Fig3.Unit process flow
Chemical Reaction Other
Chemical Equation Fig4.Chemical reaction formula
Substance 5-chloro-1,2,3-thiadiazole(5CT), Fig5
5-amino-1,2,3-thiadiazole(5AT), Fig6
Type of Accident Explosion, fire
Sequence The temperature in the reactor, which was usually around 0 °C, was unstable on the day of the accident at the 5AT manufacturing plant. Therefore, automatic operation and manual operation were repeated alternately. When the pressure of the reactor exceeded 1.5 MPaG, an operator switched to manual operation from automatic, adjusted the opening of the pressure control valve and closed the 5CT supply valve. As the operator noticed the abnormally high pressure of the reactor after going back to the control room, he returned to the site to stop the supply of liquid ammonia. The pump discharge pressure for the ammonia supply at that time was 2.0-3.0 MPaG. After a while, this pressure suddenly dropped to about 0.5 MPaG and the 5CT feed piping exploded at the same time between the 5CT supply pump and the mixing chamber.
Cause 1. The ammonia supply pressure rapidly dropped.
2. Ammonium chloride was detected from the block valve of the mixing chamber after the accident. From the two points above, the cause was considered as follows. Liquid ammonia flowed backward from the mixing chamber to the heated 5CT piping. Therefore, the 5AT production reaction progressed in the 5CT piping and 5AT or 5CT exploded due to reaction heat.
The mixing chamber: Piping equipment that mixes ammonia and heated 5CT prior to the reaction.
Countermeasures 1. The following countermeasures are taken on facilities.
1-1. Check valves, etc. are equipped so that raw material such as ammonia cannot flow backward from the mixing chamber.
1-2. Structures of piping and valves are considered so that solid by-products do not clog them.
1-3. The alarm operates when temperature and pressure deviate from the fixed range. In addition, safety equipment for emergency cut-off of the raw materials supply and emergency blowdown of products are installed.
1-4. Important valves such as a coolant supply valve and a pressure control valve can be adjusted from the control room.
2. The following countermeasures are taken on an operation control system.
2-1. A sufficient examination system is necessary when temperature and pressure of reactors are unstable, when the set value is exceeded and is on an upward trend,, including abnormal behavior of other facilities.
2-2. The operation standard in an emergency and/or in an abnormal case is decided clearly, and education and training are carried out periodically.
3. As for the substances of which there is little safety information,
the safety information such as reaction characteristics, decomposition
tendency, flammability and so on, should be obtained through the small scale experiment and study on the properties such as the heating value of a reaction, thermal instability, impact sensitivity, and so on.
Knowledge Comment Although 5CT was not well understood in those days, it was known to be a material with a potential hazard. Grasping and assessing hazards are indispensable when a potentially hazardous material is handled.
The material flows from high pressure to low pressure locations, and backflow prevention cannot be achieved without understanding this.
Background The plant had a structural defect; there was no check valve on the 5CT supply piping to the mixing chamber so that liquefied ammonia could flow backward. A backflow of ammonia was a natural phenomenon when the 5CT pressure drops upon stopping the 5CT supply pump, while the ammonia pump was working. As it is well known that ammonia and 5CT would react on contacting each other, it is necessary to adjust the facilities and its mode of operation not to contact except in the reactor.
Safety equipment such as an alarm system that warns hazardous temperature and/or pressure, and an emergency cut-off system were inadequate. There was also a structural defect such as piping that was easily clogged by ammonium chloride as a by-product. Checks of each point were not sufficient, even when the temperature of the reaction chamber was unstable and the temperature gradually rose. The standard of emergency procedure was not well established. Knowledge about the thermal decomposition hazard of 5AT and 5CT, especially decomposition behavior at a lower temperature was insufficient.
After all, a lack of fundamental knowledge on the operation and insufficient preparations against reaction hazard are the key problems.
Reason for Adding to DB Example of accident caused due to no countermeasure against a back flow inducing an abnormal reaction leading to an explosion
Scenario
Primary Scenario Insufficient Analysis or Research, Insufficient Prior Research, Insufficient Grasp of Property, Poor Value Perception, Poor Safety Awareness, Inadequate Risk Recognition, Other Reaction, Planning and Design, Poor Planning, Poor Design, Regular Operation, Erroneous Operation, Insufficient Pressure Observation, Bad Event, Chemical Phenomenon, Abnormal Reaction, Secondary Damage, External Damage, Explosion, Bodily Harm, Injury, 3 person injured
Sources Ministry of Labor. Safe section, Cases of explosions and fires and countermeasures .[A].Gas, vapor, etc. explode. Case 6, New publication. Cases of labor accidents and countermeasures, pp.210-211(1984)
Tamura Masamitsu, Wakakura Masahide, Explosion during an amination reaction of 5-chloro - 1, 2, 3 - thiadiazole. Reaction danger -Accident case and analysis-p.133(1995)
Ministry of Labor. Industrial Safety and Health Department. Safe section, Explosion during amination of 5-chloro -1, 2, 3- thiadiazole. Safety of batch process, pp.58-59(1987)
Number of Injuries 3
Physical Damage A receiving drum was seriously damaged.
Multimedia Files Fig5.Chemical formula
Fig6.Chemical formula
Field Chemicals and Plants
Author ARAI, Mitsuru (Environmental Science Center, The University of Tokyo)
TAMURA, Masamitsu (Center for Risk Management and Safety Sciences, Yokohama National University)
Scenario