Fire in a Chemicals Production Plant
The fire occurred in the unit for the production of the hexamethylenediamine (HMD). The process is based on the hydrogenation of the adiponitrile [(CH2)4(CN)2]A jet fire of hydrogen was generated during shutdown of the hydrogenation reactor. The shutdown had the purpose to the cleaning of the nickel catalyst. The event initiated in the reactor exit tank, which was collecting the reaction products: HMD and water. The liquid level in this tank was monitored by two independent instruments: a floating gauge and a gamma-radiography technique.A sudden increase of the signal indicating the level of liquid in in the exit tank caused the automatic full opening of the regulation valve and the emptying of the tank. This caused a back-flow of the hydrogen contained in the dome of the reactor into the tank through the degassing line. This line, under normal conditions, had the purpose to bring back to the reactor the residue of hydrogen dissolved in the reaction product. The hydrogen leak ((5000 Nm/h in 10 min) was detected by an over consumption of hydrogen, which induced the operator to warn the operative room. During the operation of closure of the regulation valve, the workers saw a jet fire at the fire trap of the reactor placed outside the building. After closure of the valve, the fire extinguished automatically due to the absence of hydrogen.
Event Date
January 17, 2006
Record Quality Indicator
Region / Country
Event Initiating System
Classification of the Physical Effects
Nature of the Consequences
Causes
Cause Comments
The immediate cause was a sequences of events triggered by partially wrong readings of the monitoring system (see below). The root case could be identified in operative shortcoming, which allowed functioning of a not properly adjusted instrument. The two level measuring instruments were indicating different values. The gauge indicates 100%, and this led to discharging the reaction product tank and thus to hydrogen entry through relief pipe. The emergency shutdown was based however on the reading of the radiography, and would have been triggered when coming below the 10% level signal. Unfortunately, the equipment was not (well) calibrated and was indicating 12% (instead of 0). The emergency shut down was not triggered. These two wrong measures lead to a continuous leak of hydrogen through discharging pipe. The leak was detected by an over consumption of hydrogen.
Facility Information
Application Type
Application
Specific Application Supply Chain Stage
Components Involved
Hydrogenation reactor, Production of hexamethylenediamine
Storage/Process Medium
Location Type
Location description
Industrial Area
Operational Condition
Pre-event Summary
The hydrogenation reactor was shut down for maintenance (regeneration of the catalyst)
Consequences
Number of Injured Persons
6
Number of Fatalities
2
Currency
Lessons Learned
Lessons Learned
An inspection following the accident found out that since at least three days the difference between the two measurement techniques for the level of liquid in the tank was more than 10 %. The system safeguard was based on the signal coming from the gamma-radiography technique. It is activated when its measurement show a level below 10%. The zero-shift on this instrument, however, was higher than this value. Therefore, for all this period the unit has operated without a safeguard in place. In the past, in the case of a similar fire on the fire-trap of another site, the operator had replaced the open tank with a closed one. This solution was not adopted for the unit affected by this accident, because the operator in this case preferred the adoption of preventive measures in the form of safe barriers placed before the possible place of the accident. One of the conclusions drawn by the inspection was that measures limiting the occurrence of an accidents (preventive) and measures for limiting their consequences (mitigating) are not incompatible, but complementary.
Event Nature
Emergency Action
The fire brigade had no reason to intervene.
Release Type
Release Substance
Release Duration (s)
600
Ignition Source
Detonation
No
Deflagration
No
High Pressure Explosion
No
High Voltage Explosion
No
Flame Type
Source Category
References
References
Event description in the French database ARIA
https://www.aria.developpement-durable.gouv.fr/accident/33838/
(accessed September 2020)