Fire in a chemicals production plant

The fire occurred in a unit to produce hexamethylenediamine, an organic compound precursor of polymers (HMD). The process is based on the hydrogenation of a precursor in presence of a catalyst. 

A hydrogen jet fire generated during shutdown of the hydrogenation reactor. The shutdown was part of the operative cycle, aiming at cleaning (regenerating) 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 Nm³/h in 10 min) was detected by an over consumption of hydrogen, which induced the operator to warn the operative room. When closing 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.

The INITIATING cause was a sequences of events triggered by partially wrong readings of the monitoring system. 

The two instruments installed to monitor the liquid levels in the tank were indicating different values. The gauge indicated 100%, and this led to discharging the reaction product tank and thus to hydrogen entry through relief pipe. However, the emergency shutdown was based on the reading of the radiography, which should have activated it when coming below the 10% level signal. Unfortunately, the equipment was not (well) calibrated and was indicating 12% (instead of 0). Therefore, the emergency shut down was not triggered. 

The root case could be identified in operative shortcoming, because of the inadequate calibration the of an instrument. Moreover, the leak was detected only indirectly by an over consumption of hydrogen. This, together with the unclear interlocking functions of the instruments measuring the liquid levels, suggests as well a shortcoming in the safety design of the installation.   

hydrogenation reactor

The hydrogenation reactor was shut down for maintenance (regeneration of the catalyst)

DESCRIPTION OF THE CHEMCIAL PROCESS Hexamethylenediamine or hexane-1,6-diamine, is the organic compound with the formula H2N(CH2)6NH2. It is used almost exclusively for the production of polymers, mostly for the production of nylon 66. It is produced by the hydrogenation of adiponitrile, in presence of a catalyst, according to the reaction: NC(CH2)4CN + 4 H2 → H2N(CH2)6NH2 The source of this event does not provide information on the process parameters such as temperature, pressure and reactant masses. From the technical literature, the pressure ranges from 120 C to 170 C, and the pressure is in the 200 to 350 bar. 

The values of the operative parameters are however unknown, because the plant was in programmed shutdown, and the report does not mention the actual temperature and pressure.   

Since the fire was extinguished by the closure of the regulation valve, the fire brigade did not need to intervene.

The post-incident investigation found out that the difference between the two measurement techniques for the level of liquid in the tank was more than 10 % since at least three days. 

The system safeguard was based on the signal coming from the gamma-radiography technique. It was 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, due to a similar case of fire on the fire-trap at 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 site operator preferred preventive measures in the form of safe barriers acting before the possible occurrence of an accident. 

A lesson 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. Safety barriers and mitigation measures are complementary and should both be used.