Fire in hydrogenation unit of a monomer production plant

The event occurred at a chemical plant for polyamides (SEVESO- classified). Maintenance was ongoing on an isolated unit producing a polyamide monomer (lactams), when a fire developed on the ground floor of the unit of hydrogenation of cyclo-dodecane (CDA).
A fire detector on the unit triggered an alarm 4 min after the start of the fire and the station operator put the unit in safe conditions and alerted the firefighters of the site. The fire was under control in half an hour. The nitrogen purging device of the reactors was put into service, but it malfunctioned, due to the difficulties of connection of the hose and leakage on a valve.
The unit was slightly damaged (damaged valves, melting cables), and it restarted 13 days later.

INITIATING cause has been the failure of a low-level sensor, which allowed the injection of hydrogen into the production line.

The analysis of the accident by the plant operator showed that that sensor failed to give the signal of low level in the CDA storage tank. The low level in the tank allowed hydrogen into the production line, which was shutdown. The hydrogen escaped from the manual valve at the bottom of the reactor finishing the process. This valve had already been found leaking a week before and tightened but apparently had started leaking again.
In the presence of an unidentified ignition source, the leaked hydrogen ignited. The fire was further fuelled by the opening of the reactors' automatic degassing valves, which were damaged by the initial fire. This opening increased the hydrogen supply to the unit. Unfortunately, in absence of a more detailed description of the components of the production unit, it is impossible to get a better understanding of the accidental sequence and of the role of the various components (CDA tank, production line, various valves).

ROOT CAUSE could be attributed to a series of malfunctions of critical or at least important safety-related devices, hinting at poor inspection and maintenance. A delayed fire detection and an excessively slow pressure drop in the decompression line could be considered indication of a shortcoming in safety design.

polyamide monomer unit, hydrogenation of cyclododecadiene unit

Occurred during maintenance

According to Wikipedia [https://en.wikipedia.org/wiki/Cyclododecane#Synthesis] the organic compound cyclododecane (CDA) mentioned in the ARIA report has formila (CH2)12 or C11H24 (melting point 60 C).
The CDA is a precursor to laurolactam, a monomer of polyamide, in its own a precursor to the polymer Nylon 12, polyesters, and synthetic lubricating oils.
The CDA is produced industrially through catalytic trimerisation of butadiene to cyclododecatriene, followed by hydrogenation. It is in the unit responsible for this last hydrogenation step that the fire occurred.

There were no impact on the environment.
Damage to the unit was limited to components such as valves, melting cables).
The plant restarted 13 days later.

1. Shortly before 17h - fire develops on the ground floor of the hydrogenation unit of cyclododecadiene (CDA).
2. Four min after the fire started, a fire detector on the unit triggers an alarm and the station operator put the unit in safe conditions and alerted the firefighters of the site.
3. Half an hour later the fire is under control.
4. 13 days later operations restart.