Explosion in a chemical plant, caused by accidental formation of hydrogen

In a chemical plant, a violent explosion took place during the batch production of chlobenzorex from an aqueous solution of sodium borohydride contained in a feed tank on a movable truck. The reason was the accidental production of hydrogen due to an unexpected chemical reaction.

The workshop was devastated: roof ripped out, light walls displaced, solvent pipes ruptured by the projections. The feed tank was destroyed but the remaining process equipment was not damaged.
Outside the workshop but near a door, a technician was projected against the containers by the shock wave. He sustained injuries on the ribs and was hospitalised for a month.

DETAILED ACCIDENT SEQUENCE
The sodium borohydride solution (44 kg of powder for 130 kg of solution) was prepared 2 hours before production in a closed 630 litre agitated feed tank. Since dissolving was problematic, the operating mode had been recently modified to include pre-heating of the solution by hot water surface flow (45- 55°C).
14:00 The reactor was placed under nitrogen circulation (N2) and connected to the feed tank whose heating was stopped.
14.15 The N2 inlet of the feed tank was connected to the supply hose.
14:20 The N2 supply to the feed tank started with the partial opening of the supply valves of the reactor and the valves at the bottom of the feed tank (at a flow rate of 100 l/h – P(N2) = 2.8 bar).
14:40 The N2 hose supplying the feed tank was abruptly torn out letting the gas escape though the opening. The superfluous staff was evacuated from the workshop and an emergency team installed a valve on the N2 inlet to stop the leak. The team tried to decompress the feed tank in the reactor by opening the reactor valve to the maximum and opening the feed tank valve that was difficult to access.
At this moment, the feed tank’s manhole gave in a few seconds before it exploded.

The INITATING CAUSE was the explosion of a flammable mixture inside a tank, due to the accidental formation of hydrogen.
Post-accident investigations revealed a thermal decomposition of sodium borohydride triggered by an excessively high storage temperature (50 °C due to hot water surface flow). Maintaining the temperature at 50 °C stabilised the decomposition rate in such a way that the small quantities of hydrogen formed could escape via the leaking feed tank.

When the heating was stopped, the reaction evolved in a self-sustaining mode: temperature and pressure increased under the effect of heat resulting from the decomposition, causing a thermal explosion. The inspection hole opened under a pressure of 15 to 20 bar followed by the rupture of the feed tank. The depressurisation of hydrogen in the form of a shock wave was responsible for the damage caused in the workshop.
The ROOT CAUSE were a poor knowledge of the properties of the borohydride, the absence of a critical analysis of the process and of the effect of its modifications (management of changes), and failing of considering typical production incidents consequences.

Tank with an aqueous solution of sodium borohydride

The workshop was devastated: roof ripped out, light walls displaced, solvent pipes ruptured by the projections. The feed tank was destroyed but the remaining process equipment was not damaged. Outside the workshop but near a door, a technician was projected against the containers by the shock wave. He sustained injuries on the ribs and was hospitalised for a month.

The system had been modified recently modified to improve the dissolution into water of a chemical compound. The modification consisted in pre-heating the solution. Among the several contributing causes, the absence of a critical analysis of the effect of the modifications is probably the one deserving the most the name of root cause. The plant operator failed to set up an effective management of changes and consider the whole of the unit in term of hazards and risks.