Explosion in a fertilisers plant

The vent occurred in the ammonia production unit.
A minor leak had been detected in a flange on the piping of the ammonia synthesis loop (operating at 250 bar). The company called a specialized external contractor to perform an on-stream repair of the valve. This solution was chosen because of for the limited extents of the leakage and the economic costs and efforts associated to a shut-down of a very complex system.

The on-stream repair consisted in: (1) fabrication of a special bracket in two parts adapted to the dimensions of the flange of the valve, and (2): injection of filling material in the bracket around the flange (the material had to be injected at a pressure higher than the operating pressure inside the pipe). During the repair some stud bolts broke causing the escape of a jet of a gaseous mixture of unknown amount of hydrogen and nitrogen at 250 bars. The escaping mixture exploded, killing 2 workers and damaging the valves and the piping.

INITIATING cause was the failure of a flange followed by the release of a nitrogen-hydrogen mixture.

The investigation revealed that two causes have probably contributed to the accident:

(1): the flange stud-bolts of the valve have been replaced by others made of a material not equivalent to the one originally specified;

(2): the maintenance company performing the repair have not taken into account the overpressure on the stud-bolts due to the injection of the filling material in the bracket.

Therefore the root cause could be identified in the lack of a proper risk assessment of the repairing operation.

flange, bolt, ammonia synthesis loop, valve, bracket

Following the detection of a small leak repairing works had been subcontracted, to be performed while the plant continued its operation.

Despite the fact that the automatic detection and mitigation system did not function as designed, thanks to the eagle-eyed able seaman radioing to the ship’s cargo control room, the gas combustion unit was quickly shut down, and a major fire was prevented.
The unit was immediately shut down and isolated before the crew implemented the fire prevention response plan.

After the accident, the following measures were established:
- compilation of written maintenance procedures with emphasis on safety issues;
- establishment of the sequence of operations to be followed during repair of such valves.