Explosion and a fire at an ammonia production plant

The explosion occurred while restarting the ammonia reactor. Synthetic gas (70% H2) was released at the flange of the relief valve of the turbo-compressor. The explosion was
followed by a jet fire.
The hydrogen detectors and fire alarm notified the control room, triggering immediate shutdown of the facility. Rescue crews were quickly able to control the event.

The INITIATING cause was the failure of the flange at a relief valve. Probably it had not been properly calibrated. While restarting the unit, this valve fluttered, causing vibrations that eventually led to rapid loosening of the flange nuts, which most likely had not been tightened enough at the outset.

The inspection noticed also the lack of traceability of these jointing operations (torque). Considering also the difficulties encountered in the days before the accident, it could be concluded that repair/maintenance processes were part of the root cause.

flange, compressor, detectors

Five days before the incident, the unit was put under air due to a problem on the CO2 absorption column at the start of the production of ammonia. Due to a human error, too much air entered the unit, causing the pressure to drop in the turbo-compressor and triggering the safety shutdown of the unit. The relief valve between the turbo-compressor and the methanisation reactor opened under the pressure and this opening was not noticed by the operators.
During the ensuing days, production resumed, yet an abnormal synthetic gas reading led the operator to conduct further investigations and discover that the previous valve placed under such pressure was no longer sealed: it was allowing gas to escape via a 47-m high chimney. The facility was stopped once again to allow replacing the suspected valve.
The unit was started once again. The anaerobic digestion reaction began at 10 pm; the synthetic gas turbo compressor started up at 1:30 am and the accident happened at 3:14 am on the flange of the newly-installed valve (diameter: 6″, approx. 150 mm).

The accident caused no casualties, as the technician present in the vicinity was able to escape just before the explosion, after hearing the whistling sound associated with the discharge of synthetic gas.
Physical consequences of the turbo compressor’s direct environment involved: electrical cabling, melted cladding, heavy damage to heat insulation on the pipeline. Ammonia production was shut down for more than a month.

A manual hydrogen vent was executed to reduce (in jargon 'to stabilise") the vapour pressure inside the LH2 tank. The drivers could handle the emergency alone.

The company did the following:
(1) A certification by the plant inspection unit had to be executed after the valve calibration, performed by a contractor.
(2) The jointing protocol was improved with more rigorous specifications.
(3) An additional pressure sensor was installed.
(4) The specifications for valve sealing and overhaul were strengthened.