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.
Event Date
November 29, 2006
Record Quality Indicator
Region / Country
Event Initiating System
Classification of the Physical Effects
Nature of the Consequences
Causes
Cause Comments
The immediate cause was the failure of the 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.
Facility Information
Application Type
Application
Specific Application Supply Chain Stage
Components Involved
flange, compressor, detectors
Storage/Process Medium
Location Type
Location description
Industrial Area
Operational Condition
Pre-event Summary
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).
Currency
Lessons Learned
Lessons Learned
As corrective measures, the company established that the valve calibration task was required had to pass a certification by the plant inspection unit. The jointing protocol was improved with more rigorous specifications, and additional pressure sensor was installed.
Event Nature
Emergency Action
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 internal emergency plan was not activated.
Release Type
Release Substance
Hydrogen Release Concentration (%)
70.00
Ignition Source
Detonation
No
Deflagration
No
High Pressure Explosion
No
High Voltage Explosion
No
Source Category
References
References
Event no. 32817 of the French database ARIA (accessed December 2020)