Fire of a hydrogen-rich gas mixture in a fertilizers plant

The event occurred on vent stack of an ammonia synthesis unit. The release of the gas was more specifically on the water purge line or the hydraulic guard of the vent stack. The process gas consisting in 75% hydrogen, ignited.
The fire could be extinguished the by the emergency decompression of the unit and the automatic shutdown of the synthetic gas turbocharger.

As reported with details the ARIA description (see references), the investigation performed by the plant operator has shown that a series of not-coordinated detection of fluid levels at the exit of the reactor caused an over-pressure in the chimney. This has displaced the liquid content of the hydraulic guard at the bottom of the chimney and some of the syngas passed through the purge line of the guard and ignited. The accidental scenario of syngas emissions from the purge line had not been formally identified by the operator, nevertheless was covered by the general provisions of the emergency plan.

The root cause can be identified in some design, such as the incorrect positioning of the 2 low level sensors installed in the separator, and the dimension of the withdrawal valve.

vent stack, ammonia reactor

The material damage was negligible.

Although the ARIA report (see references) does not mention any lesson learnt, the very accurate and detailed findings from the post-accident investigation (see causes) suggest the following corrective actions:
1) to improved sensors location, and the way how their signals are worked out in the safety system of the plant (new HAZOP).
20 to better understand the flow dynamics also in accidental cases and adapt the design of the unit.