Syngas fire in a ammonia plant

The incident occurred at the ammonia synthesis unit of a production plant of fertilizers (Seveso III upper tier), during the operation of start-up. The unit was restarting, when a pipe carrying syngas (73% hydrogen, pressure 135 bar and 390°C) opened completely at a weld joining a section of vertical pipe and a connecting elbow with a horizontal pipe. The released gas ignited, due to sparks produced by the projection of fixing parts of the pipe due to the internal pressure. The burning rate of the gas cloud increased explosively, producing a deflagration, a pressure wave and a flame front that damaged the thermal insulation of other pipes near leak and generated a thermal radiative field. After the explosion, the fire continued fed by the continuous release of the gas.
The Ammonia plant was shut down immediately, the synthesis loop was depressurised and inertised with nitrogen.
The plant and local firefighters were able to extinguish the fire in two hours.

The INITIATING CAUSE was the opening at a weld of a pipe carrying syngas.
For the ROOT CAUSES, the post-accident investigation was only able propose a list of possible material- and design-related factors:
1) Mechanical vibrations of the synthesis column generated by operation at high temperatures and pressures (about 140 bar).
2) Degradation to the mechanical properties of pipe materials due hydrogen attack.
3) Change in the material properties of the weld due to high temperature (a clean separation of the two welded pipe sections was found)
4) The rupture occurred at the welding between two materials with different compositions and mechanical properties (stress caused differential dilatation).
5) Temperature transients during plant startup and ammonia reactor heating.
6) The design prescribed a minimal distance between the weld and the heated components, to avoid too high temperature on the weld, It is possible that this safety distance was not large enough.
The list suggest a ROOT CAUSE related to the lack of material control, possible design shortcoming and lack of effective inspection/monitoring procedures & methods.

pipe, weld

At the time of the event, the last actions were carried out to complete the commissioning of the installation and the resumption of the production. Restarting the plant after a shutdown for a long period of time usually took 3 days, most of the phases of the technological process requiring heating and activation of catalysts in a well-defined sequence determined by procedures supervised by plant operator. According to the process diagrams recorded in the control room, the operative parameters were normal in terms of temperatures, pressures and flow rates. In the pipe, the syngas composition had a content of 73% hydrogen at pressure 135 bar and 390°C.

DESCRIPTION of the SYNTHESIS PROCESS
The Ammonia production unit had a production capacity of 350,000 tons/year and consisted of the synthesis reactor; a gas-synthesis compressor hall; air compressor, chemical and analyser halls and administrative building.
The plant was commissioned in 1975 and upgraded in 2014-2015.

Ammonia manufacturing technology uses methane gas and water vapour as raw materials. The technological process is based on obtaining the raw synthesis gas by thermocatalytic decomposition of methane gas in the presence of water vapour and synthesis of ammonia at medium pressure. After synthesis, the gas mixture is cooled to separate the liquid ammonia (storage at a pressure of 8 - 16 bar and a temperature of -29 to -31 °C). Byproducts are used in other manufacturing process based on hydrogen; nitrogen, carbon dioxide.

One employee was slightly injured at the site due to broken window fragments from the blast - no hospitalisation required).
No environmental damage as there were no releases of hazardous substances.
Deterioration of the synthesis gas supply pipe of the ammonia synthesis column.
Destruction of thermal insulation of other pipes in the fire area.
Destruction of the insulation of power lines and AMC instrumentation in the fire area.
Thermal and mechanical damage to heat exchangers and ammonia separators.
Destruction of two watch cabins for field operators in the proximity of the column.
Breaking the windows of the surrounding buildings (administrative building and building power generation).
The cost of damage on the site was estimated at more than 2M €.
The ammonia plant was closed for 2 years.
There were no off-site effects except for blast noise.

In this incidents, the preventing and mitigating safety measures in place plaid a critical role in minimising the consequences of the incident:
(1) The event highlighted the importance of the correct identification of accident scenarios in the Internal Emergency Plan and of the training exercises (an exercise with a similar scenario had been played in 2020).
(2) The existence of an Emergency Shutdown system proved its usefulness by stopping the flow of synthesis gas and isolating the portion of he pipe affected by the facture.
(3) Finally, Safety Operative Procedures for emergencies, including field operators training ensured well trained operators and a very good reaction time.
Nevertheless, the incident revealed also the following improvement needs:
(1) More focus on process safety (i.e. on the design, to ensure that material specifications fitting for all operating modes - including starting and shut down).
(2) Improved inspections / internal audit / maintenance of high-pressure components.
The following improvements needs emerged during the emergency management:
(3) More effective communication during emergency situations.
(4) The intervention of the external firefighters mode must be re-thought taking into account that in the worst-case scenario, drivers cannot drive the intervention vehicles using their heavy protective equipment on.
(5) The medical ambulance belonging to external firefighters should be equipped with compressed air breathing apparatus, because at the time of the explosion they went to the scene only with protective masks provided by the plant operator.
(6) The external firefighters must be equipped and trained with a device able to detect hazardous substance, to be able to carry out reconnaissance and measurements in several directions and establish risk areas in the direction of propagation of the toxic cloud.