Explosion in an ammonia plant

A mechanical failure in a gasification reactor caused its sudden depressurisation. The consequences were the projection of tiles of the refractory lining (the inner coating of the reactor), and the release of feed substances, partly processed substances and reaction products through the top of the reactor. Release of gas through the flange joint of the burner of the reactor, caused the rupture of the fastening bolts.
At the same time, a very loud noise was heard accompanied by the emission of high flames.

The INITIATING CAUSE was a leakage through the flange joint of the burner. Due to the expansion of the nearest bolts, the amount of leaking gases increased, causing an over-stress and deformation of these bolts and the failure of the other ones by fracture or shear cutting.

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(1): the flange stud-bolts of the valve have been replaced by others made of a material not equivalent to the one originally specified;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

coal gasification reactor

The eMARS report is a confusing in reporting the quantities of flammable materials present in the reactor and the fraction which was involved in the incident. It appears that a total of 1600 metric tons of feed and process gases and liquid were present in the reactor. Regarding the gases, the following escaped:
O2: 46 t
Oil residues: 42 t
H2: 26 t
CO: 9 t
CO2: 11 t
H2S: 9 t
These values represent also the total of the inventory, i.e. all gases were fully released and burned in the initial fire.

The reformer involved in this event used oil residues as feedstock. Due to the much higher carbon-to-oxygen ratio and the risk of coking, this type of plants does not adopt the methane steam reformer solution, based on burners heating tubes containing reactant and a catalyst. reactant and a catalyst. Withing a Gasification unit, feedstock reacts with pure oxygen in an open refractory-lined chamber. This explains the high content of oxygen and oil among the substance participating to the fire.
In the eMARS report, apparently the terms "firebox", “burner” and “combustion chamber” are used to indicate the same component: the furnace section of the gasifier.

One person was slightly injured by the projection of tiles, did not have to be hospitalised. Similarly, one person was slightly injured during the fire-fighting operation without hospitalisation.
Due to the fire started by the projection of the incandescent refractory tiles and by fuel oil, various feed pipes were damaged (oxygen, vapour, fuel-oil and refrigerated water), as well as some elements of the upper reactor structure. Small secondary fires developed in adjacent areas, inside a radius of approx. 300 - 350 m, which reaced the fence of the plant. Approximately 40 inhabitants were potential at risk.
No ecological damage and no toxic released occurred.

No toxic substance release and not environmental damage.

The emergency shut down system of the installation was immediately activated allowing to isolate and depressurise the unit involved in the accident and to shut down all production units, maintaining only the production of vapour and nitrogen which guarantee the safety of the units through inerting.
Fire-fighting was executed by the security service personnel and workers of the installation.
Offsite, the traffic around the premises was limited.

The following monitoring, control and preventing improvements were adopted:
1. Set up of a daily control routine of the explosivity in the reactor head of both reactors (he head are at the reactor exit).
2. installation of a system for the detection of leaks at the flanges of the reactors, based on temperature measurement in the flanges, connected to an alarm system.
3. Setting of the release value (set point) of the temperature (thermal) alarm of the cooling water at the reactor exit at 5°C higher than normal operation temperature (45°C), and not 25 C more as it was before.
4. Replacement of the bolts in both reactors to respect the technical specification specifications.
5. The reactor will be shut down immediately in case of leakage.
6. Regular maintenance of the burner.
7. Explosibility measurements every single hour in the reactor heads the first 12 hours following the re-start of the reactors.
9. Installation of a remote temperature monitoring system at each of the 32 probe points of the reactor, controlled remotely from the control room.
10. Setting of the of the alarm value (threshold) in the 32 points of the reactor dome at 240°C, instead of the previous 300°C.
11. Recording of the alarm events in the data-logger.
12. Set up of a verification procedure of all temperature measurement points located externally to the reactor, including performance tests during the start-up of the reactor.