Hydrogen fire at a chemical plant

The event affected a unit producing ferric chloride (FeCl2) in a SEVESO-classified chemical plant. A fire broke out in the acid etching tank of the unit. In this tank a reaction between iron metal and hydrochloric acid in solution was carried out, having as reaction product ferrous chloride and hydrogen. The hydrogen generated was swept away by a large supply air fan to maintain the gaseous sky of the tank below the low explosion/flammability level of hydrogen in air.

A few days after the accident, the operator opened and analysed the tank to determine the origin of the fire. The reason for the fire was found to be a slow combustion of hydrogen, initiated during the loading of metal. Despite the air circulation, hydrogen accumulated at the bottom of the tank due to its overfilling. The fire then spread outside the tank through the roof opening caused by the degradation of the hydrogen detector and by melting the blast doors. The fire was then alimented by the plastic components of the equipment and the surrounding piping, as well as by the oil from the hydraulic pistons of the roof door of the tank.

The INITIATING CAUSE was the development of hydrogen – air flammable mixture inside the tank, which ignited by a non-reported source.
The hydrogen enrichment was caused by the overloading of the tank with iron, which created internal zones not affected by the ventilation.

Considering also the corrective actions described in the ARIA report, the ROOT CAUSE was probably inadequate safety design and operation.

reaction tank, production of iron chloride (FeCl2)

DESCRIPTION OF THE PROCESS
The stoichiometric reaction at the basis of the process is: Fe + 2 HCl → FeCl2 + H2

No injuries. The unit restarted 6 months after the accident.
Damage to the tank upper parts, hydrogen detector and doors

23:40 The Plant safety emergency plan is initiated .
23:53 The fire is reducing
00:50 Despite a small residual still persisting, the fire is extinguished and the reaction unit stopped.
00:45 The emergency plan is called off.
01:25 The leak is sopped.

The accident suggests that the operative procedure related to the loading of the materials and the preventive measures (hydrogen detection) were inadequate.

According to the ARIA report (see references), the operator updates the risk analysis of this unit and reinforces controls on the level of scrap metal in the tank and analyses of hydrochloric acid.
It also added safety features on the low air sweep rate and high temperature in the tank and lowers the alarm threshold of the hydrogen analyser.