Explosion in a cold rolling plant

Employees were trying to open the manhole cover (600 mm diameter) of a hydrofluoric acid tank with a grinder, when an explosion occurred.
The explosion was probably caused by the presence of hydrogen formed by the acid attack of the metal. At the moment of the accident, the tank was empty, but uncleaned, and separated from the filling and ventilation pipes.

According to the eMARS report, in the course of cleaning work on the hydrofluoric acid storage tank, the residual of concentrated hydrofluoric acid remaining in the tank was diluted with water and not completely neutralized with sodium hydroxide. Subsequently, the container was sealed, after which the cleaning work was interrupted for at least two days. During this time, an explosive atmosphere formed in the tank, as a result of the reaction of the dilute hydrofluoric acid with the iron material of the container which produced hydrogen. When the cleaning work resumed, the explosive mixture was ignited presumably by the sparking due to the work with an angle grinder.

The INITIATING cause was the ignition of the accidentally formed hydrogen insider the tank.

A possible root cause of the accident would be the lack of risk assessment related to the potential formation of explosive atmosphere in the tank. Inadequate neutralisation of the residual acid and the disconnection of the tank from the venting line were contributing causes.

hydrofluoric acid tank

DESCRIPTION OF THE SUBSTANCE INVOLVED
Hydrofluoric acid is a solution of hydrogen fluoride in water.

Two employees, thrown by the blast of the explosion were killed and two others injured. The tank cover perforated the roof of the neighbouring production hall located 60 m away.

The on-site firefighters evacuated the employees and secured the site.

When a metal enters in contact with hydrofluoric acid, one of the reaction products is hydrogen.
Fe + 2HF -> FeF2 + H2
If the reaction occurs in and enclosed space, such as the tank of this incident, hydrogen gas can accumulate in the equipment and reach the lower explosive limit when the equipment is open to air. The amount of energy required to ignite the gas mixture is small and the result is an explosion that will damage equipment.

Therefore, an adequate risk assessment needs to be performed when dealing with the acid, aiming at assessing the adequacy of measures such as purging of the tank before maintenance, availability of enough ventilation, detection of flammable gases, and safety training.

The corrective action adopted by the plant was to provide suction to prevent the formation of an explosive atmosphere inside the containers. Maintenance would then take place with the exhaust air switched on.