Explosion of hydrogen at a semiconductor plant

The incident occurred when cleaning the exhaust gas-processing equipment at a semiconductor factory.
The inside of the water scrubber was washed with hydrofluoric acid. This is a normal operation aiming at cleaning the internal surfaces from the material deposited during operation. This material consists usually in silicon oxide. However, in the case of the incident, it consisted of an external layer of oxide, covering metallic silicon. After the surface silicone oxide dissolves, the silicon and hydrogen fluoride react, generating hydrogen. The above became clear after the accident. Therefore, a backflow of air during the drainage of the cleaning fluid formed a combustible a hydrogen-air mixture, which ignited and exploded.
The water scrubber was damaged by the explosion. Two persons were exposed to the solution which was scattered and were slightly injured.

The INITIATING/direct cause was the accidental formation of hydrogen from an unexpected reaction.
The ignition source was probably an exothermal and explosive decomposition of compounds.
Contributing causes were:
(i) The operation management did not understand the process ongoing in the scrubber, and did not consider the possibility of unwanted reaction
(ii) Poor discipline and enforcement of procedure. The investigation revealed that the operators were supposed to put on protective gear before work, but they did not do so because they did not handle hydrofluoric acid directly. Although a operational standard was prepared, a manager who was not a chemical specialist changed the procedure.

scrubber

The usual procedure for the cleaning of the scrubber was to wash it with hydrofluoric acid. This is a normal operation aiming at cleaning the internal surfaces from the material deposited during operation, which normally consists in silicon oxide.

Two persons were exposed to the acid solution which was scattered around the place by the explosion and were slightly injured.
The water scrubber was damaged by the explosion.

The only emergency action was the rescue of the injured worker.

Changing the structure to prevent a reverse flow of air.
Installing a flow meter and conducting enough purging with nitrogen.
Not allowing operators to change operation standards by their own judgment.
Always wearing the prescribed protective gear.