Explosion in a storage tank of a chemical plant

A deflagration occurred inside the tank at the start of grinding operations. The tank had contained sulphuric acid. All preparatory procedures had been properly carried out: neutralisation of residual sludge with carbonate, measurement of oxygen and inflammable gases at various points.

The consequences were one dead and 2 severely injured. The tank was partially destroyed.

INITIATING cause was the ignition of undetected hydrogen in a dead zone of the reactor.
Approximately 100 g of hydrogen were in n a dead zone of the tank, which had not been checked with detectors.
The hydrogen had been produced by the corrosion of the iron of the tank by the sulphuric acid.

ROOT CAUSE could be related to the inadequacy of the explosivity measurements.

sulphuric acid storage tank.

Grinding operations were starting.

The consequences were one dead and 2 severely injured. The tank was partially destroyed.

The logic recommendations which can be issued on the basis of the known facts are:
1. To Improve flammable gases concentration measurements (more locations, more accuracy)
2. To use inert gas (nitrogen) before maintenance actions