Explosion from accidental hydrogen production

This incident occurred at a company producing building elements. The process implies the production of a small quantity of hydrogen, which is usually dispersed into the environment without consequences.
A plant operator was preparing the mix of substances to produce the material for making the blocks, by blending the various components in a mixing chamber. However, the reaction did not develop as expected. When the operator opened the inspection hatch, an explosion occurred.

DETAILED ACCIDENTAL SEQUENCE
(1) During a break in production the operator decided to wash out the mixer, as prescribed by work instructions. This implied to put the equipment in manual mode and to empty out the water hopper into the pre-weigh batcher. After that, the operator forgot to refill the water and when he switched the controls back to automatic mode, the software did not signalled that the water container was empty (as the water hopper had been emptied manually, the digital readout still read ‘full’). The water was an ingredient of the mix but was also cooling the mixture.
(2) When resuming the operation of mixing, the operator noticed an “excessive mix time” alarm on the mixer panel. This alarm ceased the process and dumped the mixer contents. However, the operator noticed an inconsistency in the mixed material discharge, which was a sludge rather than a fluid concrete. He interpreted this as a blockage and followed the written procedure for dealing with this by taking manual control and introducing as much water as possible to thin and cool the mix by opening the water hopper valves. During this, a second alarm went off reading “mix temperature invalid”, indicating that the mix temperature was not falling within the recognised range of 0 - 99 °C.
(3) The operator went up to the mixer platform to investigate the reasons for the alarms and mix inconsistency. He lifted the mixer inspection hatch. He took an inspection lamp (later revealed to be faulty), the nearest one to hand, to illuminate the interior of the mixer. This caused an explosion which injured the operator, despite he was wearing the correct personal protection equipment (PPE).

The INTIATING CAUSE was the accidental production of hydrogen because insufficient water was added to the batch. This resulted in a rapid temperature increase which induced a higher production of hydrogen than the normal negligible quantity.
The lower water quantity was a mishap of the operator, who manually emptied the mixer and neglected to refill the water; when he switched the controls back to automatic, and the programme software did not highlight that the water container was empty.

Despite the presence of functioning local exhaust ventilation I the mixer, , the hydrogen gas was ignited when the hatch was opened. The most likely IGNITION SOURCE was the faulty lamp - all other lighting in the vicinity was safety, explosion-proof lighting.

This could have been related to a human mistake. However, the post-incident investigation concluded that the “operator was acting in accordance with his training and instruction and the company's written safe system of work”. Therefore, the ROOT CAUSE lies predominantly in shortcoming of the operative procedure and of the equipment design, particularly the automation part.

Building block mixer

DESCRIPTION OF THE PROCESS
The company produce building blocks called autoclaved aerated concrete (AAC). It is also sometimes referred to as aerated concrete building blocks or insulating building blocks.
(1) Sand and water are first mixed to form a slurry.
(2) Anhydrite, lime, cement and PFA powders are then dispensed into the mix.
(3) Glycol-coated aluminium powder is added in the form of a slurry which has been mixed elsewhere and maintained in a hopper and kept cool. The aluminium is added in the last few seconds before the mix is dropped into the car.
All powders are pre-weighed and dosed automatically via a computer-controlled system. The mix was then discharged into a car, from which moulds are filled.
Aluminium and water produce a small amount of hydrogen gas which usually disperses from the car into the surrounding ventilated area and out through roof vents.

The explosion resulted in temporary injuries including loss of sight, burns and cuts.

The operator was wearing safety glasses and a hard hat, which were blown off by the blast.

The company has taken a number of measures to prevent a reoccurrence including
provision of intrinsically safe torches,
introduction of daily checks of the vent valve,
minor modification to local exhaust ventilation and increased venting throughout the mixing process,
lab testing by the aluminium supplier to evaluate system safety with regard to hydrogen generation for all reaction conditions and quantities added (results were system deemed safe),
reprogramming/development of the software to improve both the safety of the operation and operator understanding of warning alarms.