Explosion caused by unintended hydrogen production at a metallurgy plan

At a soaking facility of an industrial plant in the metallurgy sector, the pipeline between a supply tank and a feed pump became clogged and the autoclave only received sporadic feed. Problems had been experienced with the feed over several days. The pipeline was flushed with water to little avail. Because no feed was received in the autoclave, no oxygen could enter to maintain the pressure of the autoclave normally.
The blockage was quickly cleared, as a result of which a large quantity of feed, and with it solids that were contained in the blockage, entered the autoclave. The autoclave had been fed with anolyte and sulphuric acid during the entire duration of the blockage. Additionally, the water used for flushing also entered into the autoclave.

A powerful explosion occurred in an autoclave. The autoclave internals and five of a total of seven mixers sustained severe damage. The internals showed signs of warping, suggesting a sudden pressure increase inside the autoclave. Similarly, the damage to and dislocation of the entire section 1 cover, gear system and motor suggest that major forces have been involved. The explosion did not damage the externals of the pressure equipment.

The INITIATING EVENT was accidental hydrogen generation and consequent explosion inside the autoclave.

Hydrogen generation is known to occur from sulphuric acid when stagnant. An alternative explanation for the hydrogen generation could be the entry of a metal-containing feed into the autoclave. The feed with higher-than-normal metal content was due to several disrupting factors.

The ROOT CAUSE could be identified an inadequate design of the feed to the autoclave, and failure in realising the increasing hazards and risks produced by the actions aiming at solving the blockage, and int taking the required preventing measures.

Autoclave, metals treatment

Attempt to remove the flow blockage had been ongoing since days.

DESCRIPTION OF THE CHEMICAL REACTION
between metals (M) and hydrogen (H2): M + H2SO4 -> MSO4 + H2

DESCRIPTION OF THE PROCESS
Tthis event misses detailed information on the ongoing industrial process.
In general, in metal processing, sulfuric acid is used for pickling (removing rust and scale), and in processes like sulfuric acid anodizing which creates a protective oxide layer on the metal, or pickling, used to clean and prepare metal surfaces by dissolving rust and scale before processes like hot-dip galvanizing or electroplating.

The autoclave internals and five of a total of seven mixers sustained severe damage. The internals showed signs of warping, suggesting a sudden pressure increase inside the autoclave. Similarly, the damage to and dislocation of the entire section 1 cover, gear system and motor suggest that major forces have been involved. The explosion did not damage the externals of the pressure equipment.

When a metal enters in contact with sulphuric acid, one of the reaction products is hydrogen. If the reaction is rapid and severe, a large quantity of hydrogen gas can accumulate in the equipment and quickly reach the lower explosive limit. The amount of energy required to ignite the gas mixture is small and the result is an explosion that will damage equipment.
The related hazards are known to industry. This link provides background knowledge, and the improvement measures adopted for this type of hazard along the years:
http://www.sulphuric-acid.com/sulphuric-acid-on-the-web/Hydrogen-Safety.htm.
Single cases are collected and analysed by this report (Friedman in Sulphur 2015, 31st Conference): http://www.sulphuric-acid.com/sulphuric-acid-on-the-web/Acid%20Engineering%20&%20Consulting%20Inc.%20-%20Leonard%20J.%20Friedman.pdf

HIAD_209 reports the explosion on a sulphuric acid tank.