Explosion in a chemical storage unit

Two workers were asked to change the piping and support of a feed line into an intermediate storage vessel for hydrochloric acid (HCl). An explosion occurred when they opened the gas line and started grinding on the support,
The flame propagated back into the tank, causing a second explosion inside. The explosion was caused by the unexpected presence of a considerable amount of hydrogen in the tank.

According tot the eMARS and ARIA report, the hydrogen was formed during a reaction between metal powder traces and the hydrochloric acid, and must have accumulated over a period of two months. The metal powder was coming from the present in the supply line of the acid into the storage tank, as result of an incomplete reduction reaction in an upstream reactor. In that reactor, the metal powder was used to remove traces of metal oxides from the acid. Since 2009, the reactor was being operated at a lower temperature than the process was designed for, resulting in a lower reaction rate. Due to this lower reaction rate, traces of unreacted metal powder were present in the feed sent from the reactor to the intermediate storage vessel. In the storage vessel, the reaction continued, producing small amounts of hydrogen, which accumulated in the top. When the piping on top was opened, some hydrogen escaped and was ignited by the sparks of the grinding.

INITIATING cause of the explosion was the accidental formation and later ignition of hydrogen in a chemical storage tank.

As explained more in details in the lesson learned, the reason for the formation of hydrogen was a sequence of operative steps taking place before the storage along the process route, having major cause the decision to operate at a temperature lower than the design value.
The risk of hydrogen formation had been considered in the reactor, which was equipped with a ventilation system, designed to remove the hydrogen generated in the reduction reaction. The storage vessel did not, probably because a presence of metal particles there had not been considered possible.
This suggest a lack of process control, inadequate risk assessment and consequently failing of putting in place the correct safeguards.

metals processing , hydrochloric acid storage tank, connecting pipes

The event took place during maintenance works.
Since years the production process was operating at a lower temperature than the design one.

The tank ripped open and was catapulted into the air, killing two workers.

According to the reports referenced, the intermediate storage vessel was never identified during risk assessment studies of the installation. The influence of the operating temperature on the reaction process was never documented, and no safe operating window for the reactor temperature was determined. Despite the existence of Management of Change procedures, no risk assessment was made for the change in operating temperature of the reactor in 2009.
A work permit was granted for the execution of the piping modification. Since no flammable mixtures were expected, no explosion prevention measures were required and taken.

According to the eMARS report, risk assessment must be performed by a multidisciplinary team (in this particular case, at least deep knowledge of chemical process and mechanical installation would be a requirements).
Management of Change procedures must also be applied to process changes, such as changes in operating temperature. The safe operating window of process parameters (pressure, temperature, flow rate...) must be determined and documented.