Sulphuric acid tanker explosion due to unintended hydrogen production

To repair a corroded flange on a 4500 litre sulphuric acid bowser outlet pipe, an angle grinder was being used to prepare the surface for welding.

Within a few seconds of starting work there was a bang and an eruption of white, or grey-yellow, smoke from the outlet pipe. The injured person (IP), who was angle grinding the pipe at the time, suffered deep burns to his right inner thigh. The IP was wearing shorts at the time of the incident.

It appears the tanker had been used exclusively for the movement of 77% Sulphuric Acid in the past. It had been open to air for about 3 or 4 weeks before the incident. The tank was opened at high and low level by opening pipe flanges.

A plausible INITIATING CAUSE for incident appeared to be the accidental formation of hydrogen inside the tank. The ignition source could have been heat, or sparks, from the angle grinder.
The ROOT CAUSE was related to the documented risk of hydrogen build-up within concentrated sulphuric acid vessels. Ingress of atmospheric moisture dilutes the acid and initiates acid attack on the steel. Hydrogen is liberated from this reaction and can accumulate to form a flammable atmosphere within the vessel.

Sulphuric acid tank, maintenance work

The accident occurred during repairing a corroded flange. A grinder was used to prepare the surface for welding.

DESCRIPTION OF THE ACCIDENTAL HYDROGEN FORMATION PROCESS
There is a documented risk of hydrogen build-up within enclosed, or poorly ventilated, concentrated sulphuric acid vessels. Ingress of atmospheric moisture dilutes the acid and initiates acid attack on the steel. Hydrogen is liberated from this reaction and can accumulate to form a flammable atmosphere within the vessel.

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The absence of any detailed forensic examination of the scene, and physical evidence, makes it impossible to be certain of the cause of the incident.
Since the lack of the required ventilation has been identified as possible root cause. a specific recommendations is made. For improved tank purging systems, and the introduction of a permit-to-work system.