Explosion at a hydrogen production unit

The explosion occurred at the hydrogen producing unit of a refinery.
A solution of potassium carbonate was being drawn off from an absorption tower to a temporary storage tank for a turnaround shutdown maintenance.
The tank exploded, due to backflow of hydrogen into the tank. The level of the solution in the tower was not checked. When a water solution in an absorption tower for carbonic acid gas is drawn off, the water level of the solution drops. This drop was ignored and the pump cavitated. Since the pump and a transfer valve had not been shut down, hydrogen in the absorption tower flowed into the tank.
The hydrogen ignited in the tank, probably due to spark at the wall surface or the solution surface. The force of the flow blew off scales, coarse particulates, and solution in the tank.
SEQUENCE
On May 23rd , 1996: operation was stopped.
7:05 on 24th; the shift changed
7:30; a meeting about turnaround shutdown maintenance was held.
About 8:00; the meeting finished and operators in charge of solution draw-off read out and collated an operation manual for twenty minutes.
About 8:10; a potassium carbonate water solution started to be drawn off into an inventory tank. As a pump cavitation occurred, a delivery valve was adjusted, but the pump continued to cavitate, so the pump was stopped and draw-off work was also suspended.
About 8:15; the tank exploded. Its roof was scattered around the sites of neighbouring offices, and office buildings and vehicles were damaged by a blast.

The INITIATING CAUSE was the explosion of a tank due to the presence of a hydrogen explosive atmosphere.


The ROOT CAUSE was a series of wrong operations by the workers. They neglected to check the liquid level of the tower and did not handle properly when the pump started cavitating. It is unclear why they did not shut it down. Moreover, residual hydrogen in the absorption tower should have been purged with nitrogen before starting the shutdown.

The guidelines for shutdown were available but were not followed. This way, the complex shutdown operation depended exclusively on manual actions and on the capacity of personnel to understand the abnormal situation. The management and the plant design could have plaid a role in this.

potassion carbonate tank

The facility was being shut down for regular maintenance.

The tank roof and the upper part of the sidewall were deformed. The explosion damaged also windows of nine buildings, their contents, and 15 vehicles.

The emergency action was limited to stopping the pressurisation test and to de-pressurising the tank.

Complex operations, and especially those of maintenance, which are not executed frequently, should not depends exclusively on the correct performance of workers, without additional safeguards or automatic controls.

The operation of draw-off operations should be improved.
The procedural handbook should be reviewed and make clearer.
Training and education on compliance with the operation manual should be carried out.