Overview
A solution of potassium carbonate was being drawn off to an inventory tank for a turnaround/shutdown maintenance activity at a refinery's hydrogen production unit. On the day of the incident, the solution level in the tower wasn't checked as it should have been, which resulted in hydrogen gas flowing back into the tank until the increased pressure caused the tank to explode. The direct cause of the incident was the workers neglecting to check the solution level in the tower. It is not known whether the potential for backflow of hydrogen gas into the inventory tank was understood beforehand or not.
Incident Synopsis
An explosion occurred due to unexpected backflow of hydrogen gas while a solution of potassium carbonate was being drawn off to an inventory tank for a turnaround/shutdown maintenance activity at a refinery hydrogen production unit.
When the water solution in an absorption tower has carbonic acid gas drawn off, the water level of the solution drops. On the day of this incident, the level drop of the water solution was ignored, causing the pump to cavitate. Since the pump and a transfer valve were not immediately shut down, hydrogen in the absorption tower flowed back into the tank. The force of the flow blew scales and coarse particulates off the tank walls, which caused sparking and ignition of the hydrogen gas
The absorption tower for carbonic acid gas, where residual hydrogen gas remains, was not purged with nitrogen gas before turnaround/shutdown maintenance was initialized due to a mistake in the standard operating procedure. Another problem was the mishandling of pump cavitation by the workers. When the pump started to cavitate, it should have been stopped immediately. It is not known why workers did not pay attention to this procedure.
On reading the operations manual, workers reconfirmed that the tower level should be checked, the draw-off adjusting valve should be slightly opened, and the block valve should be throttled beforehand.
The physical damage to the facility consisted of a deformed tank roof and upper side wall, as well as damage to the windows of nine nearby buildings, their contents, and 15 vehicles.
Hazards should be eliminated to the extent possible rather than depending on workers to follow an operations manual.
Potassium carbonate draw-off operations should be improved, and the operations manual should be revised. The manual should be reviewed by workers before maintenance activities are started. Training on mandatory compliance with the operations manual should be carried out for all staff involved in hydrogen plant operations and maintenance.