Mix-up of hydrogen and oxygen in a gas production plant

This near-miss occurred at a company producing compressed gas bottles according to customers requests. When preparing a gas mixture containing oxygen and nitrogen, by mistake hydrogen was added, instead of nitrogen. The gas lines did not have clear labelling. The mistake was discovered during analysis, when a small gas sample (less than 5 ml) ignited during flashing.

The INITIATING CAUSE was a human error in connecting the wrong pipeline.
A contributing CAUSE was the lack of clear labelling of the pipelines gases.
The root CAUSE could be attributed to management, which did not consider the risk of gas mixing up and did not adopted preventing measures such as different connector for different (incompatible) gases.

compresed gas mixture bottle

The mixed quantities were small: 0.07 kg hydrogen and 0.33 kg oxygen.

The building was evacuated.
The bottles containing the potentially explosive mixture were blown up by the police.

The following measures were adopted:
(1) Clear marking of each gas pipeline , visible at working height
(2) Clear labelling of the filling valves with the respective gas type-specific colours and labels
(3) Installation of a protective flap as a mechanical locking of the filling valve
(4) Separate and gas type-specific connections