Human error is the main cause of a recent explosion at an air separation plant (ASU) in China, a primary investigation has found.

The ASU in Yima, Henan province, exploded on 19th July, causing 15 casualties, 16 major injuries, many minor injuries and not to mention serious damages to properties on the site and the surroundings.

The primary investigation found the operator spotted a rise in oxygen level in the insulation of the cold box on 26th June, indicating there was a leak. But the operator failied to disclose the issue and continued production thinking it would be alright if the plant operated under monitoring.

A crack on the surface of the cold box was found on 12th July, but since the standby ASU was not ready for startup, the operator decided to take the risk and continue to operate the plant.

In the late afternoon of 19th July, the cold box exploded due to the amount of liquid oxygen trapped in the insulation (perlite). The evaporation of the liquid led to a rise of pressure that the cold box could not withhold anymore.

The cold box collapsed and fell onto a 500 m3 liquid oxygen storage tank. The tank was cracked and the liquid spilling out created an oxygen-rich atmosphere, leading to subsequent explosions.

The investigation has found the incident is clearly a negligence of safety operation regulations by the operator. The report is being circulated nationwide by the government and also by the gas industry so that ASU operators will be alerted to closely follow safety regulations.