Industrial Chemicals’ risk assessment ignored the explosive risk posed by hydrogen in vent gas, which was routed through the plant’s emergency scrubber at its control of major accident hazards (COMAH) regulated site in West Thurrock in Essex.
The gas ignited and triggered an explosion, destroying a tank and displacing a scrubber column. Two workers suffered minor injuries in the blast as the manufacturer was carrying out the final stage in a series of tests to produce hydrochloric acid in the plant’s new burner in 2013.
The HSE investigation found that the root cause of the explosion could be traced to the design, construction and commissioning of the second plant phase. The court was told that Industrial Chemicals had considered a potential, but unlikely, scenario where safety measures, such as controls on the burner or an existing scrubber, would result in the chlorine failing to ignite. This would result in a “chlorine breakthrough” where the element would go through the burner, potentially overwhelming the scrubber. It would then be released to the atmosphere at a height where it would not disperse before it presented a risk.
The HSE inspector commented that that Industrial Chemicals were focusing on the chlorine risk and to mitigate what they saw as a risk, they ran it through an additional, emergency scrubber because they felt it gave them an extra layer of protection … Unfortunately, what they didn’t do was recognise the presence or significance of hydrogen in the vent gas and the implications for routing that.
Hydrogen and chlorine are a very bad mixture in terms of their potential to explode and hydrogen has a very low energy required to ignite. It takes a lot less energy to make it go bang and, comparatively, you get a bigger bang compared to other elements like natural gas.
A DSEAR assessment for hydrogen would generally assume that because it’s an easy material to ignite, if it gets where it shouldn’t, it will find an ignition source. The explosion occurred when Industrial Chemicals was undertaking the first full, extended power run on the hydrochloric acid process. Over the preceding months, it had started with cold commissioning – running innocuous materials through the burner – before moving on to hot commissioning. Initially, this involved running a smaller electrical current in the burner for a short period before a second test running the burner at full capacity for a similar period.
HSE explained that Industrial Chemicals had failed to identify the composition of the vent gas and understand the hydrogen hazard. It’s a fundamental principle of the process that you should know what goes in and what comes out.
Industrial Chemicals, pleaded guilty to breaching reg 4 of the Control of Major Accident Hazards Regulations 1999. HSE recommended that the starting point for the fine should have been £2.4m with a range of £1.5m to 6m. However, the judge reflected on the fine level and decided to reduce the starting point to £2m. After considering the company’s early guilty plea, good safety record and mitigation, he reduced it to £1.2m.
