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 on 26 September 2013.
Chelmsford Crown Court heard that Industrial Chemicals had completed the first phase of its chlor alkali plant to make sodium hypochlorite (bleach) in 2011-2012. Construction on the plant's second phase -- to add hydrochloric acid capability -- had taken place in early 2013 and commissioning on the extension was nearing completion.
The court was told the first part of the production process is identical for sodium hypochlorite and hydrochloric acid: saturated brine (salt and water) is charged with an electrical current in an electrolzyer to produce hydrogen, chlorine and caustic soda. In the second part of the process, either chlorine and caustic soda are combined (with hydrogen vented up a stack) or chlorine and hydrogen are combined in a burner (with the caustic soda routed elsewhere) to produce bleach.
For hydrochloric acid, the process combines only hydrogen and chlorine. When the elements ignite, the reaction produces hydrogen chloride, which is dissolved in water to create the finished product. Oxygen is also present in the burner column and hydrogen bonds better with that element. To make sure that all the chlorine is used in the process, manufacturers use excess hydrogen. Extra hydrogen can be added or chlorine removed to produce the same result.
The Health and Safety Executive (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.
HSE inspector Andrew Saunders told IOSH Magazine: "They 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."
Saunders said that significant quantities of chlorine could only be present in the vent gas if other control measures had failed.
"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," he added. "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."
Saunders said that hydrogen risk assessments 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.
Saunders 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," he said.
He added that although the burner column was a standard piece of kit, the design and risk assessment had overlooked the fact that the additional component for the second phase -- the emergency scrubber -- was not. Although Industrial Chemicals had undertaken a HAZOP process, it had started at stage 3, missing critically important stages 1 and 2, he said.
"At stage 3, what you should have is a completed design and a design that is assumed to be safe and appropriate. At the stages before that, where you are looking at hazard identification or feed studies, it is at those earlier stages of the risk assessment, if they had had an appreciation of what the vent gas contained, they should have picked it up."
Saunders also said the manufacturer did not have a completed design because the route into the emergency scrubber wasn't added until a later date. At this stage, it failed to revisit its risk assessment and look at the implications of the changes.
Industrial Chemicals, pleaded guilty to breaching reg 4 of the Control of Major Accident Hazards Regulations 1999. The company was sentenced at Chelmsford Crown Court on 7 July (see box below for how the judge applied the sentencing guidelines).
Saunders said the defence and prosecution had disagreed over the likelihood of harm. The judge looked at the potential for harm from explosions, made a reference to the HSE's photographs and agreed that it was high rather than medium.
"A few moments before the explosion happened, one employee was taking a sample a matter of feet away from the seat of the explosion," said Saunders, who added that the company was very fortunate that the two workers only suffered minor injuries. One had a grazed knee from a breeze block, which dislodged from the control room wall. The second suffered a minor caustic burn caused by a drip from the plant moments after the explosion.
Saunders added 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.
After the explosion, the HSE served a COMAH prohibition notice. "Although it was damaged beyond use at that stage, we felt there needed to be a fundamental look at it before it was rebuilt and put back into operation," said Saunders. "They had submitted a safety report covering this plant prior to the incident which was rejected."
Industrial Chemicals has rebuilt the plant, which came online in 2014. The company has revised its approach to vent gas. "It no longer goes through the emergency scrubber. They've also moved the dedicated scrubber so that it sits at a higher level and, in the unlikely event of a chlorine breakthrough, you would get a release at a much higher level, which would dissipate before it caused anyone any problems. They also have measures in place to prevent hydrogen ignition from part of that vent gas."
Sentencing guidelines application
| Culpability: |
High |
| Seriousness of harm risked: |
Level A |
| Likelihood of harm: |
High |
| Harm category: |
1 |
| Number of workers exposed to risk: |
Four at the time |
| Significant cause of harm: |
No |
| Size of organisation: |
Large |
| Turnover: |
£100m approx. for Industrial Chemicals Group |
| Starting point: |
£2m |
| Mitigation: |
full co-operation, early guilty plea, good safety record, mitigation measures |
| Penalty: |
£1.2m plus £35,854 |