Fire and explosions in the process industries are relatively common, particularly within plants that handle combustible powdered materials.
In 2015 four people were killed after an explosion at the Bosley Mill wood treatment facility in Macclesfield.
Following the disaster, Steve Smith, HSE head of operations for the North West, said: “As the investigation progresses, HSE inspectors are working with the company to identify potential and actual safety issues around the site. HSE specialists continue to assist with the collection of evidence in order to try to establish exactly what happened.”
The vast majority of plant owners are diligent, but if you have not had enough training you will err on the safe side and over-zone, which adds considerable cost
Mike Bradley, director, Wolfson Centre for Bulk Solids Handling Technology
While it remains unclear what caused the explosion, Mike Bradley, director at the Wolfson Centre for Bulk Solids Handling Technology, suspects a catastrophic build-up of dust was to blame.
Fortunately, most fires and explosions do not end in the loss of life. Bradley says this is mostly because plants have adequate protection – typically explosion venting equipment.
According to explosion protection manufacturer Fike, venting technology works by offering over-pressure protection from industrial explosions by providing a planned pathway for the expanding gases to escape.
The right approach
Bradley says in order to protect the pieces of equipment where explosions are most likely to occur, industry follows guidance set out in the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) and the ATEX Directive.
This information is used to help contextualise the likelihood of having explosive conditions in various parts of a plant, as well as helping plant owners select equipment that is suitably rated for its inability to provide an ignition source.
Bradley says: “With DSEAR you are essentially dividing the plant into four zones: 2.0, 2.1, 2.2 and un-zoned, where you can realistically say you won’t get an explosive concentration of dust.
“An explosive concentration is actually a very thick concentration. You couldn’t breathe in it, let alone see in it – this would be inside pieces of equipment, not on the shop floor. If you have an explosive atmosphere outside the equipment, you have a serious environmental problem as well.”
In some cases, specific explosion testing of materials is needed where good test data is not available
Bill Treddenick, operations director, Lorien Engineering Solutions
According to DSEAR, the highest risk area, zone 2.0, is where you would frequently have explosive concentrations. Zone 2.1 is considered “relatively infrequent”. Bradley says zone 2.1 would typically be an area where a sack of dusty material – such as icing sugar – is loaded into a mixer.
Meanwhile, explosive conditions would only occur in zone 2.2 under “unusual circumstances”, Bradley says, while in un-zoned areas plant owners are free to use the least protective safety equipment.
Bradley says: “DSEAR is used to set the zones, while ATEX is the piece of legislation that outlines the requirements for pieces of equipment used in each type of zone. For example, category 1 equipment has a high level of protection against ignition, which makes it very expensive. Even to do paperwork for category 1 equipment would cost in the region of £50,000.”
Unfortunately, for many process plant owners in the UK, over-zoning is a major issue. Bradley says that in Europe this is not as much of an issue because engineers on the continent are typically better trained.
Bradley says: “We cut a lot of corners in the UK in terms of training. There is also a culture of job enlargement where you get nurses doing the job of doctors, for example. The vast majority of plant owners are diligent, but if you have not had enough training you will err on the safe side and over-zone, which adds considerable cost.”
However, Bradley is not suggesting plant owners should start under-zoning their facilities, but argues that saving money by avoiding over-zoning is very important.
“There is no use having a safe plant if it puts you out of business,” he says.
Reviewing the situation
For new-to-service plants, adherence to fire and explosion regulations is typically achieved through the normal design process, which includes a means to identify hazards and to review and mitigate them.
Bill Treddenick, operations director at Lorien Engineering Solutions [pictured], says it is important to understand when that review occurs.
Treddenick says: “With good planning and foresight, drawn largely from experience, this can occur early in the design process, at a point when DSEAR/ATEX compliance verification can proactively lead and inform the design. Should the compliance review occur late in the design process, or even during or after construction, compliance can still be achieved, but this will usually be more costly and will be disruptive to the planned start-up programme.”
Of course, building safety into the fabric of a plant does not guarantee a fire or explosion will never take place. Improving safety and mitigating risks remains vital throughout the entire lifecycle of a plant, Treddenick adds.
He says: “For legacy plant, we first recommend that a gap review is undertaken by a competent engineer. Once the ‘out of compliance’ plant is identified in principle, then a survey of the vulnerable plant may be completed, and a ‘Basis of Safety’ report written, together with a Hazardous Area Classification and recommended further actions. In some cases, specific explosion testing of materials is needed where good test data is not available.”
Bradley adds that it is also vitally important to keep a plant clean. As a rule of thumb, if you can write your name in dust you have a catastrophe waiting to happen, Bradley says.
He says: “If you handle solids you will get some dust but you have to keep the plant clean and invest money in cleaning. It might not be very glamorous work but it is vitally important. It’s good business to keep people safe.”
In September, oil and gas risk management software provider DNV GL said it was conducting research with explosion and fire modelling business Gexcon, investigating dust explosions at a larger scale.
The project combines Gexcon’s capability in dust explosion modelling with large-scale experimental capabilities at DNV GL’s Spadeadam Research and Testing centre, Cumbria.
DNV GL safety engineer Clara Huéscar says: “Though considerable research has been done on dust explosions, experimental work has taken place in small enclosures of less than 250 m3. The increasing use of biomass, particularly wood material, in the power generation industry is raising safety concerns around the potential of dust explosions occurring.”
The aim of the research is to produce new guidelines for industry to mitigate explosive events. DNV GL says the guidance will result in more reliable and cost effective safety designs and “a better understanding of the extent of the hazards in large volumes”.
DNV GL also announced in October that it had acquired ComputIT, a specialist computational fluid dynamics (CFD) firm that develops technology for simulating fires, explosions and gas dispersion for plants pipelines and structures.
ComputIT’s KFX (Kameleon FireEx) suite of software tools is designed as an advanced code for flares, gas dispersion, and for fire and explosion simulation, and complements DNV GL’s risk and consequences application technology, the oil and gas firm says.
ComputIT CEO Trond Evanger adds: “In the future, production has to be at lower cost, but at the same time one has to maintain a high safety level. This calls for optimised technical solutions, and our background on physical modelling and utilisation of digital capabilities, together with the specialist competence of DNV GL, will be an excellent platform for developing the future industry.”