With all the powerful equipment and dangerous substances to be found in process engineering plants, it’s hard to believe that a critical factor in major fires and explosions can be something routinely found in homes and offices.
Dust, when airborne, can turn a small fire into a catastrophic explosion. A report by the US Chemical Safety and Hazard Investigation Board found that a 2008 incident at an Imperial Sugar refinery in Georgia killed 14 people and injured 38 more when an ‘explosion was fuelled by massive accumulations of combustible sugar dust throughout the packaging building’.
Closer to home, if further back in time, the Health and Safety Executive declared that an explosion at a General Foods plant in Banbury in 1981 – which injured nine people – came after a holding bin failed, releasing custard powder that ‘ignited as a dust cloud within the building’.
Many ingredients have the potential to become combustible. Risk is present when the combustible material comes into contact with potential ignition sources
Jon Barfield, compliance consultant, Lorien Engineering Solutions
These are not isolated incidents. Data cited by the official magazine of the International Institute of Risk and Safety Management suggests that, at the start of this century, a dust explosion occurred on average once a day across Europe. A look at kst values – used to measure ferocity of an explosion – shows how serious these can be.
“A dust explosion has a kst of 200 compared to 50 in the case of a methane explosion,” says Mike Bradley, director of The Wolfson Centre for Bulk Solids Handling Technology at the University of Greenwich. “There is more fuel per volume in dust than in gas. Wood powder doesn’t displace oxygen.”
The logic can be understood with reference to the fire triangle, and indeed the explosion pentagon. These say that for fire to occur, three things are generally needed: fuel, oxygen and spark. To convert this into to an explosion, you only need add dispersion of the fuel and some form of containment.
Breaking the science down into its component parts like this gives a stark insight into how easy it is to create the conditions for disastrous incidents in process plants.
“Gases, vapours, mists and dusts can all form explosive atmospheres with air,” says Jon Barfield, compliance consultant at Lorien Engineering Solutions. “With this in mind, many ingredients have the potential to become combustible. Risk is present when the combustible material comes into contact with potential ignition sources.”
When I talk about dust explosions I say, above all else, keep the place clean. You don’t need to be an expert to understand that
Mike Bradley, director, Wolfson Centre for Bulk Solids Handling Technology
These too are frighteningly easy to come by. “There are a number of possible ignition sources that can cause an incident during manufacturing, such as hot equipment surfaces, mechanically generated sparks, build-up of static electricity and unsuitable electrical components.”
Fire suddenly seems almost inevitable, and then all you need is to get the flammable dust or vapour moving within the four walls of the plant and you are in line for an explosion.
“If there are dust layers present in an area, a small fire or explosion can cause that dust to become airborne,” warns Barfield. “Subsequently the dust cloud itself can ignite, causing a secondary explosion that can be many times the size and severity of the primary explosion.”
So, with the risk of fire and explosion so high in a process manufacturing environment, and the consequences potentially so dire, the industry has focused on methods of control or mitigation.
Building on the 19th century Davy lamp used by miners, early process explosion safety equipment started with ways of caging a fire or explosion if it did occur within a piece of machinery, preventing it from reaching the main factory and growing in scale.
“Flame-proof metal equipment was developed to allow gas in but not let a flame out,” says Keith Plumb, associate consultant at BPE. “But this was very costly and physically very heavy.
“Since then we’ve created various ways of sealing things with sand or pressure. Higher-powered machinery will be sealed up to stop flammable material getting in.”
Of course, when you’re using the machinery as part of a production line, you often need flammable material to be entering as part of the process.
“In the oil and gas sector there are obvious and enormous risks,” says Plumb. “But even in other industries you get less obvious but still flammable materials such as natural gas from the gas distribution system for power; alcohol to extract desired elements from a natural material; chemicals for reactions. Flammable materials can even be used in cleaning the plant.”
Continuing around the fire triangle, working in a true vacuum would theoretically eliminate oxygen but this is practically very hard to achieve and maintain. There are also other reactor gases such as chlorine.
Dispersion, or movement of dust, fine solid particles or gases, is also highly difficult to prevent, especially when a minor explosion can itself do the job by creating a pressure wave lifting all this flammable material into harm’s way. Containment is often something of a given as well within a manufacturing environment, meaning we are left with the need to remove the source of ignition.
One method that has come to the fore is using 24-volt instruments with such low energy requirements that an electrical spark becomes impossible. This has its merits but, unfortunately, electricity is just one of many potential means by which a spark can be created – one of 13 ways according to Plumb, including mechanical friction from faulty or overworked components such as bearings.
With such a maze of possibilities to negotiate, standards and legislation come into play to ensure best practice fire and explosion control. Bradley [pictured] emphasises too the importance of the Dangerous Substances and Explosive Atmospheres Regulations 2002.
“These stipulate you should do a specific risk assessment looking at your equipment and environment and ascribe to each area a zone according to the likelihood of dust being present in an explosible concentration,” he says. “Then you must take steps to ensure equipment you use in each zone is certified to be used in that way.”
Super-high risk environments, where flammable materials will often be present, require machinery that will not produce an ignition source even during certain failure events. “It basically has to have very little energy in it,” says Bradley.
He adds that the chance of a fuel getting in the air in quantities that could cause an explosion in staffed parts of a process plant should be eradicated to the best of human foresight.
“If you have dust on the floor you can’t say you’ve achieved that,” says Bradley. “As a rule of thumb 1mm of dust is enough for a secondary explosion. You should not be able to write your name in the dust anywhere in the factory. You should not be able to see footprints.”
So after all the technological advance, the new methods and the standards and laws, a key way to control fire and explosion risk is to put a vacuum cleaner round.
“It is not uncommon to see plants not cleaning effectively,” says Bradley. “We sometimes see there is an accident waiting to happen.”
The issue is often that while manufacturing managers may focus on technology and processes they may be culturally blind to the huge danger of dust. “You can go for years without an explosion, and you don’t see the risk,” says Bradley. “Then when it happens it is a catastrophe.
“When I talk about dust explosions I say, above all else, keep the place clean,” he adds. “You don’t need to be an expert to understand that.”