Combustible flour dust
Grain handling is a high hazard environment. Perhaps one of the most dangerous hazards to consider is the combustibility of grain or flour dust…
It seems almost absurd to think that flour, the main ingredient in something as delicious as cakes, cookies and muffins can be a deadly explosive. The reality is that fire and explosions from grain or flour dust accumulations can be catastrophic, leading to serious injuries, employee deaths, and the obliteration of entire buildings.
Why? Grain dust is highly combustible and burns where airborne flour dust or enough accumulated surface dust comes into contact with an ignition source and explodes when confined.
Such explosions should be handled with ‘kid’ gloves as they pose unique dangers such as shifting grain, fatigued structures and hidden spot fires. As long as an ignition source remains, the possibility of another explosion remains. Even water sprayed by straight stream onto smouldering grain by fire fighters could dislodge and suspend dust and cause another explosion.
Flour is a starch and, like other carbohydrates, burns very easily. Its ability to spread in the air into a cloud adds to its explosiveness, meaning that the highest risk for explosions is in facilities where flour is heavily handled.
As such grain elevators have been identified as one of the most significant sources of grain dust fires and causes for concern by elevator and mill operators. December of 1977 saw a turning point from a regulatory standpoint in the US as five grain elevator explosions in seven days killed 59 people, injuring 48 others and resulting in millions of dollars of damage.
In a nutshell, proper employee training and instruction on safe work practices are imperative to the solution.
Fortunately dust explosions in the South African grain environment are not a commonplace occurrence due to local grain management’s focus on good house-keeping practices.
Still, the importance of safe, clean work practices cannot be understated. Primary and secondary explosions can be averted in enclosed environments, such as silos and grain storage areas, by keeping the handling of screenings, bran and dust to a minimum during any process of grain handling.
Grain Storage
Grain storage entails the management of an enclosed atmosphere environment. Aeration tempers, cools and even dries grain. As such, the poor temperature conductibility of grain, combined with little or no aeration, may result in the development of hot spots and high moisture zones in the middle of the upper and lower regions of stored grain.
The balance between the moisture content of grain, temperature and relative humidity is always an important consideration in the storage and aeration of grain. Insects and fungi also impact on this balance as they are living metabolically active organisms that release heat and moisture through respiration.
Also consider oilseeds’ limited ability to hold water in comparison with starchy seeds. Oilseeds overheating beyond 90 °C due to poor storage conditions feature the ability to spontaneously combust – as has previously been the case at a Roetan-silo with a sunflower seed bin. However starch-rich grain in South Africa has never spontaneously combusted although it has happened several times with oilseeds.
Employee Training
Employee training on fire prevention is tackled by experts through advocacy of the ‘fire triangle’ that features three legs: fuel, oxygen and heat. Removing any of the three legs could prevent a fire.
Similarly dust explosions add a fourth leg – confinement. Remove any one of the four sides to the ‘explosion square’ and an explosion can be prevented.
Grain dust serves as the fuel source of fires and explosions. Unfortunately it can be found, in varying amounts, in each load of grain delivered from the farm.
Dust concentrations in mills contribute to flammability and must be in suspension for an explosion to take place. Dust simply lying on the ground is therefore not seen as an immediate threat.
According to ‘Dust Explosions in Process Industries’ by Rolf K. Eckhoff dust particles need to be smaller than 0.1mm to combust, bearing in mind that a decrease in particle size contributes to an increase in explosion risk. The dust concentration range must also be between 40 grams and 4 000 grams per cubic meter. Dust moisture content also contributes as the drier the dust the more significant the risk of ignition.
Any open flame or spark can serve as a source of ignition. Great care therefore needs to be taken in terms of moving equipment to prevent any metal rubbing or scraping against other metal surfaces that could cause a spark and ignite dust.
Sparks from cigarettes or maintenance welding and cutting equipment are potentially deadly. Electrical malfunction, choked bucket elevators or even a slipping conveyor belt could all ignite dust. Even a lightning strike poses a fire threat that could give rise to a huge explosion.
The first side of the explosion square considers fuel control. The importance of each employee’s role in this activity should be emphasized significantly. Kick the process off by implementing a hazardous dust inspection, testing, housekeeping and control program.
Cleaning Methods
Proper housekeeping is non-negotiable. Regular cleaning frequencies must be put in place for floors and horizontal surfaces to minimize dust accumulations within operating areas of the facility. Cleaning methods that do not generate dust clouds should be employed by using only vacuum cleaners approved for dust collection. Relief valves should be located away from dust deposits.
Dust-containing systems, such as ducts and dust collectors, should be designed without leakages and in a way that fugitive dusts are not allowed to accumulate in the work area. Optimal equipment function should be ensured and any leaking dust be rectified immediately.
Heat is the second side of the explosion square, and is often seen as the key source of ignition and thus imperative to explosion prevention.
Specific activities that may contribute to control of ignition sources include:
• Instituting a welding, cutting and burning permit program,
• Avoiding the storage of any combustible or flammable materials in the mill or elevator,
• Control of static electricity, including bonding of equipment to ground,
• Absolutely no smoking, open flames or sparks,
• Using only correctly classified equipment,
• Separating heated surfaces and heated systems from dusts,
• Proper selection and specification of industrial trucks,
• Use of appropriate electrical equipment and wiring methods, and
• The implementation of a scheduled equipment preventative maintenance program.
Oxygen – forming the third side of the square – is ever present and a major contributor to combustibility. Although generally beyond employee control it remains vital that all fire and explosion suppression systems that can chemically control oxygen be tested and maintained regularly.
Confinement makes up the fourth and final side of the explosion square. Ignition must take place within a confined space for an explosion to occur and is the result of the tremendous pressures built up and released instantaneously. An explosion cannot take place in open spaces however dangerous flash fires may result.
As such two major types of dust explosions have been identified – primary and secondary explosions. The first is initiated by a source of ignition, such as dust; whereas the second occurs from a blast wave caused by the primary explosion that spreads and causes dust layers in other areas to become suspended in air.
The problem is that a highly flammable dust suspension arising from the primary explosion can be ignited by the primary dust flame within milliseconds of each other.
Although confinement is generally also considered beyond the control of employees, the past couple of years have seen facilities being modified to employ explosion relief venting. Improper repair/maintenance of damaged or worn venting may prevent such devices from functioning properly and should also be checked regularly.
It is suggested that dust collectors not be located inside buildings and rooms, and where it does, they should have explosion relief venting distributed over exterior buildings walls that is directed away from employees.
OSHA further proposes that facilities have isolation devices to prevent deflagration propagation between pieces of equipment connected by ductwork, and that dust collector systems have spark detection and explosion/deflagration suppression systems.
Separator devices can also contribute to the timeous detection and removal of foreign materials capable of igniting combustible dusts.
Again, the proper training to be aware of explosion relief devices, their function and the importance of regular and proper maintenance is imperative.
Why wait for an earth-shattering event before observing, analysing, measuring, reporting and implementing a preventative safety and health management system for your company?
