Since the upper explosive limit is of little practical importance, data for this parameter is rarely available. The most violent explosions are produced when the proportion of oxygen present is not far removed from that which will result in complete combustion. The range of the explosive concentrations of a dust cloud is not simply a function of the chemical composition of the dust; the limits vary with the size and shape of the particles in the dust cloud.
Ignition of Dust Clouds Although mixtures of dust and air within the flammable range are capable of explosion, they will not explode unless they are ignited in some way. Once a source of ignition is presented to the flammable mixture, flame will propagate throughout the cloud. The mode of ignition of a dust cloud is typically a hot surface, an electrical spark or a mechanically generated frictional spark - see ignition sources.
The minimum condition necessary to initiate a dust explosion with certain modes of ignition can be measured and some results are listed below. Data is provided for comparison purposes only and must not be used for explosion protection design.
Note: Figures for explosion protection design should be taken from explosibility testing of indicative samples.
See dust explosion testing for details. Effects of a Dust Explosion The heat produced by the combustion of the dust particles in a dust explosion and any gases evolved will cause a rapid increase in pressure at the walls of the vessel containing the dust cloud. In factories it is the effect of this pressure wave on relatively weak items of plant and buildings which has caused the deaths and injuries to persons employed in handling materials giving rise to dust explosions.
Further, since the pressure wave produced by the explosion can cause further dust which may have accumulated in the plant or on internal surfaces of buildings to be thrown into suspension in air, additional fuel can be fed to the flame and a disastrous secondary explosion may follow - see photo left.
Additional consequences following a dust explosion pressure wave are: the fires that may have been started by the dust flame; the implosion effect on the plant and buildings as the pressure within these rapidly returns to normal; the compromise of emergency exit routes and emergency lighting. Because of the operating environment in grain handling facilities, the potential hazard of combustible dust can never be completely eliminated.
Moving grain generates dry dust and that grain dust is highly combustible. Sparks from friction, static electricity, rubbing pulleys, and hot work are just a few of the ignition sources potentially present within the grain handling environment.
Most grain handling and processing systems that include bucket elevators, transfer bins, hammermills, baghouses, and headhouses provide the confined space needed for dust to remain in suspension and for pressure to build, increasing the potential for explosion.
Yet, several grain dust explosions still occur every year. A key to preventing large-scale damage and injury is understanding the difference between the primary explosion and a secondary explosion. The primary explosion is frequently the result of ignition sources such as welding sparks or overheated bearings causing fuel sources such as corn or wheat dust to combust.
The primary explosion may not be large, but it often results in a dispersion of dust that can then combust, leading to a larger and more damaging secondary explosion that carries further into the facility. A common question is: How much dust is needed to cause a grain dust explosion? This concentration varies by particle size, with smaller particles more hazardous. The small particles of most grains mean that grain dust and the flour that can result from grains such as wheat should always be considered combustible.
Many improvements in design, monitoring, maintenance, and training employees have reduced the number of explosions that occur. One of the challenges of prevention is that it is often difficult to complete an incident investigation of explosion events after occurrence. Of the explosions reported from through , the ignition source and fuel source remained unidentified in nearly 70 incidents, according to information collected by Purdue University.
Other challenges influencing prevention efforts include multiple sources of dust generation within the grain facility, poorly maintained handling or conveyance equipment, and a lack of a preventive maintenance approach in grain and feed handling and processing facilities.
A lack of awareness of the significance of grain dust as a hazard by workers, supervisors and contractors is another challenge. The responsibility for overseeing details or creating awareness on these items belongs to workers and supervisors.
Safety directors in grain and feed handling facilities balance multiple hazards and, in some cases, prioritizing housekeeping and equipment maintenance can be challenging. Further, having safety standards and implementing these standards consistently can be an obstacle. The reluctance of personnel to accept the potential grain dust hazards in grain and feed handling facilities is a major barrier. Additionally, grains are handled and processed differently based on industry type.
As a result, safety requirements may vary, leading to a discrepancy in the literature and confusion about best practices for managing grain dust hazards. Advanced engineering controls such as explosion suppression systems and dust collection systems have been successful in lowering the number of grain dust explosions.
Yet, the presence of advanced control equipment can raise a false sense of security in managers, supervisors and workers. Grain dust collection and mitigation systems work well when installed and used correctly and in alignment with the process flow and design of the system.
When dust collectors are inadequately designed for the environment they operate within or process changes are made after the installation without an adequate review, the system may not protect the facility or its employees as expected. At this point, the risk of harm to human life, and of destruction of equipment or even entire buildings, is overwhelming.
Some examples of industrial processes that typically create combustible dust are blasting, crushing, cutting, grinding, milling, polishing, and sawing. Combustible dust is a common issue that affects a wide variety of industries and work environments.
The dust can gather in plain sight, but also in places where it may not be detected easily. This means that companies need to take action to maintain the safety of their workers and facilities. Having proper housekeeping routines in place is a given. The most important thing, however, is to make sure that your equipment can keep you and your colleagues safe , and effectively prevent dust explosions from causing destruction.
Nederman has a long history of making products that collect combustible dust , and that prevent dust explosions from spreading. Visit our website , and our Nederman Knowledge Center , to find out more about how we work with reducing the risks of combustible dust and dust explosions. Nederman has wide experience in providing solutions for handling potentially explosive dust - combustible dust. Contact your local Nederman experts to find out more about our products and solutions.
Nederman helps customers around the world protect their employees, their production and the environment from harmful dust, smoke and fumes. Nederman offers a wide range of products, solutions and services for industrial air filtration.
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