Moving and handling grain creates grain dust, which when combined with other components typically found in a grain handling environment, can result in grain dust explosions. Grain dust explosions are high impact events that can result in human fatalities and injuries, loss of facilities, and significant operational downtime.
The May 2018 explosion in South Sioux City, Nebraska, U.S., damaged the structure, injured two workers, and interrupted business operations. A second explosion in eastern France in June 2018 injured six and destroyed the facility. Like many grain dust explosions, the cause of the blasts in both incidents is still unknown, but given what we know about previous grain dust explosions a likely suspect is the ignition of combustible grain dust. Because a grain dust explosion has far-reaching effects, preventing these events is critical. Since 2015, Purdue University in West Lafayette, Indiana, U.S., has gathered information on grain dust explosions in the United States. The table on page 72 summarizes the explosions and known sources recorded in 2015, 2016 and 2017.
For a grain dust explosion to occur, five components must be in place: oxygen, confined space, ignition source, dispersion, and a fuel source (grain dust). The elements make up the dust explosion pentagon. Grain handling creates an environment that can result in combustion and explosion of dust. The three elements needed for a fire — grain dust as a fuel source, oxygen, and an ignition source — can create a flash fire that is dispersed within a confined space, resulting in the deadly ingredients for an explosion. Eliminating one of these five elements will prevent an explosion, but elimination of these hazards is not a simple process. Oxygen and confined spaces are part of handling grain and cannot be eliminated. Therefore, managing housekeeping related to grain dust and appropriate maintenance of equipment to prevent ignition sources from sparking and dispersing is a critical challenge. Managing grain dust and maintaining equipment is not a once-a-year task, rather, these tasks should be approached as an ongoing, scheduled, continuous improvement process.
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.
The 1987 implementation of the OSHA Grain Handling Facilities Standard reduced the number of grain dust explosions by 42% the first year because of increased attention on controlling grain dryer fires, grain dust explosions, and hazards associated with entry into bins, silos and tanks. 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? The simple answer is “no amount of dust is safe.” If the dust accumulation covers the floor so you cannot see the color of the surface (about the thickness of a piece of paper), enough dust is present to lead to an explosion. A more complex answer to the question uses a measure called the Maximum Explosive Concentration (MEC). 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.
Mitigating and preventing explosions
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 104 explosions reported from 2005 through 2017, 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. Narrow opinions such as “nothing has happened until now” or “accidents occur no matter how much you train the workers” must be overcome to successfully deliver the safety message about grain dust explosion threats faced by the grain handling and processing industry. 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. Also, outside parties who inspect facilities may not always be able to identify dust explosion hazards.
Awareness and use of these advanced controls are mostly limited to larger organizations with many employees and a dedicated safety staff. Smaller facilities often operate without any advanced dust control or explosion mitigation plan. Facilities with a smaller budget have options to minimize grain dust hazards in their facility, but they must take the hazard seriously. In all mitigation methods, the focus should be on removing one of the five components of dust explosions from the mix. Effective dust collection at the loading and unloading points is one option as is managing dust once it has entered the facility.
When loading and unloading, the focus should be to avoid turbulence of grain at transfer points to minimize dust separation from the grain stream. Another method for controlling dust is to adjust the angle of spouting to reduce grain speed as it moves from one bucket elevator to another conveyor. Grain leaks from spouting can be problematic in terms of dust accumulation and chokes can overload a bucket elevator. To minimize the risk of a primary explosion, spout liners and cushion boxes can be used to reduce the separation of dust in a spout and to allow the incoming grain to hit grain accumulated in the cushion box. On a bucket elevator, installing temperature monitors on bearings allow operators to monitor for hazard warnings and prevent the possibility of a spark from an overheated bearing.
Several options exist to prevent excessive accumulation of grain dust. First, the use of food grade mineral oil, at first handling, can effectively lower dust emissions from moving grain by making the grain “sticky.” With the addition of a mineral oil, the dust remains “stuck” to the grain, minimizing the possibility of the dust dispersing in the air. A second important prevention practice is a consistent housekeeping schedule. Whether dust accumulation is managed by dry sweeping or with more sophisticated methods like a central vacuum system, finding and collecting dust from all points in the facility is important. Vacuums and compressed air have applicable standards and regulations and have advantages and disadvantages.
Brooms, with antistatic bristles, are easy to operate and have no regulatory constraints, but they require an operator, which can be difficult to find, especially during busy periods. No matter which method is used, chasing down grain dust in hidden areas such as beams, light fixtures and ledges is critically important.
Raising awareness of hazards
The grain handling and processing industry has made good progress in preventing grain dust explosions through enhanced engineering controls and interventions, improved grain handling methods, and increased awareness of employees and managers about grain dust hazards. However, grain dust explosions still occur more than they should. For this reason, increasing the awareness and knowledge of grain dust explosion hazards, their causes, and measures to curtail the risk is important.
Until we reach the goal of zero grain dust explosions, the effort of increasing awareness of grain dust hazards, preventive tools, and best practices will continue. The grain and feed industry has made substantial progress, but the need continues for highly trained workers and supervisors to understand and apply safe practices in day-to-day operations.
Purdue University and Iowa State University have taken a leading role in raising awareness of grain dust hazards. Since 2015, Purdue University and Iowa State University have led grain dust explosion prevention workshops across the Midwest region of the United States and by request elsewhere in the United States. The workshops include topics such as dust explosion hazards, ignition sources, housekeeping practices, sources of dust, and related topics to provide a basic understanding of grain dust hazards.
An advanced workshop addresses grain handling and conveying equipment maintenance as it relates to grain dust, preventive maintenance in grain and feed handling and processing facilities, control mechanisms for use during unloading and grain handling, controls of bucket elevator operations to limit dust, and use of advanced engineering controls such as appropriate sensors and explosion vents. The target audiences for both workshops are grain and feed handling organizations, insurance field specialists, agricultural construction workers, and rural firefighters, all of whom could potentially interact with grain dust. The four-hour workshops are offered free of charge by Purdue and Iowa State due to funding from an OSHA grant. If interested please contact Professor Gretchen Mosher (firstname.lastname@example.org) or Professor Kingsly Ambrose (email@example.com).