Juan Ramirez, PhD, PE

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Dec 1, 2021

Dust explosions occur when high concentrations of fine combustible particles in the atmosphere are ignited within an enclosed space. A wide variety of materials can generate a dust explosion including corn, sawdust, coal, drugs, dyes, metals, plastics and rubber. Even materials that are not normally considered to be combustible, such as aspirin or sugar, can produce explosions when burned as dispersed dusts. Grain dust is an especially combustible hazard and can result in very violent and destructive explosions within grain elevators.

In the United States, “grain elevators” receive, store, process and ship a variety of agricultural grains. Grain elevator facilities specifically employ bucket elevators to transport grains from the bottom of the elevator to a distributor at the top. Using extremely large pulleys, the vertical conveyor belt vigorously moves thousands of cubic feet of grain per hour during normal operation resulting in the formation of a combustible dust cloud inside the bucket elevator.

Grain elevator explosions can occur when the bearings for the pulleys, which are typically mounted on the outside of the elevator casing, seize up and overheat causing the combustible dust inside the elevator to ignite. This can lead to a catastrophic explosion and significant property damage, injury, or loss of life.

Although requirements outlined in NFPA 61: Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities can help mitigate or prevent grain dust explosions, these safeguards are not often present in agricultural facilities. Instead, facility operators – whether they know it or not – rely on adequate lubrication to prevent the ignition of dusts from overheated bearings. The problem is that, when bearings aren’t properly greased, fire and explosion can occur in the grain elevator.

A Dust Hazard Analysis performed by Jensen Hughes consultants is a systematic review of potential fire, flash fire, or explosive grain dust hazards and the prevention plan to mitigate the risk. Part of the DHA consists of testing the dust using our one-cubic meter explosion chamber in order to quantify the explosibility of the dust. The lab also houses a 20-L Siwek explosion chamber, capable of characterizing dust explosibility parameters such as explosion severity and ignition sensitivity.

For example, a forensic investigation of a 2019 grain elevator dust explosion in Illinois revealed improper lubrication as the cause of the explosion. The subject grain elevator had been built and installed for 10 years prior to the accident and had been operating without any incidents. The facility had well-documented records of grease being applied to the bearings at appropriate intervals. A few months before the accident, however, the facility switched greases for all its bearings because their supplier was no longer carrying the grease they had been using.

Laboratory testing as part of the forensic investigation showed that the base oil viscosity of the manufacturer-recommended grease was higher than that of the grease being used by a factor of four. Consequently, the lubricant film between metal surfaces was much thinner than intended by the manufacturer which led to metal-to-metal contact inside the bearing. This, in turn, resulted in a bearing fire culminating in a grain dust explosion in the elevator. Fortunately, nobody was hurt in the explosion, but the facility incurred significant business interruption costs.

When a facility switches greasing materials, there is always the potential for incompatibility between the manufacturer-recommended grease and the new greasing agent. Many people in the industry rely on compatibility charts found in trade magazines and web pages to make important maintenance decisions. However, contradictions exist among some of these charts and very few provide the origin of the data or research used to establish the compatibility relationships.

The hazards associated with the change in grease could have been detected had the facility implemented a Management of Change (MOC) approach. Under the Occupational Safety and Health Administration’s (OSHA's) Process Safety Management (PSM) standard, performing MOC is actually required for select industries, but the methodology is applicable and beneficial to all facilities to minimize risks when implementing changes in process chemicals, technology, equipment, and procedures.

This incident ultimately reinforces the importance of properly lubricating grain elevator bearings and verifying that the grease being used meets requirements set forth by bearing manufacturers to prevent grain dust elevator explosions. Even though the Illinois facility was greasing the bearings at an adequate frequency and had the maintenance records to prove it, operators were unaware of the hazards presented when the facility changed the greasing agent. With the wide variety of greases available in the marketplace, the issue of grease compatibility is not straightforward and should be determined with laboratory testing.