Implement engineering strategies to reduce the risk of grain dust explosions

by Stormy Wylie
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Preventing grain dust explosions should be a grain elevator or mill manager's highest priority. Every mill manager should develop a strategy to minimize the probability of a grain dust explosion in his or her facility and identify the most likely places where a primary explosion will occur.

First, it is essential to understand the "how" and "why" of a dust explosion in order to formulate a prevention strategy. There are four basic ingredients of an explosion: fuel, ignition source, oxygen and containment. Fuel is grain dust suspended in the air at or above the minimum explosive concentration (MEC). It is not grain dust in grain, dust on floors or equipment.

Palmer (1973) published values for MECs for most dusts, including grain dust. A value of 50 g/m3 is most often used. Lesikar et al (1991) determined that a more accurate value for the MEC for grain dust is approximately 100 g/m3 depending on the grain.

To get a feel for how concentrated an MEC is, consider that the U.S. standard for worker exposure in a grain handling facility is 15 mg/m3 (total) and 5 mg/m3 (respirable). Note that an MEC is 3,000 to 20,000 times heavier than the standard.

Anyone who has been in a room with a particulate concentration of 15 mg/m3 can attest to the fact that conditions were unbearable and a dust mask was essential to remain in the room. It is unlikely that anyone has observed an MEC in the work environment of a grain elevator or mill. An MEC is so "thick" that a person cannot see their fingers moving 12 inches away. MECs do not happen in rooms where people are working, but at enclosed grain transfer points such as bucket elevator legs, enclosed belt loading points, grain falling into bins, etc.

Ignition sources of grain dust explosions can vary. In general, a dust cloud can be ignited by a source that is 390°C (734°F) (Palmer, 1973). Hot bearings and welding have been identified as a source of ignition. Motors that were not "explosion proof" have been cited in court cases. Many grain dust explosions have unidentified ignition sources. Occasionally, electrostatic discharge is blamed for the explosion, although this explanation is usually arrived at after a thorough investigation has eliminated all other sources.

A grain dust explosion is usually a series of explosions. The initial or primary explosion will generate a fire front moving about 10 feet per second and a pressure wave moving at about 1,000 fps. The pressure wave entrains layered dust into a secondary MEC, which is subsequently ignited by a relatively slow-moving fire front.

Pressures as a result of a primary explosion are typically 2 psi, which will knock down a brick wall, whereas secondary explosions can generate pressures that exceed 80 psi. It is not possible to engineer a grain handling facility strong enough to contain an explosion.

PREVENTION STRATEGIES. A number of management actions can be taken to prevent dust explosions. Most would be covered by complying with standards of the U.S. Occupational Safety and Health Administration, but there are actions and decisions regarding engineering that should be incorporated into a comprehensive grain dust explosion prevention strategy.

First, survey the facility with an engineer to identify all points where an MEC can occur, including all grain and milled grain transfer points. Require that an estimate of the mass of grain dust that potentially can be entrained in air be determined at each transfer point.

For example, a grain elevator leg may be moving 12,000 bushels of corn per hour. Feed grains can have 0.1% to 0.5%, or 2 to 10 pounds, of grain dust per ton (U.S.D.A., 1980). At 60 pounds per bu, the leg is moving 360 tons per hour or 720 to 3,600 pounds of grain dust per hour. If 10% of this dust is entrained in the air at this transfer point, 1.2 pounds of grain is sufficient dust to cause an MEC to be present for a volume of 10.9 m3 (@50 g/m3) or 380 ft3.

There are two engineering methods available to prevent an MEC. One is to install a ventilation system that removes dust from the transfer point so that the concentration of grain dust is less than an MEC. The purpose of this system is not to clean the grain or remove dust from the grain but to lower the concentration of dust at the transfer point to a level below the MEC. The other method is to add mineral oil to the grain upstream so that less of the grain dust is entrained in the air at the transfer point.

Using oil for dust suppression is a viable approach for reducing grain dust in the air at a transfer point. However, it is not a panacea.

Application of mineral oil is expensive and does not have the same level of effectiveness for all grains. Wardlaw et al (1989) reported that the effectiveness of mineral applications to grain sorghum (milo) for suppression of grain dust was not as effective as the same application rate to corn. Wheat also may exhibit characteristics that may not be conducive to using mineral oil for dust suppression.

There are some benefits of using oil for dust suppression. A mill manager is not faced with the added cost of installing an abatement system and disposing of the captured dust. However, if the mill uses oil application as the only strategy for prevention of grain dust explosions and is handling grains where the effectiveness of the oil application is questionable, there will be times when MECs exist.

The advantage of the ventilation system approach is that if the system is engineered properly, it has the same effectiveness for different grains.

Ventilation systems are basically big vacuum cleaners, consisting of hoods, conveying pipe and fan/motor systems that pull air from the grain transfer point in order to dilute the concentration of grain dust. A properly engineered ventilation system will reduce the grain dust concentration to less than the MEC; it will not reduce the concentration to zero.

The use of a ventilation system incurs additional requirements in the U.S. The emission concentrations leaving air pollution abatement systems must meet various State Air Pollution Regulatory Agency (SAPRA) regulations. Typically, those air pollution abatement systems are cyclones or bag filters.

The cost of cyclones and bag filters can vary significantly. As a rule of thumb, use U.S.$1 per cubic foot per minute for cyclones and U.S.$10 per cfm for bag filters. Because the cost of abatement systems increase with the flow rate, there is an economic incentive to use less air for those ventilation systems to minimize cost. It is essential that the engineer who designs this system uses sufficient air to maintain the concentration at the transfer point to less than the MEC.

A problem encountered by grain handling facilities using ventilation systems is the allowable emission concentration. (The dust must be removed from the ventilation air stream so as to minimize the impact on the public off property.)

The New Source Performance Standard (NSPS) published by the U.S. Environmental Protection Agency limits the particulate matter concentration leaving the abatement device to no more than 0.01 grains per dry standard cubic meter (gr/dscm), or 23 mg/m3. This means that the E.P.A. has placed a limit on the dust concentration that can be emitted from any cyclone or bag filter, and SAPRAs must insure that this limit is enforced.

This issue is controversial. The E.P.A. set the concentration limit (NSPS) for hazardous waste incinerators at 0.08 gr/dscm. The question is, why are hazardous waste incinerators allowed to emit 8 times more dust than a grain elevator?

Additionally, what does the mill do with the dust collected by the ventilation system? Do not add the dust back to the grain stream entering the leg. The bucket elevator leg is the place at which most primary explosions occur.

CYCLONES VS. FILTERS. The perception of SAPRA and many consulting engineers is that only bag filters can achieve 0.01 gr/dscm. Hence, all emitting points of an export elevator have bag filter abatement systems. However, cyclones can achieve the NSPS for grain elevators if they are designed properly.

For an elevator that has 200,000 cfm of ventilation air installed primarily to prevent dust explosions, the mill manager must spend U.S.$2 million for bag filters. The use of cyclones would cost about U.S.$200,000.

The management incentive is to use less air if it costs U.S.$10/cfm for the bag filter, which could reduce the volume of air needed at the transfer point to lower the concentration to less than the MEC. This incentive would be significantly less if the cost of the abatement system were U.S.$1/cfm.

Bag filters periodically have MECs present, while cyclones do not. This is a consequence of the normal operation of bag filters that many non-engineers do not realize.

The typical pore size of the bag filter material is 74 micrometers. Because a large fraction of grain dust is less than this size, dust particles are allowed to pass through the filter media and be emitted. However, in a relatively short period of time, a dust layer forms on the fabric surface. The particulate matter cannot pass through this layer and is captured. Hence, the operation of a bag filter requires the formation of this dust layer to remove fine dust particles.

As the dust layer grows, the pressure drops. At some point, no air is moved by the fan and, subsequently, no dust is captured from the collection point to the filter. The engineering of bag filters incorporates a cleaning cycle. When the pressure drop reaches a set point (usually 5 inches H20), indicating that the dust layer has reached the design limit, compressed air "pops" the bags, removing the dust layer and restarting the cycle.

It is likely that every time bags are cleaned in a bag filter, the captured dust falling to the collection chamber will result in an MEC. The use of cyclones would reduce the probability of a grain dust explosion.

Cyclones are perceived as being not very efficient. It is typically assumed that a cyclone will have an efficiency of 70% to 90% for abating grain dust.

The Department of Agricultural Engineering at Texas A&M University has been working on cyclone design for over 20 years. Its 1D3D cyclone is considered the best available control technology in a number of states in the U.S.

Using this cyclone, collection efficiencies can exceed 98%. For an inlet loading of 1 g/m, the emission concentration will be less than the NSPS (0.009 gr/dscm @ 98% efficiency). More importantly, it would seem logical that E.P.A. should revise the NSPS for grain elevators to 0.03 gr/dcsm, which would still be significantly less than the NSPS for hazardous waste incinerators.

The use of cyclones rather than bag filters has an additional impediment. Those grain processing management personnel who have made the decision to replace cyclones in the 1970s and spent the money for expensive bag filters may
not want to convert back to cyclones.

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