Grain Dust Handling Part II: Storage Facilities

Introduction

Of all types of dust explosions, those associated with grain dust are the most common and the most dangerous. Forty-eight percent of all dust explosions since 1900 have been associated with grain or other agricultural products. USDA statistics indicate that from 1958 to 1982, there have been 434 grain dust explosions, resulting in 776 injuries, 209 deaths, and millions of dollars in damage. Any time that grain is moved processed or handled, potentially combustible dust is generated. Air and ignition sources such as hot bearings and static electricity are inherently present in grain elevators, dryers and other basic handling equipment. Explosion and fire hazards exist anywhere that grain dust is present. Therefore, intelligent design, good safety practices, and careful housekeeping are necessary to prevent dangerous and costly accidents.

Design of Structures

Grain silos are upright cylinders made of reinforced concrete or steel. Their function is the storage and preliminary processing of many types of bulk agricultural products. A structure located at the top of the silo, known as a headhouse, elevates the grain to the top of the building. The grain then falls through a variety of devices such as scales, samplers, cleaners or other machines that are necessary for a particular product. The product collects in the silo until it is shipped off for further processing or sale. The grain or other commodity is removed from the silo from the bottom of the bin; the removal facilities are usually located in a basement. All structures associated with grain storage should be designed to relieve as much pressure as possible in the event of an explosion. To achieve this, doors, windows, explosion relief panels and lightweight construction are used. It is better to have a few doors and windows blown off than to have a heavy concrete structure explode under high pressure, hurling large, heavy chunks of concrete for considerable distance.

Safety Practices

Every facility that deals with the storage or handling of grains should have a dust suppression and collection system. Generally, a dust collection uses large filters to remove dust from the air. The air inside a storage or processing facility is forced through a filter, leaving the dust particles behind. Of course, the filters and air ducts must be carefully maintained in order for them to be effective.

Careful attention should also be paid to the electrical systems present in a grain facility, including electrical grounding of machine parts where static charges might develop. It is unlikely that a spark from a static discharge would have enough energy to ignite a dust cloud, but it is sound practice to avoid the hazard altogether. It is a good idea to construct the electrical system so that the whole system shuts down automatically if one part of it fails. Spring-return pneumatic devices can ensure shutdown even if all power to the system is lost. Lightening arrestors are also of importance for fire protection.

Grain storage and handling facilities sometimes contain other combustible materials in separate areas. Sprinkler systems should be used to protect these areas. In the event of a fire, it is important to be careful in aiming hose streams. Poorly aimed streams could stir up extra dust, increasing the explosion hazard.

Fumigants for the control of insect pests may be contributing factor to the explosion hazard. Flammable or combustible fumigant vapors might lower the minimum explosive concentration of any dust that is present during fumigation. So far, there have been no studies to prove or disprove this theory.

Housekeeping

Good housekeeping practices are the most important part of an effective safety program for grain storage and handling facilities. The best safety devices in the world cannot function adequately unless they are properly maintained. Dust ducting and other control systems should be inspected periodically to ensure that they are unclogged and operating properly. In addition, ledges, walls, floors, and anywhere else that dust can settle and accumulate should be swept clean frequently. This built up dust is the major fuel for the secondary explosions which cause most of the damage to grain facilities. Spills should be cleaned up promptly.

Investigation Tips

1. Keep in mind that the cause and origin of the explosion may be difficult to determine. Multiple sources of ignition are usually present, and it might turn out to be important to figure out which one really caused the fire. (Was the fire started by the worker welding on the silo at the time of the explosion, or the improperly maintained bearing in the headhouse?)
(National Institute for Occupational Safety and Health, NIOSH Publication 83-126 -Occupational Safety in Grain Elevators and Feed Mills)

2. Take a close look at the dust collection system. If possible, determine whether a clogged filter or some other malfunction might have contributed to the explosion.
(National Grain and Feed Association, Dust Control for Grain Elevators, chapter 13, 1981)

3. Dust explosions often cascade from one structure to another, observe the area to determine whether this phenomenon has occurred. Remember that secondary explosions are usually more damaging than the primary explosion.
(National Fire Protection Association, Fire Protection Handbook, section 5, chapter 9, 1986)

4. If a fire followed by an explosion has occurred, analyze the techniques used to fight the fire. High pressure hose streams can stir up acumilated dust and present an explosion hazard.
(U.S. National Materials Advisory Board, NAS-NAE Publication PB83-129064 -Prevention of Grain Elevator and Mill Explosions, 1982)

5. Beware of attributing the cause of a mysterious explosion to static discharge, even if the structure involved is poorly grounded. It is highly unlikely that a static electricity spark can generate enough energy to ignite suspended grain dust.
(National Fire Protection Association, Industrial Fire Hazards Handbook, p.157, 1990)

6. Grain storage facilities must be equipped with explosion relief vents. However, it is sometimes impossible to retrofit an older facility with adequate venting due to structural limitations.
(National Fire Protection Association NFPA 61B Standard for the Prevention of Fires and Explosions in Grain Elevators and Facilities Handling Bulk Raw Agricultural Commodities, chapter 3, 1989 edition.)

7. Some facilities, especially bucket elevators, may be equipped with explosion suppression devices that rapidly detect incipient explosions and immediately disperse large quantities of suppressing agent to smother the explosion. Use caution if these devices have discharged. The fire suppressant gas usually used is Halon 1011, which is toxic.
(National Grain and Feed Association, Retrofitting and Constructing Grain Elevators, pp 45-46, 1985)

Grain fires are not uncommon, but they usually do not cause severe damage to structures, nor do they usually follow grain dust explosions. However, it is very likely for a dust explosion to follow a grain fire. Fires in grain storage areas tend to smolder for extended periods. When the smoldering grain is stirred up, combustible dust is released, and the burning grain acts as an ignition source, possibly resulting in an explosion.