Hazards Of Grain Dryer Fires
Date Posted: October 13, 1998
Property loss in a grain dryer fire or explosion is twofold. There is physical damage to the drying equipment and loss of the grain. Often overlooked is the loss from an unscheduled interruption in necessary grain drying activities at the grain elevator facility. Either kind of loss can be extensive and costly; however, both can be prevented or minimized.
The conditions that can cause a fire or explosion can be reduced by following a few precautions pertaining to safe operation and maintenance of the grain drying facilities.
Use of Grain Dryers
The purpose of any grain dryer operation is to reduce the moisture content in agricultural commodities such as grain. In grain elevators, drying is practically always done with heated air.
All grain dryers work in basically the same manner. The air moving system incorporates fans or blowers to draw outside air into the grain dryer. The air is forced through a heat source or burner where the air is heated to a predetermined temperature. The heated air is then forced through the grain. The heated air picks up moisture from the grain and is discharged from the dryer. Part of the discharged air on many grain dryers is recirculated through the air moving system to increase drying efficiency.
Grain inflow and discharge are controlled in different ways, depending upon the specific type of drying system used. Continuous and batchflow are two types of grain flow into and out of self-contained dryers that most elevators use. Various types of bin dryers also are used at elevators.
The fuel source, most commonly natural or liquid petroleum (LP) gas, is always located outside the dryer. However, vaporizers, which are usually used for LP gas dryer, may be located inside or outside the dryer.
The tremendous heat of the burner is an inherent fire hazard in grain dryers. In addition, there is a readily available source of fuel, either the gas used in the burner or the grain itself. Also, grain dryers are operated for long periods of time, commonly 24 hours a day during the drying season, and their operation may be unsupervised much of the time. If any part of the system develops a problem, a major fire can result.
Many fires in grain dryers are caused by malfunctions in the burner ignition mechanisms or the fuel supply line to the burner. Either type of malfunction can set fire to the grain being dried. Some grains, such as sunflowers and grain sorghum, are more susceptible to fire than others, but all grains will burn. Grains will burn more easily if not cleaned of foreign material and grain dust. The risk of fire is decreased by cleaning the grain before it is dried.
Grain dryer fires can be prevented or minimized by properly maintaining the drying equipment and operating it in a safe manner.
A number of safeguards and automatic controls are built into grain dryers to protect against fires, if thee is a malfunction.
To operate a dryer safely, the equipment must be properly installed according to manufacturer's specifications. This includes the dryer, operating controls, emergency controls, and other equipment such as conveyors to fill and unload the dryer.
All dryers must be located outside of buildings to minimize the hazard of a fire from spreading to nearby structures. All openings on exposed surfaces of buildings within 30 ft. of the dryer should be protected or covered to prevent fire from entering buildings.
All components of the dryer should be made with noncombustible or fire-resistant materials. The interior surfaces of dryers should be easy to clean. Regularly clean the dryer to remove highly combustible grain particles from the dryer.
Access doors or openings on dryers should be constructed to allow convenient routine inspection, cleaning, and maintenance, as well as to facilitate fire fighting. The dryer should be capable of being unloaded quickly during an emergency or fire.
Primary and secondary air intakes should be screened to reduce the amount of combustible material that enters the drying chamber. Primary intake screens should be made with steel mesh measuring a maximum of 1/4 in. Secondary intake screens should be steel mesh measuring no more than 1/2 in.
Filling and discharge grain spouts and ducts to the dryer should be provided with gravity firestops to reduce the spread of fire to adjacent buildings or equipment.
All electrical equipment in grain dryers should be rated for the class and group area in which it is to be used. Generally, this would be Class II Group G for dusty areas or Class I Group D for flammable vapors.
External vaporizers are normally located between 10 and 25 ft. from the dryer and other structures. Internal vaporizers located inside the dryer structure should have a minimum design pressure of 250 ln. per sq. in. with a safety factor of 5.
Automatic Operation Controls
Automatic operation controls are the most important controls in a grain dryer operation. They should be installed, calibrated, and functioning properly.
Automatic temperature controls must be exact. Grain dryers at elevators are commonly operated with drying air temperature up to 240 deg. F. The maximum setting on the high temperature limit control should be 260 deg. F. Hot-air exhaust temperatures should be monitored and controlled for a maximum temperature of 210 deg. F. At least one heat detector should be located for each 50 sq. ft. of exhaust surface. All temperature gauges should be easily visible.
Many grain dryer fires are caused when the operating temperature is too high. Each grain has a specific maximum drying temperature. Don't exceed the recommended temperature for the grain being dried when setting the drying air temperature thermostat. Foreign material, which is usually more combustible than the grain, is best removed before the drying process begins.
Emergency Operation Controls
If the automatic operation controls fail, or some other problem develops somewhere in the system, emergency safety controls must take over. The emergency controls must shut down the system and warn the operator that a problem exists.
If for some reason the air intake system or blower stops, automatic controls must immediately stop gas flow to the burner. These controls must be checked on a regular basis.
If the ignition fails, fuel in the burner should be automatically expelled or purged for at least 1 min. or 4 equivalent air changes.
The gas in the burner is automatically ignited by a gas pilot or electrical spark. Hand-held torches should never be used for ignition.
The system should require a manual test to restart the burner.
The flow of gas or other fuels to the combustion chamber must be automatically controlled with a regulator. The main gas supply line must also be equipped with a manual positive-acting safety shut-off valve.
Audible alarms and warning lights should alert operators that an emergency condition exists. A single emergency electric disconnect switch should be provided in a readily accessible location. All employees should know the location of and how to safely operate this switch.
All emergency controls should be interlocked so that failure of one element will shut down the entire system, stop all movement of grain into an out of the dryer, and shut off the fuel supply, which will shut down the burner.
Some type of automatic fire fighting controls should be located in or near the grain dryer. One example is a sprinkler system or at least a fixed water supply available for fire fighting. The area around the dryer should have easy access for fire fighting crews and their fire hoses.
Operation and Maintenance
The best automatic operating and safety controls cannot prevent grain dryer fires or explosions. The equipment must be properly maintained through routine inspections and operated by a competent an trained employee.
Many hazards can be eliminated if:
-Proper procedures are always followed when welding inside or around a dryer.
-The inside of the dryer is cleaned thoroughly on a regular basis-- every 72 hr. for a continuous drying or 168 hr. total drying time for noncontinuous operation.
-The dryer and its components are tested annually, prior to the drying season.
A regular maintenance and inspection program would include the fuel system; operating, safety, or emergency controls; the physical structure; and the grain filling and unloading equipment. The checking procedure should include the following steps.
-Check for leaks in fuel lines, loose pipe connections, regulators, and all valves.
-Check calibration on all operating and safety gauges, thermostats, and other controls. Check fuse links on fire stops; inspect water sprinkler heads for clogging or damage; test emergency electric disconnect; check operation of electrical interlocks and alarm systems.
-Check for damaged, rusted, or worn structural members; combustion chamber; bearings; V-belts and chains and their drives; and tightness of air screens.
-Check conveyors to be certain emergency conveying systems operate properly.
Always provide fire protection devices, such as portable, all-purpose dry chemical fire extinguishers, near the grain dryer. If a fire or explosion occurs in a grain drying facility, several critically important rules must be followed. Remember that not all grain dryers are alike, and not all fires are alike so there an be no such thing as absolute fire attack rules.
Good planning will help control and minimize loss. A lost of grain dryer fire emergency procedures should be part of your company's emergency action plan. That plan should be reviewed regularly with managers, supervisors, and all employees.
The first task when a fire is suspected is to shut down the entire drying operation. This includes the flow of grain to and from the dryer.
The local fire department and probably the gas and electric companies should be notified immediately. If the grain is burning, stop the fan to slow the supply of oxygen to the fire.
The next task is to empty the dryer. The grain is an almost unlimited supply of fuel that is fairly easy to ignite. Water or chemical extinguishers work best on grain dryer fires.
Do not try to move burning grain through the grain handling system except to unload the dryer. Never move burning grain into storage buildings or elevators. Burning or smoldering rain should be piled outside where it can be soaked with water or another extinguishing agent.
Watch for secondary fires. A fire watch should be posted after a fire for at least 12 hr.
The basic guidelines in this publication should be a part of every employee's orientation program. Since grain dryers operate largely by automatic controls, an emergency could occur any time of the day or night. Every employee should be equipped to deal with a grain dryer operation or emergency problem, or at least know who to notify if a problem arises.
Written by Coopertive Extension Service, Iowa State University, Ames, IA 50011.