Preventing a dust disaster
December 01, 2005
by Melissa Alexander
By making preventative maintenance a top priority, grain handling facilities can avoid the human and financial toll that a dust explosion can inflict.
by Gary Imhoff and Suzi Fraser Dominy
(This is the second in a two-part series on preventing grain dust explosions. The first article, which was published in the November 2005 issue of World Grain, focused on the virtues of good housekeeping, the use of good managerial controls and developing written housekeeping procedures.)
There are various devices for monitoring moving parts of machinery that can be installed on equipment to forewarn us of possible problems that can cause failure or become a possible ignition source for a grain dust explosion.
In the U.S., the Grain Handling Standard requires all inside bucket elevators in facilities with more than 1 million bushels of capacity to be equipped with legs that have belt motion and belt alignment monitoring equipment. It also states that bearing temperature monitoring devices should be installed on all inside legs, with the bearings mounted on the inside of the leg casing.
Leg motion detectors are usually set to emit an audible alarm when the belt speed is reduced by 10% of normal belt speed. If the problem is not corrected and the belt speed drops by 20% of the original normal speed setting, the leg monitor device will shut the leg down.
It is recommended to immediately address a 10% slowdown warning to prevent a leg shutdown and a plugged leg. Motion detectors can be mounted on both the top and boot pulley shafts. If only one shaft is monitored, make it the boot pulley because the belt could be slipping on the head pulley.
Leg belt alignment monitoring devices are usually brass rub blocks that are installed in the leg housing on both sides of the belt. If the belt rubs on the blocks, friction causes a temperature rise on the surface of the rub block and an alarm will go off if the temperature rises above a set limit.
Rub blocks can be placed on both the up and down side of the leg and at the head and boot pulley to detect if either of those pulleys has moved on the shaft and if the belt has remained aligned.
GRAIN STREAM PROCESSING EQUIPMENT It is important to remove tramp metal objects from the grain stream prior to its entry into processing equipment. This is usually accomplished by placing magnets in the grain stream just before the grain enters the equipment. These magnets must be maintained by keeping them reasonably clean so their effectiveness is not reduced.
Magnets with large amounts of tramp metal attached lose their ability to attract additional tramp metal, especially larger pieces. A piece of metal getting into grain processing equipment can readily become an ignition source.
INFRARED THERMAL IMAGING Infrared thermal imaging is an excellent management tool that can discover possible problems before they lead to a shutdown or even overheating that could become an ignition source.
This tool is probably most useful for the scanning of electric motors and electrical components, which are not usually monitored with temperature sensing probes.
By using this tool, corrections can be made before a breakdown occurs or overheating causes a fire or explosion. Also averted is the possibility of downtime and loss of production.
PREDICTIVE MAINTENANCE You must know your equipment, how it works, usual maintenance required, and from past experience, what is most likely to break down.
You can predict when repairs may be needed by the amount of product the machinery has handled since the last repair. You can also predict when repairs are needed by listening to equipment for unusual noises and by excessive vibrations of the equipment.
Making sure the shafts, belts, bearings and motor alignment are correct will prevent additional wear on equipment.
FIRE AND EXPLOSION PREVENTION Make sure that all dust suppression systems in place are operating efficiently.
Flammables should not be stored in grain handling facilities where dust may be present. Place flammables in approved containers and properly store them.
To prevent fires, also keep combustibles from accumulating in your facilities.
Make sure that without exception, all hot work in a grain handling facility is according to standard hot work procedures.
If your facility does not have a "No Smoking" policy in place, it is time to do so now. There should be no smoking in or within 50 feet of any grain handling facility.
EXPLOSION SUPPRESSION The first approach is explosion prevention. The second approach is to protect your facilities in the event of an explosion.
Most explosion suppression devices are installed in bucket elevator legs. Grain legs move materials at high speeds, generate a large amount of grain dust, and keep the dust in a confined space. These three things are just waiting for an ignition source to start the explosion.
Explosion suppression can be accomplished by venting the explosion pressure front to the outside atmosphere through vent panels that easily exhaust the flame and pressure fronts to the outside atmosphere before they reach another area of the facility and cause a secondary explosion.
Flame arresting systems work by using an extinguishing method somewhat similar to a fire extinguisher, except they work automatically when temperatures reach a predetermined level. These systems must be properly maintained to ensure they will work properly if and when the time comes to prevent secondary explosions.
ELECTRICAL SYSTEMS Electrical systems must meet strict national requirements, according to the exposure within a given area of the facility.
A good managerial control would be to install surge protectors in electrical rooms to protect the main electrical gear from line surges or lightning strikes.
OUTSIDE CONTRACTOR SAFETY You cannot afford to let an outside contractor or repairman jeopardize your safety program.
You must have a contractor safety program in place and make sure the contractor understands and follows your procedures and has sufficient insurance coverage.
Your contractor policy must also address how to manage contractors while they are on your site. Periodic site inspections of the contractor are an important tool to assist you in making sure the contractor is properly following your procedures.
If a contractor is at your facility for an extended period of time during a construction or large repair project, contractor employee safety training is an issue that needs to be addressed. You should be notified of a contractor employee safety meeting on your site so you can have a company representative present for the meeting. You should also know how the contractor initially trains new employees. If your contractor is unsafe, you are unsafe.
EMPLOYEE TRAINING Employees should also be trained in your contractor procedures as well as all aspects of safe grain handling, including fire prevention and the use of fire extinguishers. At least two employees should be trained in first-aid and CPR.
Providing hands-on training has proven to be much more effective than video and classroom presentations.
Maintenance must be documented as being performed according with written procedures. You need to make someone accountable, usually the location manager, to make sure that the procedures are followed properly and documented. Periodic outside inspections by safety committees or insurance company loss control representatives will help ensure that the program is being adhered to properly. People will respect what we inspect.
Gary Imhoff is retired from Farmers Cooperative Company, Farnhamville, Iowa, U.S., where he served as safety coordinator. This article is adapted from a presentation he made at GEAPS Exchange 2003. He may be contacted at 1.712.465.2195 for more information.