Safety devices and training are critical to an accident-free facility; newer technologies may reduce explosion risks
by Melissa Alexander
Bucket elevators are convenient, handle grain gently and provide high capacity with relatively low power requirements, making them the system of choice at many grain facilities. But they also present a number of mechanical dangers and explosion risks, making safety issues of paramount importance.
For example, take the sad case of a maintenance fitter in a Tasmanian grain facility who lost his arm. Although the bucket elevator was equipped with some safety devices, two key safety components either were not in place or were ignored, according to the Workplace Standards Authority of Tasmania.
The fitter, to move the bucket elevator from one position to the next, used the jog button and then activated the emergency stop button to isolate power to the elevator while he changed buckets. To remove and replace the buckets, the fitter needed to reach behind the bucket elevator belt with his left arm to hold a bucket locating bolt and washer in place while loosening/tightening the nut with a ratchet and socket spanner in his right hand.
The accident occurred when the fitter pulled the belt to get a better position, and the belt rolled forward, trapping his arm between the belt and the slatted tail drum. At that point, the fitter’s arm was severely crushed and lacerated. But the fitter could not roll the belt backwards to free his arm because of an anti-roll back device, and, while attempting to pull his arm free, he amputated it above the elbow.
After investigating, the Tasmanian Workplace Standards Authority recommended in a 1999 Safety Alert that all bucket elevators must be fitted with anti-runaway devices, as well as anti-roll back devices, and that bucket elevators also must be "locked out" electrically whenever work was in progress. The gruesome case is illustrative of the need for grain managers to assure that bucket elevators are equipped with all recommended safety components and that employees are trained in their use.
The dangers associated with bucket elevators are not limited to mechanical accidents. The bucket elevator leg is a major source of grain dust explosions, according to David Jones, extension agricultural engineer at the U.S. Institute of Agriculture and Natural Resources, University of Nebraska.
The state-of-the-art bucket elevator design is to locate the bucket elevator leg outside the facility. And additional technologies, such as explosion venting and explosion suppression devices, will further decrease the potential for catastrophic damage, Jones explained in a recent research paper.
Explosion venting is the process of providing escape routes for high-pressure gases within a bucket elevator leg. Explosion vents replace a portion of the leg casing and are designed to rupture or release at relatively low pressures.
When an explosion occurs, explosion vents are ruptured, relieving the internal pressure and maintaining the structural integrity of the bucket elevator leg casing. Venting exterior legs is simple. Explosion vents are sized as a ratio of vent area to vented volume, and total vent area should be approximately 60% of the leg cross-sectional area.
Venting interior legs is not easy. Although the same design parameters are used, consideration must be given to the space where the pressure will be exhausted. Little is accomplished if the explosion is vented into a basement, gallery or work area.
Ducting is needed to exhaust the pressure outside. The ducting should have at least the same cross-sectional area as the vent, should be no longer than about 3 meters and have no bends or elbows.
The material used for vent panels is also important and may vary from aluminum to polyvinyl chloride. The vent material should have a rupture strength of no more than 1 psi, preferably less, although heavier material can be used if the bolts attaching the vent to the leg casing are designed to fail under less than 1 psi of pressure.
The vent panels, which may become dislodged from the leg casing during an explosion, should be secured to the leg with a chain or cable so they will not fall to the ground.
SUPPRESSING THE EXPLOSION.
Limiting damage from dust explosions also can be accomplished by suppression, using a system that detects the explosion early and floods the confinement with an extinguishing agent.
A grain dust explosion can be detected by optical, thermal or pressure sensors, or a combination. Optical sensors react to ultraviolet radiation from a spark or flame, but dust concentrations inside the leg can impede this sensor type.
Thermal sensors compare the air temperature at the leg outlet to the air temperature at the leg inlet. If this temperature difference exceeds a prescribed threshold, a signal is initiated.
Pressure sensors are most commonly used for detecting grain dust explosions. They use a diaphragm-type device that responds to the initial pressure front of an explosion by quickly closing electrical contacts.
As the explosion develops, both a pressure front and a flame front propagate from the ignition point. The sensor detects the pressure front, which travels 10 times faster than the flame front, and sends a signal through a control unit to a high-rate discharge extinguisher. The control unit electronically controls the detector and extinguisher, and it may provide inter-locking to the leg and other equipment.
High-rate discharge extinguishers are pressurized, rechargeable spherical containers. The extinguisher valve can be completely opened in 5 milliseconds to provide rapid flooding of the confinement with discharge velocities that can exceed 30 meters per second.
The extinguishers are filled with a suppression agent, generally dry chemical powders or halon gases. Sodium bicarbonate is generally preferred for grain handling systems because it is compatible with food, is water soluble, is stable under long-term storage conditions and is non-toxic and non-corrosive.
Halon gases have an advantage over dry chemical powders in that they completely dissipate after each discharge, eliminating the messy cleanup after a discharge of dry chemicals. But high concentrations of halon gas are toxic to humans and, if discharged inside a bucket elevator, care should be taken to dissipate the gas below harmful concentration levels.
Explosion suppression systems are best for interior legs where explosion venting is impractical. Suppression systems generally consist of eight to 10 units mounted on the head, boot and trunk sections of the leg. Explosion suppression devices also are recommended for the inlet and outlet of the elevator leg because they create a chemical barrier to prevent the flame front from spreading to other parts of the facility.
The exact number of devices and their arrangement will depend on the manufacturer’s specifications and the configuration of the particular leg. Most explosion suppression devices can be installed directly on the elevator leg casing with minor structural changes.
In certain situations, explosion suppression devices can be used with explosion venting. An exterior leg equipped with both vents and suppression devices (located at the inlet and outlet of elevator leg) can vent any explosions that might occur and prevent propagation of the flame front to adjacent equipment.
The U.K.’s Health and Safety Laboratory is conducting further research on explosion venting to improve the design of future generations of bucket elevators. The project is examining a key design challenge, estimating the venting requirements for a given structure. Results of the research will be made available upon completion.
Searching for more safety tips?
More information on bucket elevator safety is available from a variety of sources, including government safety agencies and professional engineering societies. Most bucket elevator manufacturers and suppliers can provide details on recommended or required safety devices along with relevant regulations, standards and good practices.
The Grain Elevator and Processing Society offers a video that provides a brief overview of hazard monitoring systems, explosion venting, suppression, and other safety guidelines. Contact GEAPS by phone at 1.612.339.4625; Fax: 1.612.339.4644; E-mail: firstname.lastname@example.org; or on the Internet: www.geaps.com.