Keep it safe
February 01, 1999
by Teresa Acklin
Regular maintenance, insoections needed to keep bucket elevators operating safely.
Every bucket elevator, when it is running, is a potential bomb just waiting for an ignition source, says Bob Moser, a mechanical engineer and executive vice-president for InterSystems, Inc., Omaha, Nebraska, a U.S.-based manufacturer of equipment for the grain industry.
Mr. Moser, a frequent speaker at industry forums and lecturer on bucket elevator safety, said the best way to defuse the bomb was to maintain a rigorous and regular maintenance and inspection program.
A bucket elevator is, in simplest terms, a conveyor that moves grain vertically instead of horizontally, Mr. Moser said. Buckets, which are available in many capacities and materials, are bolted onto a continuous band made of either rubberized cotton or PVC. These buckets scoop grain from the boot (the inlet) and move it to the head (discharge).
“I always tell people that we are in the moving business,” Mr. Moser said. “We bring material in, move it, store it and sell it.” The concept of a bucket elevator has been around since ancient Egypt. But modern bucket elevators move grain at higher speeds and higher capacities than ever before. The equipment itself is not inherently dangerous, Mr. Moser said, but the seeds of danger are there.
Dust explosions usually occur at grain transfer points, such as bucket elevators or belt conveyors, where small dust particles become dislodged from kernels as fast-flowing grain hits any surface. There are four elements required to create an explosion: fuel, oxygen, containment and an ignition source. Fuel (in the form of grain dust), oxygen and containment are always present in a bucket elevator; the addition of the ignition source is what makes the machinery so dangerous, Mr. Moser said.
“Grain is airborne when it leaves the buckets and you have a lot of small, micron-size particles stirred up,” he said. A hot belt, a failed bearing or an electrical spark may be all it takes to ignite the dust and cause an explosion.
As recent events have shown, grain dust explosions can be deadly. An explosion at a grain elevator in Haysville, Kansas, U.S., last June demolished the facility, killing seven persons and injuring 10 others. No one was injured in a dust explosion at Fisherman Islands Grain Terminal in Brisbane, Queensland, Australia, in January 1998, but the blast damaged the steel over-silo gallery and the conveyor and tripper inside.
In the past 10 years, there have been an estimated 148 grain dust explosions, resulting in 21 deaths, 122 injuries and U.S.$104 million in damage, Mr. Moser said. Half of those explosions originated in bucket elevators, he said.
Loss of life is the most costly result of an unsafe bucket elevator, Mr. Moser said. But there are other costs as well. Downtime from an accident can cost a company anywhere from U.S.$350 per hour to U.S.$34,000 per hour, he estimated. Then there is the cost and time to rebuild a facility, he said. Add to that the cost of injuries, increased insurance premiums and training new employees.
REGULAR MAINTENANCE NEEDED.
The bucket elevator leg is one of the largest producers of dust in a grain-handling facility and is the most dangerous place for concentrations of dust, Mr. Moser said. Many elevator legs are equipped with inadequate dust collection systems, he added.
Managers of bucket elevators should be aware of all the regulations concerning this machinery, he stressed. U.S. Occupational Safety and Health Administration regulations require that bucket elevators use static conductive belts with an inspection port and have externally mounted bearings, motion detecting devices and belt alignment devices. Some states also have adopted National Fire Protection Association regulations, which require that exterior bucket elevator legs have explosion relief vents in the head and casing. Many companies also have their own engineering requirements or specifications.
Mr. Moser advised that exterior legs be used whenever possible. But this is not the solution to avoiding an accident, he said. “Preventing dust explosions in a bucket elevator takes meticulous maintenance and means constantly checking equipment for speed, alignment, temperature and so on,” he said.
Most manufacturers have recommended maintenance programs for their equipment, Mr. Moser said. These should be followed regularly and faithfully.
Many safety features have been built into modern bucket elevators, including externally mounted bearings, motion detection and belt alignment devices, underspeed rotation switches, run-back prevention devices, replaceable linings and quick clean-out plates in the boot. Conventional wisdom also says that explosion vents should be placed every 6 cubic meters of elevator leg with 1 square meter of light relief panel, designed to open automatically under only slight pressure.
Mr. Moser also offered these suggestions for keeping a bucket elevator running safely:
Maintain the proper angle in discharge areas to keep material from building up in the boot and head areas.
Use replaceable linings to avoid excessive housing wear.
Consider a gravity take-up, which keeps the belt operating at the optimal tension all the time.
Keep the pit area dry. A wet pit can lead to corrosion, would could cause a lack of structural integrity and allow dust and material to escape.
Make sure equipment is labeled properly. A facility that is not equipped with safety labels is a potential safety hazard.
Attach the elevator legs to one structure to keep it straight and plumb.
Buy a tapered hub pulley; without one the pulley can move off line and cause belt misalignment.
Install clean-out panels in the boot; the drop-down type are easier to open. The clean-out panels should be used as much as weekly to prevent material from building up in the boot section.
Maintain proper hopper height. A hopper that is not positioned at the correct height to clean out the boot can cause a choke in the leg.
Clean-up shovel pockets can keep workers from disconnecting spouts or having to use a section hose for cleaning. Inspection ports must have a protective mesh.
Install a tramp iron magnet to avoid pieces of metal floating around in the bucket elevator.
Large inspection doors on the up side and down side are recommended. These should be large enough to allow for bucket replacement and inspection.
Dual casing will reduce interior volume, which in turn reduces airborne fuel.
Use hammered joints in the casing area where flanges come together. If the sections are not aligned just right, hammer the sheet metal back to keep the belt and cups from catching on it.
Head sections should be properly contoured on the hood so that material is directed to the discharge. Flat spots on the hood will encourage back-legging, which puts unneeded pressures on the elevator system.
Make sure there are lagging inspection doors and pressure relief vents.
Use slide lagging on the head pulley, which provides better traction and can be easily replaced.
Use vented buckets to ensure proper bucket discharge.
Make sure the belting is adequate for the temperature and moisture conditions. The belting also should be at least one inch wider than the nominal cup size to keep buckets from catching protruding edges.
Check the belt for cuts or splits and make certain that it is running straight.
In a new bucket elevator, the belt can stretch from 1% to 2% within the first few months of operation. Proper tension is essential for safety.
Be sure that the guarding is in place and that safety labeling is visible and undamaged.
Options including electrical overload protection, auto-lube systems, dust pick-ups, dust suppression systems, belt alignment switches, temperature monitors, plug switches and pulley scrapers can keep a bucket elevator running clean and choke-free.
Consider a 200% V-belt drive, which will operate the elevator with one-half of the belts missing. There have been instances when burning V-belts have been the cause of a fire and explosion.
Complete the checklist in the operations manual from the supplier.
If in-house safety is not available, hire a consultant or safety expert.