Why do bucket elevators cause so many dust explosions?
Abstract
A bucket elevator holds a dust cloud in both legs during normal operation and has moving parts that can overheat: a slipping belt, a misaligned belt rubbing the casing, failing bearings, tramp metal. It also connects silos and conveyors, so an explosion in it spreads.
Because a bucket elevator brings all the conditions for a dust explosion together in one confined, connected machine. Both legs carry a dust cloud in normal operation, the belt, pulleys and bearings can generate frictional heat when something goes wrong, and the elevator links intake pits, silos and conveyors, so an explosion inside it can spread through the plant. OSHA's grain handling guidance states plainly that hazards inside bucket elevator legs are the source of many grain elevator fires and explosions.
Short answer
- Fuel and dispersion: buckets scoop product at the boot and throw it out at the head, so the up-leg and the down-leg contain a dust cloud whenever the elevator runs. HSE guidance notes that both legs may hold an explosible cloud in normal operation.
- Ignition: mostly mechanical: a choked leg or slipping belt heated by the still-turning head pulley, a belt running off-track and rubbing the casing, an overheated bearing, tramp metal or loose buckets striking the casing, and static on non-conductive belts.
- Confinement and connections: a long, narrow casing lets the flame accelerate, and the elevator feeds silos and conveyors through which an explosion can travel.
- Prevention: belt speed, belt alignment and bearing temperature monitoring that stop the drive; conductive belts; magnets for tramp metal; no jogging of a choked leg; good housekeeping and dust-tight casings.
- Protection: vents at the head and near the boot (and along long legs) or suppression, isolation at inlet and discharge, and siting outdoors where possible.
Why the inside of a leg is so dangerous
A bucket elevator lifts product on buckets fixed to a belt that runs over a driven head pulley and a boot pulley, inside two casings. Product enters at the boot, is scooped up, and is discharged at the head by centrifugal force. Each step raises dust, and fine dust also rides down the return leg. Where the grain or powder is dusty, the concentration in parts of the leg can lie inside the explosible range for long periods, which is why the inside of a leg is often classified as Zone 20. OSHA guidance notes that proper aspiration of the leg can lower the dust concentration below the lower explosive limit, which shows how close to that limit an unaspirated leg can be.
The casing is long, narrow and closed: an ideal tube for a flame to accelerate in. A small ignition at the boot can grow into a much more violent explosion by the time it reaches the head, and the elevator then passes it on to the silo or bin it feeds and back to the intake.
The usual ignition sources
| Ignition source | How it develops | Typical safeguard |
|---|---|---|
| Belt slip | The leg chokes or the belt loses tension; the driven head pulley keeps turning under the stalled belt and heats it until it smoulders | Belt speed monitor that stops the drive; never jog to clear a choke |
| Belt misalignment | The belt runs off-track and rubs the casing or the pulley frame | Belt alignment monitors at head and boot, or a self-aligning system |
| Hot bearings | Worn or poorly lubricated bearings inside or partly inside the casing overheat | Bearings outside the casing, or temperature or vibration monitoring |
| Tramp metal and loose buckets | Impacts and friction sparks; damaged buckets catch the casing | Magnetic separation upstream, bucket inspection, access for cleanout |
| Static electricity | Non-conductive belts and isolated metal parts charge up | Conductive belts, earthed head pulley and shaft |
| Hot or smouldering product | Embers from dryers or grinding carried into the leg | Temperature control upstream, spark detection where relevant |
HSE guidance in the UK identifies frictional heating as the cause of a number of elevator explosions. The US grain handling standard turns the safeguards into specific duties. For inside bucket elevators at grain elevators it requires:
- bearings mounted outside the casing, or inside bearings monitored for temperature, vibration or similar;
- a motion detector that shuts the elevator down before the belt speed has dropped by more than 20 % of its normal operating speed;
- a belt alignment monitor that alarms, or a system that keeps the belt tracking;
- belts and lagging bought after 30 March 1988 to be conductive, with a surface resistance of no more than 300 megohms;
- no jogging to free a choked leg, and access at the head and boot for inspection and cleanout.
Elevators with an operational fire and explosion suppression system protecting at least the head and boot, or with dust control that keeps the concentration inside at least 25 % below the lower explosive limit, are exempt from the bearing, motion and alignment duties. Grain elevators storing less than one million bushels may replace the motion and alignment devices with a daily visual check of bucket movement and belt tracking. Outside the grain sector these are recognised good practice for any combustible product.
Protecting the elevator when ignition still happens
Monitoring reduces the chance of ignition; it does not make an explosion impossible. HSE guidance states that explosion vents at the head and as close as practicable to the boot, generally within 6 m of it, usually protect an elevator handling dusts with a Kst of 150 bar·m/s or less, provided each vent is at least as large as the cross-section of the leg (both legs for a vent at the head). Long elevators may need additional vents along the legs, and dusts with a higher Kst need a specific design. Because vents are hard to duct safely to the open air from inside a building, HSE prefers elevators sited outdoors; indoors, suppression at the head and boot is the usual alternative. The inlet and the discharge into the silo or conveyor need isolation, and a vent burst or detection should stop the drive.
Example: A flour mill finds that the speed monitor on an indoor elevator trips several times a week and has been bypassed. The investigation shows belt stretch and worn head pulley lagging. Retensioning the belt and relagging the pulley removes the cause; the trip is restored and tested by simulation.
Common mistakes
- Bypassing nuisance trips. A speed or alignment trip that keeps operating is reporting a real problem; a slipping belt is an ignition source.
- Clearing a choke by repeatedly restarting. Jogging heats the belt exactly where the cloud is densest.
- Leaving inspection doors open or casings leaking. Dust escapes into the building and becomes fuel for a secondary explosion.
- Testing monitors only on paper. Speed, alignment and temperature trips must be proven by simulation to stop the drive.
Related on excodex
- Dust collectors, silos and bucket elevators: where dust explosions start
- Dust explosion protection: venting, suppression, isolation and containment
- Is a grain silo or flour mill a dust explosion zone?
- Why is a secondary dust explosion usually worse than the first?
- Standards space: ISO 80079-36
Standards and further reading
- OSHA 29 CFR 1910.272, grain handling facilities, paragraph on inside bucket elevators: osha.gov
- OSHA 29 CFR 1910.272 Appendix A, guidance on bucket elevators, bearings, belts and leg aspiration: osha.gov
- HSE, HSG103 Safe handling of combustible dusts (elevator venting and siting): hse.gov.uk (PDF)
- ISO 80079-36 and ISO 80079-37, non-electrical equipment for explosive atmospheres (ignition hazard assessment of mechanical equipment such as elevators).
- NFPA 660, standard for combustible dusts and particulate solids (now includes the former agricultural and food processing requirements of NFPA 61).
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.