Bulk Engineering, BULKtalk, Safety

BUlktalk: conveyor fires

Conveyor belt fires are reported regularly in various forums from around the world. Steve Davis explores how Australian bulk handling sites can avoid them.

Conveyor belt fires are reported regularly in various forums from around the world. Steve Davis explores how Australian bulk handling sites can avoid them.

Conveyor belt fires can result in injuries and cause significant damage and loss of production. Absence of fires on your site to date is no guarantee for the future unless good preventive measures are taken on a continuous basis.

Conveyor belt fire causes are well understood and are therefore the result of a lack of the systems and practices required to prevent them.

Fire prevention has three components: design, which should mitigate potential causes of fire; operation and maintenance, which should manage ongoing fire risks; and housekeeping, which should mange external fire risks. Design of the conveyor system should include analysis of perceived risks, including the risk of fire. Potential fire risks should be identified, and mitigation strategies included in the design. The design intent of fire mitigation must be clearly identified to personnel involved in the day-to-day operation and maintenance of the system, and personnel must implement and maintain strategies and equipment. Regular visual inspection of the conveyor system for issues known to cause fires and other problems reduces risks. Housekeeping, including clean-up of material and other spillages, correct storage of all equipment after use and general evaluation of territory reduces risk of fires and improves likelihood of safe egress.

As with any fire, three components are necessary for a fire to occur. Conveyors generally operate in an atmosphere containing oxygen, the first component necessary to sustain a fire. The conveyor itself provides the second component, the fuel for a fire. Conveyor belts are manufactured from natural, or more likely a synthetic rubber, fabric carcasses, and rubber noodles for steel cord. Most components of a conveyor belt are flammable, although the extent of flammability varies significantly. Some of the conveyor components, such as skirts and cleaners, idler rolls and pulley lagging might be rubber or polyurethane, each with different degrees of flammability. The ignition temperature for rubber is quoted between 300 and 400 degrees Centigrade. Typical mineral oil lubricants in bearings and gearboxes have a flash point as low as 160 degrees Centigrade. Some bulk materials are flammable.

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The third component necessary for a fire is an ignition or heat source. Given that fuel and oxygen are always present for conveyors, we must look to potential ignition sources as the methodology to prevent fires. Except for any deliberate setting of a fire, it should be possible to remove or replace all types of ignition source. In addition, we have several methods available to detect the potential of a fire before it occurs. 

If we look at the belt conveyor as a machine, the typical ignition sources for a belt fire are friction and mechanical failures. Friction arises whenever the belt rubs against something fixed, or that should be rotating but has stopped. Is the most likely fire the result of a seized rubber lagged pulley or idler, where the belt rubs against fixed rubber and ignites the pulley? If your conveyor has lightweight idlers, is the idler material flammable from rubbing friction? Polymer idlers appear to wear away before this could happen. Could fire be caused by rubber or metal skirting or a wear liner that has been dislodged and become jammed against the pulley or the belt or both? Constant motion from the belt against any non-moving item can create a local fire. 

Could fire be caused by a belt rubbing against steel structure, a chute or other component and heating the metal? A short cycle belt could potentially get hot enough to burn from this, however longer cycle belts typically cut through steel structure, or the steel destroys the edge or cover of the belt. Neither of these are good for the conveyor and should be rectified as soon as possible. It is possible that continuous belt rubbing against steel will heat the steel until it is hot enough to ignite the belt cover. We have all seen idlers and pulleys which, when seized, are worn through until there is a knife edge. This could become a friction ignition source, and often cause serious damage to the belt covers.

Most conveyors have an electric motor and gearbox, and all pulleys are mounted on grease lubricated rotating bearings. A fire engineer thought motors the most likely ignition source for a conveyor fire, followed by grease in pulley bearings if the bearing starts to seize and overheats. Brakes are a potential ignition source and reach high temperatures for a short period. Dragging brakes will remain hot during operation. 

Considering external ignition sources, we look first for storage and spillage of flammable materials near the conveyor such as oil or grease. These should be removed as they are easy to ignite and then transfer fire to the belt. Pipelines carrying flammable liquids near conveyors could leak and create a fire risk, especially if separation is small such as using the conveyor gallery as a pipe rack. Electrical faults are common fire starters, and we commonly use conveyors as cable racks.

Several bulk commodities are flammable, such as coal, sulphur, and biomass. These may add fuel to a conveyor fire, or if burning prior to conveying may be the ignition source for a fire. Sulphur, coal, and other dusty materials that are susceptible to ignition from static and other sparks maybe easily ignited on or near a conveyor. Conversely, if a conveyor does burn, it is likely to ignite any spillage from flammable bulk materials. 

Conveyors that discharge these materials into a bin need special attention as the bin contents can ignite or explode from static and blow back to the conveyor.

Other potential ignition sources include mobile hydraulic equipment, hot work maintenance on nearby equipment. Hot work near isolated conveyors has been known to start a fire chain, for example hot work set fire to a rubber lined screen underpan which then ignited the feed conveyor belt. We have prohibited smoking on most sites, but could vapes initiate a fire?

The number of fires we see indicates that we are not perfect in removing ignition sources, so we should now look at reducing the consequences. Consequences of a conveyor fire include potential injuries, smoke, and chemical inhalation from burning rubber and other chemicals. Fires can burn hot enough to destroy all electrical, mechanical, and structural components of a conveyor and may spread rapidly if the conveyor is not stopped at the earliest opportunity. Conveyor fires are most destructive when the conveyor continues to move, and the fire spreads the full length of both carry and return sides.

Most conveying systems are single stream with no redundancy, and as such there is likely to be substantial downtime for inspections, assessment, demolition, and rebuilding. Alternate methods of material transport may be required at high cost.

Safety in design must consider safe egress from all locations in the conveyor system, with a strategy of dual redundant pathways. 

Longer conveyor galleries often cannot comply with regulatory expectations regarding maximum distance to exit point, and this must be considered with application for exceptions if required. Egress may need to consider provision for stretchers. In the event of a fire, emergency services may require specific access to the site and the structures, plus access to a defined water supply. Water supply to elevated gantries may require additional tanks and pumps to meet emergency services’ needs.

Mitigation strategies include choice of belt cover material. Fire Resistant Anti-Static (FRAS) belt is available at a premium over conventional belt. This belt is formulated to ground static charges and to resist fire. The cover will still burn but should self-extinguish if there is no external flame. Use of this belt should be discussed with suppliers, as there are several grades available. I have seen several installations where FRAS rubber belt has been changed out for “standard” rubber to save money. Static earthing is improved by using earthing whiskers onto the belt covers.

Selection of components will impact risk. Cheap idlers with poor sealing arrangements may reduce cost, however in a wet and / or dusty environment bearing failure will be higher leading to more seized rolls. Pulley bearings should be fitted with excellent seals and correct relubrication addressed. Select the correct belt for the duty. Australia has some of the best products and guidance on conveyor component selection and has developed a wide range of monitoring devices for preventing fires.

There are a wide variety of sensors available that can detect temperature, heat, flame, smoke, vibrations, and chemical emissions. Heat sensing wires can be run down both sides of a conveyor to pick up any change in temperature that could indicate a hot bearing or the start of a fire. Temperature sensors are used to detect temperature trends in bearings, motors, and gearboxes. Vibration detectors can be mounted directly to bearing housings. We have systems that trend the condition of idlers along a conveyor and identify failure. 

The key with all monitoring is to understand what the outcome means and how the system should react. Most of this can be managed by the automated control system, however operators should understand what is occurring especially if there are manual inputs required. Monitoring provides a condition trend from a baseline. The trend for conveyor components will typically vary with ambient conditions, load cycles and deterioration. We want to keep the system running but must stop the system when there is a risk of injury and damage. This means we can accept and monitor a gradual trend change, either cyclic or continuous deterioration to a planned maintenance point. 

When there is a rapid change in the trend, such as with any indication of a fire, we need to plan the action in advance. An immediate stop for a conveyor, including up and downstream feeds, and initiation of fire mitigation processes seems the most likely course.

After identifying a fire, personnel must evacuate, so an alarm system and exit strategy was included in the design risk outcomes and fire management plan. The design will also incorporate methodology to restrict and extinguish a fire as quickly as possible. 

A call to the local fire brigade is necessary. Fire control systems can make this call automatically.

Immediate solutions to fires include: no provisions except those required by the local fire brigade, i.e. let the fire burn as all personnel have been evacuated or, provide localised permanently connected fire systems for fire management. It is common practice to install a dedicated fire water system with fire hose reels and hydrants. Unless site personnel have been trained in using these hoses and have suitable PPE to protect against heat and fumes, this should be left to the professionals.

For some conveyors, where there is a need to provide a safe exit, where the bulk material is flammable, or where there is a desire to protect equipment, we install deluge systems. These either form an extension of the fire water system or can be independent. Foam deluge is an option to water. Deluges are designed to exclude oxygen and cool the fire. Where the conveyor carries flammable bulk, the deluge can wash burning bulk from the conveyor and spread a fire further. Design and installation of deluge systems should be more than just conveniently located piping with available spray nozzles.

Finally, fire management protocols and systems must be always available and require testing and maintenance. Adaptation is necessary if maintenance of the conveyor or the fire system changes the protocols in place, for example, if the water supply must be isolated or an egress walkway blocked, change in use, etc. Australian conveyors may be older than 50 years in their original design, with multiple alterations and upgrades. Is the fire management system still suitable? 

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