Martin Engineering
A fire is one of the worst catastrophes in any manufacturing facility, but operations involving large conveyor networks are at particular risk of large-scale fire events and the hazards they pose to worker safety and company profitability. There are a number of underlying reasons. For one, the nature of bulk material conveying generates dust that can serve as fire fuel and even an explosion hazard. Controlling fugitive material in these operations is an ongoing challenge. Second, conveyors often don’t receive the maintenance they need. They are often huge, complex structures that are only serviced when a component fails. If not addressed promptly, a seized idler or off-center belt can create sufficient frictional heat to cause ignition.
Friction contact can not only cause belt and component damage, but can also lead to conveyor fires.
In addition, the size and speed of modern conveyors present the ability to spread even a modest fire outbreak over great distances in a very short time, and conveyor fires can be very difficult to extinguish. The belt is a long structure, and a fire can spread all along this extended route with frightening speed. If it’s not spotted in time, burning material can even be transferred from one conveyor to another. The belt path will often run around or through a variety of enclosures and structures. They all carry the risk of fire inside them, where it’s very difficult to access. In many operations, elevated conveyor systems can also prevent firemen from reaching a blaze.
If proper measures for prevention or suppression are not taken, there is a chance of substantial destruction. Flames can be conveyed throughout the facility quickly, putting the entire operation and everyone in it in jeopardy. Beyond the obvious workplace safety issues, recovery from the insurance, production loss and legal consequences could take years. In underground applications, the danger is far worse. With enough friction and heat, many conveyor belt constructions are capable of self-sustained fire propagation, whether they are made from natural materials, synthetics or a combination of the two.
Once ignited, belt fires tend to produce a thick, black smoke resembling a tire fire, which is often a far worse hazard to personnel than the flames themselves. Being near a burning conveyor belt can expose workers to carbon monoxide poisoning and toxic gases, such as hydrogen cyanide (HCN) and sulfur dioxide (SO2), as well as oxygen depletion and intense heat. Smoke and heat can limit visibility, preventing access and escape, and it can take days to completely extinguish the event. If a burning belt should split under tension, the burning ends can fly apart, leading to multiple fires at different locations.
Long, complex conveyor systems and difficult access can make firefighting next to impossible.
Causes of Belt Fires
Although heat caused by friction is the principal trigger of conveyor fires, there can be several underlying causes. Seized components such as rollers, brakes or bearings are a common source, often worsened by fugitive material that acts as an abrasive and/or fire fuel. Carryback on pulleys can also cause belt slip, creating additional friction. Static electricity can trigger a fire as well, particularly when significant amounts of airborne dust are present. It has been estimated that frictional and static ignitions may account for up to 40 percent of all belt fires. [Fig.1]
Fig. 1 – Number and causes of belt fires from 1986-1991. Reported in the HSE Topic report, “Safe Use of Belt Conveyors in Mines.
To create a fire there must be three elements: Oxygen, heat and fuel (aka, “the fire triangle”). One factor that makes conveyor fires so hazardous is that the belt itself can become fuel. In fact, some experts would argue that there is no such thing as a non-flammable belt, particularly when accompanied by a combustible material.
For an explosion to initiate, there must be the elements of the fire triangle plus dispersion of combustible dust, vapor or flammable gas within a confined space. Many explosions start as localized flash fires caused by the ignition of a distributed fuel and oxygen in an open area or large enclosure. Often a flash fire will cause a significant dispersion of combustible dust, creating conditions for a secondary ignition that may result in an explosion or spreading of the localized fire. Many common bulk materials that seem inert may be easily ignited in dust form or give off flammable or combustible vapors when heated. Some materials, such as brown coal, can even spontaneously ignite.
Explosion Pentagon / Combustion Triangle
Friction
The source of friction can often be traced back to either the discharge zone or the loading zone, or both. Inadequate belt cleaning in the discharge zone can lead to dust and carryback on the return side of the belt, causing a fouled tail pulley to run under a stalled belt, which creates tremendous heat. A fouled tail pulley can also cause the belt to enter the loading zone unevenly. As a result, cargo dropped from the transfer chute can settle on one side of the belt, causing the belt to drift out of alignment and come in contact with the structure, resulting in high temperatures caused by friction that can ignite a fire.
Dust emissions of any kind (combustible or noncombustible) originating from the loading or discharge zones have a tendency to foul rolling components, leading to failure of the seals and contamination in the bearings, eventually causing the roller to seize. uous frictional contact with a seized idler or the roller face can cause a loaded belt to exceed safe operational temperatures. It can also potentially result in extreme wear on the belt, degrading the main fire-retardant layer and exposing the more heat-sensitive materials found in the belt’s inner construction.
Controlling Fugitive Material
Without proper belt cleaning, chute sealing and belt tracking, spillage that collects around the loading/discharge areas and along the belt path can damage moving components, restrict access by fire crews and potentially act as fuel. Significant amounts of dust can also collect on the walls of the loading zone chute. If this occurs, a spark can potentially create an explosion. Additionally, common maintenance within the chute such as removal of the wear liner using a cutting torch often requires confined space entry by workers. If the inside of the chute is not adequately cleaned, this task could pose a serious hazard.
Conveyors can spread even a modest fire over great distances in a very short time.
When examining conveyor equipment, one should approach the task with the same critical mindset as an inspector from local, regional or national authorities. Before concentrating on specific components of the conveyor, safety professionals recommend reviewing the system as a whole, taking into account the combustibility of the material being conveyed and how the areas around the conveyor could be impacted. Operations personnel should also assess:
- Spillage levels and cleaning schedules
- The conveyor’s proximity to work stations
- How the conveyor design (enclosures, narrow walkways, etc.) could create a hazard for employees • Safe storage of flammable liquids
- Compliant signage
Finally, take a common sense look at the current state of all fire suppression equipment. Conveyor designs are ever[1]evolving and are often retrofitted with new equipment based upon changes in production and volume. In older facilities, fire suppression systems were most likely designed for the conveyor that was in place years — if not decades — earlier and may no longer be effective in the event of a fire on the current system.
Considerations for Conveyor Fire Prevention
Often, the potential return on investment (ROI) for fire prevention equipment isn’t recognized until managers are inspecting burnt rubble and negotiating with insurance adjustors. However, safety-minded operators understand that the same equipment they would implement for increased efficiency, such as belt cleaners and impact cradles, can also help prevent fires. Impact cradles and support cradles help reduce fugitive material by providing a flat, stable belt surface that facilitates effective sealing.
Transfer chute design is a critical element of fugitive material control, with sufficient settling zones and confinement. New raised chute designs also allow for external wear liners that eliminate the need for confined space entry or torch removal. When combined with improved skirt seal designs, spillage and dust emissions are drastically reduced, which protects rolling equipment from fouling and seizure. The decrease in fugitive material reduces the collection of potential fuel around the conveyor and requires considerably less labor for cleanup.
Modern belt trackers with troughed idlers or gripping return idlers are so sensitive that they detect mis-tracking the moment it happens and correct the belt path immediately, significantly reducing the chance of a friction fire due to belt drift. This also minimizes spillage from uneven cargo loads along the belt path. A heavy-duty belt cleaner system featuring modern primary and secondary cleaners increases the volume of discharge and limits the amount of carryback and fugitive dust. Along with limiting spillage, tail pulley health is improved, and there is less fouling of rolling components.
Conveyor belt fires often release dense black smoke that resembles a tire fire.
Solution
Improving the fire retardance of conveyor belting is just one of the ways that an operation can reduce the risk of conveyor fires. The development of fire-retardant conveyor belting has made an important contribution to safety, but even fire-retardant belts can burn in the presence of another fuel. These potential fire hazards can only be minimized through high standards of conveyor installation, maintenance and cleanliness.
Operators should regularly contain airborne dust at transfer points; remove accumulated dust from beams, pipes, conduits, equipment and fixtures to minimize spillage from belts. Damaged rollers must be identified so that they can be replaced as soon as possible before the damage leads to frictional heating that can result in a fire. Operators should use flame resistant grease and other lubricants. Fire detection and suppression systems must be tested according to industry and regulatory standards.
Conveyor belt alignment is also an essential prerequisite to belt safety. Belts must be properly centered to prevent frictional heating where the belt rubs against the conveyor structure. Another significant fire prevention measure is control of ‘hot work’ permits and procedures (for employees and outside contractors) and restrictions on the placement of flammable liquids and gases.
It is the employer’s responsibility to protect workers from one of the most potentially hazardous pieces of mechanical equipment at any facility: conveyors. Knowing the risks of fire, conducting periodic inspections, and striving to implement the steps and equipment needed for a safer workplace should be the goal of every operation. Experience has shown that clean conveyor systems, good fire-fighting capabilities and ongoing maintenance form the best defense against fire.
