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Burning Questions: A Belt Fire Safety FAQ for Your Facility
Elevators and feed mills could hardly function without belts for conveying product and transferring power, but like every useful tool, belts come with significant safety issues. NIBA-The Belting Association has provided this FAQ that addresses fire and explosive atmosphere issues you face with belts at your facility.

What specifically is meant by “flame retardant?”

In the U.S. a flame resistant belt is one that meets the flame resistant standards of the U.S. Department of Labor, Mine Safety and Health Administration, Schedule 2G. It presumes a mine belting application.

Note: For the purposes of this discussion, the following terms are synonymous:

Flame Resistant
Flame Retardant
Fire Resistant
Fire Retardant

Will a flame resistant belt burn?

Yes. In some circumstances, any flame resistant belt will burn and even propagate flame. If the defenses against fire built into the belt are overwhelmed, the belt will burn.

Are there different types of flame resistance?

Yes, depending upon the governing body involved. Each such body will define fire resistance or flame retardance by some set of test criteria that may involve an honest attempt to simulate their own most pressing problems, an honest difference of opinion and economic and political considerations.

Test criteria currently used can include some or all of the following:
1. Small scale flame tests.
2 Large scale (gallery) flame tests.
3. Drum friction.
4. Electrical resistance tests.

What criteria does MHSA use in Schedule 2G to define flame resistance?

MSHA uses a small scale flame test. The code of Federal Regulations, Title 30, currently describes a test method and acceptance criteria for approval of conveyor belts used in underground coal mines. The test method defines: number of specimens tested, specimen orientation, ventilation conditions, ignition source, and exposure time. Measurements are made of the afterflame time and afterglow time of each specimen. Acceptable performance is an average afterflame time of less than 60 seconds, and an average afterglow time of less than 180 seconds. This type of test generally distinguishes between belts that will continue to burn once the ignition source is removed and those that are self-extinguishing.

How about large scale flame tests?

If the defenses against fire built into a belt are overwhelmed, the belt will burn.

Some authorities have argued that small scale flame tests are too limited by scale (approximately 500 watts of energy) and that they are not designed to provide information on flame spread rate. Large scale flame tests are an attempt to put a full-sized belt (full width and varying lengths up to 60 meters) into a simulated mine gallery under so-called “real end-use” conditions. An ignition source in the range of 100 kilowatts or more is used. Conditions of acceptance are that the belting shall be self-extinguishing after the flame is removed and that some length of the belt sample shall remain undamaged across its full width.

Does frictional heating have a role in fire resistant considerations?

Both the small scale and large scale tests involve a flame heating source. In conveyor/elevator applications, frictional heating is frequently cited as the sources of belt fires. This may be the result of a frozen idler, a poorly tracked belt, or slippage between the belt and the drive pulley.

Typically, a belt sample is wrapped around and held fixed while applying pressure to a revolving drum. The test is run to destruction or limited to specific time interval. An acceptable belt will show no sign of flame or glow and the drum will not exceed a set temperature. For example, in Canada the temperature limit is 325 degrees Centigrade.

What is the best definition of fire resistance/flame retardance?

Because of the application, economic, and political considerations that go into any definition of fire resistance, there is no best definition.

Be aware of any specific fire resistant specifications which are mandated for the end-use belt application under consideration, whether mandated by governmental decree, or industry custom. Do consult with your belting manufacturer.

What are the sources of static electricity we need to be on guard against?

Static electricity is defined as electrical charge at rest. It can be generated by the triboelectric effect and can be accumulated by conductive and inductive charging.

When two surfaces in close proximity are moved relative to one another, a static charge is generated--the triboelectric effect. Conveyor and elevator installations are classic examples. In operation, the belt surface is continually leaving the pulley surface, generating static electricity. The surfaces can be similar, dissimilar, conductive or non-conductive and static electricity will be generated. Varying the types of surfaces can vary the amount of static charge generated (this is relatively insignificant), but the generation of static electricity cannot be eliminated.

Further, as the conveyor or elevator continues to operate, the static charge will continue to accumulate and increase unless it is bled off in some manner. In a grain elevator application, probably the greatest source of static electricity is the movement of grain down a chute that has been lined with urethane or UHMW polyethylene. The individual grain particle which is now charged will carry that charge onto the conveyor or elevator belt where it very well may be accumulated.

How can we control static electricity?

In a conveyor or elevator belt installation, we cannot control static by eliminating generation. However, accumulation and storage of static electricity can be controlled. First of all, we must keep in mind that static electricity is being continuously bled out of the system by the surrounding atmosphere. This is not an insignificant effect.

For example, a person walking across a carpet can generate 35,000 electrostatic volts at a relative humidity in the 10-20% range. That same person walking across that same carpet would only exhibit 1,500 electrostatic volts if the relative humidity were in the 65-90% range, due to the high bleed off of static charges by the humid air.

Accumulation and storage of static charges can be controlled by making the entire system sufficiently conductive and contiguously and properly grounded. This means that the belt, pulley lagging, pulley, bearing, structure, and electrical ground must all be connected electrically. It also means that conductive grease would be essential in the bearings.

What are the harmful effects of static electricity?

The effects of static electricity stem from the voltage accumulated, a subsequent electrostatic discharge resulting in an electromagnetic pulse, and discharge current.

Certain electronic components and assemblies are voltage sensitive -- 20 volts being sufficient in some cases to cause damage.

Other electronic gear (such as memory systems) can be upset by an electromagnetic pulse which is the result of an electrostatic discharge. Other gear can experience failures which are power, and hence current, dependent.

Finally, the heat of a spark discharge can ignite a gaseous or dusty explosive atmosphere, provided the spark discharge energy exceeds the minimum ignition energy requirement. It should be noted that an explosive atmosphere of certain types of starch dust can be ignited with a minimum ignition energy of 20 millijoules (Reference: “Explosibility of Agricultural Dusts,” Murray Jacobsen, John Nagy, Austin R. Cooper and Frank J. Ball, Bureau of Mines, R.I. 5753--1961).

How much conductivity is enough?

Extensive studies by the British National Coal Board (BNCB) from 1950 to 1966, reported by Barclay and summarized by Norman, found that belts with a surface electrical resistance of 1 x 109 ohms or under did not retain a static charge when run on a typical grounded conveyor. Those with a surface electrical resistance of 6 x 109 ohms and greater did retain static charges. As a result of this work, the B.N.C.B. concluded that a maximum of 3 x 108 resistance was a safe condition in new conveyor belts in underground coal mines.

What standards are generally accepted?

In the United States, OSHA has also selected 300 meg-ohms (300 x 106 or 3 x 108) as the definition of “static conductive” for grain applications.

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