Conveyor belt fire safety

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CONVEYOR BELT SAFETY IN UNDERGROUND COAL MINING

Conveyor belts represent a high risk, because they have the ability to spread a fire over long distances.

Fires on belt conveyors are mostly ignited by mechanical failures like frozen idlers, which is even more dangerous in combination with coal dust.

The fire safety requirements are similar for the most hard coal mining countries. Examples are shown below.

Drum Friction Test The drum friction test simulates a belt slipping over a jammed pulley, or a pulley rotating under a stationary conveyor belt. This measures whether the surface temperature remains under a required maximum after a specific time and under a specific tension. Generally a rubber conveyor belt can produce a pulley surface temperature of up to 500°C (930°F). The visual appearance of flame or glow is not permitted.
Surface Resistance Test An electrostatic charge may build up on the conveyor belt surface and iginite a mixture of flammable gases and air. Therefore the surface resistance of the conveyor belt covers has to be below 300 MOhms.
High-Energy Propane Burner Test Conveyor belts must not propagate fire. In order to determine whether a conveyor belt fulfills this requirement, it is ignited by a propane burner. After the ignition source has been removed, the flames must self-extinguish within a certain time frame or within a certain distance. A typical sample for this test is 2 m long at full width.
Laboratory Gallery Test A small scale version of the Propane Burner Test, first describe in DIN 22118
Spirit Burner Test A small piece of the conveyor belt is held over a spirit burner flame. After a certain time the burner is removed. The duration of flame and glow has to be within a specified time limit.
Some countries have even more stringent requirements, for instance regarding the belt's toxicity, hygiene or cover roughness. The exact specifications can be found in the relevant standards.
It is obvious that not every single belt can be tested as per the above procedures. However, there is an easy, cheap and efficient way to check whether the belt supplied is in accordance with the agreed fire safety requirements. This kind of finger print method was developed in Europe: The Limiting Oxygen Index Test(LOI) This method is an effective and inexpensive quality control. It determines the minimum concentration of oxygen that will support combustion of a specific belt sample. The result of the approved belt must comply with any supplied belt of the same type.
The requirement in the United States, in foce until the end of 2009, is quite simple: Bunsen burner test A small piece of a belt is held over a Bunsen burner flame and the duration of the belt's afterglow must be less that a specified maximum time. This test, which was implemented as per the 1969 Federal Coal Mine Health and Safety Act, is similar to a standard that was in force in Europe until the mid 1970's for underground conveyor belts (fire resistant, grade K or S). Later, these conveyor belts have only been allowed to be used in above ground applications.
The new test apparatus as per MSHA 30 CFR 14. Laboratory-scale flame test The belt sample size is 9 x 60 in (229 x 1524 mm).
A comprehensive description of international safety standards has been published here: http://www.msha.gov/Beltair/OtherSubmits/Kusel%20of%20Phoenix%20Conveyor%20Belts.pdf In December 2007 the Final Report of the US Technical Study Panel recommends the use of fireproof conveyor belt: http://www.msha.gov/BeltAir/BeltAirFinalReport122007.pdf A presentation on an underground coal mining steel-cord conveyor belt after 15 years of operation can be viewed here: http://www.youtube.com/watch?v=3vBTfYd1T6c

Note: Fatalities in the Aracoma Alma Mine in West Virginia have focused again on conveyor belt related fires and the devastating impact it has on miners' safety. On December 31, 2008, the MSHA has published new requirements (30 CFR 14). As of December 31, 2009, new belts need to have a higher flame resistance. After another 10 years, all belts supplied until 2009 have to be replaced.
In contrary to what was expected, viz. that the USA are going to implement safety requirements on the international level - which have been in force for many years in Europe, Australia, Canada, India, China etc. - they can apparently be met with conventional material (f.i. SBR), just adding more fire retardants to the rubber compound. This is frustrating from the safety viewpoint. And this slight enhancement in fire resistance is on the account of lower wear resistance. A sketch of the new test apparatus is in the chapter above.