UNDERGROUND SAFETY

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 ignite 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 described 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.

Note: Some countries have even more stringent requirements, for instance regarding the belt's toxicity, hygiene or cover roughness.

LIMITING OXYGEN INDEX (LOI/OI)

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, the LOI, was developed in Europe.

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.

LABORATORY-SCALE FLAME TEST

The belt sample size is 9 x 60 in (229 x 1524 mm)

Requirements as per MSHA 30 CFR 14.

In USA only.

FIRE SAFETY REQUIREMENTS AS PER DIN EN 12881 (GENERAL USE)

Fire Safety Requirements as per DIN EN 12881 (general use)
Cat. Application Surface resistance Drum Friction Ignition Determination
Proc. Flame Glowing Load Time

Max.
drum temp.

Summary
of six test specimens
Max. for each test specimen Covers
1 General use, risk only through electrostatic discharge. ≤ 300 MΩ Not required Not required Not required
2A  As for category 1, additional hazard from small open flames on the cover stock (additional causes of fire). ≤ 300 MΩ Not required 45 s 15 s With Not required
2B  As for category 2A, the additional risk is smaller, open flame on the carcass. ≤ 300 MΩ Not required 45 s 15 s With / without Not required
3A  As for category 2A, additional hazard of local heating due to friction. ≤ 300 MΩ A1 No Permitted Constant
343 N
1 h No 45 s 15 s With Not required
3B  As for category 3A, there is an additional risk due to small, open flame on the carcass. ≤ 300 MΩ A1 No Permitted Constant
343 N
1 h No 45 s 15 s With / without Not required
4A  As for category 1, additional risk of fire spreading caused by additional fire sources. Secondary safety device? ≤ 300 MΩ Not required       Undamaged piece over entire width of a 100 mm length
4B  As for category 4A, additional hazard of local heating due to friction. Secondary safety device? ≤ 300 MΩ A1 No Permitted Constant
343 N
1 h No       Undamaged piece over entire width of a 100 mm length
5A  As for category 4B, there is however an increased risk of local heating due to friction. Secondary safety device? ≤ 300 MΩ A2 No Permitted Max.
1,715 N
2.5 h No       Undamaged piece over entire width of a 100 mm length
5B  As for category 5A, with an additional risk from glowing. Secondary safety device? ≤ 300 MΩ A2 No No Constant
343 N
2.5 h No       Undamaged piece over entire width of a 100 mm length
5C  As for category 5B with an additional risk when operating in a potentially combustible atmosphere. Secondary safety device? ≤ 300 MΩ A2 No No Constant
343 N
2.5 h 400° C       Undamaged piece over entire width of a 100 mm length

Note: The higher the category, the higher the fire safety requirements. Since the differences are huge, it is mandatory to indicate the category and not only the standard.

A similar classification is given in DIN EN 14973 specifically for general underground use:

Class A  For general use, the only hazard being limited access and means of escape

Class B1  Same as class A, but potentially flammable atmosphere. No secondary safety devices installed

Class B2  Same as class A, but potentially flammable atmosphere. With secondary devices installed.

Class C1  Same as class B1, plus combustible dust or material conveyed. No secondary devices installed (the strictest class).

Class C2  Same as class B1, plus combustible dust or material conveyed, and additional fuel sources. Secondary devices installed.

Strict requirements for conveyor belts are also as per DIN EN 22109 (textile) and 22129 (steel cord) for hard coal underground use.

A comprehensive description of international safety standards has been published here:
http://www.msha.gov/Beltair/OtherSubmits/Kusel%20of%20Phoenix%20Conveyor%20Belts.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