Page 405 - Advanced Mine Ventilation

374 Advanced Mine Ventilation

22.6.1 Detection of Belt Fires

The old technique for detecting belt fire is known as point-type heat sensors (PTHSs).
They are simply a bimetallic sensor that measures a rise in temperature above a base of

57 C (134.6 F). Because of the low ambient temperature in mines, the fire volume has
to be large to activate the alarm. By then it may be difficult to contain it.
Current technology to detect a belt fire depends on carbon monoxide concentration
in the air. It works in conjunction with PTHS but can detect a fire as far as 1500 ft
away. Usually, an audible alarm sounds and it can activate a water spray system
that extinguishes the fire.

22.6.2 Preventing Belt Fires

Preventing belt fires requires the following measures:
1. Use belts that can withstand heat without catching fire.
2. Clean all spilled coal promptly to avoid belt rubbing on coal.
3. Prevent slippage of belt by maintaining adequate tension.
4. Maintain the idler/rollers. Damaged rollers should be removed and replaced promptly.
5. Check for misalignment and tension periodically.

Current regulations require all the above precautionary measures. Slippage switches
are installed at belt drives that can deenergize the drive motor if the roller is turning
without belt movement. Most conveyor slip switches are designed to work on one
of the three principles:
Magnetism,
Centrifugal force, or
Photoelectricity.
The most common device is a slip switch proximity sensor. They detect any inter-
ruption in the magnetic field by targets installed on a rotating shaft or a roller. Another
device monitors the voltage and current for the main drive motor. Such data are
communicated to the mine office and corrective actions are immediately taken.
Electrical sources of fires near a belt conveyor account for 13% of ignitions
and fires. Cutting and welding on belt structure contributed 10% of reported belt
fires. These fires can be minimized by proper training and maintenance of equip-
ment. Safe work practices must be developed and workers should be annually
refreshed.

References

[1] Elfstrom RH. Explosion in No. 26 colliery, Glacier Bay, Nova Scotia, on February 29,
1979. Canadian Department of Labour; 1980. p. 122.
[2] Thakur PC, Davis JG. How to plan for methane control in underground mines. Mining
Engineering 1977:41e6.