Coal Conveyors - Fire protection guidelines - Including PRB Coal

radiators as defined by Planck. E.g. for C02 formation a peak occurs at 4.3 microns which may be compared with specific narrow bands where little emission occurs. Other types filter shorter wavelengths, similar to spark detectors (e.g. 0.7 to 2.7 microns), and apply analysis to detect “flame flicker”. These techniques monitor for volatile fluids and gases after “flash point” and are not suited for close range monitoring of moving bulk materials such as coal when on conveyor belts.

¨ IR Black Body Emission Detectors

These employ IR filters that select longer wavelengths and are “blind” to the visible spectrum. They can detect both the high energy emissions from very hot / glowing embers, and those from abnormal but relatively low temperature bulk coal transiting the monitored belt area.

It is this type of detector that is best suited to Coal Conveyor monitoring.

Response Summary

Coal is a good thermal insulator and therefore buried hot/burning coals will raise the belt load surface temperature by a relatively small amount. Similarly, coal in its pre-ignition cycle has an abnormal but relatively low temperature.

Whilst some detectors may be able to respond to burning coal (550ºC to 1000ºC / 1000ºF to 1800ºF ), the correct type of black body monitor will also respond to an abnormal but non-ignited conveyor load (100ºC to 200ºC / 200ºF to 400ºF ). Spark, Ember & Flame detectors will not respond to the latter condition.

Detectors must be blind to “visible radiation” from both local illumination units and reflected sun light.

It is not a requirement that detectors should be immune from being directly pointed at the Sun; it being a massive IR energy source.

Obviously the detector should anyway be directed at the conveyor for best performance. (See following)

Installation Performance

The Detector when mounted above the conveyor at a height of between 1.0 and 1.5 metres (3ft - 5ft), should be able to cover the full width of the belt with its optical system.

The response of the detector depends upon the size and surface temperature of the object. Typically an alarm must be given if an object of area 0.025 sq m (40 sq inches) at a temperature of 140ºC to 190ºC (280ºF to 380ºF) passes beneath the detector at a speed of up to 6m/s (20 ft/s).

The system must also detect glowing coals of 30mm diameter (1 sq inch in area) and less.

Detector Location

The infrared detector should be mounted at the loading end of each conveyor such that on detecting a fire the belt could be stopped:-

¨ Where the fire would be within a water spray deluge curtain.

¨ Before the conveyor can discharge the fire to another belt or silo, etc.

The Infra-red Detector is configured to automatically stop the conveyor and raise an alarm.

At this stage the water spray can be initiated :-

¨ Automatically by the IR Detector.

¨ Manually, after an operator has observed the belt.

¨ Automatically after a time delay, if not overridden by the operator.

¨ Automatically by LHD detection.

As these “stop zones” will be remote from the coal plant control room, they could be monitored by CCTV cameras.*

* If it is opted to always automatically initiate water spray from the IR Detection equipment then CCTV is un-necessary.

In addition to the arrangements described above for belt “loading” points, an IR Detector should be installed just prior to the conveyor system discharge to silos or hoppers.

The detector must be located sufficient distance back from conveyor end in order to be able stop the detected “hazard” within a water spray zone and before discharge of the belt.

In the case of very long conveyors due consideration should be taken of the fact that abnormal heat can be aided to develop into ignition by the movement of the coal in air and therefore it may be prudent to install additional intermediate IR detection units.

Special Considerations

Coal conveyor tunnels and housings are an extremely dusty environment. IR Detectors employed must accommodate “air cleansing” for optical lenses.

The same consideration applies to CCTV cameras and there will be an economy in sharing a clean air supply.

All equipment must be able to withstand the rigors of “housekeeping hose-down” arrangements.

For further information see :-