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It has been established by Patol Limited, that each of the above requires a different form of fire detection to provide a reliable, fast response and trouble free system. 

Conveyor Systems transporting Powder River Basin Coal (PRB Coal) are especially at risk.
The unique nature of PRB Coal and its "self ignition" characteristic presents an even greater need to install Fire Protection of an appropriately effective form. 

It is the object of this data sheet to indicate general solutions, however it should be emphasized that any specific installation/site/plant must be addressed individually in order to obtain the correctly engineered system. 

Also see : COAL CONVEYOR FIRE PROTECTION - GUIDELINES

2.0  STATIC FIRE ON THE CONVEYOR

In the conveyor structure, material (such as coal) which falls from the moving belt, or an accumulation of settled dust from the transported substance, provides a ready source of combustible matter. PRB Coal can in fact self ignite in a very short time period

A mechanical fault in the bearing of a roller, or friction caused by the build up of material around a roller, or a friction fault between the belt and a stalled roller, may result in a heat source sufficient for initiation/combustion (fire). 

The ignition criteria for each material being transported must be considered. Some materials, such as Powder River Basin Coal (PRB Coal), may have an combustion risk without a fault generated heat/ignition source.

Many different types of fire detector have been tried for material conveyors over the years including--

In every case, experience has shown that these devices are either  unsuited to the environment producing unwanted alarms due to dust or fog, or are so insensitive that a fire can propagate and cover many metres of the length of the conveyor before they are operated.

Patol Limited's solution to the above is Linear Heat Detecting Cable - L.H.D.C. - Linear Heat Detector - LHD

Tests have been carried out to check the response time of  Linear Heat Detecting Cable - L.H.D.C. in a number of different locations within the conveyor housing. 

As a minimum, a detection "run" should be installed above the centre of the belt at a height 1.0 to 1.5 metres. Duplexing of this "run" , especially with sideways separation, enhances both responsivity and coverage and is recommended. Ideally detection "runs" should also be sited immediately above the return roller on each side (beneath the bottom plate on most conveyor systems), however, experience has shown that rigorous cleaning regimes, especially on coal conveyors, can cause damage to the L.H.D.C. when installed in this location.

If the conveyor housing has a pitched roof and contains more than one conveyor it is recommended that a detection "run" is installed close to, but not directly in, the apex.

The above arrangements have been employed on a number of large Power Station installations and have been found to be trouble free.

The zoning of the conveyor is dependent upon whether the system is for detection only or is used in association with an "electrically actuated" sprinkler or waterspray system.

As an indication of relative performance, L.H.D.C. located on a conveyor, as detailed above, responded within two minutes to a small coal fire with a surface area of approx. 0. 1 sq metres, where a conventional sprinkler bulb took over 12 minutes to respond to a timber fire of surface area of approx. 1.5 sq metres.

There have been situations where conventional sprinkler heads have failed to prevent the spread of a belt fire in a Power Station conveyor system due to the delay in operation.

The use of L.H.D.C. has become an  established industry standard and is of  particular import in harsh environments where other types of detectors would be constantly raising unwanted alarms. When combined with  "electrically actuated" sprinklers  L.H.D.C. provides the ultimate in reliability with rapid response of the protection system.

It must be appreciated that L.H.D.C.  is only recommended for a static fire either on a conveyor belt or in the conveyor housing. Due to the rapid movement of conveyor belts (up to 6metres per second) L.H.D.C. does not provide an adequate form of detection for a moving belt fire. (SEE FOLLOWING)

3.0  IMPORTED FIRES ON MOVING BELTS

A number of years ago research was conducted such as to find  the ideal "moving" belt fire detector. This being undertaken when it became obvious that existing systems employing thermocouples and heat detectors in hoods were unable to detect anything other than a very large fire.

It was clear that the reason why Linear Heat Detectors and thermocouple hoods were not able to detect small fires moving at speeds of up to 6metres per second was that they relied upon convected and radiated heat. Even the very low thermal mass of the thermocouples used in the hoods (working on the differential between hot and cold junctions) cannot produce reliable results.

In order to gain sufficient sensitivity it was clear that a radiation detector which operates on an emissions principal, as opposed to  thermal effect, would be the only reliable device with sufficient sensitivity. The result was the development of an infra-red pyrometer based system which has been employed in a series of trials.

The device developed is a quantum infra-red detector designed with parameters that incorporate the principals laid down by both Wien's & Stefan's laws of physics. Simplistically, the device is tuned for wavelengths which enable it to detect required black body emissions yet is virtually blind to direct and reflected radiation from the sun. Conventional flame detectors have proven to be unsuitable as they only monitor diffusion flames within predetermined flicker frequencies and are "blind" to the black body radiation from smouldering fires.

The Patol Limited 5000 Series Infra-red Detector is mounted above the conveyor at a height typically of between 1.0 and 1.5 metres such that its optical system can cover the full width of the belt. The response of the detector depends upon the size and surface temperature of the object. Typically the alarm can be given if an object of 250mm by 250mm at a temperature of 100 degrees centigrade passes beneath the detector at a speed of up to 6metres per second. The system can also detect hot coals as little as 25mm (1inch) in diameter.

This reponcivity encompasses the detection of PRB Coal in its self combustion phase but before ignition.

The detector will respond to a hot spot beneath the surface, as well as a flaming fire, provided that the temperature/area relationship at the surface is within the alarm level.

The infrared detector should be mounted at sufficient distance back from the discharge end of the conveyor such that on detecting a fire the belt would be stopped :-

The 5000 Series Infra-red Detector would be used to raise an alarm and stop the conveyor.

At this stage, the L.H.D.C.run above the belt would then be able to detect the fire, confirm the alarm and where necessary actuate the waterspray system.

Where the environment in the location of the infrared detector is particularly prone to airborne dust then it is recommended that the air purging accessory is fitted to the detector in order to keep the optical system clean. Tests have shown that the detector can still raise an alarm when a relatively small fire moving at 4 metres per second is viewed through a cloud of coal dust.