Sensors & Transducers


Conveyor system protection

January 2013 Sensors & Transducers

Conveyor systems are not new. However, the fire protection of conveyors has not stayed in line with the advances in belt transfer technology over the years: measures such as thermocouple hoods and ember detectors have not been proved as effective as was hoped.

Now, AST can provide an effective fire detection and protection system for conveyors of all forms. A fire condition within a conveyor system may be considered to be:

* A static fire on the conveyor belt, or within the conveyor mechanism.

* An imported fire on a moving conveyor belt.

It has been well established by fire engineers around the world, that each of the above requires a different form of fire detection to provide a reliable, fast response and trouble free system. It is the object of this article to indicate general solutions; however it should be emphasised that any specific installation/site/plant must be addressed individually in order to obtain the correctly engineered system.

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. A mechanical fault in the bearing of a roller, or friction caused by the build-up of coal around a roller, or a friction fault on the belt itself, may result in a heat source sufficient to initiate combustion.

Tests have shown that coal is not readily ignited by hot objects such as metal falling from welding or cutting operations. It is also evident from tests that even a mass of several kilograms of red hot metal when placed on top of or into coal dust or ‘fines’ will not impart sufficient energy to cause smouldering (smouldering is considered to be the increase in temperature of a slowly increasing volume of coal over a long period). Many kilowatts of energy are required to raise the temperature of coal to the point where smouldering occurs leading to gases being generated which subsequently ignite. It is evident then, that a mechanical fault in the bearing of a roller, or friction caused by the build-up of coal around a roller, is not a sufficient heat source to set the coal alight.

However, coal is not the only substance to be transported by conveyor and the ignition criteria for each material must be considered. Many different types of fire detector have been tried for material conveyors over the years including:

* Collective reflectors (thermo-couple hoods).

* Ionisation and optical point smoke detectors.

* Point heat detectors.

* Point flame detectors.

* Obscuration detectors employing infrared beams.

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.

AST’s solution is a linear heat detecting cable (LHDC). Tests have been carried out to check the response time of an LHDC 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 of 1-1,5 metres. Duplexing of this run, especially with sideways separation, enhances both response time 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 LHDC 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 water spray system.

As an indication of relative performance, LHDC located on a conveyor as detailed above, responded within two minutes to a small coal fire with a surface area of approx. 0,1 square metres. By contrast, a conventional sprinkler bulb took over 12 minutes to respond to a timber fire of surface area of approx. 1,5 square 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 LHDC has become an established industry standard and is of particular importance in harsh environments where other types of detectors would constantly be raising unwanted alarms. When combined with electrically actuated sprinklers it provides reliability with rapid response of the protection system.

However, it must be appreciated that LHDC 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 4 metres per second) LHDC does not provide an adequate form of detection for a moving belt fire.

Imported fires on moving belts

A number of years ago research was conducted to find the ideal moving belt fire detector. This was undertaken when it became obvious that existing systems employing thermocouples, heat detectors in hoods, flame detectors and ember detectors 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 4 metres per second was that they relied upon 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 infrared pyrometer based system which has been employed in a series of trials. The device is a quantum infrared detector. Simplistically, it 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.

The AST Pyroscan3 radiation detector is mounted above the conveyor at a height of 1-1,5 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 250x250 mm at a temperature of 100 degrees centigrade passes beneath the detector at a speed of up to 4 metres per second. The system can also detect hot coals as little as 25 mm in diameter.

The detector will respond to a hot spot beneath the surface, as well as a flaming fire, provided that the temperature at the surface is within the adjustable 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:

* Before the conveyor can discharge the fire to a hopper, another conveyor etc.

* Where the fire would be beneath a protection system such as a water curtain.

The radiation detector is used to raise an alarm and stop the conveyor. At this stage, the LHDC run above the belt would then be able to detect the fire, confirm the alarm and where necessary actuate the water spray system.

Where the environment in the location of the infrared detector is particularly prone to airborne dust, it is recommended that an 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.

For more information contact Grant Wilkinson, Alien Systems & Technologies, +27 (0)11 949 1157, grant@astafrica.com, www.astafrica.com


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