FIRE DETECTION

Despite the most stringent measures to prevent fires, even the best managed sites can experience fire events from time to time.

The faster that these are detected and addressed (extinguished) the lesser will be the cost due to plant damage and down time.

A fire condition within a conveyor system may be considered to be :-

¨ A "static" fire - On a stationary conveyor belt, or within the conveyor mechanism / housing.

¨ A "moving" hazard - Hot or burning coal imported onto a traveling conveyor belt.

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

It is the object of this guideline note 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.

Static Conveyor Fire

In the conveyor structure any coal which falls from the moving belt, or an accumulation of settled coal dust, is a potential hazard.

A mechanical fault in the bearing of a roller, or the friction between a seized roller and the belt, will result in a build up of heat which can be sufficient to ignite the belt, when the belt stops, on its own.

The introduction of coal makes a fire  highly probable.

Many different types of fire detector have been tried for conveyors. These attempts, some going back many years, include:-

¨ Collective reflectors (thermo-couple hoods)

¨ Ionization & optical point smoke detectors

¨ Point Heat Detectors

¨ Point Flame Detectors

¨ Obscuration detectors employing infra-red beams

¨ Pneumatic detectors with fusible bulbs

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 static conveyor before they are operated.

The true solution has been determined to be Linear Heat Detection. (LHD)

 

 

 

 

 

 

 

 

LHD takes a number of forms, the three principal ones being :-

¨ Electrical - Analogue

¨ Electrical - Digital

¨ Optical - Analogue

In all of these cases the LHD comprises a robust ‘Cable’ that will ‘Annunciate’ an alarm if any portion of the LHD experiences an abnormal temperature.

Being a sealed ’Cable/Wire’ the LHD is not prone to the water & dust problems that cause conventional smoke / heat / beam detectors to be ineffective in the rigorous environment of a coal conveyor housing/enclosure.

Whilst each supplier of LHD will have their own recommendations regarding location and mounting of the detector, all types respond to heat. Tests have been carried out to determine the optimum location for the LHD.

To protect the upper side of the conveyor itself a detector "run" should be installed above the centre of the belt at a height of 1.0m to 1.5m (3ft-5ft).

Normally this is achieved by the use of a steel catenary support wire to which the LHD is affixed .

Ideally LHD "runs" should also be sited at the conveyor sides such as to detect fire heated air ‘spilling’ upwards around the belt edges from events occurring on the underside. The detector is mounted above the return roller (belt) on each side.

However, experience has shown LHD when installed in this location can be prone to damage from rigorous cleaning regimes and maintenance activity such as roller changes.

Each specific conveyor arrangement must be considered in order to determine the optimum mounting method. The mechanical arrangement of some conveyor truss work can provide a suitable mounting without further protection such as slotted steel trunking or LHDC Armour.

The LHD must always be mounted (by clips or spacers) such as to be in the air flow. It must not be directly mounted to steelwork which would act as a heat sink and retard response.

In some cases, especially with knowledge of site operating procedures, it may be deemed that LHD side runs are not practical.

Enclosed conveyors should have extra  detection at ceiling height to detect PRB dust ignition on cable trays, pipes etc located above the conveyors. With a pitched roof enclosure LHD should be located near the apex.

Rectory Road,  Padworth Common,  Reading,  Berkshire. UK     RG7-4JD

Tel: +44 (0) 118 9701701     Fax: +44 (0) 118 9791700     Email: info@patol.co.uk

Fire-protection guidelines for

conveyors transporting coal - 2