Fire Zoning & Extinguishing

The design of the fire detection system must take into account the level of alarm definition required to provide personnel with adequate indication of event location.

Some LHD equipment types provide ‘Distance Monitoring & Display” units which indicate the exact length into the LHD installation that the initiating ‘fire event’ has occurred.

Normally each conveyor is sub-divided  into fire zones. Automatic initiation of Water Spray extinguishing by the LHD (if this has been implemented) is likely to be the defining factor regarding zone size due to :-

¨ Hydraulic limitations related to water delivery. (Pump & pipe sizing)

¨ The nature and efficiency of the water spray delivery.

¨ Consideration of the “clean up” activity needed after a conveyor loaded with coal has been deluged. Coal slurry  handling problems must be minimized.

Response Tests - Suppression

As an indication of relative performance between water spray operation that is initiated solely by self activating sprinklers, and that electrically initiated from LHD, tests have shown :-

When located above a conveyor at 1.0m (3ft) height LHD responded within 2 minutes to a small coal fire with a surface area of approx. 0.1 sq m. (1 sq ft)

A conventional sprinkler bulb took over 12 minutes to respond.

When combined with an "electrically actuated" sprinkler system, LHD provides the optimum rapid response protection system.

It must be appreciated that LHD is only recommended for detecting fires on a stationary conveyor belt or within the housing.

Fires on belts, which can be moving at up to 6m/s (20ft/s), can not transfer sufficient heat energy during the rapid transit to make LHD effective. (see following)

 

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 systems existing at that time, employing thermocouples and heat detectors in “heat collecting hoods” were unable to detect anything other than a very large fire.

The reason why these methods can not detect small fires moving at speeds of up to 6m/s (20ft/s) is that they rely  upon convected and radiated heat. Insufficient heat energy can be transferred, even to the very low thermal mass of the thermocouples, to produce reliable results.

The solution is to monitor for infra-red  “black body” emissions, as opposed to imparted thermal changes.

Infra-red emissions occur for all materials. The wave length spectrum and intensity of this IR depends on the material’s temperature, and for solid bodies such as coal is determined by the Laws of Physics formulated by Planck, Stefan, Boltzmann & Wien.

Planck’s Law defines the spectrum and level of IR emissions for a ‘black body’ at any given temperature.

 A great variety of infra-red detectors are available for fire protection. However, whilst all do monitor for ‘infra-red’ emissions they have very different uses and operational performances. It is essential that the correct type is employed.

Infra-red monitors fall into four principal categories :-

¨ Thermal Imaging Systems

This equipment employs an infra-red camera and produces a thermal image (photograph) which may be either directly observed or computer analysed for anomalies. Thermal imaging is a very useful tool for monitoring static bunkers and silos; however, it is not usually practical for conveyors, as imaging scan times are too slow for fast moving belts.

¨ Spark Detectors

These are designed to detect sparks and very small visibly glowing embers. These types monitor for short IR wave lengths that are at or near the solar band and are mainly intended for installation in ducts where there is a zero level of ambient lighting. These types are not solar blind and the IR filters (e.g. 0.4 to 1.6 microns) do not permit detection of IR emissions from relatively low temperature abnormalities such as pre-ignited coal.

¨ Flame Detectors

Many of these types monitor for the peak IR emissions, associated with specific gases, that  occur during combustion. These emission spectra are not the same as for “black body”

LHD & Catenary Wire Support

 

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 - 3