Flame Detectors

Flame detectors are optical fire detection devices, which are able to detect infra-red and/or ultra violet radiation given off from a flaming fire.

Flame detectors are ineffective for slow smouldering fires where traditional smoke detectors would be more suitable however flame detectors will generally respond far quicker to rapidly developing fires such as combustible gases and liquids etc.

Flame detectors come in many sizes and variations but generally fall into three groups.

Infra-red Flame Detectors

I.R. Flame Detectors respond to flaming fires emitting light in the infrared area of the spectrum (modulating at 5 to 30 cycles per second). I.R. Flame Detectors can respond to a fire condition in typically less than 50 milliseconds and are designed to detect hydrocarbon fires whilst ignoring potential false alarm hazards such as arc welding, nuclear radiation and x-rays.

The sensor usually incorporates a delayed response, selectable in the range 3-30 seconds, to minimise responses to non-fire sources of radiation. In this way alarms are only generated by a sustained, flickering sources of I.R. radiation - e.g. fire.

The sensitivity of Infrared Flame Detectors is affected by the distance of the device from the fire source such that, if a distance is doubled the fire has to be four times as large to be detected. The trigger delay is therefore adjusted to suit the installation conditions shorter delays for lower mounting, longer delays for higher mounting.

High Risk Areas

For high-risk areas, a detector may be set with a low delay threshold for greater sensitivity and shields used to eliminate possible causes of false alarms.

Some detectors designed for very fast response do not have the 'flicker' discrimination feature but instead have two sensors with different spectral responses in order to distinguish an actual fire from other sources of IR radiation.

Ultra Violet Flame Detectors

U.V. Flame Detectors are extremely fast and capable of detecting flaming fires emitting light in the ultra violet area of the spectrum. Typically, U.V Flame Detectors can respond to a fire in less than 10 milliseconds and are used in high hazard applications such as petrochemical plants, munitions factories and other areas where flammable or explosive liquids or solids are handled or stored.

Ultra Violet Flame Detectors can be used in explosive atmospheres and some are capable of being directed at specific hazards.

Models can have angular fields of view ranging from 90 to 180 degrees. Sensitivity is usually factory set for the application.

U.V Flame Detectors should not be used around arc welding, as they will respond to the UV light given off by the welding process.

Ultra Violet - Infra-red Flame Detectors

UV-IR Flame Detectors respond to flaming fires emitting light in both the ultra violet and the infrared area of the spectrum.

UV-IR Flame Detectors require both the UV and the IR sensors to alarm simultaneously thereby making them highly resistant to common sources of false alarms such as welding, x-rays, lightning, artificial lighting and interrupted hot body radiation.

Maintenance

Due to the adverse environmental conditions and contaminants (oil, petrochemicals, dust etc.) in which these detectors are often fitted, regular, frequent and thorough maintenance by competent operatives is essential.

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