21 Dec A flame detector for all reasons

Flame detectors use optical sensors to detect flames due to fire and are key components of a fire detection system in for example, the oil, gas and petrochemical industries. A variety of sensor technologies are available. In this article Dr. Daniel Waldron, Research Engineer with Fire Fighting Enterprises Ltd. explains how one particular approach to flame detection can create a device that is capable of detecting all types of burning fuels. This eliminates the need for matching a detector type with a certain fuel, reducing the complexity and cost of selecting a specific detector for a specific fuel type.

Most flame detectors work by looking at the energy emitted by the hot by-products of a fire. Of these by-products, one of the most common is carbon dioxide (CO2), which many fuels produce when burned. The hot CO2 produces substantial amounts of energy at a wavelength of 4.3 microns, which an infrared sensor can detect. The simplicity of this approach makes for very dependable detectors, but leaves open a key weakness – what happens when a fuel does not contain carbon?

Flames are naturally turbulent, exhibiting modulation or ‘flicker’.

An opportunity missed
Many manufacturers have made complex, expensive detectors that add different sensors into the unit, allowing the unit to detect multiple fuel types. This approach does overcome some of the issues with a single waveband detector, but misses the opportunity to make a much better detector. Instead of relying on multiple sensors to look at different areas, why not design a unit where every sensor sees across a wide spectrum? The use of multiple sensors then directly increases the performance and reliability of the unit. By pursuing with this philosophy in mind, it is possible to retain the sensitivity of the flame unit, whilst increasing its resistance to false alarms.

The power of spectrum

Every object in the universe is continually emitting and absorbing heat energy in the form of infrared
radiation. The hotter the object, the more energy it emits. Importantly, the type of energy emitted also changes. In the same way as a bulb filament becomes hot and finally incandescent, you can say the same of a flame. As the flame gets hotter, it will start producing infrared light first, before producing visible light and finally, ultraviolet light. This characteristic light produced by a flame is one of two main criteria that the detector uses to make a decision. The other is looking for the flicker of a flame.

Nature does not make straight lines
If you carefully watch a piece of wood burn in a fire, you will notice that the flames do not flow smoothly – rather they flicker, roll and move, producing light that has an element of randomness to it. This natural flame is turbulent, the variation in fuel and access to air creating the observed changes. FFE’s Talentum detectors only accept light that has this natural variation (what we have termed ‘flicker’), allowing the detectors to easily discriminate between a real flame and a false source.

Note that a synthetic flame (such as that from a Bunsen burner or small lighter) produces a smooth flame without flicker – the detector ignores these sources, going into fire only once a natural fire is detected.

The combination of the two characteristic components of a flame (amount of energy emitted and characteristic flicker) makes for a far more rigorous detection decision, whilst additionally covering all fuel types.

The false alarm resistance of the detectors, combined with their reliability and rapidity of detection makes Talentum ideal as a fire detection solution in an aircraft hangar.

False alarm resistance
One of the most signifcant benefits of this approach to flame detection is in the systems’ resistance to false alarms. The specialist infrared sensors at the heart of the detectors cross-over one another so that the system becomes in effect, double or triple knock (i.e. two or three sensors have to be activated together to signal a fire) and the fire safety team can be confident that it is not a false activation. This is especially important in the types of industry that these units are installed in. Within critical applications/industries such as power generation, oil and gas installations, recycling, industrial manufacture and other hazardous environments, the cost of false activations can be prohibitive. Reliability and confidence in the alarm is a serious consideration and treated with the same importance as accurately detecting the fire in the first place.

Sulphur flames are very challenging to detect as they produce a very limited light spectrum.

One detection technology for any fuel
Whilst false alarm resistance is key, a device that can detect flames from any form of combusting fuel is also very important. This includes common, carbon materials such as coal, cotton, paper, aviation fuel, petrol and other standard materials as well as detecting fuels that have traditionally been very difficult to sense, including items such as sulphur, hydrogen, fluorine and magnesium. In a situation where there is doubt as to the nature of your risk, or you are detecting multiple fuel types within a single area, broad-spectrum detectors offer the user an excellent solution.