By Dr. Daniel Waldron

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. This article 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.

What is different?

The technology behind Talentum flame detectors is unlike that of any other detector in the market.  Whilst other detectors concentrate on a single area of the infrared light spectrum, these detectors examine a much broader range, whilst also looking for the characteristic flicker of a flame.  This approach is difficult to implement, but creates a detector with excellent all-round performance, sensitive to all fuel types and at a lower cost per unit than standard flame units.

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?

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.  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.

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).  This approach makes sure that when a Talentum device signals a fire you can have confidence 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.

The Talentum range comprises three different detectors, all based around the same core concept, but with a differing resistance to false activations.  These are the IR2, IR3 and UV/IR2.

UV/IR2

The UV/IR2 offers the best detection speed, flexibility and false alarm resistance of any detector in the range.  With sensing elements in the ultraviolet and near infrared region, it looks at the complete energy characteristics of a flame.  This allows it to discriminate between a flame and a wide variety of false alarm sources, including sunlight, welding, artificial lighting and lightening.  The only drawback to the UV/IR2 is that, within a very dirty environment the UV signal can drop, making the detector less sensitive and therefore requiring a larger flame to activate the detector.  For this reason, the IR3 was developed.

IR3

Designed with dirty environments in mind, the IR3 is a detector that can cope with the high levels of local pollution.  Relying on three IR sensors to function, the infrared light of the flame can easily penetrate through the majority of dust and dirt that builds up in heavy, industrial environments.  The three sensors allow the detector to disregard some false sources such as lighting and indirect sunlight.   As such, these detectors are usually our first choice for industrial applications, where constant maintenance can be difficult and the environment challenging.

IR2

This model is the most basic in the range, covering the near infrared region with its two sensors, both of which must activate to signal a fire.  This model has the least false alarm resistance, but can detect a fire very rapidly, making it ideal for detecting flames on units such as conveyors or other internal processing systems, where external  light sources are kept to a minimum (Talentum also offers specialist machine fire detection).  However, for the majority of applications, the UV/IR2 or IR3 are more suitable.

Completing the job

With all models, an extensive list of accessories and housings allows them to be installed in the most hazardous and challenging environments.  This includes 316-grade stainless steel housings for high corrosion environments, Exd flameproof and Intrinsically Safe (IS) detectors for hazardous applications and weather shields for external use.

One detection technology for any fuel

Whilst false alarm resistance is very important, what really makes the technology behind these detectors stand out is its sheer flexibility in flame detection.  The UV/IR2 offers the user a device that can detect flames from any form of combusting fuel.  This includes common, carbon materials such as coal, cotton, paper, aviation fuel, petrol and other standard materials. The UV/IR2 has also detected fuels that have traditionally been very difficult to sense, including items such as sulphur, hydrogen, fluorine and magnesium.  Whilst other manufacturers may offer several solutions to cover the full range of fuel types, the Talentum UV/IR2 can sense all of them.  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, these broad-spectrum detectors offer the user an excellent solution.  You can have confidence your life safety equipment will work, whatever your flame risk.

Dr. Daniel Waldron is Product Manager at FFE Ltd.