Hi everyone. I’m Jonathan Saint with Net Safety and I want to talk about ways to prevent false alarms with flame detectors. I wrote on this topic in a recent edition of Petroleum Review. If you’d like to see the whole article, click HERE.
Most flame detectors function using optical systems like ultraviolet (UV) and infrared (IR) spectroscopy. Almost all flames produce heat, carbon dioxide, carbon monoxide, water, carbon and other products of combustion, which emit visible and measureable UV and IR radiation. These spectral emission “by-products” are what flame detectors sense to quickly and accurately determine the presence of a fire. These same emissions from non-flame sources cause nuisance false alarms and plant shutdowns.
Today’s optical flame detector options include single wavelengths of UV and IR, integrated UV/IR sensors, and more advanced units that offer triple wavelength IR sensors. The performance and advantages of each of these systems vary a lot.
A huge consideration in the selection of a flame detector is the potential for false alarms. False alarms are generally not the result of an issue with an instrument but rather its response to non-flame radiation sources that fall within their field of view. There are two basic types – natural and man-made. Natural sources include rain, lightning and sunlight while man-made source examples are artificial light sources, welding, and radiation from heaters and machinery. And falling into these two primary sources are four primary types: solar-blind UV; window contaminates; non-modulated IR; and modulated IR sources.
With non-modulated sources of radiation, the energy is constant over time or varies at an extremely slow rate. Examples of these are IR energy emitted from heaters, lamps and heat from the sun. Additionally, there’s a small amount of IR radiation emitted from all objects which is constantly present in any detector’s field of view. To overcome this, the majority of flame detectors available on the market today are designed to only detect modulated IR radiation sources – a key characteristic of flames. With modulated sources, characterized by varied and sporadic energy or as a combination of non-modulated sources, identification can be very challenging. Examples of these false sources are heated emissions, moving lights, signals or combinations of non-modulating sources being altered by objects moving back and forth in front of them in between the source and the sensor (vehicles, personnel, or fan blades for example). This is overcome by the use of multi-bands which can distinguish on the IR spectrum between flames and other sources of radiation.
While UV detectors work well in sunlight, other factors in outdoor applications may negatively affect them. UV sensors are designed to monitor solar-blind UV, the band of UV energy that is blocked by the ozone layer in the upper atmosphere. Powerful sources of this energy wavelength are commonly produced in industrial settings by halogen lamps, arc welding and even lightning. Additional bands can be employed, or combined UV/IR detectors will overcome almost all of these sources of inference.
Finally, window contamination will negatively affect the detector’s performance and can cause the instrument to go into fault mode. Attenuating energy sources will hit or deposit on the window face of the detector as well as accumulate on external reflectors used for automated visual integrity testing. Water droplets, condensation, snow and ice are powerful absorbers of IR energy that can be delivered in random scales and intensity and are a well-known source to trigger false alarms or faults when combined with modulated energy sources like direct sunlight. UV radiation is also easily absorbed by a range of oils, smoke, carbon and specific gases. Engineers need to be aware of the presence of vapors such as hydrogen sulphide, benzene, ammonia, ethanol, acetone and others when selecting a flame detector for their application.
There’s no perfect flame detection system for every application – all have challenges. But understanding the type of fire to be detected, the environmental conditions surrounding the installation, and the required performance makes the choice of flame detection technology a much more manageable decision. A detection solution that allows for field sensitivity and time delay settings will help mitigate the more challenging false alarm sources by allowing users to fine-tune their instruments in situ for optimal performance.