Few industrial technologies are as ubiquitous as flame detection. Most facilities need flame detectors as their first line defense in protecting people and property. However, just knowing you need a flame detector isn’t enough to make an informed decision. Below are a few principles of flame detection that will contribute to your understanding of this much needed technology.
Today’s flame detectors utilize optical technologies to detect flames. Flames are known to emit electromagnetic radiation in the infrared (IR), visible light, and ultraviolet (UV) wavelengths depending on the fuel source. Because of this, optical flame-sensing technologies have been developed utilizing UV, UV/IR and multi-spectrum infrared. The effectiveness of a given flame detector is dependent on the size of the area to be monitored, the detection technology, and the nature of the flame’s fuel source. Further, these optical sensors rely on a clear line-of-sight to detect radiation emitted by a flame. This forces the operator to also be mindful of potential obstructions and blind spots in the detection area.
When selecting flame detection for your plant, consider whether the potential flame you are looking to detect will be hydrocarbon-based or non-organic. With hydrocarbon flames, a combination of methane and oxygen produce hot CO2 and H2O gases. This type of fire creates a UV signal reading at 0.2 microns, and an IR signal reading at 2.7 microns and 4.5 microns. Hydrogen flames only produce water molecules and therefore do not produce the same CO2 peak that a hydrocarbon flame is known to produce. As an example, the Rosemount 975UF ultra-fast ultraviolet infrared flame detector can detect hydrocarbon-based fuel and gas fires, hydroxyl and hydrogen fires, as well as metal and inorganic fires. The above chart will guide your selection of technology.
False alarms are a continuous challenge in flame detection. Environmental conditions such as sunlight, lightning, hot objects, and other non-flame sources can cause false alarms. It is critical that the flame detector is able to distinguish between an actual fire and a false radiation source. Outdoor applications must contend with the visible range of sunlight, which covers 0.3 to 0.8 microns. A detector that reacts to direct or reflected sunlight is clearly not appropriate for these applications. UV detectors generally detect energy below solar emissions (0.185 to 0.260 microns) and can be a suitable choice for outdoor applications because of their extremely fast response and wide field of view; but UV/IR and triple IR options offer higher immunity to potential false alarms from high-energy bursts from reflective surfaces. The Rosemount 975UR UV/IR flame detector incorporates a special logic circuit that helps prevent false alarms caused by solar radiation.
Flame detection technologies effectively recognize a flame at different distances. It is important to utilize a detector that can operate within your required detection range. Likewise, optical flame detectors come with a “field of view” range similar to the lens on a camera. Taking the field of view range into account will help determine how many detectors are needed to cover an installation or a specific area within the installation. For example, the Rosemount 975MR multi-spectrum infrared flame detector detects fuel and gas fires at long distances. This product will detect a 1 ft2 (0.1m2) gasoline pan fire at 215 ft (65 m) in less than 5 seconds.
How do you use flame detectors to achieve optimum coverage?