34 Safety
Selecting the Right Flame Detector
Edward Naranjo, Product Manager, General Monitors Tel: +1-949-581-4464
Email:
info@generalmonitors.com • Web:
www.GeneralMonitors.com
Te petrochemical and other hazardous industries require continuous flame monitoring equipment to prevent catastrophic fires. To select such equipment, the users should understand the principles of flame detection and review the types of detectors available. Armed with this knowledge they will be able to match the appropriate flame detector to process and site performance requirements and to the type of hazard whose consequences the instrument is designed to mitigate.
Typical Flame Hazards
The range of potential flammable hazards is expansive and growing as materials and processes become more complex. Increasingly sophisticated flame sensing technologies with embedded intelligence are required to detect the most common industrial fuels:
• Alcohols • Diesel • Gasoline • Kerosene • Jet Fuels • Ethylene
• LNG/LPG • Hydrogen • Paper/Wood • Textiles • Solvents • Sulphur
Principles of Flame Detection
Most flame detectors identify flames by so-called optical methods like ultraviolet (UV) and infrared (IR) spectroscopy and visual flame imaging. Flames are generally fueled by hydrocarbons, which when supplied with oxygen and an ignition source, produce heat, carbon dioxide, and other products of combustion. The intense reaction is characterised by the emission of visible, UV, and IR radiation (Fig 1). Flame detectors are designed to detect the absorption of light at specific wavelengths, allowing them to discriminate between flames and false alarm sources.
Flame Sensing Technologies
There are four primary optical flame-sensing technologies in use today: ultraviolet (UV), ultraviolet/infrared (UV/IR), multi-spectrum infrared (MSIR), and visual flame imaging. They are all based on line-of-sight detection of radiation emitted in the UV, visible, and IR spectral bands by flames. Technologies may be selected to suit the requirements of flame monitoring applications, including detection range, field of view, response time, and particular immunity against certain false alarm sources.
ULTRAVIOLET VISIBLE INFRARED Figure 2 – Examples of UV/IR Flame Detectors UV Flame Detectors
UV detectors, such as the General Monitors Models FL3101H and FL3111, respond to radiation in the spectral range of approximately 180-260 nanometers. They offer quick response and good sensitivity at comparatively short ranges (0 – 50 ft). Because they are susceptible to arc welding, halogen lamps, and electrical discharges like lightning, they tend to be sited indoors. Thick, sooty smoke can also cause failures due to attenuation of the incident UV radiation.
UV/IR Flame Detectors
When a UV optical sensor is integrated with an IR sensor, a dual band detector is created that is sensitive to the UV and IR radiation emitted by a flame. A combined UV/IR flame detector, such as the General Monitors Models FL3100H or FL3110 (Fig 2), offers increased immunity over the UV detector, operates at moderate speeds of response, and is suited for both indoor and outdoor use. As with UV detectors, however, the detection range of these instruments may be reduced by heavy smoke.
Multi-Spectrum Infrared Flame Detectors
SUN’S ENERGY REACHING THE EARTH
Multi-Spectrum IR flame detectors use multiple infrared spectral regions to further improve differentiation of flame sources from non-flame background radiation. These flame detectors, such as the General Monitors Model FL4000H (Fig 3), are well suited to locations where combustion sources produce smoky fires. They operate at moderate speed with a range of up to 200 feet from the flame source — both indoors and outdoors. These instruments exhibit relatively high immunity to infrared radiation produced by arc welding, lightning, sunlight, and other hot objects that might be encountered in industrial backgrounds.
300nm 400nm Figure 1: Emission Energy Spectrum. Wavelength OCTOBER / NOVEMBER 2013 •
WWW.PETRO-ONLINE.COM 800nm 2-5µm
Figure 3 – General Monitors’ FL4000H MSIR Flame Detector incorporates Neural Network Technology to discriminate between a real fire and false alarm source.
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