Hi everyone. I’m Larry McGee and today I want to talk about a significant development in optical flame detection. Optical flame detectors have historically been fitted with a visual integrity (VI) test feature in order to detect the presence of accumulated material on the lens that would prevent the device from detecting flame – obviously a pretty critical alarm.
Most of the principle VI schemes have employed an internal light source that projects a beam through the lens and into a metal reflector from which the beam is reflected back through the lens and onto the primary sensor where it verifies the integrity of the optical path – maybe! Problems arise because the reflectivity of the metal reflector can be compromised by accumulations of airborne materials and corrosion, thus, the reflector must be maintained on a regular basis. In addition, the lens can have distributed deposits of material that actually block flame signals, but allow the test beam to pass through normally giving a false impression of integrity. Another known issue occurs when reflectors become misaligned and the beam bouncing back to the sensor actually misses, creating a fault condition.
The solution to the first issue, with regard to maintaining reflectors for the purpose of visual integrity testing, comes from the technology developed by Net Safety. Our Triple Infrared (IR) detectors employ a system of visual integrity testing that does not use external metal reflectors, greatly reducing VI faults that are often caused by problems with metallic reflectors. The Net Safety technology involves three beams of multi-wavelength infrared energy that are directed through the lens and reflected off the front surface of the lens and back onto the sensing elements. This reflection is caused by the difference in refractive index at the face of the lens where the sapphire lens material ends and atmosphere begins. The amount of energy reflected from a clean, IR transparent lens is known and measured during each VI test cycle. When material that reflects infrared energy (which can obscure the passage of infrared energy into the sensor and reduce its flame detection sensitivity) is deposited on a lens, there is an increase in the amount of energy reflected back into the sensors. This increase in reflected energy is detected and, at a predetermined level, causes the VI fault to trip. With this unique technology we completely eliminate the external reflector from the VI process. This makes verifying optical integrity much more resistant to the VI fault condition, whether by obstruction or reflector misalignment.
In practice, particularly in the oil and gas industry, it is rare to find accumulations of material on the sapphire lens that will block the flame detection capability of the Net Safety Triple IR flame detector. This is in stark contrast to an ultraviolet (UV) flame detector where even a thin, invisible film of oil will fully obscure the flame detector. The materials present in these facilities are predominantly hydrocarbon based materials which are substantially transparent to the wavelengths of infrared energy that are detected and analyzed to identify flame in the detector’s field of view. Testing has revealed that a 1 to 2mm layer of thick, black, petroleum based material has little effect on the flame detection capabilities of the IR instrument. While such a coating is startlingly obvious to the human eye and should be removed as a matter of course during routine maintenance, it has little effect on IR flame detection and hence does not trigger the VI fault condition. While many materials that typically occur in industrial applications, and that could be expected to coat the lens of a standard flame detector, have been tested, none have been found which will substantially obscure flame detection and not be detected by this advanced Triple IR VI verification technology.
Thank you so much for visiting the blog. If you have questions on any aspects of fire or flame detection technology, please leave me a comment here.