Not all flame and gas hazards in facilities are the same, therefore the detection technology chosen should be specific to the type of hazard to be detected. How do users select technologies to see certain fires or detect certain gases? The intent of this paper is to give a broad introduction to the selection and use of flame and gas detector technologies.
We will answer questions such as these: When should a flame-detector solution use infrared vs. ultraviolet detectors? Which gas-detector technologies are appropriate for flammable gases, hydrocarbon gases, and toxic gases?
One size does not fit all
A safety engineer at an LNG facility reviews two flame detectors: an uncertified infrared (IR) detector and an FM-approved triple IR detector. Which should she choose? Meanwhile, a plant manager, considering gas detectors for his offshore facility, researches which type of detector – electrochemical or semi conductor – is proven to sense hydrogen sulfide accurately. What criteria should he use? Today’s flame and gas detection technology should be applied specifically to the hazard or hazards to be detected. One type of detector technology might be more suitable than another in a specific situation.
In general, a hydrocarbon fire emits CO2, carbon, water, and heat (IR). Certain flame detectors see these types of combustion products. But those same flame detectors, for example, might not see SO2 that is produced by a sulfurfueled fire – therefore, that sulfur-fueled fire might go unseen.
Because not all fires are the same, a flame detector must be matched to the type of fire fuel that it is expected to see. Various technologies and algorithms enable detectors to be sensitive to certain fuel fires. These technologies primarily use the following emissions spectra for optical flame detection:
- infrared (IR)
- ultraviolet (UV)
- visible spectrum (CCTV - based devices)
In choosing flame detectors, it is important to recognize that they consist of window(s), optical sensor(s) – such as Geiger Muller and/or thermopiles, and electronics in a suitable enclosure. To differentiate a fire from a non-fire event, detectors include special optics and processing algorithms.
- The optics filter out the non-fire spectra emissions to minimize false alarming. For example, sunlight should be filtered so it doesn’t cause alarms.
- Sophisticated algorithms analyze the optical signals and determine whether the detector is seeing a fire or non-fire event. These algorithms are, in most cases, patented and closely guarded.
This is why different manufacturer’s detectors have different performance, although they might use the same technology.
Flame detection technologies
The technologies widely used today are based on UV and IR sensors, and combinations of UV and IR sensors. CCTV is an emerging technology being used in some flame or smoke detection applications.
UV-based detectors – The first flame detection technology used, UV detectors provide good response to a broad spectrum of hazards. Lightning, the sun, and arc welding can cause false alarms. Also, radioactive sources can generate false alarms because UV detectors are based on a Geiger- Mueller tube. The performance of UV detectors might be impaired by certain chemicals that attenuate UV light, such as oils, silicones, ethanol, and ammonia. Consult your detector manufacturer for a comprehensive list of UV-attenuating chemicals.
IR-based detectors – IR emissions are generated by any material that is above absolute zero, such as the sun, a person, or any black body. IR detectors can be prone to false alarms from chopped or modulated IR sources. Carbon-based fueled fires are strong emitters in the IR spectrum, therefore single-IR and dual-IR detectors are suitable for their detection.
Multiple IR (MIR) – The MIR (or triple IR) sensor flame detector (Figure 1) is a newer technology that is displacing the other technologies in many applications, due to better performance and fewer false alarms. These detectors have the greatest on-axis detection ranges (>200ft) of any technology.
UV/IR – Because the combination UV/IR detector reduces false-alarm issues, it has been popular. But it is being displaced by the new generation multi-IR detectors.
CCTV – An emerging technology, there are a few CCTV flame detectors available for specific fuels. CCTV devices use visible light for flame detection, rather than the spectral emissions of the products of combustion. Due to the use of visible light for flame detection, their performance can be affected adversely by ambient lighting conditions.
Selecting a flame detector
After identifying the fuel source of the fire to be detected, review the performance standards with NRTL (Nationally Recognized Testing Laboratory) verification. Specifically, in Europe the performance standard for optical flame detectors is EN54-10 ‘Flame Detection – Point Detectors,’ or ANSI FM 3260 in the USA.
The standard test fuel in most standards is n-Heptane. But because that might not be the fuel in your hazard, confirm that the selected detector will see the hazard in a specific application. The detector manufacturer should provide documented, third-party verification for cones of vision, size, and response times to various fuels.
Maintenance: Because flame detectors are optical devices, they depend on the cleanliness of their lenses, which can be obscured by insects, ice, oil mists, and other environmental contaminants. Many detectors have optical integrity tests that verify the lens cleanliness and signal failure. Some contaminants can be handled by detector innovations, such as heaters to displace rain and ice. In addition, some manufacturers provide built-in calibrated test lamps so you are assured they are operating correctly.
Field of View: Many manufacturers claim various cones of vision from 120° to 90° and maximum on axis distances now beyond 200ft. The US and European standards both define the off-axis limit to be where 50% of the on-axis detection capability is lost. See figure 2 for an example cone of vision.
Gas detection may measure several characteristics of gases, for example toxicity or flammability. To detect a particular gas successfully requires determining a unique attribute for the target gas that can be sensed and measured.
- Flammable gases are measured as a percentage of their Lower Explosive/Flammable Limit (LEL/LFL). For example, Methane LEL is 5% in air. Toxic gas detectors in general are measured in parts per million (ppm).
- Hydrocarbon gases can be detected either by their combustibility or IR absorption properties.
- Toxic gases are more difficult to detect due to their various chemistries and unique characteristics. The primary toxic sensor technologies used are solid state and electro-chemical, although there are emerging technologies that use optical sensors.