The detection of fire is based on a number of properties including smoke emission, a rise in temperature and optical radiation emissions. The most rapid and consistent technique of identifying a fire in large spaces or outdoor environments is optical flame detection. Emission of visible (Vis), ultraviolet (UV) or infrared (IR) radiation forms the basis of this kind of detection.
Definition of a Flame
When a hydrocarbon flame undergoes combustion (oxidation reaction), carbon dioxide (CO2) and water (H2O) are produced, in an excited state (*):
2 C6H14 + 19 O2 + Energy ⇒ 12 CO2*+ 14 H2O*
By photon emission at particular wavelengths, these molecules return to their basic state.
CO2* ⇒ CO2 + hυ (2.9 to 4.3µm)
H2O* ⇒ H2O + hυ (2.8µm)
Figure 1. Hydrocarbon fire of a typical emission spectrum
Different Types of Detectors
The different types of detectors are:
Ultraviolet (UV) Detectors
UV radiation with a wavelength below 300nm alone is picked by an ultraviolet detector. The sensor includes a photo-tube with an anode and cathode arranged in roughly a 300V potential difference.
Infrared (IR) Detectors
These pyroelectric sensors are capable of detecting thermal radiation and show sensitivity to changes in the received light signal. A lithium/tantalum crystal is linked with a field effect transistor or an OP amp. A spectral gate or an optical filter chooses a specific spectral band or wavelength (2.9µm, 4.3µm, 4 to 5µm…).
The flame emits random flickering in the infrared band, which is perceived by the crystal to generate a signal to be processed by a low frequency bandpass filter (1-20 Hz), before a microprocessor interprets the signal. It is more economical and consistent than an ultraviolet detector.
UV / IR combined detectors
This is a combined detector integrating gate AND between "UV channel" and "IR channel" technologies to enable superior rejection of false alarms and a very good detection over long distances.
UV / IR² combined detectors
The integrated UV/IR detector, which deploys a performant UV sensor, has the disadvantage of a limited distance of detection (detection range) due to the infrared portion. By using further information in the IR band with a second sensor, the detection range is improved and these are significantly immune to false signals.
Figure 2. UV/IR² Combined detector
Multi-IR detectors eliminate UV and multiply data in the IR domain. These devices depend on the CO2 IR band at 4.4µm. The H2O IR band (2.9µm) is used by some devices for detection of hydrogen or ammonia fires. These detectors have a very low false alarm rate and a long detection range.
Figure 3. Multi-IR detector
Detection by Imaging
This method is also common for flame detection. The basis is image processing issued from CCD matrices. These were restricted to the visible range; however, some now have infrared matrices with a spectral filter like the one used in conventional multi-IR detector for enhancing sensitivity to fire.
Almost all devices include adequate protection modes under regulatory requirements (ATEX / IEC / FM / UL / CSA…) and classic information outputs signal (4-20 mA / relay / ModBus / ...).
From a metrological point of view, optical flame detectors have specific features. The sensitivity of an optical flame detector is affected by:
- The fuel – all combustions do not generate the same quantity of CO2 or H2O molecules.
- The fire size – in geometric terms the amount of signal obtained by the detector decreases as the square of the distance between them.
- In case the fuel is gaseous, the nozzle diameter is not sufficient, this needs to be completed by data related to flame height or flow.
- reabsorption of CO2 radiation in the optical path, the attenuation by smoke, rain, fog, wind speed on the fire, the presence of interfering sources may also interfere.
Figure 4. Sensitivity parameters
This is normally linked with a specific fire as it normally decreases with the distance between the device and the fire. It significantly varies from one technology to the other and can vary from below one second to thirty seconds based on technologies, manufacturers and fires.
Cone of Vision
This is a highly delicate feature but critical in terms of device positioning. Manufacturers provide values between 90° and 120° on the horizontal axis, at certain times less on the vertical axis, because of the optical elements essential for self-test.
Figure 5. Cone of vision
False Alarm Immunity
This is critical as the financial consequences are significant in case of accidental activation of the extinguishing system.
Choice of Technology
Based on the following criteria the right choice of detector can be made. These include:
- The fuel should not have hydrocarbon molecules. The ideal device would be one configured on UV or H2O emission-based infrared.
- The risk source is at a short or a very long distance.
- The work place is a closed environment, where quick accumulation of smoke may take place or vapors or other chemical compounds may be present.
- The work area is outdoors. Here single UV technology is very risky.
- The coverage area includes hot equipment with chances of strong air convection and the presence of carbon dioxide. In order to enhance resistance to false alarms, UV technology (UV, UVIR, UVIR²) can be used.
- The alarm triggering must be very short (<1 sec).
- “Friendly” fires like a flare is in the field of view of the device, live or by reflection, and the signal is strong enough to activate a traditional detector.
Installation and Setup
The installation needs to consider the device cone of vision to cover the area that needs to be monitored especially with recoveries between detectors. Also, the devices are normally installed in a high location resulting in a “shadowed” area at the bottom of the detector.
It is better that the detectors are not installed using the IR bands on pipes that can wobble with a strong wind, for instance, so that a background signal is generated that cannot interfere with the detection.
The key part of the optical flame detectors enables the user to fine-tune the sensitivity and the time delay before triggering an alarm, depending on the type of risk to be monitored and the environment.
Figure 6. Installation and setup
There is a normative framework for other device aspects beyond the hazardous area equipment standards.
EN 54-10 - Detection and Fire alarm systems - Flame detectors - Point detectors.
Even though this is not mandatory, the evaluation of devices based on IEC 61508 standard is more often required by users. Beyond the definition of reliability parameters (SFF, PFD, MTBF, MTTR), certification covers the software, both in its structure and in its development process. In addition, a Safety Integrity Level (SIL) number is determined for the safety function provided by the device.
This information has been sourced, reviewed and adapted from materials provided by Teledyne Gas & Flame Detection.
For more information on this source, please visit Teledyne Gas & Flame Detection.