Flame detector

A flame detector is a fire detector that is activated by the electromagnetic radiation emitted by flames. The common types sense ultraviolet (UV) or infrared (IR) radiation, and some units combine both. Unlike a smoke detector or a heat detector, a flame detector does not wait for combustion products or hot gases to reach it. It responds to radiation that travels in a straight line from the flame, so it must have a clear view of the fire.

Flame detectors often start fast-response extinguishing or explosion suppression systems, which makes a reliable response within the required time essential. The SFPE Handbook gives the example of infrared detectors that must find a 1 m² pool fire of jet fuel in a 20 m high aircraft hangar. UV detectors give the fastest response of the detectors used to trigger suppression of gas explosions. The range of a flame detector depends on the size of the fire, the distance, the angle to the detector's axis and the absorption of radiation by the air. Designers therefore work with manufacturer curves of fire size against distance, which are based on the inverse square law. For analysis and design, the International Fire Engineering Guidelines (2005) refer designers to the relevant standards, handbooks and manufacturer data.

How flame detectors work

Every fire detector senses one or more fire signatures. The SFPE Handbook lists these signatures as radiation, invisible and visible products of combustion, a rapid change in temperature and a high temperature. Smoke detectors respond to the products of combustion and heat detectors respond to temperature. Ultraviolet and infrared detectors are the only types in that list that respond to radiation. A flame detector is therefore one part of a fire alarm system that can sense a fire at a distance, before any heat or smoke reaches the ceiling.

The radiation that a flame detector senses is the same thermal radiation that transfers heat from a fire. Radiation is emitted by the surface of a hot body, which can be a solid, a liquid or a gas body such as a flame or a smoke layer. At the temperatures that matter in fires, most of this radiation lies in the infrared and part of it in the visible range; ultraviolet usually plays a small role in heat transfer. Gases between the fire and a receiving surface can absorb part of the radiation.

Fire detection devices operate in only one of three bands of the spectrum. The SFPE Handbook gives ultraviolet as 0.1 to 0.35 micrometres, visible light as 0.35 to 0.75 micrometres and infrared as 0.75 to 220 micrometres. Its table of fire signatures assigns the ultraviolet detector to radiation with a wavelength of 1700 to 2900 ångström, that is 0.17 to 0.29 micrometres. A sensor reacts only to specific wavelengths, so the fuel must emit radiation within the sensor's band. An infrared detector with a single sensor tuned to 4.3 micrometres, the emission peak of carbon dioxide, cannot be expected to detect a fire of a fuel that contains no carbon.

The three bands used by flame detectors, with the ultraviolet detector band and the 4.3 µm carbon dioxide emission peak. Based on Schifiliti et al. (2002), pp. 4-4 and 4-30.

Types and selection

The International Fire Engineering Guidelines name ultraviolet and infrared flame detectors as the two main examples. Combination ultraviolet/infrared optical flame detectors also exist; they were among the devices tested in large US Navy aircraft hangars, 15 m and 22 m high. Fires of jet fuel in those tests ranged from 100 kW to about 33 MW.

Selecting a sensor depends on the fuel, the growth rate of the fire, the ambient conditions and the control or extinguishing function that the detector must start. The designer must also consider absorption of radiation by the air, other sources of radiation in the area, the size of the spark, ember or fire to be detected, and the distance to the sensor. In the hangar example of the SFPE Handbook, infrared detection at 4.3 micrometres suits the carbon-based jet fuel. Infrared detectors were also expected to cover a larger area per device than ultraviolet detectors and so to cost less in that case.

Sources of radiation that are not fires can cause false alarms. Resistance to such false responses can be obtained by filtering, by using several sensors in one unit, such as a two- or three-channel detector, or by both. A system can also use coincident detection, in which two or more detectors must operate before an alarm is accepted. In explosion suppression, the choice between pressure sensors and ultraviolet detectors depends on the required response time and on concerns about spurious activation and about the detectors' view being blocked.

Detection distance and the inverse square law

The radiant power that reaches a sensor falls quickly as the distance to the fire grows. Standard practice for the design of radiant energy detection is based on curves of fire size against distance, which are derived from the inverse square law. The SFPE Handbook gives the relation as:

$$S=\frac{kP{e}^{-\zeta d}}{d^2}$$

Symbol	Description	Unit
$S$	radiant power reaching the detector	W
$k$	proportionality constant for the detector	–
$P$	radiant power emitted by the fire	W
$\zeta$	extinction coefficient of air	m−1
$d$	distance between the fire and the detector	m

The handbook prints the exponential term without its minus sign. The minus sign used here follows from the handbook's own statement that humidity and dust absorb radiation and so limit the radiation that reaches the sensor. Manufacturers test a sensor with several fuels and plot the normalised fire size against the normalised distance. The resulting curve gives the largest distance at which the sensor consistently detects a fire of a given size, usually stated in square metres of burning area.

As a worked example, the hangar case in the SFPE Handbook uses a detector that sees a 1.0 m² fire at 40 m on its axis. In clear air, with the absorption term equal to one, halving the distance to 20 m makes the received power four times larger, because it scales with one over the distance squared. The other way round, a fire that emits only half the radiant power reaches the same signal only at 40/√2, about 28 m. Real air absorbs part of the radiation, so these clear-air values are upper limits for the reach of the detector.

Relative radiant power at the detector against distance in clear air, set to 1 at 40 m. Halving the distance gives four times the power. Based on Schifiliti et al. (2002), pp. 4-31 to 4-32.

The handbook then applies typical reductions to the detection distance of the hangar detectors, which have a field of view of 45 degrees. A 30% loss of sensitivity at the edge of the field of view reduces the distance for the 1.0 m² fire from 40 m to 28 m. A further loss of 3% at an ambient temperature of 50 °C gives about 26.8 m, and another 3% at 90% relative humidity gives about 25.6 m. The largest viewing distance in the layout at 45 degrees is 20 m, so the design is considered valid. Had the distance dropped below 20 m, a different layout or different devices would have been needed.

Infrared flame detectors in an aircraft hangar: the detection distance for a 1.0 m² fire falls from 40 m on axis to about 25.6 m at the edge of the view, still above the 20 m needed. Based on Schifiliti et al. (2002), pp. 4-31 to 4-32.

Design of a flame detection system

Fire detection systems serve goals of life safety, property protection, business protection and environmental protection. Detectors may also start other fire protection systems, such as special extinguishing systems and smoke control systems. When a flame detector activates a fast extinguishing or control system, such as an explosion suppression system, it must respond reliably within the required time.

The SFPE Handbook describes the design in a fixed order. First a detector type is chosen that matches the fuel. Then the possible sources of interfering radiation are identified and a device is selected that resists them. Next, the manufacturer's data give the mounting height and lateral distance limits for the critical fire size. The layout must then account for all obstructions so that every part of the space is watched. Finally, the reductions in sensitivity due to viewing angle, temperature and humidity are checked against the actual viewing distances.

The International Fire Engineering Guidelines describe a flame detector as a point that receives radiation from a flame with a specific flame temperature and emissivity. The intensity of the radiation it receives can be calculated with the methods for radiation from fires in the same guidelines. The sensitivity varies with the direction of the incoming radiation, and this off-axis sensitivity must be considered in design. A method for estimating flame height is also needed for the operation of flame detectors. In New Zealand, the fixed detector criteria of C/VM2 (second edition, 2025) do not cover flame detectors. Its table gives activation criteria only for heat detectors, smoke detectors and sprinklers.

Applications

The sources describe flame detectors in large hangars, in process plants and in explosion suppression systems. The SFPE Handbook uses an aircraft hangar of 50 m by 80 m with a 20 m ceiling, where a fuel spill fire must activate a fixed suppression system. In the chemical process industries, smaller leaks may be detected by gas or flame detectors, if these are installed. A quantitative risk assessment should then establish the position, effectiveness and reliability of these detectors. It should also describe in detail how gas and flame detection start the emergency shutdown system.

In explosion suppression systems, detectors, a power supply and control system and fast extinguishing units work together. Dust explosion applications usually use pressure sensors, while gas explosion applications can use pressure sensors or ultraviolet detectors. Of these, ultraviolet detectors give the fastest response. The control system includes interlocks that monitor the system itself, shut down processes and trigger alarms when the system activates.

Detectors in such plants may be installed where a flammable gas atmosphere can form, and one way to prevent an explosion there is to exclude ignition sources. Low-power equipment can be designed so that even its worst fault cannot release the minimum ignition energy of a given gas; such equipment is called intrinsically safe. Where this is not possible, the equipment can be housed in an explosion-proof enclosure that stops a flame from spreading out of it. The fuel oil tank room of World Trade Center 7 is an example. All its electrical devices were explosion proof, suitable for Class 1, Division 1, Group C in the hazardous area classification of the New York City building code.

Limits

A flame detector is mainly a line-of-sight device. A fire hidden behind equipment, stored goods or structure may not be seen, which is why the layout must consider all obstructions. Sensitivity decreases as the fire moves away from the optical axis: a fire detectable at a given distance on the axis may not be detectable at the same distance 30 degrees off the axis.

The surroundings also matter. Humidity and dust absorb radiation, and a higher ambient temperature can lower the relative sensitivity of the sensor. Even a small decrease can affect the response to the expected fire. Other radiation sources in the area, or air that absorbs a large fraction of the radiation, may change the type, location and spacing of the sensors.

The detector also has to match the fuel. A sensor that reacts only to a narrow band, such as the carbon dioxide peak at 4.3 micrometres, will not detect fuels that do not emit in that band. Finally, the performance data of flame detectors come largely from manufacturers. The International Fire Engineering Guidelines therefore advise taking relevant standards, handbooks and manufacturer data into account in analysis and design.

See also

• Deluge system
• Beam smoke detector
• Multi-sensor detector
• Detector activation time
• Aspirating smoke detection

References

• Barry, T. F. (2002). Quantitative risk assessment in chemical process industries. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 5-176–5-213). National Fire Protection Association; Society of Fire Protection Engineers.
• Beyler, C. L. (2002b). Flammability limits of premixed and diffusion flames. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-172–2-187). National Fire Protection Association; Society of Fire Protection Engineers.
• Grill, R. A., & Johnson, D. A. (2005a). Documentation of the fuel system for emergency power in World Trade Center 7 (NIST NCSTAR 1-1J). National Institute of Standards and Technology.
• Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
• Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
• National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Schifiliti, R. P., Meacham, B. J., & Custer, R. L. P. (2002). Design of detection systems. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-1–4-43). National Fire Protection Association; Society of Fire Protection Engineers.
• Stroup, D., Lindeman, A., McGrattan, K., Peacock, R., Overholt, K., Joglar, F., LeStrange, S., & Montanez, S. (2016). Verification and validation of selected fire models for nuclear power plant applications: Supplement 1 (NUREG-1824, Supplement 1; EPRI 3002002182). U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research; Electric Power Research Institute.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
• Zalosh, R. (2002). Explosion protection. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-402–3-421). National Fire Protection Association; Society of Fire Protection Engineers.