Flame-Actuated Detectors

Flame-actuated detectors are optical devices that "look at" the protected area. They

Figure 7-13.\Infrared flame detectors.

generally react faster to a fire than nonoptical devices do.

INFRARED FLAME DETECTORS.\ Figure 7-13 shows two typical infrared (IR) flame detectors. IR flame detectors respond directly to the IR, modulated (flickering at 5 to 30 cycles per second) radiation from flames. The sensor design usually incorporates a delayed response, selectable in the range of 3 to 30 seconds, to minimize responses to nonfire sources of radiation. Thus, alarms are caused only by sustained, flickering source of IR radiation.

The IR flame detector is ineffective for smoldering or beginning fires. It is used where possible fires would develop quickly (fuels, such as combustible gases and liquids, or loose cotton fiber), and it is capable of protecting a large area if it is mounted high on a ceiling or wall (30 to 50 feet).

The sensitivity of IR detectors to a fire is affected by the distance of the device from the fire. For example, if the distance is doubled, the fire has to be four times as large to be detected. To maintain immunity to possible nonfire sources of alarms, you should usually select longer response delays (10 to 30 seconds) for low (8-foot) ceiling mounting. Shorter delays, in the range of 3 to 10 seconds, are used when detectors are mounted on higher ceilings. For high-hazard areas, the detector can be mounted on a low ceiling and a low delay setting used to obtain sensitivity and fast response. Shields to eliminate possible false alarm sources from the field of view of the detector are sometimes used, especially in a high-sensitivity application of the device.

Some detector models designed for fast response do not have the "flicker" discrimination feature, but instead have two sensors with different spectral responses. These sensors are used to distinguish between an actual fire and other sources of IR radiation.

Glowing ember detectors are nondiscriminating and fast acting. Ambient light levels must be maintained below 20 footcandles. Location and shielding are important for this type to avoid false alarms caused by incandescent lamps and sunlight.

ULTRAVIOLET FLAME DETECTORS.\ The ultraviolet (UV) flame detector is extremely fast and is used in high-hazard applications, such as aircraft maintenance areas, munitions production, and other areas where flammable or explosive liquids or solids are handled or stored. The detector responds to UV radiation not visible to humans. Figure 7-14 shows a typical UV detector. The detector and circuitry may be in a single housing or in separate housings. They act

Figure 7-14.\Ultraviolet flame detector.

together as a normally open switch that becomes momentarily closed, causing an alarm, when UV radiation enters the detector viewing window. Response time is typically less than 25 milliseconds for an intense UV source. Some models have a built-in short time delay (3 seconds, nominal) to reduce responses to lightning and other momentary events.

Frequently, separate relay contacts are provided for immediate and delayed alarm outputs, adjustable up to 30 seconds. A visual indicator, visible through the viewing window, usually indicates detector actuation.

The UV detector is capable of use in explosive atmospheres, and some models have swivel mounts for directing them at specific hazards. Various models have angular fields of view ranging from 90 to 180 degrees. Sensitivity is usually factory set for the application.

TESTING FLAME-ACTUATED DETEC-TORS.\Flame-actuated detectors should be inspected monthly for physical damage, accumulation of lens deposits, and paint. A spot of paint on a lens can prevent the detector from "seeing" a critical area in the protected space. Remove or protect the detectors when painting is being done.

Be sure that auxiliary functions of the flame detection system are deactivated before testing is done unless these features are intentionally being tested. Before the test, inform the fire department and persons who would hear the alarm.

False alarms or failure to detect during a test may be caused by environmental factors or the aiming of the detector. During the monthly inspection, check that detectors are not blocked and that lenses are shielded from direct rays of the sun and other sources of IR, such as welding equipment, in the case of UV detectors.

If a detector has a clean lens but fails an operating test, make adjustments and/or perform other field maintenance, following the manufacturer's instructions. Obtain field service by a factory-trained technician or return the equipment to the manufacturer for repair.

Infrared Detectors.\On IR detectors (fig. 7-13), the dark spot or dome at the bottom center of each IR device is the lens. Detector lenses must be kept clean to ensure the earliest possible detection of a fire. Test at least one detector in each initiating circuit (zone) monthly. Follow a rotation schedule so that all detectors are tested semiannually.

A small soldering iron held 6 inches in front of a glowing ember detector can serve as a heat source for testing. A 250-watt IR heat lamp several feet from the detector can serve as a flame substitute in testing an IR flame detector.

Ultraviolet Detectors.\Keep UV detector lenses totally clean. A gradual buildup of contaminants frequently found in high-hazard spaces (oil, gasoline, petrochemicals, salt, and dust) block UV radiation. A layer thin enough to be undetectable to the human eye can cause a UV detector to be completely blind. Clean lenses according to the manufacturer's instructions.

A test feature designed into some detectors allows for checking the optical integrity of the device. A small UV source inside the detector housing is shielded from directly illuminating the sensor. Local or remote operation of a test switch deactivates alarm circuits and illuminates the test lamp. The test lamp rays then pass through the front window to the sensor. Detector response to the test indicates that the window is clean and that the sensor and electronic circuits are operational.