What Makes Up a Fire Protection System (based on Det-Tronics white paper,click on the title to view the article)
A fire protection system is comprised of several subsystems that can include, but are not limited to: flame, smoke and gas detection; notification and/or suppression activation; and a controller that receives the inputs from the detection devices, makes decisions and initiates appropriate action or actions.
Effective protection is based on the materials and fuels present, the processes involved, the environment, and other control measures present. Based on these variables, an effective detection and suppression system may require multiple technologies to effectively detect the hazards.
Flame detectors
NFPA 72 describes a flame detector as “a radiant energy-sensing fire detector that detects the radiant energy emitted by a flame.” Flame detectors are line-of-sight devices that can employ several sensing technologies: ultraviolet (UV), infrared (IR), ultraviolet/infrared (UV/IR) and multi-spectrum
infrared (MSIR).
There are many factors to consider when selecting the type, quantity, location, and spacing of flame detectors, including:
• Matching the spectral response of the detector to the spectral emissions of the fire or fires to be detected.
• Minimizing the possibility of spurious nuisance alarms from non-fire sources inherent to the hazard area.
NFPA 72 addresses these selection factors in Chapter 17:
• 17.8.3.2.1 The location and spacing of detectors shall be the result of an engineering evaluation that includes the following:
1) Size of the fire that is to be detected
2) Fuel involved
3) Sensitivity of the detector
4) Field of view (FOV) of the detector
5) Distance between the fire and the detector
6) Radiant energy absorption of the atmosphere
7) Presence of extraneous sources of radiant emissions
8) Purpose of the detection system
9) Response time required
Smoke detectors
A smoke detector detects the particles produced by combustion using a variety of technologies. These can include ionization, cloud chamber, photoelectric light obscuration, photoelectric light scattering and video image detection. Smoke detectors should be located and spaced
in anticipation of airflow from sources likely to present fire risks, but without resulting in unwarranted alarms. NFPA 72 describes the requirement this way:
17.7.1.9 The location of smoke detectors shall be based on an evaluation of potential ambient sources of smoke, moisture, dust, or fumes, and electrical or mechanical influences, to minimize nuisance alarms.
Combustible gas detectors
NFPA 72 describes a gas detector as “a device that detects the presence of a specified gas concentration.” Detectors are intended for specific types of gas or vapor and should be chosen accordingly, as specified in NFPA 72:
• 17.10.2.4 The selection and placement of the gas detectors shall be based on an engineering evaluation.
Gas leak detection technologies can include:
• Acoustic detection for gas leaks — ultrasonic sensors detect leaks based on noise patterns
• Infrared detection along a line of sight — detects gas along a path providing a large area of detection
• Point detection of a combustible or toxic gas by electrochemical, catalytic, or infrared technologies — gases come into contact with the detector, causing the detector to activate.
Fire protection system controller
A safety system controller (SSC) receives and interprets input from multiple detectors and makes executive decisions regarding notification and further activity including suppression. While the primary function of detectors is to detect a hazard and then signal to alert that an event has occurred, it is also essential that detectors are capable of maximizing false alarm rejection – so they do not alarm to non-hazardous events, such
as arc welding. NFPA 72 defines a nuisance alarm this way: “An unwanted activation of a signaling system or an alarm initiating device in response to a stimulus or condition that is not the result of a potentially hazardous condition. In some cases, in order to prevent nuisance alarms, the SSC may discount information from a single detector if it is not confirmed by other detectors in the area.
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