LEARNING SPECIAL HAZARD DETECTION TYPES

Transcription

1 LEARNING SPECIAL HAZARD DETECTION TYPES JUNE 2013 COPYRIGHT CLIMATEC

2 SPECIAL HAZARDS DETECTION TYPES EXPLAINED STAGE ONE DETECTION - ACTIVE AIR SAMPLING\\INCIPIENT AND IN- VISIBLE SMOKE STAGE AIR SAMPLING DETECTOR An Air Sampling Detector consists of a piping or tubing distribution network that runs from the detector to the area(s) to be protected. An aspiration fan in the detector housing draws air from the protected area back to the detector through air sampling ports, piping, or tubing. At the detector, the air is analyzed for fire products. Capable of detecting a fire at its earliest stage - can detect the presence of particles that are created in the very early stages of combustion, even before smoke is visible Air Sampling Smoke Detectors are capable of detecting a fire at its earliest stage. Traditional spot-type detectors passively detect smoke or heat in their immediate area. Air Sampling Smoke Detectors actively pulls room air through a piping network to its detection chamber where it can detect the presence of particles that are created in the very early stages of combustion, even before smoke is visible. A fan continually drawing air into a pipe network attached to a detector unit, and passes the air through a dual stage filter to remove dirt and particles larger than 25 microns, sending the clean air to a laser detection chamber for smoke detection. The detector measures the light scatter caused by any smoke and processes the detector signal and presents the smoke level graphically. LASER SYSTEMS PARTICLE COUNTING A fan continuously draws air from the protected area through the detection chamber. Once inside, particles pass through the laser beam while the

3 photo collector measures reflected light from the particles. The collector counts the number of discrete particles to determine the level of smoke concentration. A particle size discriminator prevents large particles, such as dust, from contributing to the smoke concentration. used in sophisticated facilities where particle concentration is absolutely critical Unlike other high-sensitivity smoke detection system, particle counting doesn t use filters. Instead, it relies on the same advanced particle counting techniques used in sophisticated labs and clean room facilities where particle concentration is absolutely critical. Because they detect both the number and size of particles, it can actually distinguish particles of combustion from dust and other airborne contaminants that routinely trigger nuisance alarms in traditional spot detectors. STAGE TWO SPOT SMOKE DETECTION\\VISIBLE SMOKE These systems use devices that respond to the smoke particles produced by a fire. They operate on the ionization, photoelectric, cloud chamber, or other smoke particle analysis principle of operation. Spot-type smoke detectors use either the ionization principle of operation or the photoelectric principle. These systems are intended for early warning. Some are

4 designed for installation in ventilation ducts. (See NFPA 72, National Fire Alarm Code.) Properly installed, smoke detectors can detect smoke particles in very early stages of fire in the areas where they are located. The selection of a particular detector or mixture of detectors should be made by a fire protection specialist and based on building and fire-load conditions. IONIZATION SMOKE DETECTION The principle of using a small amount of radioactive material to ionize the air between two differentially charged electrodes to sense the presence of smoke particles. Smoke particles entering the ionization volume decrease the conductance of the air by reducing ion mobility. The reduced conductance signal is processed and used to convey an alarm condition when it meets preset criteria. Ionization smoke detection is more responsive to invisible particles (smaller than 1 micron in size) produced by most flaming fires. It is somewhat less responsive to the larger particles typical of most smoldering fires. Smoke detectors that use the ionization principle are usually of the spot type. Photoelectric light-scattering smoke detection is more responsive to the visible particles produced by most smoldering fires PHOTOELECTRIC LIGHT-SCATTERING SMOKE DETECTION The principle of using a light source and a photosensitive sensor arranged so that the rays from the light source do not normally fall onto the photosensitive sensor. When smoke particles enter the light path, some of the light is scattered by reflection and refraction onto the sensor. The light signal is processed and used to convey an alarm condition when it meets preset criteria. Photoelectric light-scattering smoke detection is more responsive to the visible particles (larger than 1 micron in size) produced by most smoldering fires. It is somewhat less responsive to the smaller particles typical of most flaming fires. It is also less responsive to black smoke than to lighter colored smoke. Smoke detectors that use the light-scattering principle are usually of the spot type.

5 STAGE THREE FLAME DETECTION\\FLAMING FIRE Flame detectors are solutions for virtually any application where fire may result in a large loss of capital equipment and where risk to personnel is high. Industrial and commercial applications include oil and gas pipelines, turbine enclosures, off-shore platforms, automotive manufacturing facilities, aircraft hangers, munitions plants, nuclear facilities, and many, many more. These systems use devices that respond to radiant energy visible to the human eye (approximately 4000 to 7000 angstroms) or to radiant energy outside the range of human vision [usually infrared (IR), ultraviolet (UV), or both]. Flame detectors are sensitive to glowing embers, coals, or actual flames with energy of sufficient intensity and spectral quality to initiate the detector. Infrared (IR) flame detectors work within the infrared spectral band. Hot gases emit a specific spectral pattern in the infrared region, which can be sensed with a thermal imaging camera (TIC) a type of thermo graphic camera. False alarms can be caused by other hot surfaces in the area. A typical frequency where single frequency IR flame detector is sensitive is in the 4.4 micro meter range. Typical response time is 3-5 seconds. Ultraviolet (UV) detectors work with wavelengths shorter than 300 nm. These detectors detect fires and explosions within 3 4 milliseconds due to the UV radiation emitted at the instant of their ignition. False alarms can be triggered by UV sources such as lightning, arc welding, radiation, and sunlight. In order to reduce false alarms, a time delay of 2-3 seconds is often included in the UV Flame detector design.

6 UV and IR flame detectors compare the threshold signal in two ranges in AND configuration and their ratio to each other to confirm the fire signal and minimize false alarms. STAGE FOUR HEAT DETECTION\\INTENSE HEAT HEAT-RESPONSIVE DEVICES ARE EITHER A LINEAR HEAT OR A SPOT HEAT DETECTION TYPE. LINEAR HEAT DETECTION These systems are relatively low cost. They cannot detect small, smoldering fires. Line-type detectors can be installed in a relatively inconspicuous manner by taking advantage of ceiling designs and patterns. (See NFPA 72, National Fire Alarm Code.) The air temperature surrounding a fixed temperature device at the time it operates usually is considerably higher than the rated temperature, because it takes time for the air to raise the temperature of the operating element to its set point. This is called thermal lag. A fire protection professional can assist you with determining the best application of specialty products like these. There are numerous applications where Linear Heat Detection is used such as Refrigerated Storage (Freezers), Warehouse, Bridges, Conveyors, Chemical and Gas Storage Facilities, just to name a few. A fire protection professional can assist you with determining the best application of specialty products like these.