Description of Fire Detection Systems and Detectors

Description of Fire Detection Systems and Detectors Points : Description of Fire Detection Systems and Detectors, Fusible Bulb (FB) Detector, Raise of Temperature Rise Detector, Heat Sensitive Cables, Thermal Expansion Type Detector, Radiation Detectors, Smoke Detectors, Flammable Gas Detector, Smoke and heat detectors, Heat-sensitive fire detection canopies (1) Fusible Bulb (FB) Detector Five different temperature ratings covering the 60°C to 140°C range are used. When subjected to a specific temperature, the bulb shatters to release either compressed air (detector) or high pressure water (sprinkler).

(2) Raise of Temperature Rise Detector ROTR is a mercury switch which operates an alarm and/or initiates release of compressed air, Halon gas or high pressure water dependent upon the system. Two interconnected inert:gasfi1led bulbs, one shielded from direct heat, contain a flow restricting orifice or a column of mercury. (3) Heat Sensitive Cables Are suspended along cable trays, as shown by Fig. to detect an increase m surrounding temperature. The detector cable has conducting cores insulated from each other by a heat- sensitive negative temperature. The outer sheath is usually made from a distinctively-coloured-high temperature PVC compound to safely withstand localized fire. Oze type of c.3ble exhibits a response to temperature according to the percentage of its total length exposed to heat. Typically, -the temperature rise, above a specified minimum ambient temperature of 40°C, to nitiate an alarm condition ranges form 25°C over the entire length of the cable to 75°C over only a one meter length. Thus, it is not normally practicable to extend or shorten standard lengths of cable, sirtce each is matched to its monitoring unit. As a general rule, cable heat detection schemes are used to initiate alarms only. Indication is displayed at ire control panel.

(4) Thermal Expansion Type Detector Utilizes the fact that metals expand when hated to open an electrical contact and hence initiate the alarm. Bimetallic strip is used to enhance the amount of expansion obtained. The coefficient of expansion of both gases and liquids is greater than that of lids, so potentially they are more sensitive as heat detectors. One such application is the uartzoid bulb is sprinkler system which consists of a liquid-filled bulb blocking the utlet of the sprinkler head. The liquid expands more than the glass so when heated it will thatter on reaching a predetermined temperature. Once the bulb has broken, the sprinkler head will discharge. (5) Radiation Detectors In addition to light, fires generate infrared and ultraviolet radiations which are less affected by smoke than normal white light. By employing a photoelectric cell with infrared filter or an ultraviolet sensor, the presence of flames can be detected in spite of the smoke generated. The signal from the photoelectric cell or u.v. sensor can be arranged to trigger the alarm. These devices suTer from the disadvantage that they will react to infrared or ultraviolet radiations from other scsurces and hence are prone to spurious alarms. This disadvantage is overcome in a combined smoke and heat detecting sy{em in which a modulated infrared beam is transmitted across the area to be potected Smoke and heat haz,e interfere with the beam and this is detected in the receiving unit which initiates the alarm. (6) Smoke Detectors There are three types of smoke detectors utilizing ionizing radiations, Light scatter and obscuration. An ionization detector works on the principle that ions are absorbed by smoke particles, thus when smoke enters a chamber which contains an ionized gas some of the ions are absorbed by the smoke particles and the. current flow in the chamber is reduced. In practice a radiation source is used to maintain a pre- determined level of ionization in both a sealed and open chamber. Both chambers will therefore pass the same current. When smoke enters the open chamber the ionization is reduced and less current flow in that circuit producing electrical imbalance which is detected and amplified to initiate the alarm. A smoke detector with ionization chamber is illustrated in Fig. A light-scattering smoke detector works on the theory that smoke will scatter a beam of light. A photoelectric cell is fitted in a chamber at right angles to a light source and in the fire-free condition receives no light. However, when smoke enters the chamber the light is scattered and same is detected by the cell which triggers the alarm. In an obscuration detector the light beam is directed at the photoelectric cell and when smoke enters, less light is received by the cell and the alarm is raised. A smoke detector based on the principle of photoe electric principle is illustrated in Fig.

(7) Flammable Gas Detector A flammable gas detector is designed to measure the amount of flammable gas in the atmosphere and relate it to the upper and lower flammable limits. The gas mixture is drawn over a catalytic surface where oxidation, i.e., combustion, takes place. The combustion causes a rise in temperature of the surface, which is measured by a decrease in its electrical resistance. The instruments have to be calibrated for the particular gas of interest but for petrol vapours, pentane or heptanes are used as the reference gas. The readings are usually displayed in terms of percentage of the lower explosive limit. While heat and smoke detectors are commonly used, infrared and ultraviolet actuated detectors are ‘used only in, special cases. These operate by the action of Infrared/ultraviolet let rays emitted by the fire on a photo sensitive element, which forms part of electric circuit connected to the main control panel, Liner/line detectors are also used in special areas. These are of particular use for cable gallery/tunnel, conveyors and the like in Power Stations and elsewhere. The detectors ate installed, mostly under the ceiling spaced according to specifications/code of practice (J.S. 2175/1977 I.S. 11360/1985 etc.) Certain minimum numbers of detector are electrically connected to a loop which is termed as a “zone” control ‘Panel’. All the zones in the premises to be protected are connected to the main control pane! installed at the Control Room located at a special selected area. (8) Smoke and heat detectors These are located on ceilings of risk areas, they have neon indicators to identify which device has operated. Smoke detectors may be of either the ionization type (Fig) oi the photo-electric cell type, though thermostat and electrical fusible link types are also in general use. Generally, such detectors initiate a discharge of Halon gas in computer rooms. Mechanical fusible links are often employed to close fire dampers in ventilation ducting. (9) Heat-sensitive fire detection canopies Mounted over selected coal con veyors, detect rate-of-rise in temperature; any fire will activate the sprinkler fire protection system, and stop the conveyor. Optical methods of detecting conveyor fires are prone to spurious operation caused by clouds of coal dust created when dry coal is being handled. In one design of heat canopy, a group of compensated electrical detectors, incorporating exposed and shielded elements, is used. The shielded clement is covered by a silicone compound, which has a larger thermal mass than the exposed element In the event of fire, the hot gases trapped under the canopy, together with the radiated heat, warm each exposed element to create a small voltage difference between the shielded and exposed element. Since group of elements are connected in series, the resultant voltage, is used to actuate the detection system.