Fire Protection Systems PDF

Fire Protection Systems • Chapter 15 OBJECTIVE 2 Describe the components and operation of a typical fire detection and alarm system in an industrial setting. FIRE DETECTION AND ALARM SYSTEMS Fire detection devices are necessary so that automatic or manual fire suppression can be initiated. Other f...

Fire Protection Systems • Chapter 15 OBJECTIVE 2 Describe the components and operation of a typical fire detection and alarm system in an industrial setting. FIRE DETECTION AND ALARM SYSTEMS Fire detection devices are necessary so that automatic or manual fire suppression can be initiated. Other fire protection systems (for example, automatic fire doors for the compartmentalization and protection of escape routes) should also be activated upon fire detection so that occupants have time to move to safe locations, typically outside of the building. Fire alarm and detection is covered in NFPA 72 National Fire Alarm and Signaling Code. One concern about rapid initial fire growth is that it can reduce the time available, after detection, for these life and property saving responses. Therefore, detection systems must be designed to reflect the building's features, occupants, and fire safety features. Smoke is often the first indicator of fire, so a system of automatic smoke detectors should be used. However, in certain locations, detectors that sense heat or rate of heat increase may be more appropriate due to the types of fires likely to occur in those areas. Regardless of detector type, it is important that each area of the building is assessed. The purpose of this assessment is to gather information on response time after the fire is detected in relation to other lethal or other highhazard conditions that could develop during that time. It may not be necessary for alarms to be linked directly to detector locations, but they should be designed to systematically inform occupants. This might include the use of central annunciator panels and monitors, voice messages to provide instructions, and remote alarms directed to supervised stations or fire departments. All of these features will impact the response time and the efficiency of that response. HEAT DETECTORS Heat detectors are the oldest type of automatic fire detection device, beginning with the development of automatic sprinklers in the 1860s. Heat detectors are generally located on or near the ceiling and respond to the thermal energy released from a fire. Heat detectors respond either when the detecting element reaches a predetermined fixed temperature, or they respond to a specified rate of temperature increase. In general, heat detectors are designed to operate when heat causes a change in a physical or electrical property of a material or gas within the detector. Some heat detectors only initiate an alarm and have no extinguishing function. They have the lowest false alarm rate of all automatic fire detector devices but are also the slowest at detecting fires. A heat detector is best suited for fire detection in a small, confined space where rapidlybuilding high-heat fires are expected, in areas where ambient conditions do not allow the use of other fire detection devices, or where the speed of detection is not a prime concern. A sprinkler can be considered to be a combined heat-activated fire detector and extinguishing device when the sprinkler system is provided with water flow indicators that are connected to the fire alarm control system. Water flow indicators detect either the flow of water through the pipes or the subsequent pressure change upon activation of the system. 3rd Class Edition 3 • Part A2 777 Chapter 15 • Fire Protection Systems Operating Principles of Fixed-Temperature Heat Detectors Fbced-temperature heat detectors are designed to alarm when the temperature of the operating element reaches a specified point. The air temperature at the time of alarm is usually considerably higher than the rated temperature. This is because it takes time for the air to increase the temperahire of the operating element to its set point. This condition is called thermal lag. Fbced-temperature heat detectors are available to cover a wide range of operating temperatures, from about 57°C and higher. Higher temperature detectors are also necessary so that detection can be provided in areas normally subjected to high ambient (non-fire) temperatures, or in areas zoned so that only those detectors in the immediate area operate. Fusible Element Type Eutectic metals (aUoys ofbismuth, lead, tin, and cadmium that melt rapidly at a predetermined temperature) can be used as operating elements for heat detection. The most common use is the fusible element in an automatic sprinkler, as shown in Figure 2. The fasing (melting) of the element allows the cover on the orifice to fall away, water to flow in the system, and the alarm to be initiated. Figure 2 - Automatic Sprinkler Head Fusible element Eutectic metals, used as solder to secure a spring under tension, may also be used to actuate an electrical heat detector. When the element fuses, the spring action doses contacts and initiates an alarm. Detectors that use eutectic metals cannot be restored; either the device or its operating element must be replaced following operation. Bimetallic Type When two metals with different coefficients of thermal expansion are bonded together and then heated, differential expansion causes bending toward the metal with the lower expansion rate. This action causes an open contact to close. The low expansion metal commonly used is Invar, an alloy of 36% nickel and 64% iron. Several alloys ofmanganese-copper-nickel, nickel-chromiumiron, or stainless steel may be used for the high-expansion component of the bimetal assembly. Bimetals are the operating elements of a variety of fixed-temperature detectors. These detectors are generally one of two types: the bimetal strip and the bimetal snap disc. As it is heated, a bimetal strip deforms in the direction of the contact point. With a given bimetal, the width of the gap between the contacts determines the operating temperature: the wider the gap, the higher the operating point. 778 3rd Class Edition 3 • Part A2 Fire Protection Systems • Chapter 15 fS The operating element of a snap disc device is a bimetal disc formed into a concave shape in its unstressed condition, as shown in Figure 3. Generally, a heat collector is attached to the detector frame to speed up the transfer of heat from the room air to the bimetal. As the disc is heated, the developed stresses cause it to suddenly reverse curvature and become convex. This provides a rapid, positive, snapping action that closes the alarm contacts. The disc itself is not usually part of the electrical circuit. AU heat detectors that use bimetal elements are self-restoring after operation. In other words, when the ambient temperature drops sufficiently below the operating point, the disc snaps back to its original shape and opens the alarm contact. Figure 3 - Bimetallic Snap Disc Fixed-Temperature Detector (Bertold Werkmann/Shutterstock) 3rd Class Edition 3 • Part A2 ^ z_ 779 i^ Chapter 15 • Fire Protection Systems Rate Compensation Detectors A rate compensation detector is a device that responds when the temperature of the surrounding air reaches a predetermined level, regardless of the rate of temperature rise. Figure 4 shows a typical spot-type detector with a tubular, metal outer shell that expands lengthwise when heated. Internal contact points will dose at a certain point during expansion. A second metallic element (the low expansion struts) inside the tube exerts an opposing force on the contacts to hold them open. The forces are balanced so that, with a slow rate of temperature rise, there is more time for heat to penetrate to the inner element. This inhibits contact closure until the entire device has been heated to its rated temperature. However, with a fast rate of temperature rise, there is less time for heat to penetrate to the inner element. In this case, the element exerts less of an inhibiting effect, so contact closure is obtained at less than rated temperature, thus initiating an earlier warning. Figure 4 - Spot-Type Rate Compensation Detector Low expansion struts Brazed in.., , /- Nams plate sealing head |/ /-insulator Sealed end j\ Contact points \ ^_f / '"""Lead wire clamp Adjusting screws Wall' outer shell Glass beads. II Mo'untir insulation nermaucseai head Rate of Rise Detectors A fire rapidly increases the air temperature in the space above the flames. Fuced-temperature heat detectors will not initiate an alarm until the air temperature near the ceiling exceeds the designoperating point. The rate of rise detector, however, will function when the rate of temperature increase exceeds a predetermined value, typically around 7°C to 8°C per minute. Rate of rise detectors are designed to compensate for (and are therefore not affected by) the normal changes in ambient temperature, which are less than 6.7°C per minute. These temperature changes are expected even when there is no fire. In a rate of rise detector, air is heated in a tube or chamber, which causes it to expand and increase the pressure inside the tube or chamber. The pressure exerts force on a diaphragm, which closes the alarm contacts. If the tube or chamber were sealed, slow increases in ambient temperature or a drop in the barometric pressure would cause the detector to initiate an alarm regardless of the rate of temperature change. To overcome this, pneumatic detectors have a small orifice, which vents the pressure that would otherwise occur. The vents are sized so that, during a rapid temperahire rise, the rate of expansion exceeds the venting rate and the pressure is allowed to rise. When the temperature rise exceeds 7°C to 8°C per minute, the pressure is converted to mechanical action by a flexible diaphragm. There are two types ofpneumatic heat detectors: line detectors and spot detectors. 1. Line Type The line detector consists of metal tubing in a loop configuration that is attached to the ceiling or side wall (near the ceiling). Lines of tubing are normally spaced no more than 9.1 m apart, no more than 4.5 m from a wall, and with no more than 305 m of tubing in each circuit. A minimum of 5% or 7.6 m (whichever is greater) of each tube circuit must be within each protected area; otherwise, sufficient pressure will not develop to activate the detector. In small areas where there is insufficient tubing exposed to the area, air chambers or rosettes of tubing are often used. These features act like spot-type detectors by providing the volume of air required to meet the minimum 5% or 7.6 m exposure. A line-type rate of rise detector will activate when either a rapid heat rise occurs in one area of exposed tubing or when a slower heat rise takes place in several areas where tubing on the same loop is exposed. 780 X 3rd Class Edition 3' Part A2 Fire Protection Systems • Chapter 15 ^ Figure 5 shows a line-type detector. Air in the tube is heated by the fire, which causes an increase in tube and chamber (labelled as C in Figure 5) pressure. The pressure acts on two diaphragms (labelled C in Figure 5), causing them to expand and complete the electrical alarm circuits at the contacts (labelled D in Figure 5). If the tube was completely sealed, slow increases in ambient temperature or a drop in barometric pressure would cause the alarm to initiate, regardless of the rate of temperature change. These false alarms are overcome by using a small orifice (labelled F in Figure 5) to vent the pressure during slow increases in temperature or a drop in barometric pressure. Figure 5 - Line-Type Rate of Rise Detector ^J C^"-' ^'/ '^>^ c ^ ^ ^* '-T "B^ ^=;F 4. — J ^ ^: -^J 2.Spot Type The same pneumatic principle is used to close contacts within spot-type detectors. However, the spot type differs from the line type in that the spot type contains all of the air in a single container rather than in a tube. This means the spot detector only reacts to temperature rises in a smaller, localized area within the room. 3rd Class Edition 3 • Part A2 781 ^ Chapter 15 • Fire Protection Systems Combination Detectors Combination detectors contain more than one element that responds to a fire. These detectors may be designed to respond to the changes in either individual element or to the combined effect of both elements. An example of the former is a heat detector that operates on both the rate of rise and the fixed-temperature principles. The advantage with this detector is that the rate of rise element will respond quickly to a rapidly developing fire, while the fixed-temperature element will respond to a slowly developing fire when the detecting element reaches its set point temperature. The most common combination detector uses a vented air chamber and a flexible diaphragm for the rate of rise function, while the fbced-temperature element is usually a spring restrained by a eutectic metal (a metal alloy with a relatively low melting point). When the fixedtemperature element reaches its design operating temperature, the eutectic metal melts and releases the spring, which closes the contacts. Figure 6 illustrates a combined rate of rise and fixed-temperature device. Air is supplied to the chamber, A, and it slowly escapes through the vent, B. With a slow temperature rise, enough air escapes through the vent to prevent a pressure increase in the chamber. However, a rapid temperature rise causes the pressure in the chamber to increase until the diaphragm, C, closes the alarm contacts, D and E. Fixed temperature operation occurs when the fusible alloy, F, melts, releasing the spring, G, which pushes on the diaphragm and causes both contacts to close. Figure 6 - Spot-Type Combination Rate of Rise, Fixed-Temperature Detector Electronic Spot-Type Thermal Detectors A thermoelectric effect detector is a device that uses a sensing element that consists of a thermistor, or multiple thermistors. These thermistors produce a change in electrical resistance in response to an increase in temperature. This resistance change is monitored by associated electronic circuitry, and the detector responds when the resistance changes at an abnormal rate (rate of rise type) or when the resistance reaches a specific value (fixed-temperature type). Rate of rise detectors use two thermistors. One is exposed to changes in atmospheric temperature. Wlien the temperature rapidly changes, as in a fire, the temperature of the exposed thermistor increases faster than the temperature of the unexposed reference thermistor. This variation in temperature changes generates a net change in resistance and causes the detector to initiate the alarm. Most rate of rise detectors are designed with a fixed-temperature backup feature so that, should the temperature rise be slower than 8°C per minute, the detector will operate when the exposed thermistor has reached a predetermined fixed temperature. 782 3rd Class Edition 3 • Part A2 Fire Protection Systems • Chapter 15 f£ SMOKE DETECTORS A smoke detector will detect most fires much more rapidly than a heat detector. Smoke detectors are identified by their operating principle. Two of the operating principles are the ionization principle and the photoelectric principle. The ionization principle provides a somewhat faster response to high energy (open flame) fires, since these fires produce a large amount of smaller smoke particles. The photoelectric principle responds faster to the smoke that is generated by low energy (smoldering) fires, since these fires generally produce larger smoke particles. The sensors are available as photoelectric, ionization, or a combination of the two. lonization Smoke Detectors lonization smoke detectors are usually of the spot type, as shown in Figure 7. A small amount of radioactive material ionizes the air in the sensing (measuring) chamber. This ionization renders the air conductive and permits an electric current to flow through the air between two electrodes that form a circuit with a battery. This flow of electricity gives the chamber an effective electrical conductance. When smoke particles enter the detector, they decrease the electrical conductance of the air by attaching themselves to the ions and causing a reduction in ion mobility. When the conductance is below a predetermined level, the detector responds with an alarm. Figure 7 - lonization Smoke Detector Basic contact spring Clean air Reference chamber Reference plate-, Reference r-source Support ^, __. .Terminal screw Smoke Detector PC board Alarm indicator Radioactive material Detector cover Measuring J chamber Measuring source Measuring chamber ^iuTh Tinnuff .iiinnr 3rd Class Edition 3 • Part A2 783 ?& Chapter 15 • Fire Protection Systems Photoelectric Smoke Detectors The presence of suspended smoke particles (generated during the combustion process) affects the passing of a light beam through the air. This effect can be used to detect the presence of a fire in two ways: • Obscuration of light intensity over the beam path • Scattering of the light beam 1. Light Obscuration Principle Obscuration is a recognized term for an interruption or reduction in visibility. Therefore, smoke detectors of this design consist of a light source, a light beam gathering system, and a photosensitive device (receiver). When smoke obscures part of the light beam, the light reaching the receiver is reduced and this initiates the alarm. Figure 8 illustrates a common obscuration smoke detector, most of which are the beam type and are used to protect large, open areas. The light source is located at one end of the area and the photosensitive device at the other. Figure 8 - Obscuration Smoke Detector Clear air Light source \ Receiver Light beam -.-^Smoke particles- 2. Light Scattering Principle When smoke particles enter a light path, scattering of the light results. Smoke detectors may use this photoelectric light scattering principle. As shown in Figure 9, these detectors are usually of the spot type. They contain a light source and a photosensitive device, which are positioned so the light rays do not normally fall onto the sensor. However, when smoke particles enter the light path, they scatter the light and cause it to strike the sensor. The presence of light on the sensor activates the alarm. The light sensor device is usually a photodiode or a phototransistor. Figure 9 - Scattering Smoke Detector Scattered light 784 3rd Class Edition 3' Part A2