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The Automatic Sprinkler
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What are automatic sprinklers? How do they fight fire? Why are they needed? Where should they be located? The more you know about sprinklers and how they operate, the better your chances of avoiding a large loss.
Over the last five years, the average fire loss to an adequately sprinklered FM Global-insured location was less than US$600,000; the average fire loss to locations needing sprinklers was US$3.2 million. A study of these losses has demonstrated that auto- matic sprinkler protection is your best defense against fire.
Having performed consistently for more than 100 years, automatic sprinklers are the most efficient “fire- fighters” in the world. They’re also your first line of defense against fire. Yet, some people fail to understand their importance. Excuses for not installing automatic sprinklers vary. Among the most popular: ■ A fire won’t happen here. ■ The fire service or brigade
is near. ■ Fire protection costs too much. ■ Insurance will cover it.
Each excuse gambles that a devastat- ing fire loss will not happen. Or, even if it does, business won’t be affected. In reality, choosing not to install sprinklers where they’re needed cre- ates the potential for disaster.
This brochure is made available for informational purposes only in support of the insurance relationship between FM Global and its clients. This information does not change or supplement policy terms or conditions. The liability of FM Global is limited to that contained in its insurance policies.
Rapid Fire Growth
10 seconds 1 minute, 50 seconds 3 minutes, 30 seconds
These successive photos illustrate how quickly a fire can grow, often faster than it takes personnel to respond. If automatic sprinklers had been available, they would have responded within the first two minutes, before the fire could grow out of control.
Fire Develops Fast Usually, a fire starts small and can remain nearly invisible for lengthy periods while it incubates. Once it is large enough to see or trigger an alarm, and starts to grow, the time be- tween a small fire and an inferno can be measured in seconds. As a result, sprinklers are needed to control or suppress the fire before it rages out of control.
When sprinklers operate, water is discharged and an alarm is sounded. The sprinklers concentrate the water directly in the area of the fire without being hindered by heat, smoke or toxic gas, as a firefighter would be. If provided with a strong enough water supply, only sprinklers over the fire and in the immediate area will oper- ate. By operating in the early stages of fire development, sprinklers use less water and control the fire more
effectively than hose streams applied much later. In a building without sprinklers, the fire is uncontrolled. By the time the fire service has been called and arrives on the scene, heat, flames and smoke will be intense and easily could exceed manual firefighting capabilities. FM Global statistics show that fire damage in an unsprinklered building is far more extensive and costly, and business downtime is much longer than when a properly designed sprinkler system is in place.
What Is an Automatic Sprinkler? Automatic sprinklers are installed at regular intervals in an extensive net- work of piping and valves. This sys- tem feeds water to the sprinklers. The sprinklers discharge water as soon as their thermal-sensing elements are heated to their rated temperatures.
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Sprinklers are installed in one of three positions: upright, pendent or sidewall. Sidewall sprinklers, depending on the type, can be installed vertically or horizontally.
The three main parts of a sprinkler are the orifice, the deflector and the thermal-sensing element.
The Orifice The orifice is the opening through which water is discharged. Its size varies, depending on the type of sprinkler. The orifice size determines the sprinkler’s discharge coefficient. Sprinklers may have discharge coeffi- cients of 2.8 (40), 5.6 (80), 8.0 (115), 11.2 (160), 14.0 (200), 16.8 (235), 22.4 (315) and 25.2 (360)
gal./min./psi1/2 (L/min./bar1/2). The flow from a sprinkler is equal to the discharge coefficient multiplied by the square root of the water pressure at the sprinkler.
The Deflector With a standard installation, water is discharged uniformly below the plane of the deflector in an umbrella- shaped spray pattern. The design of the deflector determines the shape and characteristics of the spray pattern.
The Thermal-Sensing Element The thermal-sensing element oper- ates the sprinkler. There are two types: fusible element (or solder element) and glass bulb.
When the sensing element of the fusible element-type sprinkler reaches its operating temperature, the solder melts, permitting the sprinkler to open. Water under pressure is dis- charged from the sprinkler’s orifice, striking its deflector and spraying downward in an umbrella-like pattern onto the fire.
The glass bulb-type sprinkler head consists of a bulb filled with liquid. As the liquid expands from the heat of the fire, pressure in the bulb builds until the bulb bursts, permitting the sprinkler to open.
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Where Are Sprinklers Needed? Sprinklers are needed wherever combustible construction material or occupancies exist; wherever any activity might fuel, ignite or spread a fire; wherever something might be damaged by fire or products of combustion. Few places exist where sprinklers are not needed.
How Do Sprinklers Fight Fires? Alarm Response A sprinkler system sounds an alarm whenever water is discharged from one or more sprinklers. Specifically, the local alarm—the bell outside the building—sounds, alerting anyone in the area to the discharge. At the same time, depending on the system, an alarm also may sound at an on-site monitoring/supervisory station or an off-site central station alarm facility or municipal dispatcher. Of course, personnel should call the local public fire service at the first alarm or sight- ing of fire.
Sprinkler Response Control and suppress. These two key words best describe how sprin- klers respond to fire. Control-mode sprinklers control the fire. Water discharged from control-mode sprin- klers wets burning surfaces to a lim- ited degree, but, more importantly, wets unburned surfaces imme-diately beyond the fire area, reducing fire intensity and preventing further fire spread.
In addition, the sprinkler discharge cools the area near the roof or ceil- ing, preventing structural damage as well as preventing sprinklers far from the fire area from operating need- lessly.
Suppression-mode sprinklers work by applying water at high momentum directly into the fire plume while it is still small, effectively stopping the fire before it really gets started. As a result of this fast response and lim- ited fire growth, fewer sprinklers will operate and the amount of fire, water and smoke damage may be reduced.
Types of Sprinkler Systems Available Inside a sprinklered building, the piping system may comprise one or more of the following types:
Wet Pipe The wet-pipe system is the most common. All piping is filled with wa- ter under pressure. When one or more
sprinklers operate, water is immedi- ately discharged out of each opened sprinkler and continues flowing until a control valve is shut off after the fire has been extinguished.
Dry Pipe The dry-pipe system is used in unheated buildings or other areas where water could freeze in the pip- ing. Piping contains air or nitrogen under enough pressure to hold water back at a valve located in a heated area or enclosure. When a sprinkler operates and begins releasing air, the drop in air pressure automatically opens the valve. Water then flows into the piping to the sprinklers.
The time lag between sprinkler operation and water discharge makes dry-pipe systems somewhat less efficient than wet-pipe systems. A quick opening device can help reduce this time. The device can be an exhauster, which helps remove the air, or an accelerator, which enables the dry-pipe valve to open faster.
Potential Results of a Fire
Without Sprinklers ■ Hose streams may not be able to reach the fire. ■ There may be delay in attacking the fire; partitions may block access. ■ Heat and smoke may keep firefighters out. ■ Building structural integrity may be compromised resulting in collapse.
With Sprinklers ■ Sprinklers protect all areas. ■ Sprinklers promptly attack fire anywhere. ■ Sprinklers work in conditions where humans cannot. ■ Sprinklers help maintain structural building integrity.
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Preaction Like a dry system, the preaction system contains no water; water will not enter the piping until an accept- able automatic detection system acti- vates within the protected area. When the detector activates, an automatic water-control valve releases water into the piping, allowing it to flow from any sprinkler that subsequently opens. The detection system provides an early alarm before any sprinkler operates.
Deluge The deluge system is used to pro- tect hazards when water must be discharged immediately over a large area. Like the preaction system, it
In any well-protected property, sprinklers are needed:
Over: ■ Proc