Fire sprinkler heads

Question 15.3 How Does an Automatic Fire Sprinkler Head Operate ... 

Fire-detection devices such as flame-sensing or ioni2ation-interference types operate much more rapidly than sprinkler heads and are used extensively both as alarms and to activate fixed fire-extinguishing systems. 

Sprinkler systems are an effective way to contain fires. The system consists of an array of sprinkler heads connected to a water supply. The heads are mounted in a high location (usually near ceilings) and disperse a fine spray of water over an area when activated. The heads are activated by a variety of methods. A common approach activates the heads individually by the melting of a fusible link holding a plug in the head assembly. Once activated, the sprinklers cannot be turned off unless the main water supply is stopped. This approach is called a wet pipe system. These systems are used for storage areas, laboratories, control rooms, and small pilot areas. Another approach activates the entire sprinkler array from a common control point. The control point is connected to an array of heat and/or smoke detectors that start the sprinklers when an abnormal condition is detected. If a fire is detected, the entire sprinkler array within an area... 

Determine the fire water requirements (gpm, number of sprinkler heads, and pump horsepower) to protect an inside process area of2000 ft2. Assume that the sprinkler nozzles have a 0.5-in orifice and that the nozzle pressure is 75 psig. 

The principal difference between a pre-action system and a standard dry pipe system is that in the pre-action system, the water supply valve is actuated independently of the opening of the sprinklers. The water supply valve is opened by the operation of an automatic fire detection system, which immediately allows water to enter the overhead piping. Then, when the fire generates enough heat to fuse the sprinkler head, water is immediately discharged on the fire as with a wet pipe system. This system reduces the inherent time lag of a dry pipe system, but can still be used in unheated areas. An example of a pre-action system is shown in Figure 7-26. 

Any suitable fire detection system can be utilized to actuate the control valve. Since any other fire detection device is likely to operate faster than a sprinkler head, it is common for waterto have reached the sprinkler by the time... 

Deluge protection can be applied over the entire hazard area with open sprinkler heads located at various floor levels or fixed water spray systems can be applied specifically on the equipment to be protected. Specific water spray application is preferred because it provides better equipment cooling and reduces the water wasted due to wind and fire draft. 

Fire detectors—dry-pilot head and pneumatic rate-of-rise heat detectors are the most frequent devices used. A dry pilot head detection systems uses 165°F (74°C) V2-in sprinkler head with air pressure maintained in the piping. When the sprinkler opens the air is released resulting in the sprinkler valve opening. Other types of detectors include ultraviolet, infrared flame detectors, or thermostatic cable heat detection. 

Fusible link A connecting link device that fuses or melts when exposed to heat. Used in sprinkler heads, fire doors, and ventilators. 

Q Britannia metal is harder than pewter. This tin-antimony alloy s properties can be varied by the addition of zinc, copper, lead, or bismuth. o When heat from a fire melts the Wood s metal plug in a sprinkler head, water that was held back by the plug is freed. 

Sprinklered Area an area that has an oveihead system designed and installed according to NFPA 13, "Standard for the Installation of Sprinkler Systems" to spray water down from sprinkler heads during a fire. 

If a fire occurs in one of the zones a fusible link will fail, causing the pressure control deluge valve (PCDV) to open and the main fire water pumps to start. Water will flow out of the sprinkler heads in that zone only. The PCDV can also be tripped manually. 

The functioning of these sprinklers can be initiated by an electronic detection system, or by heat-sensitive valves at each sprinkler head. When a fire is detected, a spray (or more recently, a mist) of water is emitted. The water absorbs the heat of the fire through a phase change to steam and thus arrests the combustion cycle. In certain appHcations where oil fires are expected, such as in restaurant kitchens, a special foam or gel replaces the water in the system. 

There must be at least one fire department cormection through which a fire department can pump water into the sprinkler system. When a large number of sprinkler heads have become involved in a fire, or the fire department has connected to the same supply to service their own hoses, the flow of water through the sprinkler pipes may be reduced so that the required volume and pressures caimot be met. In such cases, the fire department can boost the pressure and volume through this connection, preferably from separate water mains not being used in their own fire suppression efforts. 

A variation of the dry-pipe system is the deluge system. The sprinkler heads are continuously open and water is prevented from entering the system by a deluge valve. When a fire is detected the valve to the water supply opens and water flows into the system and out of aU the sprinkler heads. This not only wets the immediate area of a small fire, but the entire area to which the fire may spread. This type of system is usually chosen when the contents of the space are unusually hazardous. A variation on this system is the pre-action system in which the sprinkler heads are not open, but when a fire is detected the deluge valve opens and water is supplied to the sprinkler heads. The water entering the system causes an alarm to be sounded. When the heat causes the sprinkler heads to fuse, or open, water is discharged onto the fire. 

Heat is the most obvious choice of a characteristic by which a fire can be automatically recognized. In the section on fire suppression systems, the fusible links in the sprinkler heads represented one type of heat detector. Alloys have been developed that will have reproducible melting points. When the temperature at the detector site exceeds the melting point of the alloy, contacts are allowed to move so that the device can either make or break a circuit, just as with a manual alarm system. There are plastics which can perform in the same manner. Fixed temperature systems are very stable and not prone to false alarms, but are relatively slow to respond. There are several other versions of these fixed temperature detectors, including bimetalhc strips, where the differential rate of expansion of two different metals causes the strip to flex or bend to either make or break the contact. Others depend upon the thermal erqransion of hquids. 

The water supply to B6596 consists of a 6-inch riser in the mechanical equipment room that supplies water to the fire protection wet-pipe sprinkler system. Sprinkler heads have internal fusible elements that open the sprinkler flow orifice at a temperature of 165°F. Fire alarm pull stations and fire extinguishers are located in both the main facility and the Chapel. 

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