Fire Sprinkler Systems
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A fire sprinkler system is an active fire protection measure, consisting of a water supply, providing adequate pressure and flow rate to a water distribution piping system, onto which fire sprinklers are connected.
Operation
Each closed-head sprinkler is held closed by a heat-sensitive device called a fusible link. The link applies pressure to a seal which prevents water from flowing until the ambient temperature around the sprinkler reaches the design temperature of the individual sprinkler head. Each sprinkler activates independently when the predetermined heat level is reached. The design intention is to limit the number of sprinklers that operate to only those above the fire, thereby concentrating the available water from the water source over the point of fire origin.
Types of Fire Sprinkler Systems
Wet pipe systems
Wet pipe sprinkler systems are installed more often than all other types of fire sprinkler systems. They also are the most reliable, because they are simple, with the only operating components being the automatic sprinklers and (commonly, but not always) the automatic alarm check valve . An automatic water supply provides water under pressure to the system piping. All of the piping is filled with water. Until sufficient heat is applied, causing one or more sprinklers to fuse (open), the automatic sprinklers prevent the water from being discharged.
Dry pipe systems
Dry pipe systems are generally used in spaces in which the ambient temperature may be cold enough to freeze the water in a wet pipe system, rendering the system inoperable.
Water is not present in the piping until the system operates. The piping is pressurized with air, at a "maintenance" pressure which is relatively low compared with the water supply pressure. To prevent the larger water supply pressure from forcing water into the piping, the design of the dry pipe valve (a specialized type of check valve) intentionally includes a larger valve clapper area exposed to the maintenance air pressure, as compared to the water pressure.
When one or more of the automatic sprinklers is exposed to sufficient heat, it operates, allowing the maintenance air to vent from that sprinkler. Each sprinkler operates individually. As the air pressure in the piping drops, the pressure differential across the dry pipe valve changes, allowing water to enter the piping system. Water flow from sprinklers needed to control the fire is delayed until the air is vented from the sprinklers. For this reason, dry pipe systems are usually not as effective as wet pipe systems in fire control during the initial stages of the fire.
Deluge systems
"Deluge" systems are systems that have open sprinklers, i.e. the heat sensing operating element is removed during installation, so that all sprinklers connected to the water piping system are open. These systems are used for special hazards where rapid fire spread is a concern, as they provide a simultaneous application of water over the entire hazard.
Water is not present in the piping until the system operates. Because the sprinkler orifices are open, the piping is at ambient air pressure. To prevent the water supply pressure from forcing water into the piping, a deluge valve is used in the water supply connection, which is a mechanically latched valve. It is a non-resetting valve, and stays open once tripped.
Because the heat sensing elements present in the automatic sprinklers have been removed (resulting in open sprinklers), the deluge valve must be opened as signaled by a specialized fire alarm system. The type of fire alarm initiating device is selected mainly based on the hazard (e.g., smoke detectors, heat detectors, or optical flame detection). The initiation device signals the fire alarm panel, which in turn signals the deluge valve to open. Activation can also be manual, depending on the system goals.
Pre-Action Systems
Pre-action sprinkler systems are specialized for use in locations where accidental activation is undesired, such as in museums with rare art works, manuscripts, or books.
Pre-action systems are hybrids of wet, dry, and deluge systems, depending on the exact system goal. There are two sub-types of pre-action systems: single interlock, and double interlock. The operation of single interlock systems are similar to dry systems except that these systems require that a “preceding” and supervised event (typically the activation of a heat or smoke detector) takes place prior to the “action” of water introduction into the system’s piping due to opening of the pre-action valve (which is a mechanically latched valve). Once the fire is detected by the fire alarm system, the system is essentially converted from a dry system into a wet system. Or, if an automatic sprinkler operated prior to the fire being detected by the fire alarm system, water will be allowed into the piping, and will discharge water from the sprinkler.
The operation of double interlock systems are similar to deluge systems except that automatic sprinklers are used. These systems require that both a “preceding” and supervised event (typically the activation of a heat or smoke detector), and an automatic sprinkler activation take place prior to the “action” of water introduction into the system’s piping.
