The type of system that should be used depends chiefly on the temperature maintained in the building, damageability of contents, expected propagation rate of a fire, and total fire load.
In the United States the wet-pipe sprinkler system is the most common and affordable sprinkler system available. In consideration of the approximately $1.50/ sqft installation cost, minimal maintenance costs, and the impressive record for reliability, wet-pipe sprinkler systems should be every engineers first choice in sprinkler protection. The wet-pipe sprinkler system is clearly established as the workhorse of the fire protection industry.
Unless out of service, wet-pipe sprinkler systems are always water filled. Consequently, building temperature must be maintained above 40F to prevent freezing.
Other than a gate valve and an alarm valve or shot-gun riser assembly, there are no devices between the water supply and sprinklers.
To indicate the flow of water as a result of an operating sprinkler or broken pipe, a local alarm bell on the exterior of the building being protected is required.
For a wet-pipe sprinkler system this alarm feature is accomplished in one of two ways. In the past it was more common for engineers to specify the installation of an alarm-check valve in the main supply pipe, i.e. system riser. The alarm-check valve (Fig. 14.15) is a swing check valve with an interior orifice that admits water to an alarm line onto which a water-motor-driven gong is attached. To help differentiate between a water pressure surge and a legitimate water flow, a retard chamber is often used. The retard chamber acts to delay pressure surges so they subside prior to causing nuisance alarms. In lieu of a water-motor-gong, and in all cases on slick wet-pipe systems, a vane-type water-flow indicator can be installed and connected to an electric bell to give notification of water flow. Among the advantages of using a vane-type water-flow switch are that most models include a variable time delay to serve as the retard function and they can easily be monitored as a fire alarm device. Figure 14.15 shows a typical alarm-check valve.
Where a wet-pipe sprinkler system is installed but small unheated areas such as truck docks or attics exist, an antifreeze system, normally a subsystem to a wetpipe system, will be employed. In most instances, when the capacity of an antifreeze system exceeds 40 gallons, the cost of system maintenance becomes prohibitive and a dry-pipe system is more appropriate.
An antifreeze system consists of an antifreeze U-Loop (Figure 14.16) which includes an indicating control valve, antifreeze solution test ports and drain connection and a check valve or backflow preventer to restrict the migration of antifreeze from the antifreeze side of a system to the wet-pipe side. Since the waterside of an antifreeze U-loop is subject to freezing, the U-loop must be located in a heated area. The operation of a sprinkler on an antifreeze system is identical to that of a wet-pipe system; however, rather than water flowing from the sprinkler immediately, it is first the antifreeze solution, followed by water. While it may be of concern, the antifreeze solutions currently permitted by NFPA 13 are tested for their ability to control fire and they do not detract from the characteristics of water as an extinguishing medium.
Given todays increasingly stringent environmental regulations, the installation of backflow prevention devices are often required on antifreeze systems to prevent antifreeze from flowing into wet-pipe sprinkler systems and endangering potable water supplies. The presence of a backflow preventer in an antifreeze system causes special problems with respect to excess system pressures and where a reducedpressure backflow (RPV) preventer is used, proper maintenance of antifreeze solution concentrations. Where backflow preventers are included in antifreeze system design, expansion chambers must be used to absorb the excess pressures that may build up on the antifreeze side of the system. Where RPVs are employed, the system owner or person in charge of antifreeze system maintenance must be aware that the antifreeze system solution may change over time if antifreeze bleeds from the system through the RPVs interstitial zone.
In locations where it is impractical to maintain sufficient heat to prevent freezing and the area is too large to be protected by an antifreeze system, dry-pipe systems are often specified. A dry pipe system is similar to that of a wet-pipe system;
however, it normally contains air under pressure instead of water. In a dry-pipe system a normally high water pressure is held back by a normally low air pressure through use of a differential type dry-pipe valve. This valve employs a combined air and water clapper (Fig. 14.17) where the area under air pressure is about 16 times the area subject to water pressure. When a sprinkler activates, air is released to the atmosphere through the sprinkler orifice, allowing the water to overcome the pressure differential and enter the piping. On smaller systems, riser mounted air compressors are used to maintain the air pressure such that the dry pipe valve does not operate as a result of small pressure losses over time. Floor mounted air compressors or plant air systems are typically used for air pressure maintenance on
larger dry pipe systems.
In dry-pipe systems of large capacity, the relatively slow drop in air pressure when a single head or a few heads are activated is overcome by use of an accelerator or exhauster. The former is a device, installed near the dry-pipe valve, to sense a small drop in pressure and transmit the system air pressure to a point under the valve clapper. The additional air pressure on the bottom side of the clapper causes it to move into the open and locked position faster than it would otherwise; therefore, water reaches the open sprinklers with less delay.
While a dry-pipe system is intended for use in areas of 40F or less, the drypipe valve must be installed in a heated area or enclosure since there is water in the piping up to the valve and priming water in the valve itself.
The indication of water flow in a dry pipe system is accomplished in a manner identical to that of the wet-pipe system; however, if the option of the electric bell is desired a water pressure switch must be employed. Since a dry-pipe system is normally empty, when a sprinkler operates water rushes into the piping. If a vanetype water-flow indicator were installed, the rushing water could dislodge the vane and cause an obstruction in the sprinkler piping.
Preaction sprinkler systems are used where the presence of water, except in emergencies, is unacceptable or where a dry-pipe system is necessary and the additional expense of a detection system can be justified. The water in these systems is controlled by a preaction deluge valve, which is operated by an integrated fire alarm system consisting of heat or smoke detection devices installed throughout the same area the preaction system protects. There are two basic types of preaction systems:
Single Interlock. These systems admit water to their piping upon actuation of an associated detection system. Their primary benefit is that system piping or sprinklers can be damaged or removed without accidental water discharge. Single Interlock systems are commonly used in computer rooms and sometimes museums although wet-pipe systems are normally adequate.
Double Interlock. A combination of a single interlock preaction system and a dry-pipe system, these are filled with compressed air that is capable of holding the water pressure at the preaction deluge valve back until the air pressure is released.
Water only enters the piping of a double interlock system after the associated detection system operates and the systems air pressure has been purged. The double interlock system is most frequently used for the protection of refrigerated cold storage / freezer warehouses where a false activation would result in frozen pipes and long periods of business interruption. Often times double interlock sprinkler systems are wrongly specified for the protection of high dollar areas such as computer rooms, museums, etc., where the protection of wet-pipe systems or single interlock preaction systems are adequate.
Since preaction sprinkler systems rely on a fire alarm system, and in the case of the double interlock system, the dry-pipe principle, they are the least reliable of sprinkler systems and require the greatest amount of maintenance.
These systems are identical to that of single interlock preaction systems except none of the sprinklers have operating components or caps. Like the sprinkler systems depicted in the movies, the operation of a deluge system results in water flowing from all system sprinklers simultaneously. As with the preaction system, a preaction deluge valve controls the water in these systems. Since deluge systems are often installed in harsh environments where smoke or heat detectors are prone to failure, pneumatic or mechanical means are commonly employed for valve operation.
Operation of a pneumatically controlled preaction deluge valve can be by means of a pilot line of small-diameter pipe on which are spaced automatic sprinkler heads at suitable intervals. These heads can be augmented when necessary by use of a mechanical air or water-release device, which operates on the rate-of-rise principle as well as the fixed temperature of the sprinkler heads. Other pneumatic means include small copper air chambers, sensitive to rate-of-rise conditions connected by small-diameter copper tubing to the release mechanism of the valve. In some instances specialized infrared or ultraviolet flame detectors may be used to activate a deluge system. The nature and extent of the hazard and the surrounding ambient conditions always determines the kind of detection required.
While a small two or three sprinkler deluge system may be used to protect an isolated industrial hazard, a large deluge system having as many as one-thousand sprinklers may protect an aircraft hangar, chemical plant, or a portion of a plant where process vessels and tanks containing flammable materials are located. Deluge systems are only justified for the protection of areas where the probability of a fire is likely and the fire growth potential is extreme.
The types of sprinkler systems referenced above are intended for fire control and on a limited basis fire suppression; however, sprinklers can also be used for exposure protection. In instances where buildings are located too close to one another or to an adjacent fire hazard, such as a combustible liquid storage tank, a hybrid sprinkler system can be specified to prevent the spread of fire from a fire area to an exposed building. Such systems have open nozzles directed onto the wall, windows, or cornices to be protected. The water supply may be taken from a point below the inside-sprinkler-system control valve if the building is sprinklered, otherwise from any other acceptable source, with the controlling valve accessible at all times. The system is usually operated manually by a gate valve but can be made automatic by use of a deluge valve actuated by suitable means on the exposed side of the building. The distribution piping is usually installed on the outside of the wall with nozzles provided in sufficient numbers to wet the surface to be protected.