Back to the Basics:
Wet & dry sprinkler systems By Todd Stevens, CFPS
T
he fire protection industry continues to grow and evolve every day. Engineers, designers and local authorities are confronted with new challenges that
involve unique applications and special hazard systems. Some of these systems include deluge, preaction, foam or clean agent suppression. Wet systems are, by far, the most common fire protection systems installed; dry systems are the second most common. We often overlook the basic design principles of these systems. In this article, I will go back to the basics and review the equipment and operation of wet and dry systems. NFPA 13, Standard for the Installation of Sprinkler Systems provides the minimum requirements for the design and installation of automatic fire sprinkler systems1 the article.
and will be referenced throughout
Wet systems A wet system, as defined in NFPA 13, is a sprinkler sys-
tem employing automatic sprinklers attached to a piping system containing water and connected to a water supply so that water discharges immediately from sprinklers opened by the heat from a fire2
. A wet system is used in
areas that can be maintained above 40° F (4° C). Providing immediate water discharge onto a fire will help to control it quickly and will help prevent it from spreading, which explains why a wet system is the most commonly used. A wet system is fairly simple, but there are some options
to consider when designing the equipment involved. We start to build the components that make up a wet system at the point where the water supply enters the building. The installation of the water supply and components such as backflow preventers are based on local authority require- ments and NFPA 24, Standard for the Installation of Private Fire Service Mains and Their Appurtenances. The sprinkler riser is the bridge between the under-
ground and interior piping. This is the point where we sup- ply the cross mains, feed mains and branch piping that make up the wet system piping network. It is at the sprin- kler riser where we begin to make decisions on the com- ponents needed. There are two typical types of wet system sprinkler risers to choose from; a riser alarm check valve assembly or a ported check valve with a flow indicator.
Riser check valve assembly We use an alarm check valve on the riser to lock water pressure into the system and prevent the water in the
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sprinkler piping from traveling back into the water supply. Trim piping around the check valve is used to activate an alarm, which can be a mechanical or electrical device, during a water flow condition. The alarm check valve is equipped with an inlet, an outlet and multiple tapped boss- es.
Two of these bosses are for pressure gauges that show
the supply side and the system side water pressure. Two are located in the back of the valve for a bypass line; this is for low flow conditions with less than one sprinkler and
Continued on page 46
Wet System Riser: This detail is showing a Riser Check Valve Assembly complete with retard chamber, pressure switch and water motor gong. Shown in a flow condition, you can see the path of the water within the trim piping activating the alarms. Courtesy of Viking Corporation.
May 2011
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