MECHANICAL CONTRACTING
Back to the basics: Wet and dry sprinkler systems
BY TODD STEVENS CFPS 54
and local authorities are confronted with new challenges that involve unique applications and special hazard systems. Some of these systems include deluge, reaction, 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 systems and will be referenced throughout the article.
T
Wet systems A wet system (Fig. 1), as defined
in NFPA 13, is a sprinkler system
he fire protection industry continues to grow and evolve every day. Engineers, designers
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 fire . 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, making a wet system good fire protection. 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 requirements and NFPA 24, Standard for the Installation of Private Fire Service Mains and Their Appurtenances. The sprinkler riser is the bridge
between the underground and interior piping. This is the point where we supply the cross mains, feed mains and branch piping that make up the wet system piping network. It is at the sprinkler riser where we begin to make decisions on the components 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
An alarm check Fig. 1. A wet system
valve on the riser locks water pressure into the system and prevents the water in the 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 bosses. 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 prevents the clapper from opening unnecessarily. The other two bosses are for a main drain and an alarm line port. The main drain is located so that the entire system above the alarm valve clapper can be drained. The alarm line is connected to trim piping, continuing on to the alarm devices. When a sprinkler operates, water flows through the check valve and pushes the clapper open. Water then enters into the alarm line and will activate an alarm pressure switch or a water motor gong, sending a signal that there is water flowing in the system. A retard chamber is an important
option that can be added to the alarm line. The biggest nuisance for any sprinkler system is a false water flow alarm. A retard chamber provides a mechanical delay prior to an audible alarm, either mechanical or electrical. It is used when there is variable pressure in the water supply. If variable pressure or water surges are present, false alarms may occur in the system. The retard chamber is a one gallon
tank that absorbs water surges prior to their reaching the alarms. As soon as water enters the retard chamber, it is drained through a restricted orifice in the bottom. If there is a sprinkler flow condition, the chamber will fill more quickly than it is drained, allowing the water to fill the retard chamber and continue on to the alarm pressure switch or water motor gong. Multiple retard chambers can be added to a system with large water surges. The water motor gong is a
e Continued on p 56
phc june 2011
www.phcnews.com
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