Wet & dry sprinkler systems Continued from page 44
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
requiring someone to call local authorities if it is ringing. Although this alarm is not used as often as it was in the past, some local officials or end users may require that it be installed. If this alarm is required, it is important to specify an alarm check valve.
Ported check valve Technology has allowed us to utilize electrical devices
and fire alarm systems to help provide the required alarm signal upon water flow. If we take advantage of this tech- nology, we can provide a sprinkler riser that is slightly less complicated than a full riser check valve assembly. A port- ed check valve will include the following: a check valve to lock pressure into the system, pressure gauges above and below the check valve and a main drain connection. This check valve does not include a tapped boss for the
alarm line. The required alarm trim piping can be elimi- nated and a vane type water flow switch installed in the piping, above the check valve. This is an electrical switch with a plastic paddle installed through a hole in the pipe. When the paddle is pushed forward by a water flow con- dition, it will create an electrical alarm signal, which can be sent to a fire alarm panel or directly to an electric bell. As discussed, some water supplies may create surges
and cause false alarms. The water flow device includes a retard or delay setting built into the switch, preventing the signal from being sent until the paddle is held forward, by water flow, for a set length of time. Both of these riser assemblies are acceptable configura-
tions to NFPA 13. It is important to review which type is required by the local authorities and which may be the most cost effective. Many engineer’s specifications include an alarm riser
Dry System Riser: This detail is showing a Dry Pipe Valve with trim. Some of the components shown include a dry valve, air maintenance device, pressure switch and accel- erator. Courtesy of Viking Corporation.
enters into the alarm line and will activate an alarm pres- sure 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 sys- tem is a false water flow alarm. A retard chamber provides a mechanical delay prior to an audible alarm, either mechani- cal or electrical. It is used when there is variable pressure in the water supply. If variable pressure or water surges are pre- sent, false alarms may occur in the system. The retard chamber is a one gallon tank that absorbs water
surges prior to 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 mechanical device typically installed on the outside wall of the sprinkler riser room. This is a water-driven alarm with no electrical connection,
Page 46/Plumbing Engineer
check valve, which may not be necessary if a water motor gong is not used. A ported check valve can provide the same principles but at less cost for both the equipment and installation. Engineers should review their specifications to make sure that they are looking for the proper equip- ment necessary for each project.
Dry systems A dry system, as defined in NFPA 13, is a sprinkler sys-
tem employing automatic sprinklers that are attached to a piping system containing air or nitrogen under pressure, the release of which (as from the opening of a sprinkler) permits water pressure to open a valve known as a dry pipe valve; the water then flows into the piping system and out the opened sprinklers3
. A dry system is going to be consid-
ered for areas where the temperature cannot be maintained above 40° F (4° C). It is important, however, to make sure the dry pipe valve assembly and equipment is installed within a heated environment. The dry pipe valve is a fairly simple concept, but, like the wet riser, offers a few options to consider when specifying the equipment.
Dry pipe valve The dry valve, installed at the sprinkler riser, creates the separation of the air pressure in the piping network and the water supply. These valves generally work on a differen- tial principle. This means that, if a valve has a 6:1 differ-
Continued on page 48 May 2011
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