MECHANICAL CONTRACTING e Continued from p 54
mechanical device typically installed on the outside wall of the sprinkler riser room. This is a water-driven alarm with no electrical connection, 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
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utilize electrical devices and fire alarm systems to help provide the required alarm signal upon water flow. If we take advantage of this technology, we can provide a sprinkler riser that is slightly less complicated than a full riser check valve assembly. A ported 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 eliminated 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 condition, 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 configurations 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 check valve, which may not be necessary if a water motor gong is not used. An easy riser 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 equipment necessary for a particular project.
Dry systems A dry system (Fig. 2), as defined in
NFPA 13, is a sprinkler system 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 sprinklers . A dry system is going to be considered 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 differential principle. This means that, if a valve has a 6:1 differential, you can hold back 6 psi of water pressure with only 1 psi of air pressure above the clapper. When a sprinkler operates, and the air pressure is lowered, the water pressure below the valve will eventually overcome the differential and push the clapper of the dry valve open. This will allow the water supply to enter the piping network and be discharged from the sprinklers that have operated. This often brings the
question from engineers, how much air pressure is required to keep the dry valve in the set position? Using the 6:1 rule as an example, simply take the water supply pressure, divide it by 6 and add 15 to 20 psi for a cushion. Multiple manufactured dry valves may have a variety of differentials: It is important to review the manufacturer’s technical data, which often provides the proper air supply requirements, based on your water supply pressure. When a sprinkler operates in a
dry system there will be a delay in full water flow, due to the travel time from the valve to the open sprinkler. NFPA 13 discusses requirements for maximum water delivery in dry systems based on the size of the system . Accessories called accelerators are available to assist the speed of the valve
operation. Accelerators can help redirect air pressure in the system to lower the differential of the valve faster, which in turn speeds the operation of the valve. When putting a dry valve into
service, you often need to open the face plate of the valve and reset the clapper assembly inside. Newer valves can be reset externally, eliminating the need to open the valve for any reason. After the clapper is set and the face plate is replaced, you are ready to introduce air into the system. Compressed air or nitrogen can be used to pressurize the dry pipe system. Riser mounted compressors are often used on smaller systems; however, tank mounted compressors with air maintenance devices are suggested for most applications. An air maintenance device helps
to maintain the proper air pressure in the piping and uses the supply
Fig. 2. A dry system
within the tank to introduce more air to the system in the event of a small loss. Another option to consider is dryers installed on the air supply to remove any excess moisture in the piping. The trim piping installed on the dry valve offers the necessary pressure gauges, drains and pressure switches for water flow conditions. Other details of these systems are
specific to various manufacturers. I suggest that you take the time to review the equipment and installation requirements applicable to your system. ;
Todd Stevens, CFPS, is a national
technical representative for The Viking Corporation.
phc june 2011
www.phcnews.com
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