Back to the Basics: Wet & dry sprinkler system
The 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 and will be referenced throughout the article.
A wet system, as defined in NFPA 13, is a sprinkler system 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 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
We use an alarm check valve on the riser to lock water pressure into the system and prevent 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 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, 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 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. 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 equipment necessary for each project.
A dry system, 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 sprinklers3. 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 total volume of piping4. 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 within the tank to introduce more air to the system in the event of a small loss. Another option to consider are 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.
1. NFPA 13, 2010 edition, Section 1.1.1
2. NFPA 13, 2010 edition, Section 3.4.10
3. NFPA 13, 2010 edition, Section 3.4.5
4. NFPA 13, 2010 edition, Section 7.2.3.
Todd Stevens, CFPS, is a national technical representative for The Viking Corporation.