A wet pipe sprinkler riser is required to have specific components to perform as required per NFPA 13, Standard for the Installation of Sprinkler Systems. A wet pipe system riser typically contains either an alarm check valve assembly or a “shotgun” riser assembly which are used to maintain system pressure and provide waterflow notification. Both styles of riser provide their own benefits and disadvantages that should be considered when designing a wet pipe system.
Historically, Alarm Check Valves provided notification of waterflow before electrical waterflow alarms were used. These valves controlled the flow of water to a bell called a water motor gong, which is discussed further below. An Alarm Check Valve consists of multiple components that make up the entire valve assembly. These components include the following:
- Check valve clapper – The clapper is the component that maintains water pressure on the system side of the alarm check valve assembly. Once a pressure drop on the system side occurs from either an opened sprinkler head or other form of waterflow, the alarm check valve clapper will open and provide the system with water from the water supply. This clapper also controls the flow of water to the waterflow alarm.
- Pressure Gauges – Pressure gauges are devices that indicate the current pressure with the sprinkler system. On an alarm check valve assembly, there is usually a pressure gauge on the system side and one on the water supply side of the clapper. Typically, the pressure on the system side of the clapper is greater than the water supply side of the clapper.
- Pressure Switch – A pressure switch is a type of waterflow alarm, which simply detects the force of water pushing against it. When the clapper opens inside the Alarm Check Valve, water is allowed to push up against the pressure switch, sending an alarm signal to the building’s fire alarm panel. The fire alarm will then notify occupants and transmit a signal to a central station monitoring service or directly to the fire department.
- Water Motor Gong – A water motor gong is a mechanical bell – driven by water – that provides localized notification when water is actively flowing through the system. Water motor gongs can be seen on older buildings as a large, flattened dome, cast iron bell mounted on an exterior wall. Gongs will always have a drain immediately beneath them. When the Alarm Check Valve clapper opens during a water flow, a pipe is uncovered which allows water to flow to the gong. This water strikes a water wheel inside the gong, spinning the wheel and driving the hammer inside the gong. The spent water then runs freely out the drain to the exterior of the building. Water motor gongs are seen less in newer system installations as they have been replaced by electric-powered bells.
- Retard Chamber – A retard chamber is used to create a mechanical delay to mitigate false alarms caused by water surges in a system. The retard chamber is a small tank (typically 1 gallon) located between the Alarm Check Valve and the pressure switch and water motor gong piping. The tank must first fill with water before traveling through the trim piping and activating the notification devices, causing a mechanical time delay. This delay helps mitigate false alarms caused by water surges in the system by preventing immediate activation of the notification devices.
One benefit of an Alarm Check Valve is that it provides mechanical notification through the associated trim components, such as the water motor gong. Electrical power is not required to initiate an audible alarm signal, water pressure alone drives the alarm. This provides an alternate, backup alarm signal beyond the required fire alarm components. The necessity of a water motor gong is typically enforced by the AHJ as they are not required on wet pipe systems by NFPA 13. If a water motor gong is required by the AHJ to be installed for a system, an Alarm Check Valve assembly should be specified for the project.
A shotgun riser assembly has a similar function to an Alarm Check Valve – it maintains system pressure and initiates alarm signals. The main difference in these two styles is that a shotgun riser assembly does not contain the associated trim components that are provided on an alarm check valve assembly, such as a pressure switch, retard chamber, or water motor gong. The shotgun assembly is provided with a simple check valve (which is often the system’s backflow preventer and not a separate check valve), pressure gauge, and a waterflow alarm device. The waterflow alarm is typically a paddle-type switch, rather than a pressure switch. The paddle-type alarm has a plastic arm located inside the pipe. When water flows, it moves the paddle, activating an electrical switch mounted on the pipe. This sends an alarm signal to the building’s fire alarm system, notifying a central station monitoring company or the fire department. Shotgun riser assemblies are far simpler than an Alarm Check Valve, and do not require the associated trim components which usually helps to reduce installation costs.
It is important to understand the differences in the types of risers when designing and specifying the components for a new wet pipe system. Each type has benefits and downsides that should be considered on a project specific basis. If you have any questions on which type of riser assembly would be best fit for your project, please contact our office at firstname.lastname@example.org.