NFPA guidelines for installing components that supply and maintain pressurized air to dry sprinklers
A dependable and properly maintained supply of pressurized air is key to the reliable operation of dry sprinkler systems. But the extra equipment—including air compressors and air maintenance devices— needed to achieve optimum levels makes installation more time-consuming and complex.
In this post, we will take a look at best practices for installing the unique components of a dry sprinkler system that supply and maintain the pressurized air in the pipes.
Check out previous installments in the series:
Are you looking to buy parts for your building’s sprinkler system? QRFS offers a range of dry sprinkler heads available on special order. Simply give us a call at 888.361.6662 or email firstname.lastname@example.org.
Extra parts make installation of dry systems more complicated
Dry sprinkler systems are used when the pipe in a wet sprinkler system can’t be heated to prevent freezing, risking blocked or burst pipes that would render the wet system inoperable. Dry sprinkler systems are filled with pressurized air or nitrogen until the heat from a fire opens a sprinkler head. When that occurs, the loss of air pressure in the piping causes a special dry valve to trip, releasing water into the pipes and out the open sprinklers onto the fire.
Filling the system with pressurized gas that won’t cause the pipes to freeze in cold temperatures requires extra components not found in wet systems—and NFPA 13: Standard for the Installation of Sprinkler Systems sets strict guidelines for installing these parts to ensure the systems work as intended. Let’s take a look at some of the main air supply components and NFPA’s installation requirements.
Dry sprinkler components that maintain air pressure and supply
NFPA 13 defines its use of “air” as air, nitrogen, or other approved gases for a dry sprinkler system. The air supply must come from a reliable source, and A.126.96.36.199.1 permits it to be “generated on-site or from storage containers sized to provide a reliable supply for at least 6 months of expected maintenance use.” These gas-storage containers also must have a low-pressure alarm that alerts onsite personnel when they need to be refilled (188.8.131.52.3).
While air supplies may be provided by plant air systems, most often an air compressor feeds clean, dry, pressurized air or nitrogen into the system. The amount of air pressure needed to keep the dry valve from opening must be maintained on dry sprinklers at all times until the system is activated (184.108.40.206). How much and how long the compressor runs directly relate to the amount of leakage in the system—often a direct result of how tightly fittings are installed.
NFPA 13 offers the following guidelines for air leaks:
From the 2019 Edition of NFPA 13
220.127.116.11.2 The permitted rate of air leakage shall be as specified in 28.2.2, which details the dry pipe and double interlock preaction system air test.
18.104.22.168 In addition to the standard hydrostatic test, an air pressure leakage test at 40 psi (2.7 bar) shall be conducted for 24 hours. Any leakage that results in a loss of pressure in excess of 1 ½ psi (0.1 bar) for the 24 hours shall be corrected.
22.214.171.124 Where systems are installed in spaces that are capable of being operated at temperatures below 32°F (0°C), air or nitrogen gas pressure leakage tests required in 28.2.2 shall be conducted at the lowest nominal temperature of the space.
126.96.36.199 Pipe or tube specifically investigated for suitability in dry pipe and double interlock preaction system(s) and listed for this service, shall be permitted to be tested in accordance with their listing limitations.
Smaller sprinkler systems may use riser-mounted compressors to combat minor leaks, which are less expensive but tend to be noisier than tank-mounted compressors. Tank-mounted compressors also have some downsides to consider—including higher costs and a need for additional floor space—but are preferable for most applications.
Here’s why: The tank acts as a pressurized reserve with a larger capacity, reducing the frequency that the compressor needs to run to supply the system. Lower ambient noise is another benefit, as mounting the compressor to the tank vibrates the tank instead of the building’s piping. In fact, General Air Products released a “Quiet Series” tank-mounted compressor to further reduce noise complaints.
NFPA 13 doesn’t distinguish between types of compressors, but it does require the compressed air supply to come from a source that’s always available (188.8.131.52.1) and can restore normal air pressure to the sprinkler system within 30 minutes (184.108.40.206.2). The sole exception: compressors that help protect refrigerated spaces maintained below 5°F (–15°C), where normal system air pressure can be restored within 60 minutes (220.127.116.11.3).
NFPA 13 also offers installation guidelines for the air supply connections that link the air compressor to the dry valve. Installers should use the type of connection recommended by the manufacturer and approved by the authority having jurisdiction (AHJ) while considering the pressures, temperatures, and vibrations that the connections and its adjacent equipment endure. A.18.104.22.168.1 says that flexible hoses may be suitable that are capable of withstanding the expected vibration, maximum pressure of 75 psi or greater, and a maximum temperature of 150°F (66°C) or higher.
Other installation requirements include:
22.214.171.124.1* The connection from the air supply to the dry pipe valve shall not be less than 1∕2 in. (15 mm) in diameter and shall enter the system above the priming water level of the dry pipe valve.
126.96.36.199.2 A check valve shall be installed in the air filling connection.
188.8.131.52.2.1 A listed or approved shutoff valve of either the renewable disc or ball valve type shall be installed on the supply side of this check valve.
NFPA 13 (184.108.40.206) also calls for the installation of an approved relief valve between the air supply and shutoff valve that’s “set to relieve pressure no less than 10 psi (0.7 bar) in excess of system air pressure provided in 220.127.116.11.1 and shall not exceed the manufacturer’s limitations.”
Relief valves alleviate excess pressure due to potential thermal expansion issues. Relief valves that are approved, or “listed,” are tested and certified by a qualified organization such as FM Global or UL (e.g., UL 1478A).
Proper installation of automatic air maintenance devices prevents accidental triggers
Automatic air maintenance devices are essential for maintaining proper air pressure in dry sprinkler piping. Installed downstream from the air compressor, they reduce the possibility of false valve trips due to tiny leaks that slowly reduce a dry system’s air pressure by providing a continuous but restricted air supply to the pipes. They also ensure the pressure doesn’t climb too high, which can damage the dry valve and prevent it from opening during a fire.
The air maintenance device automatically causes the air compressor to cut in and out to achieve the right balance. It prevents the air supply from adding air at too fast a rate, which could stop the dry valve from releasing the water during a fire.
NFPA 13 requires dry systems to have a dedicated, listed air maintenance device (18.104.22.168.3.1) except in systems where “the air compressor supplying the dry pipe system has a capacity less than 5.5 ft3/min (160 L/min) at 10 psi (0.7 bar).” (22.214.171.124.2)
126.96.36.199.2 Where stored nitrogen or other approved gas is used, the gas shall be introduced through a pressure regulator and shall be in accordance with 188.8.131.52.
NFPA 13 offers the following guidelines for air maintenance device installation:
184.108.40.206.1* Unless the requirements of 220.127.116.11.2 are met, where the air supply to a dry pipe system is maintained automatically, the air supply shall be from a dependable plant system or an air compressor with an air receiver, and shall utilize an air maintenance device specifically listed for such service and capable of controlling the required air pressure on, and maximum airflow to, the dry pipe system.
18.104.22.168.2 Where the air compressor supplying the dry pipe system has a capacity less than 5.5 ft 3/min (160 L/min) at 10 psi (0.7 bar), an air receiver or air maintenance device shall not be required.
22.214.171.124.3 The automatic air supply to more than one dry pipe system shall be connected to enable individual maintenance of air pressure in each system.
126.96.36.199.4 A check valve or other positive backflow prevention device shall be installed in the air supply to each system to prevent airflow or waterflow from one system to another.
188.8.131.52.5 Where an air compressor is the dedicated air supply, it shall be installed in accordance with NFPA 70, Article 430.
184.108.40.206.5.1 The disconnecting means for an automatic air compressor shall not be a general-use light switch or a cord-and-plug connected motor.
Consistent air pressure is essential in dry sprinkler systems
In order to operate reliably, dry fire sprinkler systems require a clean, dry supply of air or nitrogen that’s carefully maintained at the proper pressure. By taking the time to understand NFPA requirements and installing these devices correctly, you can ensure that your dry system stands ready to protect people and property from fires in freezing temperatures.
Check out the next installment in this series: Guide to Dry Sprinkler Systems, Part 5: Daily, Weekly, and Monthly Inspections.
If you’re looking to buy components for your dry pipe sprinkler system, QRFS offers a range of dry sprinkler heads available on special order. Simply give us a call at 888.361.6662 or email email@example.com and we’d be happy to help.
Note: This post was updated to accurately reflect that NFPA 13 does not require air maintenance devices to be solely “listed by Underwriters Laboratories (UL).”