A dry pipe sprinkler system is one in which pipes are filled with pressurized air or nitrogen, rather than water. This air holds a remote valve, known as a dry pipe valve, in a closed position. Located in a heated space, the dry-pipe valve prevents water from entering the pipe until a fire causes one or more sprinklers to operate. Once this happens, the air escapes and the dry pipe valve releases. Water then enters the pipe, flowing through open sprinklers onto the fire.
Dry pipe systems are installed at locations where the ambient temperature is may be cold enough to freeze the water in a wet pipe system. The chilling temperature may render the system inoperable.
Dry pipe systems are most often used in unheated buildings, in parking garages, outside canopies attached to heated buildings (in which a wet pipe system would be provided), or in refrigerated coolers.
Dry pipe systems are the second most common sprinkler system type. In regions using NFPA regulations, dry pipe systems cannot be installed unless the range of ambient temperatures reaches below 40F
Operation - The piping network is filled with air above the water supply pressure to prevent the water to flow into the piping. The design of the dry pipe valve (a specialized type of check valve) results in a higher force exerted on top of the check valve clapper.
When one or more of the automatic sprinklers are operated due to heat, it allows the air in the piping to vent from those Sprinklers. Each sprinkler operates individually. As the air pressure in the piping drops, the pressure differential across the dry pipe valve changes, allowing water to enter the piping system.