Dry pipe systems use automatic sprinklers that are attached to a piping system containing air under pressure. When the air is released from the opening of the sprinklers, the water pressure overcomes the lessened air pressure and opens the dry pipe valve. The dry pipe valve is designed so that a moderate air pressure prevents a higher water pressure from opening the valve.
Once the valve is opened, the water flows into the piping system and out the opened sprinklers. This system provides protection in areas subject to below freezing temperatures. Piping must be drained of water or foam solution following operation to prevent freezing.
Typically, upright sprinklers are used to allow drainage. Otherwise, approved dry pendent sprinklers must be used to prevent freezing. Dry pipe systems are inherently slower in operating and should be considered less desirable for applications using AFFF.
