What characteristic defines a dry pipe sprinkler system?

A dry pipe sprinkler system is characterized by pipes that are filled with air or nitrogen, preventing water from being present in the pipes until a fire triggers the system. When the sprinkler head is activated by heat from a fire, it opens, allowing the pressurized air to escape and creating a drop in pressure that activates the dry pipe valve. This valve then opens and fills the pipes with water, allowing for effective fire suppression.

The essential feature of this system is that water is not stored in the pipes under normal conditions, making it particularly useful in areas prone to freezing temperatures, as it avoids the risk of water freezing and blocking the pipes. In contrast, systems that continuously have water flowing through the pipes are known as wet pipe systems, which differ significantly from dry pipe systems.