Which of the following best describes a dry-pipe sprinkler system?

• A. The pipes are always filled with water to ensure immediate discharge.
• B. The pipes are kept empty until a head is activated, then filled with water.
• C. The pipes are filled with nitrogen and water is released slowly by diaphragm valves.
• D. The pipes are kept filled with pressurized air or nitrogen and release water into the pipes when a head is activated.

Answer: (D)

Dry-pipe sprinkler systems keep the distribution piping dry by filling the pipes with pressurized air or nitrogen, not water, until a head is activated. The water supply is held back by a dry-pipe valve, and when a sprinkler head is triggered, that valve opens and water rushes into the air-filled pipes, pressurizing them with water up to the activated head. This arrangement prevents freezing in unheated spaces and introduces a brief delay as the pipes fill after activation. That description matches the idea of pipes kept under pressure with air or nitrogen and water being released into the pipes when a head is activated. The other descriptions don’t fit: a system with pipes always filled with water describes a wet-pipe system; a system kept empty until activation and then filled with water misses the essential dry-pipe gas pressurization; a nitrogen-filled system with water released slowly by diaphragms isn’t how standard dry-pipe systems operate.