Explain the difference between "wet" and "dry" pipe systems as per NFPA 13.

Wet pipe systems and dry pipe systems are two primary types of fire sprinkler systems outlined in NFPA 13. The defining characteristic of a wet pipe system is that it is always filled with water. This means that when a sprinkler is activated due to heat from a fire, the water is immediately available to discharge and suppress the flames. This design provides a faster response time, making it highly effective in many applications where freezing is not a concern.

In contrast, dry pipe systems do not contain water within the pipes; instead, they are filled with pressurized air or nitrogen. The presence of air pressure keeps the valves closed until a sprinkler head activates by heat. When a sprinkler is triggered, the air pressure drops, the dry pipe valve opens, and water rushes into the piping to reach the activated sprinkler. This design is beneficial in environments where temperatures may fall below freezing, as it prevents water from freezing within the pipes.

Understanding that wet pipe systems are always water-filled allows for quicker activation, making them preferable for many indoor applications, while dry pipe systems offer solutions for areas subject to low temperatures and potential freezing conditions highlights the intrinsic differences in their design and operation.