Components of a Wet-Pipe Fire Sprinkler System
The two main types of building fire sprinkler systems are wet-pipe and dry-pipe. Wet-pipe systems always have water in the pipes to the fire sprinkler heads, whereas the water only flows into dry-pipe systems when there is a fire.
Wet-pipe sprinkler systems have few components and should not be installed where they might freeze.
The National Fire Protection Association publishes NFPA 13, “Standard for Installation of Sprinkler Systems.” This publication tells how to design sprinkler systems for buildings and details the components that are acceptable to include in the systems. For example, only new sprinklers are allowed, and pipe, either ferrous or copper, must comply with specific American National Standards Institute (ANSI) and American Society for Testing and Materials (ASTM) standards.
Sprinklers for residential systems come in six styles that provide different options for mounting them. Each style is available with ½- or 3/4-inch threads for attaching them to the water supply pipes. Some styles, called concealed, mount flush with the finished ceiling leaving only a small U-shaped copper piece exposed, while others completely protrude into the living space. A third style called flush is quite unobtrusive with only a small white dome visible when it is installed. Ceiling-mounted sprinklers are called pendants. Horizontal sidewall sprinklers mount to the wall near the ceiling and have deflectors that spread the spray out and down. Sprinklers come with different K-factors that help to match them to different locations in a room, and different places in a building, so they will emit enough water at enough pressure to extinguish a blaze.
Residential sprinkler system pipe is typically steel, brass or copper, but CPVC is also used. Steel pipe used for sprinkler systems will be schedule 10 or schedule 40, brass and CPVC will be schedule 40, and copper will usually be Type M, although Types K and L are also used. Engineers consider the characteristics of each kind of pipe and how each degrades from heat, corrodes and responds to abuse when they are selecting the pipe for sprinkler systems. Other factors include the methods for joining the pipe, cost, hydraulic calculations and maintenance.
The fittings connect one section of pipe to another and allow the pipe to turn corners without having to be bent. They also supply the connecting points for the sprinklers. The fittings must match the type of pipe and the pipe threads must match the fitting threads. One type of fitting is a tee that provides a branch line from a main line. Elbows turn pipe at corners and couplings fasten lengths of pipe together. A fitting called a back flow preventer stops water in the sprinkler system from flowing backward if pressure drops.