Water supply and distribution system design, sizing, and materials
Design basis
This domain tests sizing and material selection for potable water distribution. Start with the water purveyor's static pressure, meter size, and minimum residual pressure. Calculate developed length from the meter to the farthest fixture, add equivalent length for fittings and valves, and include elevation loss at 0.433 psi per foot of rise. Size pipe so demand fixture units, available pressure, and velocity limits are satisfied. Keep velocities moderate to limit noise and erosion; use pressure-reducing valves where static pressure is excessive and provide thermal expansion control on closed systems.
Materials and protection
Copper, PEX, CPVC, and galvanized steel have different joining, support, and temperature limits. Do not bury or sleeve materials contrary to manufacturer instructions. Separate dissimilar metals with dielectric unions or approved transition fittings. Protect against freezing, UV, and mechanical damage. Provide valves at the service, fixtures, and equipment; install backflow protection at cross-connections; and arrange piping to drain at low points.
Worked example
A meter supplies 60 psi static. The highest fixture is 20 ft above the meter: 20 x 0.433 = 8.66 psi lost to elevation, leaving 51.34 psi before friction. If the fixture needs 15 psi residual, the allowable friction loss is 51.34 - 15 = 36.34 psi. The designer compares that allowance with the pressure loss for the selected size and developed length, including fittings and the meter. If the loss exceeds the allowance, increase pipe size or reduce equivalent length.
Traps
Using only plan length and ignoring fittings; forgetting elevation; sizing by fixture count alone; omitting a PRV or expansion tank; connecting copper directly to steel; ignoring manufacturer limits for PEX or CPVC.