Water Supply and Distribution Systems
From main to fixture
Water enters through the service, passes the meter, feeds the building main, then branch lines and fixture supplies. Sizing is driven by total WSFU, developed length, and available pressure. Minimum sizes are tabulated: branch supplies are commonly 1/2 inch; mains and services are larger and increase with load. Velocity must stay low enough to limit noise and erosion.
Pressure control
Three pressures matter: static (no flow), residual (while flowing), and elevation loss (0.433 psi per foot of rise). Where main pressure is excessive, a pressure-reducing valve protects the system. Any closed system (PRV, check valve, backflow preventer, or meter with a check) needs a thermal expansion tank, because heated water has nowhere to expand.
Distribution details candidates miss
- Water hammer arrestors at quick-closing valves (flushometers, solenoid valves, dishwashers).
- Vacuum breakers or backflow devices at hose bibbs, irrigation, boiler feed, and chemical connections.
- Isolation valves on each fixture group and at the water heater.
- Slope and support: piping must drain and be supported at code intervals; Arkansas freezing conditions require burial below frost depth or insulation.
- Hot water: insulate hot lines, consider recirculation, and keep dead legs short.
Traps the exam sets: assuming pressure stays constant as flow increases (it drops); forgetting elevation loss on a multi-story building; using the drainage table for supply sizing; ignoring that a new PRV creates a closed system.
Worked example: A meter reads 62 psi static. The top-floor flushometer is 35 ft above the meter. Elevation loss = 35 × 0.433 ≈ 15.2 psi; residual at the fixture before friction ≈ 46.8 psi - subtract pipe friction to check the 15 psi minimum needed for flushometer valves.