Distribution System Components, Materials, and Design
Pipes, valves, and appurtenances
Know the common materials and where each fits. Ductile iron (often cement-lined) is strong and used for larger and higher-pressure mains; PVC (AWWA C900/C905) and fused HDPE are common for distribution and trenchless work; concrete cylinder pipe serves large transmission; copper is typical for service lines. Legacy asbestos-cement and lead services require special handling and replacement planning.
Valves do specific jobs. Gate valves are for isolation, not throttling. Butterfly valves isolate large mains and can be used for some control. Check valves prevent reversal and are not isolation valves. Pressure-reducing and altitude valves control zone pressure and tank filling; air-release and vacuum valves protect high points and prevent collapse during draining. Hydrants, meters, corporation stops, curb stops, and service lines complete the system.
Design concepts: minimum main size for fire flow (commonly 6 in.), design velocity around 2–5 fps with an upper limit near 10 fps, cover below frost, thrust restraint at bends, tees, and reducers, and reasonable valve and hydrant spacing so a break can be isolated with limited customer impact. Internal roughness (Hazen-Williams C-factor) drops as pipe ages, raising head loss.
Traps: using a gate valve to throttle flow; counting a check valve as an isolation point; ignoring thrust restraint at a reducer; forgetting that air valves are needed at high points.
Worked rule: velocity = flow ÷ area. A 6-in. main (area 0.196 ft²) carrying 500 gpm (1.11 cfs) runs about 5.7 fps - acceptable but near the upper design range.