Distribution System Design, Layout, and Hydraulics
Core concepts
A Class III operator must be fluent in how water moves and how the pipe network is arranged to move it. Understand design flow categories (average day, max day, peak hour, fire flow) and how they drive main sizing. Know pipe materials and their C-factors (Hazen-Williams roughness), because a lower C means more head loss for the same flow. Master the hydraulic concepts: pressure vs. head (1 psi ≈ 2.31 ft of head), velocity (keep design velocity roughly 2–5 ft/s in transmission mains, avoid excessive velocity that scours and raises head loss), and the minimum residual pressure conventions (commonly 20 psi at peak demand and during fire flow). Understand network layout - grid vs. tree (branch) systems, looping to eliminate dead ends and improve reliability, and the role of valves in isolating segments.
Traps to watch
Questions love to test unit conversions and the difference between pressure and head. Remember head is in feet, pressure in psi, and the 2.31 factor converts between them. Another trap: confusing static pressure (no flow) with residual/flowing pressure. Also watch for questions about fire flow being additive to domestic demand, and about dead-end mains causing water-quality problems, not just pressure problems.
Worked example
A tank surface sits 115.5 ft above a service connection. Static pressure = 115.5 ÷ 2.31 ≈ 50 psi. If fire flow analysis requires 20 psi residual at that point, the allowable head loss in the path is (50 − 20) × 2.31 ≈ 69 ft. That budget is what you compare against calculated friction loss.
Rule of thumb
1 psi = 2.31 ft of head; design for ≥20 psi residual and keep velocities in the 2–5 ft/s band.