Water Distribution: Pressure, Storage, and Public Health Protection
Water Distribution tests operation and maintenance of mains, storage, pumps, valves, hydrants, meters, and cross-connection control in island systems. Hawaii's systems range from large municipal grids to small county and private systems. The exam emphasizes pressure, disinfectant residual, public health protection, and documentation because distribution is where treated water is most exposed to contamination and loss.
Core rules and concepts
- Pressure. Convert head to pressure with psi = feet × 0.433. Many systems target a minimum of about 20 psi under normal and fire-flow conditions; negative pressure can allow contamination.
- Storage. Tanks provide fire flow and equalization but must turn over to avoid stale water. Check altitude valves, level controls, mixing, and residual in dead zones.
- Cross-connection control. Use air gaps, reduced-pressure zone assemblies, double-check valve assemblies, and pressure vacuum breakers. Containment protects the public main; isolation protects internal plumbing. Test devices on schedule.
- Main breaks and repairs. Isolate, notify affected customers and regulators as required, repair, flush, disinfect, sample for coliform, and issue a boil water notice when required.
- Flushing. Aim for self-cleansing velocity, roughly 2.5–3 ft/s, and flush dead ends and low-flow areas. Dechlorinate before discharge when required.
Worked example
A 12-inch main, 1,000 ft long, must be flushed. Radius = 0.5 ft; area = π(0.5)² = 0.785 ft²; volume = 0.785 × 1,000 = 785 ft³; gallons = 785 × 7.48 = 5,872 gal. At 100 gpm, flush time = 5,872/100 = 58.7 min. The reasoning matters: use radius, convert cubic feet to gallons, then divide by actual flushing flow.
Traps that cost marks
Using diameter instead of radius in pipe volume; mixing feet of head with psi without the 0.433 factor; flushing too slowly to scour; forgetting chlorine demand in new mains; skipping backflow testing; and ignoring pressure surges after valve or pump changes.
Key points
- psi = feet of head × 0.433; low pressure is a contamination risk.
- Storage tanks need turnover, mixing, and residual checks.
- RPZ, DCVA, PVB, and air gap are different backflow protections.
- Pipe volume uses radius: πr² × length × 7.48 gal/ft³.