Distribution Systems and Storage
Getting water to the tap
Distribution questions blend hydraulics with public health. Start with the conversion: 1 ft of water = 0.433 psi, so 100 ft of static head is 43.3 psi. Subtract friction loss (Hazen-Williams, using the pipe's C factor) and elevation change to find pressure anywhere in the system. Systems typically hold at least 20 psi under all conditions - including fire flow - because low pressure invites contamination.
Components and problems
- Valves (gate, butterfly, check, pressure-reducing), hydrants, meters, and blowoffs are the working parts; poor valve exercising is a classic failure.
- Water hammer comes from rapid velocity change; surge tanks, slow-closing valves, and air chambers control it.
- Cross-connections are the biggest health risk. Match the device to the hazard: air gap or RPZ for high hazard, double check for low hazard, vacuum breakers for hose bibs.
- Storage provides fire flow and equalizes demand, but long water age degrades residual and raises TTHM. Mixing, turnover, and inspection matter.
- Corrosion control (pH, alkalinity, phosphate inhibitors, Langelier index) protects pipes and lead service lines.
Traps
Candidates use tank elevation as customer pressure without subtracting friction loss, or confuse a check valve with an RPZ. Residual disappears first at dead ends and closed valves.
Worked example
You need 60 psi at a building 20 ft above the tank base, 1,000 ft away, with 10 ft of friction loss. Required head = 60 ÷ 0.433 = 138.6 ft. Available head above the building = tank height − 20 − 10, so the tank must sit roughly 169 ft above the building floor.