Pumping, Storage, and Distribution System Interface
A pump operates where its performance curve crosses the system curve. Total head equals static lift plus friction plus velocity head, and friction rises with the square of flow. The affinity laws for a fixed impeller: flow varies with speed, head with speed squared, power with speed cubed - so a small speed change is a large power change. Impeller trimming follows the same pattern.
Cavitation occurs when NPSH available falls below NPSH required; symptoms are noise, vibration, and pitting, and the cure is more suction head or less suction lift/loss. Watch the pump suction, not the discharge.
Conversions: 1 psi = 2.31 ft of water, so 1 ft of head = 0.433 psi. Water horsepower = (gpm x head, ft) ÷ 3,960; brake horsepower = water horsepower ÷ efficiency. Positive-displacement pumps suit high head at low flow and chemical feed, where they meter accurately.
Storage and interface: clearwells provide disinfection contact time; elevated tanks and standpipes supply pressure and equalization. Long storage means high water age, lower residual, stratification, and nitrification risk in chloraminated systems. Distribution pressure must stay above the regulatory minimum (commonly 20 psi), and backflow protection - air gap, RPZ, double check - protects the main. Water hammer from fast valve closure is controlled with slow-closing valves, surge tanks, or accumulators.
Traps: quoting water horsepower when brake horsepower is asked; omitting efficiency; using static head instead of total head; and mixing up NPSH available and required.
Worked example: 500 gpm at 120 ft with 70 percent efficiency: whp = (500 x 120) ÷ 3,960 = 15.2 hp; bhp = 15.2 ÷ 0.70 = 21.6 hp.