Pumping, Storage, and Pressure Management
Pumps, tanks, and pressure control
Pumps operate where the pump curve meets the system curve. Shutoff head is maximum head at zero flow; runout is maximum flow at low head. Cavitation occurs when suction conditions fall below required net positive suction head (NPSH), causing noise, vibration, and damage; the fix is better suction, lower temperature, or a different pump - not simply a bigger motor. Affinity laws predict changes: flow varies with speed, head with speed squared, and power with speed cubed. VFDs match output to demand and reduce surge.
Storage performs several jobs: equalizing daily demand, supplying fire flow, providing emergency reserve, and maintaining pressure. Elevated tanks set pressure by water level; ground storage needs booster pumps. An altitude valve stops tank filling when the tank is full. Hydropneumatic tanks use compressed air for pressure in small systems. Tank turnover and mixing matter - long stagnation degrades residual and can promote nitrification or biofilm. Surge (water hammer) follows rapid valve closure or pump trips; slow valve operation, surge tanks, and proper air valves reduce it.
Traps: reading a pump curve only at its best-efficiency point; confusing static pressure with residual pressure under flow; assuming tank pressure stays constant as level drops; ignoring NPSH margin.
Worked rule: a tank water surface 150 ft above a service point gives 150 ÷ 2.31 ≈ 65 psi static. Under flow, residual pressure will be lower by the head loss in the piping.