Hydraulics, Sizing, and System Design
The sizing equation
Total dynamic head (TDH) equals vertical lift from the pumping water level to the discharge point, plus drawdown, plus pressure head, plus friction. Convert pressure to feet with psi x 2.31, so a 60 psi cut-out is about 139 ft of head. Get friction from pipe and fitting tables using actual flow and pipe diameter, then add a small allowance for fittings and valves. The duty point is where the pump curve crosses the system curve; select so that point falls in the middle of the curve, not at shutoff or runout. Pump capacity should not exceed well yield, and the pump intake must stay submerged at maximum drawdown while remaining above the screen.
Traps
Sizing on cut-in pressure instead of cut-out is the most common error. Forgetting friction, long horizontal runs, and elevation changes understates head. Mixing psi and feet mid-calculation causes large mistakes. Selecting a pump that out-pumps the well causes drawdown below the intake, cycling, and motor overheating. Ignoring tank volume leads to short cycling.
Worked example
Static water level is 65 ft, drawdown 15 ft, friction 18 ft, and the switch is 40/60. Use cut-out: 60 psi x 2.31 = 138.6 ft. TDH = 65 + 15 + 18 + 138.6 = 236.6 ft, plus about 10 percent for fittings, roughly 260 ft. A pump delivering the target flow near 260 ft of head is the correct selection, confirmed against the well yield.