System head from pipe + fittings + elevation → pump power & NPSH
System head, the system curve, the duty point, and how brake/motor horsepower follow from TDH
Start here to size from the system, then branch to the specialized pump calculators in the order a real selection follows.
Build the total dynamic head (TDH) from the system: static head from elevation, pressure head from any vessel pressure difference, friction head in the straight pipe (Darcy-Weisbach), and minor losses from fittings (Crane TP-410 K-factors for elbows, tees, valves, entrances and exits). TDH at the design flow is the head the pump must deliver. Brake horsepower is then Q×TDH×SG/3960 divided by pump efficiency, and the motor is the next NEMA size above BHP×service factor (the nameplate is a shaft-output rating; motor efficiency sets the electrical input, not the frame).
The system curve plots required head versus flow: a constant static part (elevation plus pressure head) plus a dynamic part (friction plus minor losses) that grows with the square of flow. The pump operates where its own head-flow curve crosses the system curve — the duty point. This calculator reports the system curve at 0 to 125 percent of the design flow.
Each fitting adds a minor-loss head of K times the velocity head V² over 2g, where K = fT × (L/D) from Crane TP-410 (for example L/D ≈ 30 for a 90° standard elbow, 8 for a gate valve, 340 for a globe valve). The calculator sums all fittings into the minor-loss head.
NPSH available equals the absolute pressure at the suction source minus vapor pressure (converted to feet), plus the static suction head (positive if the source is above the pump, negative if it is a lift), minus suction-line friction. Keep NPSHa at least 2 to 3 feet above the pump NPSHr.