Constant-Pressure Well VFD Setpoint and Speed

The head a well pump must produce is three terms: the static lift to the water, the friction through the pipe, and the pressure setpoint it is holding. Only the FRICTION term is speed-sensitive, and it falls with the square of flow. THE CUBE LAW DOES NOT APPLY, and expecting it is the error this exists to correct. A variable frequency drive saves on the cube of speed when the head is friction head -- a circulating loop, a duct system, a pool. On a well holding a pressure setpoint, static lift and the setpoint are constants that do not care how fast the pump turns, so cutting the flow by three quarters may cut the speed by only a few percent. The pump is doing lifting work rather than friction work, and lifting work does not go away when you slow down. A constant-pressure system is worth buying for the pressure it holds -- steady pressure at every fixture, no pressure tank cycling, no water hammer at the shower when the washer fills -- and it should be sold on that rather than on an energy saving that a friction-dominated system would deliver and this one will not. AND THE DRAWDOWN IS WHAT PRODUCES THE COMPLAINT. Static lift is not a constant: pumping draws the water level down, so the lift at design flow is greater than the lift at rest, and it grows with the flow being drawn. A pump curve that comfortably covers the standing condition can fail to reach the setpoint at high flow with the level drawn down -- and the system then loses pressure at exactly the moment demand is highest, which is the fault that gets reported and the one a static calculation never predicts. Where the required head exceeds what the pump can make, no control strategy recovers it: the answer is a different pump, a lower setpoint, or storage. This computes head and speed from entered figures. It does not select a pump or read a pump curve (the affinity relation here assumes the pump follows its curve, and a real curve is not a perfect square law), size a drive or a motor, compute power or energy (which needs the curve and the motor and drive efficiencies at each speed), model the well's drawdown against rate -- that is a step test -- address minimum flow, thermal protection at low speed, or the cooling flow a submersible motor needs, or evaluate pressure tank sizing and control settings. The pump manufacturer's curve, the well's own test data, and the pump installer govern.

total head = static lift + friction + setpoint (psi x 2.31 ft/psi); friction is the only speed-sensitive term and falls with the square of flow, and the speed follows N2 = N1 x sqrt(H2/H1).

Constant-pressure well pump operation on the pump affinity relations. The pump is assumed to follow its curve; a real curve is not a perfect square law.

The affinity relations and one exact pressure conversion.

Estimate. AHJ and licensed professional govern.

Field names used by the API: static_lift_ft, friction_at_design_ft, design_flow_gpm, setpoint_psi, reduced_flow_gpm, full_speed_rpm, drawdown_at_design_ft, pump_max_head_ft, setpoint_head_ft, head_at_design_ft, head_at_reduced_ft, speed_at_reduced_rpm, speed_reduction_pct, head_with_drawdown_ft

• Pump follows its curve a real curve is not a perfect square lawthe manufacturer's curve
• No power or energy computed that needs the curve plus motor and drive efficiencies at each speeda pump selection program
• Drawdown is entered modelling it against rate is a step testthe well's own test data