Pump Meets Pipe
A centrifugal pump converts shaft rotation into fluid pressure and flow. But the actual flow delivered depends not only on the pump but on the piping system it serves — pipe lengths, diameters, fittings, valves, and elevation changes all create resistance. The operating point is where the pump's capability exactly matches the system's demand, found graphically at the intersection of the pump curve and the system curve.
The Pump Curve
Pump manufacturers characterize each pump with an H-Q curve showing head versus flow at a fixed speed. At shutoff (Q = 0), all energy goes into pressure (maximum head). As flow increases, more energy goes into kinetic energy and internal losses, reducing the available head. The curve shape depends on impeller geometry — radial impellers give steep curves, mixed-flow impellers give flatter ones.
The System Curve
The system curve H_system = Hs + KQ² has two components. Static head Hs (elevation difference plus any pressurized vessel pressure) is constant regardless of flow. The dynamic component KQ² represents friction and minor losses, which scale with the square of velocity (and hence flow). Closing a valve increases K, shifting the system curve up and reducing flow; opening it does the opposite.
Efficiency & Energy
Pump efficiency peaks at the best efficiency point (BEP) — the flow rate the pump was designed for. Shaft power P = ρgQH/η rises steeply with flow. Operating far from BEP wastes energy and shortens pump life through vibration and cavitation. Variable-speed drives (VSDs) shift the pump curve to match varying demand, maintaining efficiency across a range of operating conditions. This simulator overlays pump and system curves to find the operating point and reports the efficiency and power consumption.