Flow and head axes
Flow is normally on the horizontal axis and total head on the vertical axis. Confirm units before comparing the curve with a system requirement.
Head-capacity curve
For many rotodynamic pumps, produced head decreases as flow increases. Curves apply to a stated speed, impeller diameter and fluid condition.
System curve and duty point
Static head remains when flow is zero; friction grows strongly with flow. The intersection of system and pump curves is the expected duty point, subject to control and system variation.
Efficiency island and BEP
Efficiency contours identify operating efficiency and the best efficiency point. Reliability depends on operating region, not only the peak efficiency number.
Shutoff and runout
Shutoff is the zero-flow head at the left end of the curve. Runout is the high-flow end. Neither should be treated as a normal duty point without manufacturer guidance.
Impeller diameter and speed
A family chart may show multiple trims or speeds. Confirm the exact curve for the proposed configuration; do not combine points from different trims.
Series and parallel
Series pumps add head at a common flow; parallel pumps add flow at a common head in an idealized view. The actual combined operating point still depends on the system curve and controls.
Power and NPSH curves
Check shaft/input power across the operating range and confirm motor non-overload. Review NPSHR/NPSH3 and required margin separately from head performance.
How to verify a candidate
Use the current manufacturer curve or certified test data, match speed and impeller, plot all expected system cases, and check the allowable or preferred operating region.
Worked example
A curve gives 53 m at 100 L/min and 31 m at 200 L/min. Linear screening at 120 L/min gives 48.6 m. If the system requires 48 m, the margin is about 1.3%; the full curve and operating tolerance deserve close review.
Sources
- Hydraulic Institute — Pump FAQs
- Hydraulic Institute — Combined Pump & System Curves
- Hydraulic Institute — Pump Principles and NPSH