What a manufacturer's curve sheet actually shows
A centrifugal pump curve sheet packs four pieces of information into one chart:
1. Head vs. flow (H-Q) — the headline curve. Multiple lines, one per impeller diameter. 2. Efficiency contours — closed curves connecting points of equal efficiency. The Best Efficiency Point (BEP) sits inside the innermost contour. 3. NPSHr vs. flow — a separate line, usually at the bottom, climbing rightward. 4. Power vs. flow (BHP) — power draw, sometimes plotted as discrete points at each impeller trim and rated speed.
A single curve sheet, well read, tells you whether a pump fits a duty point.
H-Q: the line everyone looks at
The H-Q curve slopes downward — head decreases as flow increases. The shape varies by impeller type:
- Steep (radial-flow impeller) — head changes a lot for a small flow change. Good for systems where flow needs to stay constant despite varying head (e.g., constant-pressure water service).
- Flat (mixed-flow or axial-flow impeller) — head changes little even when flow doubles. Bad for parallel operation (instability) but fine for single-pump duty where the system curve crosses cleanly.
For multiple impeller trims, manufacturers usually plot curves at trim sizes labeled by diameter in inches. Each trim is its own H-Q line. The narrower the trim range published, the more the manufacturer wants you to use one of the published trims rather than splitting the difference.
What the H-Q curve doesn't tell you:
- Wear-related head degradation over the pump's life. Worn pumps deliver less head at any flow, the curve essentially shifts down. Plan for 5-10% degradation over a 10-year service life.
- Performance with non-water fluids. Hot water (vapor pressure issues), viscous fluids, slurries — all reduce head capability. Use the HI viscosity correction or the manufacturer's viscous-service curve.
Efficiency: the contours
Efficiency contours are typically labeled at 5% increments (50%, 55%, 60%, ...). BEP is the highest-efficiency point on any impeller diameter — usually 70-85% for industrial centrifugal pumps.
Reading the efficiency contour:
The intersection of your operating point (H, Q) with the efficiency contour tells you the wire-to-water efficiency. A pump operating at 70% efficiency delivers (Q · H · SG) / (3960 · 0.70) horsepower at the shaft per unit hydraulic output. The motor adds another efficiency factor; wire-to-water includes that.
The Best Efficiency Point (BEP):
The single highest-efficiency point on a given impeller diameter. Each diameter has its own BEP — they form a curve called the best-efficiency line.
Why operating near BEP matters:
- Maximum efficiency = minimum operating cost
- Minimum radial and axial loads on the pump shaft = longer bearing/seal life
- Most stable hydraulic operation
- Lowest vibration and noise
Operating far from BEP (especially below 50% of BEP flow or above 110% of BEP flow) causes:
- Reduced efficiency (more electricity for the same work)
- Higher radial loads → premature bearing/seal failure
- Increased recirculation cavitation risk → impeller damage
- Higher vibration → labor costs
The Hydraulic Institute defines two zones:
- Preferred Operating Region (POR): approximately 70-120% of BEP flow. Continuous operation is acceptable; pump life is at its design value.
- Allowable Operating Region (AOR): approximately 50-130% of BEP flow (varies by impeller type and Ns). Operation is acceptable but pump life is reduced; vibration limits may be approached.
Outside AOR: don't run continuously. Brief excursions (cold-start, low-flow during demand sag) are OK but the operating plan should not include the AOR boundaries as steady-state operating points.
NPSHr: the curve everyone glosses over
NPSHr (Net Positive Suction Head Required) is a property of the pump only. The number on the curve sheet is the NPSHa value at which the pump experiences a 3% head drop — i.e., the threshold of cavitation.
The 3% rule is a measurement convention. Pumps don't operate cavitation-free at NPSHa = NPSHr. They operate with 3% performance loss, which means *cavitation is already happening*. The Hydraulic Institute recommends an NPSH margin of 1.5× NPSHr (or NPSHa - NPSHr ≥ 3 ft, whichever is larger) for continuous service.
NPSHr always rises as flow rises (left to right on the curve sheet). At BEP the NPSHr is the design-cycle value; near runout NPSHr can be 2-4× the BEP value. Operating off-design to the right amplifies suction problems.
What the NPSHr curve doesn't tell you:
- Cavitation type. NPSHr only captures the bulk-flow cavitation that causes the 3% head drop. Recirculation cavitation (which damages impellers without affecting bulk performance) can occur at NPSHa = 2× NPSHr in some pumps.
- Effect of temperature. Cooler water has lower vapor pressure, raising NPSHa — but most curves are tested at standard conditions (60-80°F). For hot service, get the manufacturer's hot-water curve.
BHP / power: the curve they often hide
Power draw (brake horsepower) is plotted as either a separate line at each impeller trim or as discrete power points overlaid on the H-Q chart. The shape depends on the pump's specific speed (Ns):
- Radial-flow pumps (Ns < 2,000): power rises slowly as flow rises, peaks just past BEP, falls toward runout.
- Mixed-flow pumps (Ns 2,000-5,000): power profile is flatter — fewer surprises at runout.
- Axial-flow pumps (Ns > 9,000): power actually drops as flow rises. Counter-intuitive but a real consequence of impeller geometry. The implication: motor sizing for radial pumps is at runout; for axial pumps it's at shutoff.
How to read it:
1. Find your operating point's flow on the x-axis. 2. Project up to the BHP line for your impeller trim. 3. Read shaft horsepower.
For motor sizing, take the maximum BHP across the AOR (not just at design point). Then apply service factor (typically 1.10-1.15 for standard motors).
What the curve sheet doesn't show
These are the most commonly-asked-after pieces of information that aren't on the standard curve:
- Pump weight and dimensions — separate datasheet
- Mechanical seal selection — separate datasheet
- Bearing replacement intervals — vendor service manual
- Minimum continuous flow — sometimes shown as a vertical line, sometimes only in the service manual
- Maximum allowable working pressure (MAWP) — separate datasheet
- Connection sizes and flange ratings — separate datasheet
Always cross-reference the curve sheet with the full datasheet (or call the rep). Selections based on curve-sheet-only data lose the constraints that matter most for installation.
Three patterns that should make you pause
When reading a curve sheet, three signals warrant additional scrutiny:
1. A very flat H-Q curve at your operating point
If the system curve crosses the pump curve at a near-tangential angle, small changes in friction (a slightly clogged strainer, summer-day fluid temp differences) translate to large changes in operating point flow. The pump is hydraulically over-flexible for the duty.
Fix: pick a pump with a steeper H-Q, or steepen the system curve with a control valve.
2. Operating point > 105% of BEP flow
You're heading into the runout zone. NPSHr is rising fast; BHP is near its peak. The operating point is sensitive to wear-related curve degradation — a 5% head drop and you're off the right end of the curve.
Fix: pick a larger pump (one whose BEP is closer to your duty), or trim the impeller to bring BEP back toward duty.
3. NPSHr at duty point > 75% of NPSHa
Margin is tight. Any deterioration (vapor pressure increase from temperature rise, suction strainer fouling, foot valve restriction) crosses the margin into cavitation.
Fix: redesign the suction piping or pick a low-NPSH pump.
How the calculator handles it
When you select a pump in the Headloss Calculator, the curve panel displays:
- H-Q curve at the rated impeller diameter (plotted against your system curve)
- BEP marker + AOR shading
- NPSHr at the operating point
- BHP at the operating point + maximum BHP across the AOR (for motor sizing)
- Distance-from-BEP indicator (a percentage and a color flag)
The flag turns yellow at >110% / <70% of BEP and red at >130% / <50%. If you see red, the pump is hydraulically wrong for the duty regardless of how the catalog "matched" it.
References
- Hydraulic Institute. *ANSI/HI 14.3 — Rotodynamic Pumps for Design and Application.*
- Hydraulic Institute. *ANSI/HI 1.3 — Rotodynamic Centrifugal Pumps for Design and Application.*
- Karassik, I. J., et al. *Pump Handbook,* 4th ed. McGraw-Hill — pump curve interpretation chapter.
- ANSI/HI 9.6.3 — *Rotodynamic Pumps Guideline for Operating Region* (defines POR and AOR limits formally).