When your existing pump finally calls it quits, time is money. Whether you just pulled a tired end-suction unit out of the pit and copied the model number into Google, or you’re on-site holding drawings that specify an exact pump, you still have to answer the same question: “Will the new pump hit my required flow and head without creating new problems?”
That answer lives on the pump curve. Below is a quick, engineer-approved refresher on how to read a centrifugal pump performance curve, rewritten for two kinds of Pump Stop customers:
- Replace-in-kind shoppers — maintenance pros and DIYers who want a one-for-one swap and need to be sure the new curve overlaps the old one.
- Spec-driven pros — contractors, facility managers, and OEMs working from plans that already specify a make and model.
Pro tip: Enter the part number or the performance data in our Pump Stop Finder. We’ll surface the exact match or the closest cross-reference instantly.
1. Check the Curve Header
The header lists model, speed (RPM), frequency (60 Hz in North America), and suction/discharge sizes. Double-check these against the label on your old pump or your spec sheet first. If the header doesn’t match, nothing else will.
Why it matters for replacements — Matching nozzle sizes avoids pipe-fitting surprises and keeps hydraulic losses close to the original design.
2. Plot Your Flow-vs-Head Point
The bold, arcing lines map capacity (GPM or m³/h) on the X-axis to total dynamic head (ft or m) on the Y-axis. Find your required duty point—for example, 40 GPM at 50 ft—and see which impeller trim line (say, 3.75 in) intersects that point.
Replace-in-kind shoppers: If your old pump used a 3.50 in impeller but the new one needs 3.75 in, note the change—it may affect motor load.
Spec-driven pros: Verify the specified model’s standard trim covers your design point so you avoid custom impellers.
3. NPSHr: Protect Against Cavitation
The NPSHr (Net Positive Suction Head required) line runs near the bottom of the curve. Your system must supply at least this much head (NPSHa) at the pump suction to prevent cavitation. Make sure your suction conditions comfortably exceed this value, especially with hot or volatile fluids.
4. Power Lines and Motor Sizing
Power curves show brake horsepower (BHP) for water. If you’re pumping a heavier liquid (specific gravity > 1.0) or anything thicker than ~50 cP, multiply the plotted BHP by the specific gravity. Always select a non-overloading motor—one that can handle every point on the curve without tripping overloads.
Quick check: Compare the new model’s BHP at your duty point with the nameplate HP of the motor you plan to reuse. If it’s higher, bump the motor size or choose a different impeller trim.
5. Respect Minimum Continuous Flow
Operating below the minimum flow line lets liquid overheat inside the volute, warping the casing or seizing bearings. Plan to keep the pump above this threshold, or route a small bypass line back to the source.
6. Aim for the BEP Efficiency Island
Concentric efficiency contours show how much hydraulic horsepower your pump turns into useful work. Aim to land your duty point near the Best Efficiency Point (BEP)—the sweet spot where vibration, shaft deflection, and energy bills are lowest.
Formula:
BHP = (GPM × Head (ft) × Specific Gravity) / (3960 × Efficiency)
Using the Pump Performance Curve
Follow these four steps to apply the chart to your pump-selection process:
- Locate your duty point. Find the required flow (GPM) on the X-axis, move up to the curve for your impeller size, then read across to the left Y-axis for the delivered head (ft).
- Check NPSHr vs. NPSHa. The dashed NPSHr line shows minimum suction head—ensure your system’s NPSHa exceeds it by at least 3 ft to avoid cavitation.
- Size your motor. On the right Y-axis, read BHP (for motor load) and Efficiency (%) at your duty point. Choose a motor that won’t overload and aim to run near peak efficiency to lower energy costs.
- Confirm minimum flow. Never operate left of the chart’s “min flow” end-point without a bypass or throttling valve to protect the pump.
7. Quick Reference Guide
| Step | Replace-in-Kind Customer | Spec-Driven Customer |
|---|---|---|
| Header Check | Match model & port sizes to old pump label. | Confirm model matches plan submittal. |
| Duty-Point Plot | Verify new curve overlaps old duty point. | Validate design point hits the curve. |
| NPSHr vs. NPSHa | Ensure plant suction head exceeds curve value. | Document margin for commissioning report. |
| Power & Motor | Compare BHP to existing motor; resize if needed. | Select NEMA motor per spec. |
| Minimum Flow | Plan bypass if operating at throttled flow. | Confirm control valves maintain safe flow. |
| Efficiency | Keep duty point near BEP for lower electric bills. | Highlight energy savings in project ROI. |
8. Three Ways Pump Stop Makes Buying Easy
- Instant Cross-Reference – Enter any competitor part number; we’ll show the exact match or approved equivalent.
- Curve Library – Every product page features downloadable curves, manuals, and 3D CAD files.
- Live Pump Pros – Call, chat, or email. Our in-house engineers will double-check your curve math and motor sizing before you order.
Final Checklist Before You Order
- Confirm inlet/outlet sizes, voltage, and frame dimensions.
- Verify duty point is within the new curve.
- Check NPSHa ≥ NPSHr + 3 ft safety margin.
- Match or upsize motor horsepower for the selected impeller.
- Review minimum flow requirements in your control strategy.
Need Help Right Now?
Call our Pump-Stop Hotline at (781) 281-8905. We’ll get you a same-day price, lead time, and curve packet—so you can get back to moving fluid, not chasing specs.
Pump-Stop: the fastest way from part number to perfectly-matched pump.
