1. What is the performance curve
The performance curve is the pump's "data sheet" — the set of graphs describing its hydraulic behavior across all possible flows for a fixed speed and a specific impeller diameter. Unlike a positive displacement pump, where flow is practically proportional to speed (independent of discharge pressure), in the centrifugal pump flow depends on discharge pressure: the higher the installation backpressure, the lower the delivered flow.
The horizontal axis always represents flow (Q, in m³/h or L/s). Plotted on this axis are four curves: head (H, in meters), absorbed power (P, in kW), required NPSH (NPSHr, in meters) and hydraulic efficiency (η, in percent). In FB Bombas catalogs, curves are obtained with water at 20°C on a hydraulic test bench, with calibrated instrumentation and acceptance class documented in the test sheet.
2. Q × H curve — flow and head
The Q × H curve is the most well-known — it describes the head the pump delivers for each flow. In radial centrifugal pumps (FBCN/FBOT), it has a decreasing shape from left to right: head is maximum at zero flow (shutoff head) and drops progressively as flow increases. This decreasing shape is the basis of the system's hydraulic self-control: increased flow demand is partially compensated by head drop, bringing the system to a new equilibrium point.
The pump's actual operating point in the installation is the intersection between the pump Q × H curve and the system curve (the total piping friction loss as a function of flow, added to the geometric head). Selecting the pump correctly means ensuring that this intersection falls within the preferred range (POR) — ideally near BEP, which we will see in section 4.
3. Q × P, Q × NPSHr and Q × η curves
The Q × P curve describes the power absorbed by the pump at each flow. In radial centrifugal pumps, power grows with flow up to a maximum plateau and may even decrease slightly near runout. Important: minimum power occurs at shutoff (zero flow), which is why pump startup should be done with the discharge valve partially closed to limit starting current and protect the motor.
The Q × NPSHr curve describes the minimum pressure required at the pump inlet to avoid cavitation. It has an increasing shape: NPSHr grows with flow, minimum at shutoff and maximum at runout. Therefore, at high-flow operating points, the NPSH margin (NPSHa − NPSHr) needs to be re-validated. The Q × η curve describes hydraulic efficiency — bell-shaped with peak at BEP. Before BEP, efficiency grows with flow; after BEP, it decreases. Operation outside BEP means energy waste.
4. BEP, POR and AOR — Hydraulic Institute
The Best Efficiency Point (BEP) is the flow at which hydraulic efficiency is maximum — it is the design point for which the impeller was conceived, with minimum hydraulic losses and balanced radial and axial loads on the shaft. Operating at BEP means: maximum energy efficiency, lowest vibration, lowest internal recirculation, lowest radial thrust on the mechanical seal and bearings, and longest component service life.
The Hydraulic Institute (HI) defines two ranges around BEP in the ANSI/HI 9.6.3 guideline. The Preferred Operating Range (POR) is the range recommended for continuous operation, with vibration and loads within design limits — typically between 70% and 120% of BEP for low specific speed pumps, with the exact range varying with the model’s specific speed.
The Allowable Operating Range (AOR) is broader and defined by the manufacturer for each model — continuous operation is permitted, but with elevated vibration, greater internal recirculation and reduced seal and bearing life. Operation outside AOR (very low flow or prolonged shutoff) is dangerous: overheating from internal dissipation, severe recirculation, rapid seal failure.
5. How to read the curve in practice — step by step
With the four graphs understood, reading the curve to verify a selection always follows the same sequence. The starting point is the design flow — the only figure that comes from the process, not from the pump:
- Mark the design flow (Q) on the horizontal axis — the value required by the process.
- Go up to the Q×H curve of the impeller diameter under consideration and read the head: it must be equal to or slightly above the calculated total system head at that point.
- Check the distance from BEP on the Q×η curve: the operating point must fall within the POR stated for the model (typically 70–120% of BEP). Outside this range, evaluate another impeller diameter, speed or model.
- Read the absorbed power on the Q×P curve — at the operating point and also at runout: the motor must have margin to the end of the curve, so it does not overload if flow increases.
- Read the NPSHr at the operating point and compare with the calculated installation NPSHa — the NPSHa must exceed the NPSHr with a safety margin.
- Check off-design scenarios — startup with a partially closed valve, future operation at higher or lower flow: in all of them the point must remain within the AOR.
6. Operating points example — representative FBCN pump
The table below presents five operating points on a representative medium-size FBCN pump (DN80, ~220 mm impeller, 1,750 rpm), exemplifying the simultaneous variation of head, efficiency, power and NPSHr as a function of flow. Values are illustrative of typical FBCN Series behavior; the actual specific curve of each model is obtained on a bench and provided in the technical catalog.
| Point | Q (m³/h) | H (m) | η (%) | P (kW) | NPSHr (m) | Range |
|---|---|---|---|---|---|---|
| Shutoff | 0 | 38.0 | 0 | 4.0 | 1.5 | Outside AOR |
| 50% BEP | 30 | 34.5 | 58 | 4.8 | 1.8 | AOR (not POR) |
| BEP | 60 | 30.0 | 76 | 6.5 | 2.8 | POR (ideal) |
| 120% BEP | 72 | 25.5 | 70 | 7.2 | 4.2 | POR (upper limit) |
| Runout | 85 | 18.0 | 55 | 7.6 | 6.5 | Outside AOR |
7. FBCN Series hydraulic envelope
Considering the 53 FBCN Series models from FB Bombas (43 standard DN25-150 + 10 large capacity DN200-300), the hydraulic envelope covers flows from about 2 m³/h (small models at 1,750 rpm) up to 2,200 m³/h (large-capacity models), with head from 4 m to 135 m.
Each individual model covers a slice of this envelope, and selection starts from identifying the (Q, H) quadrant where the duty point sits, choosing the model whose BEP is closest to that point and whose POR encompasses the duty.
In situations where the duty falls between two models, it is possible to adjust the impeller diameter (impeller trimming) to shift BEP — a standard technique in application engineering. FB Bombas maintains the actual curve test on hydraulic bench to validate the operating point after trimming, ensuring vibration, NPSHr and absorbed power are within design.
8. The curve in fire pump service (NFPA 20)
In fire protection service, curve shape stops being an engineering preference and becomes a code requirement. Following the NFPA 20 envelope (harmonized in Brazil by NBR 16704), the pump must deliver 100% of design flow at design pressure; maintain, at 150% of flow, at least 65% of design pressure; and cap shutoff pressure at 140% of design pressure.
It is the same POR/AOR reasoning covered above, applied to the worst case: a real fire with more open heads than the calculation predicted.
In practice this rules out steep or humped curves: a fire pump is a centrifugal pump selected so the entire curve fits the envelope — and the proof is the bench test of the actual pump supplied, not the generic catalog curve. That is the criterion applied to FB Bombas FBFS packages, with the curve certified at the three points (0%, 100% and 150%) and a documented FAT for inspection.