How to Read a Centrifugal Pump Performance Curve: BEP, POR, AOR and Operating Points

1. What is the performance curve

The performance curve is the pump's hydraulic data sheet: it shows what the pump delivers from zero flow up to maximum flow, at a given speed and impeller diameter. In a centrifugal pump, flow and pressure depend on each other: against a high system pressure the pump delivers less flow than against a low one.

In a positive displacement pump, the volume displaced per revolution is fixed and the theoretical flow is proportional to speed; the delivered flow falls short of it by internal leakage (slip), which varies with pressure and decreases as viscosity increases.

Pressure, in this case, is whatever the system resistance imposes: with the discharge blocked, it builds until a relief valve lifts, a pipe or fitting ruptures or the motor stalls.

Flow (Q, in m³/h or L/s) runs along the horizontal axis; head (H, in m), absorbed power (P, in kW or cv), net positive suction head required (NPSHr, in m) and efficiency (η, in %) are plotted over it.

Before reading any number, check the speed, impeller diameter and reference liquid in the legend: viscosity changes head, flow, efficiency, NPSHr and power (the correction follows ANSI/HI 9.6.7), and impeller material also matters — FB Bombas Engineering requires deducting, for the FBCN Series, 3 percentage points from the curve efficiency for a stainless steel impeller with a passage width of up to 12 mm and 2 points for a passage of 12 to 15 mm.

Finally, a catalogue curve is not a test of the pump supplied: the documents of each supply are defined in the proposal.

2. Q × H curve — flow and head

The Q × H curve is the one most used to describe performance: at each flow it shows the total head — the energy increase per unit weight of liquid, equal to the difference between total discharge head and total suction head. Because it is given in head rather than pressure, it is independent of the liquid density.

The shape of the curve depends on the impeller specific speed; in a radial centrifugal pump head generally falls as flow increases, but not every curve peaks exactly at shutoff: the ANSI/HI 9.6.3 guideline covers curves with a “droop” and with a “dip” among the factors that affect the allowable region. Read the model curve instead of assuming its shape.

The operating point in the installation is where the pump's Q × H curve crosses the system curve, which adds the static head to the piping head loss at each flow.

Good selection makes that crossing fall within the preferred operating region (POR), preferably near BEP (section 4): the pumping system sourcebook of the U.S. Department of Energy (DOE), prepared with the Hydraulic Institute, notes that choosing a pump whose BEP is close to the system's normal operating range can yield significant operating cost savings.

Because a centrifugal pump's flow depends on the system, it can be regulated by a throttle valve, a bypass, speed control or multiple pumps; a throttle valve creates a pressure drop and reduces the flow that passes through it.

3. Q × P, Q × NPSHr and Q × η curves

The Q × P curve shows absorbed power at each flow and is the basis for selecting the driver. In a radial centrifugal pump it typically rises continuously with flow, so the mechanical load is lowest at zero flow.

That is why the DOE sourcebook notes that starting the pump with the downstream valves closed and opening them soon after it reaches speed can reduce electrical stress on the motor and its controller.

In an axial pump the relationship is reversed — power falls as flow increases — and when soft-starting, the downstream valves stay open until the pump is up to speed. The shape of the curve depends on the construction, and startup follows the assembly procedure, bearing in mind that shutoff must not be prolonged (section 4).

The Q × NPSHr curve shows the minimum NPSH needed to achieve the specified performance at each flow, at the stated speed and with the stated liquid; it does not mark the onset of cavitation.

The Hydraulic Institute defines NPSH3 as the NPSHr at which the first-stage total head drops by 3% due to cavitation and notes that noise, vibration, loss of head or efficiency and erosion can occur even with margin above NPSHr; that is why ANSI/HI 9.6.1 recommends an NPSHa above the published NPSHr.

Check the margin across the whole intended range: the DOE sourcebook points out that cavitation usually occurs at the far right of the curve, at high flow, but similar damage also appears at low flow. The Q × η curve shows efficiency, and the flow at which it peaks is BEP.

4. BEP, POR and AOR — Hydraulic Institute

At BEP, hydraulic efficiency is at its maximum, the liquid enters the impeller vanes and the diffuser without shock, and the flow is uniform, free of separation and well controlled.

According to the DOE sourcebook, performance and service life are optimized around this flow, which depends on the inlet, impeller, casing and speed; it is also where radial bearing loads are lowest.

Because the life of ball and roller bearings is an inverse function of the cube of the load, a pump whose BEP is close to the normal operating range significantly extends the interval between bearing replacements. Even so, BEP is a selection reference, not a guarantee: suction, materials, sealing, power and system requirements are checked separately.

The Hydraulic Institute covers both regions in the ANSI/HI 9.6.3 guideline, which defines the POR according to the pump specific speed.

The preferred operating region (POR) is the range of flows on either side of BEP within which hydraulic efficiency and operational reliability are not substantially degraded, with low flow-induced vibration and internal hydraulic loading; depending on specific speed, it ranges from 90–110% to 70–120% of BEP flow, so 70–120% does not apply to every centrifugal pump.

The allowable operating region (AOR) is the flow range, at rated speed and with the impeller supplied, in which the pump may operate with acceptable service life, as limited by cavitation, heating, vibration, noise, shaft deflection, fatigue and similar criteria, and it is set by the manufacturer.

HI notes that intermittent operation within the AOR typically causes no issues; that does not make the AOR automatic permission for continuous operation at any point.

5. How to read the curve in practice — step by step

Once the four graphs are understood, checking a selection always follows the same sequence. It starts from process and installation data — flow, head, liquid, temperature, suction conditions and operating regimes — and compares these requirements with the curve of the proposed configuration:

• 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 to BEP on the Q×η curve: the point must fall within the POR stated for the model, which depends on specific speed. Outside it, consider another impeller diameter, another speed or another model.
• Read the absorbed power on the Q×P curve at the operating point and at the highest flow the system can impose: since power in a radial centrifugal pump typically rises with flow, the driver needs margin up to the end of the intended range. For the FBCN Series, Table 2 (MTEC-03/00) gives the motor power reserve: approx. 20% (minimum 1.5 cv) up to 2 cv, 15% up to 20 cv and 10% above 20 cv.
• 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 the scenarios away from the rated point — operation at lower or higher flow and future system expansion: in all of them the point must stay within the AOR defined by the manufacturer. Startup follows the assembly procedure.

6. Illustrative combined-reading example

The table uses hypothetical numbers to show how the curves are read together; it does not represent an FBCN, a test or a product selection.

At the points with flow, power is calculated as P = ρ·g·Q·H/η, with Q in m³/s, ρ = 1,000 kg/m³ and g = 9.81 m/s², and rounded; at shutoff, where η = 0, the value is only illustrative. The permitted regions come from the configuration documents and are not defined in this exercise.

7. FBCN Series hydraulic envelope

For the FBCN Series (MTEC-03/00), FB Bombas Engineering gives the series limits: sizes from DN25 to DN300, flow up to 2,200 m³/h, head up to 135 m, temperature up to 260 °C and speed up to 3,500 rpm.

These are series limits, not limits of each size, and they do not apply at the same time: 3,500 rpm, for example, is allowed only on the smaller sizes, while the larger ones go up to 1,750 rpm (Table 1).

Table 1 (MTEC-03/00) lists 54 sizes — 43 with dimensions published in Table 7 and 11 with dimensions on request — and sets for each one the minimum and maximum operating flow as fractions of Qot, the optimum flow: 0.1 to 1.1 Qot for the sizes on bearing frames 30 and 40, and 0.15 to 1.1 Qot from frame 50 upward.

Each size covers a slice of the envelope; selection starts from the duty point (Q, H) and looks for the size whose BEP is closest to it and whose POR contains it.

When the duty point falls between two sizes, adjusting the impeller diameter is common. Trimming reduces the diameter and, with it, the tip speed; since the head generated depends on that speed, the whole curve moves down and to the left.

The affinity rules estimate the effect, but ANSI/HI 14.3 addresses the limitations of their use for diameter and speed changes, and the estimate does not replace the curve for the diameter supplied.

The FBCN (MTEC-03/00) also limits impeller tip speed by material — up to 40 m/s for cast iron, 60 m/s for ductile iron and carbon steel and 80 m/s for stainless steels — which bears on the choice of speed.

At FB Bombas, the documents of each supply, such as the Q×H curve for the diameter supplied, are defined in the proposal.

The FBCN Series curve sheets are published in the links below, at 1,750 rpm and at 3,500 rpm. Each sheet covers one size at one speed: the Q×H curves for the impeller diameters, with the efficiency lines, the absorbed power and the NPSHr, valid for clean water at 20 °C, as the legend states.

Take the point (Q, H) to the sheet of the candidate size and run the check in section 5; with a viscous liquid, the water curve needs the correction mentioned in section 1.

Then send FB Bombas the point, the liquid, the temperature and the available NPSHa, and engineering confirms the size, the impeller diameter and the sealing in the proposal.

8. The curve in fire pump service (NFPA 20)

In fire service, the shape of the curve stops being an engineering preference and becomes a code requirement.

NFPA 20 standardizes that shape with three criteria for the centrifugal pump, as ASPE summarizes: delivering at least 100% of rated flow at rated pressure; at least 65% of rated pressure at 150% of rated flow, the overload capacity that prepares the pump for a surge in demand during a fire; and shutoff pressure limited to 140% of rated pressure for any type of pump, in the words of the standard.

The Hydraulic Institute gives the shutoff range as 101% to 140% of rated pressure and adds that, under FM requirements, it must not be less than 99% of the maximum pressure developed by the pump: the highest point of the curve is at most about 1% above shutoff.

These are fire-service criteria of their own, not to be confused with the POR and AOR.

In selection, the whole curve must fit the envelope, not just the rated point, and the proof is the curve of the pump supplied, not the catalogue curve.

NFPA 20 requires a copy of the manufacturer's certified shop test curve for comparison with the field acceptance test, and the installed pump must match it at all flow conditions, including those tested, within the accuracy of the instruments; in the sample procedure in the annex, the important test points are shutoff, rated flow and 150% of rated flow.

In FB Bombas FBFS sets, the main pump is from the FBCN Series, and the Q×H curve of the supply is defined in the proposal.