Every centrifugal pump quotation should come with a performance curve, and a buyer who cannot read it is relying entirely on the vendor’s word that the pump fits the job. A single chart usually carries three curves stacked over the same flow axis: head against flow, efficiency against flow, and NPSH required against flow. This guide walks through what each one means, how they relate to your actual system, and what to check before you accept a quote.
What a pump curve actually shows
The main curve on a centrifugal pump datasheet plots head (in metres or feet) on the vertical axis against flow rate (in m³/h or gpm) on the horizontal axis, measured at a fixed impeller speed and diameter. For a well-behaved centrifugal design the curve slopes downward from left to right: head is highest at zero flow (shutoff) and falls as flow increases, because more of the impeller’s energy goes into moving liquid rather than building pressure. A single pump casing is usually sold with a family of curves, one for each impeller diameter the casing accepts, so the first thing to check on a vendor curve sheet is which specific diameter line is being quoted against your duty point, not just which casing size.
The duty point: where the pump curve meets the system curve
A pump curve on its own does not tell you where the pump will actually operate — that depends on the system it is connected to. Plot a system resistance curve (static lift plus friction losses, which rise roughly with the square of flow) on the same axes, and the point where the two curves cross is the duty point: the flow and head the pump will settle at once installed. Because the system curve shifts whenever a control valve throttles, a strainer fouls, or a parallel line is brought on or off duty, the real operating point moves along the pump curve over time even though the pump curve itself never changes. A quotation that only shows a single design point without the surrounding curve shape gives you no way to judge how the pump will behave as conditions vary.
Best efficiency point and operating regions
The efficiency curve, plotted on the same chart, rises from zero at shutoff to a peak and then falls again as flow increases toward the end of the curve. That peak is the best efficiency point (BEP) — the flow at which the impeller’s internal flow paths are most hydraulically balanced, and not coincidentally, the flow at which the pump runs smoothest and lasts longest. API 610 formalizes how far from BEP a pump is allowed to run with three nested bands:
| Region | Typical range of BEP flow | What it means |
|---|---|---|
| Preferred operating region (POR) | 70% - 120% | Where the pump should spend most of its operating life for full design service life |
| Rated operating region | 80% - 110% | The tighter band the rated (guarantee) point itself should fall within |
| Allowable operating region (AOR) | Manufacturer-defined | The outer hydraulic limits the pump can run at without immediate damage, set by vibration limits rather than a fixed percentage |
These bands are not arbitrary paperwork. Operating persistently outside them raises radial shaft thrust, increases vibration, and shortens bearing and seal life — the pump may still run, but it will not last as long or as reliably as one operating inside its preferred region. When a vendor’s curve places your normal flow near the edge of the allowable region rather than inside the preferred one, that is a legitimate reason to ask for a different impeller trim or a different pump entirely, not something to accept because the price was attractive.
Rise to shutoff and curve shape
Curve shape must be assessed against the system and the specified operating range. A continuously rising head characteristic may be required by the project, particularly where the control arrangement or parallel operation makes stability important. Consult KSB's characteristic curve explanation for the plotted quantities, and require the supplier to identify the applicable acceptance criterion and standard edition. Do not assume that every pump configuration has the same rise-to-shutoff requirement or that the shape alone proves system stability.
NPSH required vs NPSH available
The suction-performance curve must identify the manufacturer's criterion and reference point. NPSH3 specifically denotes the test condition at which head has fallen by three percent; this is not the onset of cavitation. Required NPSH should therefore be read with its stated basis. Available NPSH depends on the actual installation and liquid conditions. The system designer and supplier must establish an appropriate margin across the intended operating range, including relevant changes in source level, temperature and losses.
The affinity laws: how curves shift with speed
A pump curve is only valid at the speed and diameter it was plotted for. If a variable-speed drive changes the pump’s speed, the affinity laws describe how the curve moves: flow scales directly with speed, head scales with the square of speed, and absorbed power scales with the cube of speed, for a fixed impeller diameter. Trimming the impeller diameter instead of changing speed follows an approximate version of the same relationships, but manufacturers generally only stand behind it for a modest trim — commonly cited as up to roughly 5-10% off the original diameter — beyond which the impeller’s internal geometry no longer scales cleanly and efficiency drops faster than the simple ratios predict. A vendor offering a heavily trimmed impeller to hit an odd duty point is worth a direct question about whether the resulting curve was actually tested or only calculated from the affinity laws.
Running pumps in parallel or series
Curves also have to be read differently once more than one pump is involved. Two identical pumps running in parallel do not double the flow at a given head — the combined curve is built by adding the two pumps’ flows together at each head value, and because the system curve keeps rising with flow, the actual gain in combined flow is smaller than doubling, often noticeably so on a system with significant friction loss. Two pumps in series do the opposite: their combined curve adds head together at each flow value, which suits a high-head, comparatively low-flow duty better than a single larger pump would. Reading a parallel or series arrangement off a single pump’s curve, without plotting the combined curve against the actual system curve, is a common way to overestimate how much a second pump will actually add — and it also means each pump’s individual operating point shifts away from where it would sit running alone, which is worth checking against the preferred operating region for both pumps, not just one.
What to check on a vendor’s pump curve
- The duty point (or your normal and rated flows) falls inside the preferred operating region, not just the allowable region
- The curve rises continuously to shutoff, with the minimum rise-to-shutoff the applicable API 610 edition requires
- NPSHr at your rated flow, plus the required margin, sits comfortably below your calculated NPSHa
- The curve corresponds to the actual impeller diameter quoted, not a nominal or maximum-diameter curve for the casing
- Any impeller trim or speed change applied to reach the quoted point is stated, and whether the resulting curve was tested or calculated
- The performance test method (HI 14.6 or ISO 9906) and the API 610 edition referenced are both stated on the datasheet
Sourcing a centrifugal pump and want the curve checked against your system before you commit? Send us your pump datasheet and Oillinko will circulate it to potential manufacturers and review every returned curve against your duty point, NPSH margin and operating region before it comes back to you. Browse our pumps & rotating equipment category, or see our guide to pump factory acceptance testing and commissioning for what happens once a curve is accepted and the pump goes to test.
Frequently asked questions
What is the difference between NPSH required and NPSH available?
NPSH required (NPSHr) is a property of the pump itself, published by the manufacturer with its stated performance criterion. NPSH3 specifically denotes the value at which total head has fallen by 3%; it is not the onset of cavitation. NPSH available (NPSHa) is a property of your installation: the suction pressure actually presented to the pump, calculated from source pressure, elevation, friction losses and vapor pressure at the pumping temperature. NPSHa must stay above NPSHr, with margin, across the whole operating range you intend to run at, not just at the design point.
Why does running a pump far from its best efficiency point cause problems?
A centrifugal impeller is hydraulically balanced only near its best efficiency point (BEP). Away from BEP, flow inside the volute or diffuser becomes uneven, which increases radial thrust on the shaft, raises vibration, and accelerates wear on bearings, seals and wear rings. Running well below minimum continuous flow also raises internal recirculation and temperature rise, which can flash the pumped liquid at the impeller eye. API 610 addresses this by defining a preferred operating region around BEP rather than leaving it to judgment.
Can I reuse one pump curve if I change the motor speed or trim the impeller?
Only within limits, and by applying the affinity laws rather than reusing the original curve directly: flow scales linearly with speed, head scales with the square of speed, and power scales with the cube of speed, for a fixed impeller diameter. Trimming the impeller diameter follows an approximate version of the same relationships, but manufacturers generally only warrant it as reliable for a small trim — within the limits confirmed for that particular impeller — beyond which efficiency no longer scales predictably and a new test curve is the only reliable source.
Which API 610 edition should a pump enquiry specify?
Specify the edition and purchaser supplements required by the project. Ask the manufacturer to state the applicable curve, test and NPSH acceptance criteria, including any deviations. A newer publication does not automatically change an existing contract; changes require the responsible technical and commercial approvals.
