A centrifugal pump curve usually arrives as one sheet with a head line on it and a few numbers in the corner. The number that gets quoted back — so many cubic metres an hour at a rated head — is a single point on that sheet. Everything that decides whether the pump survives at that point is drawn somewhere else on the same page, or is missing from it altogether.
This is not another explanation of how to find the operating point. It works from two facts — that a curve is a test record valid only under four printed conditions, and that its slope is a selection criterion rather than decoration — and turns them into the six lines a selection file has to answer before a pump is accepted.
Key takeaways
- A centrifugal pump curve belongs to one model, one speed, one impeller diameter and one test liquid. Change any of the four and the sheet is describing a different machine.
- A typical sheet carries four curves on one flow axis: head, efficiency, shaft power and NPSH required. Each one can veto a selection on its own.
- The motor is sized at the end of the curve, not at the duty point, because shaft power is highest near the runout.
- Slope is a selection criterion. A flat curve turns a small backpressure change into a large flow change, and a drooping curve can cross the system curve twice and make the pump hunt.
- Specific speed explains the shape. Standard impellers run from 500 to 10,000, and the 2,000 to 3,000 band usually gives the highest efficiency.
- Minimum flow is a published number, and an unequal pair of pumps in parallel can push the smaller one below it.
- The sheet assumes water, one speed, one diameter and a new impeller. Viscosity, wear and a trim all move the curves, and a trim is only predictable to about 10 percent of diameter.
What a Pump Curve Sheet Actually Is
A curve sheet is a test record, not a catalogue of possibilities. It describes what one pump did on a test stand, under conditions usually printed in small type at the top of the page.
One sheet, four conditions
A centrifugal pump curve plots the flow rate as a function of head. The values on it are valid only for four things: the pump model and its hydraulic design, the rotational speed, the impeller diameter, and the liquid, which is almost always water. Change any one of them and the sheet stops describing your pump.
The trap is that a curve is usually sent without its conditions in the filename. A sheet labelled only with a model number may have been drawn at 1,450 rpm with one impeller diameter, while the unit on the quotation is the 2,900 rpm variant of the same casing. The numbers on it are then not approximate. They belong to a different machine, and no amount of reading precision recovers them.
| Condition on the sheet | What happens when it does not match |
|---|---|
| Pump model and hydraulic design | The curve cannot be borrowed from another model. Two pumps with the same duty are not interchangeable documents. |
| Rotational speed | Flow scales with the speed ratio, head with its square, power with its cube. |
| Impeller diameter | Every diameter has its own curve. A reading taken between two curves is not a valid operating point. |
| Test liquid, normally water | Viscosity and specific gravity change head, flow, efficiency and NPSH required together. |
The family behind the line
Behind every single curve is a family. Manufacturers use a coverage chart to describe the performance characteristics of a family of pumps, and the designation numbers on that chart usually encode the inlet size, the outlet size and the impeller size in that order. Within one casing, several impeller diameters can be fitted, and each diameter has its own head curve, its own power curve and its own efficiency island. Iso-efficiency lines are drawn across the set to show how efficiency behaves at different flows for each diameter.
This is why “the curve for this pump” is an incomplete question. The answer only exists once a diameter is fixed, and the diameter is part of the selection rather than an afterthought. A pump that is right at one diameter is oversized at the next size up and short of head at the next one down.
On a metal pump the range of diameters is produced by machining the impeller down. Trimming is a machining operation on the diameter, and whether it is available for a plastic impeller depends on how that impeller was made. That belongs in the enquiry, not in the assumption.
A point that is not on a curve is not an operating point
Louisiana Tech’s course notes put the rule plainly: points that are not on a head-flow curve are not valid operating points. A duty of 20 m³/h at 25 m is only real if it falls on the diameter curve you actually have. If it sits in the gap between two curves, neither impeller holds it, and interpolating between them produces a number that no pump will deliver.
This single rule catches most of the optimistic selections. A data sheet that quotes a flow range is describing the span of the family, not a promise at your head. Our polypropylene centrifugal pumps are offered from 0.75 to 7.5 kW and 12 to 35 m³/h at rated head; which point inside that range is yours is a selection question that the curve has to answer, not something the range itself settles.
Four Curves, Four Separate Vetoes
The head line is the one that gets quoted, but a centrifugal pump curve carries four curves on the same flow axis. A typical sheet also plots efficiency and brake horsepower against flow, plus NPSH required. Each of the four can stop a selection, and each stops it for a different reason.
Head and flow: the one everybody reads
Read the head the pump produces at your duty flow and compare it with the head your installation consumes at that same flow. If the pump’s head is below the system’s, the operating point does not exist on this curve. The pump will run further to the right, at a higher flow and a lower head, until the two lines cross at a point nobody chose.
Efficiency: how far the duty point sits from the best efficiency point
Efficiency is zero at shut-off, peaks at the best efficiency point and falls on both sides. The BEP is the flow at which operating efficiency is highest and radial bearing loads are lowest. Two anchors are useful when a sheet arrives without a marked BEP: peak efficiency for a centrifugal pump is commonly in the 60 to 80 percent band, and the peak sits near 60 percent of the pump’s maximum flow, not at the middle of the printed axis.
How far a duty point may sit from that peak is a manufacturer’s published range rather than a universal number. A working figure of about 10 percent either side of the BEP is typical, but it is a figure to confirm on the sheet, not to assume across a range.
Power: size the motor at the end of the curve
Shaft power rises with flow on most radial-flow pumps, which means the highest power the motor will ever see is at the right-hand end of the curve, not at the duty point. Normal practice is to size the motor for the end-of-curve horsepower rather than for the duty point. A pump whose motor is matched exactly to the duty power, with a valve that someone later opens fully, will overload at the point where it is working hardest.
NPSH required: worst at the right-hand end
NPSH required is the suction head the pump needs in order not to cavitate, and it rises as flow rises, so its worst value is also at the right-hand end. Checking it at duty only repeats the motor-sizing mistake. What happens when the margin runs out, and why the vapour and gas cases behave differently, is set out in pump cavitation.
| Curve | What to read on it | What it can veto |
|---|---|---|
| Head and flow | Head at your duty flow, on your impeller diameter | Head below system demand: the duty point does not exist |
| Efficiency | Efficiency at duty, and its distance from the BEP | A duty point outside the preferred operating region |
| Shaft power | Power at duty, and power at the end of the curve | A motor sized at duty that overloads at runout |
| NPSH required | NPSH required at duty, and at maximum flow | Insufficient suction margin at the right-hand end |
Shape Decides Stability: Flat, Steep, Drooping
Two pumps can both be correct at the duty point and behave completely differently a metre of pipe away from it. The difference is the slope of the head line, and slope is chosen as deliberately as the duty point.
A flat curve turns a small backpressure change into a large flow change
A flat pump curve shows that the response to a small decrease in backpressure is a large increase in flow. In one worked example from the DOE sourcebook, at 160 ft of head and 250 gpm, a 10 ft rise in system backpressure drops the flow by 100 gpm. A system with float valves, changing tank levels or a partly blocked strainer will move that pump around whether or not anyone intends it to.
A steep curve is a choice, not a compromise
A steep line changes head a lot for a small change in flow, so the flow stays where it was put. That suits a fixed-flow duty with a large static component, where the system’s demand does not wander. It is not a worse curve, only a curve for a different installation.
A drooping curve can cross the system curve twice
Some curves rise again at low flow instead of falling continuously, a shape that applies mainly to pumps with low specific speed. Because the system curve also rises towards low flow, the two lines can intersect at more than one point, which occasionally leads to instability. A pump in that region will hunt, repeatedly adjusting its output as it searches for a stable operating point. The Louisiana Tech notes state the same condition more briefly: a positive slope may be unstable for certain system loss curves.
| Shape | Signature | Risk | Where it is acceptable |
|---|---|---|---|
| Flat | Large flow change for a small head change | System backpressure noise becomes flow noise | Stable backpressure, or speed control |
| Steep | Large head change for a small flow change | Flow falls away quickly when demand rises | Fixed flow with a high static component |
| Drooping | Head rises again at low flow | Two intersections with the system curve; hunting | Only where that flow band is never entered |
Specific Speed Explains the Shape and the Width of the Operating Region
If the slope is the symptom, specific speed is the cause. It is a single number that predicts the shape before the curve is seen, and it also predicts how wide the safe operating band will be.
The formula, and what it is measured against
Specific speed is defined as Ns = n√Q / H3/4, with n in rpm, Q in gallons per minute and H the total head per stage in feet. In mechanical terms it is the impeller speed needed to produce one gallon per minute at one foot of head. In this form the units do not cancel, which is why the number is comparable only within one system of units and should never be quoted without them.
What the number tells you before you see the curve
The range for standard impellers runs from 500 to 10,000, and the table at the end of this section collects what the number predicts. The practical reading is that the fastest unit in a range is not automatically the most economical one, and that the drooping curves which hunt are mostly a low specific speed phenomenon.
The operating region is not the same width on every pump
The preferred operating region is the flow band around the BEP within which hydraulic efficiency and reliability are not substantially degraded. The allowable operating region defines the precise limits for minimum and maximum flow. How wide the preferred band is depends on specific speed: Hydraulic Institute guidance puts it anywhere from 90 to 110 percent of BEP flow to 70 to 120 percent, depending on the pump. A single “70 to 120 percent” rule applied to every pump is a simplification that can be wrong in both directions.
Why the width matters is bearing load. Bearing life is an inverse function of the cube of the load, so a pump running outside its preferred band does not lose a little life, it loses life quickly. Which impeller geometry produces a given specific speed is a separate subject, covered in types of centrifugal pump.
| Statement | Source |
|---|---|
| Ns = n√Q / H3/4, with Q in gpm, H per stage in ft, n in rpm | DOE sourcebook |
| Standard impellers run from 500 to 10,000 | DOE sourcebook |
| Pumps between 2,000 and 3,000 usually have the highest efficiency | DOE sourcebook |
| Above 3,000, pumps tend to be less efficient at higher speeds | DOE sourcebook |
| Drooping curves apply mainly to low specific speeds | DOE sourcebook |
| The preferred operating region runs from 90 to 110 percent up to 70 to 120 percent of BEP flow, depending on specific speed | Hydraulic Institute |
The Minimum Flow Rating, and What Parallel Operation Does to It
Two limits do not appear anywhere on a centrifugal pump curve: how little flow the pump can take, and what happens when a second pump is put beside it.
Minimum flow is a published number, not a rule of thumb
Below a certain flow the fluid leaving the impeller recirculates inside the casing instead of moving out of the discharge, and the energy that would have gone into the system heats the liquid instead. Manufacturers list the minimum flow rates for their pumps, which makes this a specification to be read rather than a figure to be guessed. When a duty point sits close to that number, the pump needs a recirculation line or a different selection.
Adding a second pump does not double the flow
Parallel pumps share a suction and a discharge. Their composite curve is built by adding the flows of each pump at the same head. Unless the system curve is completely flat, bringing a second pump on line does not double the flow rate: the increase is always less than double, and how much less depends on how steep the system curve is. The steeper the system, the closer the second pump comes to being an expensive standby.
The unequal pair
Parallel pumps are normally identical so that load is shared. When two different sizes are paralleled, the largest pump dominates the system and can force the other to operate below its minimum flow rating. If an unequal pair cannot be avoided, both curves have to be reviewed together to confirm that no pump ends up under its own minimum. Two pumps that were each correctly selected on their own can still be a bad pair.
| Arrangement | Composite curve | What you gain | What it costs |
|---|---|---|---|
| Parallel | Add flows at the same head | More flow | The steeper the system curve, the smaller the gain; unequal pumps risk the smaller one falling below minimum flow |
| Series | Add heads at the same flow | More head | Interstage piping and pressure rating become their own design problem |
| Single pump with speed control | The whole curve moves up and down | Continuous adjustment | Efficiency at reduced speed is lower than the sheet implies |
What the Curve Assumes That Your Installation May Not
Everything above assumes the sheet still describes the machine. Four assumptions decide whether it does, and three of them are invisible on the page.
Water, one speed, one impeller, new condition
The values on a curve are valid for the pump model and design, the speed, the impeller size and the fluid, which is typically water. The first three are fixed by the selection and can be checked against the nameplate. The fourth, condition, is not on the sheet at all, because what was tested was a new pump with new clearances.
Viscosity moves all four curves
The Hydraulic Institute states plainly that liquid viscosity affects total head, flow, efficiency, NPSH required and power, and refers the correction to ANSI/HI 9.6.7. That means a water curve is not a slightly conservative estimate for a viscous liquid. It is the wrong curve, and it is wrong in the optimistic direction on all four lines at once: less head, less flow, less efficiency and less suction margin than the sheet promised.
Wear moves the curve down, and a trim is not unlimited
A worn pump still carries its nameplate, but it no longer has its curve. A centrifugal pump curve describes a new machine; the one in the sump has clearances that have opened. The gap shows up first as a head that no longer reaches the shut-off value the sheet gave at zero flow, and later as a duty flow that has to be trimmed down to hold the same pressure. What moves and why is covered in impeller wear.
Trimming is the usual correction for a pump that turned out oversized, and it is a correction with limits. The DOE notes that trimming is slightly less effective than buying the next smaller impeller from the factory, and that the affinity relationships used to predict the result are not strictly accurate because of nonlinearities in flow. NESC adds a practical bound: diameter changes greater than 10 percent are hard to predict, because the relationship between the impeller and the casing changes. A trim is a fine adjustment, not a way to turn one pump into a different one.
What the Selection File Must State
A curve becomes a decision when the file around it answers six questions. A missing line is a check that was never made, and it usually surfaces as a commissioning problem rather than a paperwork one.
Six lines that turn a curve into a decision
| What the file must state | Why it cannot be left out |
|---|---|
| The model, speed, impeller diameter and test liquid the curve belongs to | Without them there is no way to tell whether the sheet describes the pump being bought |
| Head, efficiency, power and NPSH required at the duty point | Four curves, four vetoes: omitting one leaves a check undone |
| Shaft power at the end of the curve | The motor is sized on this figure, not on the duty point |
| The minimum flow, and the preferred and allowable operating regions | Together these decide whether the pump may run at low load for long periods |
| The viscosity correction basis, if the liquid is not water | A water curve cannot be used unmodified for a viscous product |
| Whether the impeller is moulded or machined, and which intermediate diameters exist | This decides whether the duty point can be adjusted later at all |
Why a polypropylene impeller makes one of them harder
The last line is the one that changes with material. On a metal impeller the range of available diameters is a machining question with a known answer. On a moulded plastic impeller the diameters are the ones the tooling produces, and a trim may not be offered at all. That does not make the pump worse; it makes the question more important, because the pump cannot be adjusted afterwards as freely.
The practical consequence is that a polypropylene pump is better selected close to its final duty in the first place. Where a metal unit might be bought oversized and trimmed down, a plastic unit is better bought at the right diameter, which means the duty has to be settled before the order rather than after commissioning. The four numbers that drive that selection are set out in our polypropylene pump selection guide.
Frequently Asked Questions
What is the difference between BEP, POR and AOR?
The best efficiency point is a single flow: the one where efficiency peaks and radial bearing loads are lowest. The preferred operating region is the band around it where efficiency and reliability are not substantially degraded. The allowable operating region is the wider band, defined by the manufacturer, within which the pump can run with acceptable life. BEP is a point; the other two are ranges, and neither is the same width on every pump.
Why is the motor sized at the end of the curve rather than at the duty point?
Because shaft power on most radial-flow pumps keeps rising as flow rises. The duty point is where the pump is meant to run; the end of the curve is where it will run if a valve is opened fully or a strainer is cleaned. Sizing the motor on the duty point leaves nothing for that possibility, and the overload appears exactly when the pump is working hardest.
What does a drooping centrifugal pump curve mean?
It means the head line rises again at low flow instead of falling continuously. Because the system curve also rises at low flow, the two can intersect at more than one point, and a pump in that region will hunt as it searches for a stable operating point. Drooping is mostly a low specific speed characteristic, and the usual answer is to keep the pump out of that flow band.
What is specific speed used for in pump selection?
It predicts the shape of the curve and the width of the operating region before the curve is seen. Standard impellers run from 500 to 10,000, the 2,000 to 3,000 band usually gives the highest efficiency, and above 3,000 pumps tend to lose efficiency as speed rises. It is also the number behind the drooping curves that can hunt.
Can a water performance curve be used for a viscous liquid?
No. Viscosity affects total head, flow, efficiency, NPSH required and power together, so a water curve is not a conservative approximation for a thick product. The correction is defined in ANSI/HI 9.6.7, and a pump selected from an uncorrected water curve will deliver less of everything than the sheet promised.
Does trimming the impeller always reduce running cost?
It reduces the energy added to the fluid, which is the point of it. It is also less effective than buying the next smaller impeller from the factory, the prediction is only reliable for modest diameter changes, and it cannot be applied to every impeller. For a moulded plastic impeller the question is whether a trim is available at all, which is why the diameter is better chosen correctly at the start. A range of PP water pump duties is selected this way, from the duty backwards.
Sources
- U.S. Department of Energy, Industrial Technologies Program, and the Hydraulic Institute, Improving Pumping System Performance: A Sourcebook for Industry, 2nd ed. The performance curve definition, the efficiency and brake horsepower curves, the coverage chart and pump designation numbers, the impeller diameter curves and iso-efficiency lines, the flat-curve sensitivity example, the drooping curve and hunting, the specific speed formula and its 500 to 10,000 range, the 2,000 to 3,000 efficiency band, the minimum flow requirement, the unequal parallel pump condition, the bearing life relationship, and the impeller trimming limits. energy.gov
- Satterfield, Z., P.E., Reading Centrifugal Pump Curves. National Environmental Services Center, West Virginia University. The typical efficiency band, the working limit on deviation from the best efficiency point, the practice of sizing the motor at the end of the curve, and the 10 percent limit on predicting impeller diameter changes. actat.wvu.edu
- Louisiana Tech University, Pump Performance Curves and Similarity Laws, Chapter XI course notes. The four conditions under which curve values are valid, the rule that points off the head-flow curve are not valid operating points, the location of peak efficiency, and the instability of a positive slope against certain system curves. www2.latech.edu
- Hydraulic Institute, Pump Curves, Pump Fundamentals. The preferred and allowable operating region definitions and the dependence of preferred region width on specific speed, the composite curves for pumps in parallel and in series, the statement that a second parallel pump does not double flow, and the reference of viscosity correction to ANSI/HI 9.6.7. datatool.pumps.org
