Centrifugal pumps and pump curves: the operating point, best efficiency point, affinity laws, cavitation and NPSH, and running in the wrong place
Centrifugal pumps from the operating and maintenance side: reading a pump curve and a system curve and finding the operating point, the best efficiency point, the affinity laws and what speed control saves, cavitation and NPSH, the failures that come from running far from the best efficiency point, and the maintenance tasks. With a drawing of pump and system curves at two speeds.
Key takeaways
- A centrifugal pump runs where its pump curve crosses the system curve. Change either curve, by speed, impeller, valve position or pipework, and the operating point moves.
- Pumps live longest near their best efficiency point. Far to the left they recirculate, heat and load the shaft. Far to the right they draw more power and risk cavitation.
- The affinity laws make speed control powerful: flow follows speed, head follows speed squared, and power follows speed cubed.
- Cavitation happens when the suction head available falls close to the head the pump requires. It shows as gravel noise, falling head and a pitted impeller inlet.
- Many pump reliability problems are operating problems. A pump that keeps failing seals and bearings is often running far from its best efficiency point.
A process water pump has been through four mechanical seals in a year. Each failure is recorded as a seal failure, and each new seal is fitted carefully. Nobody checks where on its curve the pump runs. It was sized for a future expansion that never came, and it spends its life throttled back to a third of its design flow, recirculating inside its own casing and shaking its seals apart.
This guide covers centrifugal pumps from the operating and maintenance side: how to read a pump curve and a system curve, the best efficiency point, the affinity laws and speed control, cavitation and NPSH, the problems that come from running in the wrong place, and the maintenance tasks. It sits beside the guides to slurry pumps, shaft alignment and bearing failure analysis. The curves shown are illustrative.
The pump curve and the system curve
A pump does not choose its flow. It runs where its curve, the head it can develop at each flow, crosses the system curve, the head the piping needs at each flow. The system curve starts at the static lift and rises with the square of the flow as friction grows. In the illustration the full speed pump meets the system at about 200 m³/h and 36 m, just left of its best efficiency point at 220 m³/h.
Slowing the pump to 80 per cent of full speed lowers its curve, and the operating point moves down the system curve to about 136 m³/h and 27 m. The same flow reduction by throttling would push the pump up its full speed curve and away from its best efficiency point while burning the difference across the valve.
| Term | What it is | Notes |
|---|---|---|
| Head (H) | The energy the pump adds, in metres of the liquid being pumped | Falls as flow rises along the pump curve |
| Flow (Q) | Volume pumped per unit time, in m³/h or L/s | Set where the pump curve meets the system curve |
| Efficiency (η) | Hydraulic power out divided by shaft power in | Peaks at the best efficiency point |
| Best efficiency point (BEP) | The flow at peak efficiency | Where the pump runs smoothest and lasts longest |
| Power (P) | Shaft power the pump draws | Sizes the motor, usually checked at the end of the curve |
| NPSH required (NPSHr) | The suction head the pump needs to avoid cavitation, rising with flow | Must stay below the NPSH available, with a margin |
| System curve | The head the system needs at each flow: static lift plus friction, rising with flow squared | Changes when valves move, lines scale or levels change |
A pump runs where its curve meets the system's. Most pump problems are a pump running in the wrong place on its curve.
The affinity laws and speed control
The affinity laws say how a pump's curve moves with speed. Because power falls with the cube of speed, a pump with a variable speed drive that runs at 80 per cent speed draws only about half its full speed power. Where the static head is a large part of the system head, the savings are smaller, because the pump still has to lift the liquid, but speed control still keeps the pump closer to its best efficiency point than a throttling valve does.
| Quantity | Relation | Meaning |
|---|---|---|
| Flow | Q₂ = Q₁ × (N₂ ÷ N₁) | Flow changes in proportion to speed |
| Head | H₂ = H₁ × (N₂ ÷ N₁)² | Head changes with speed squared |
| Power | P₂ = P₁ × (N₂ ÷ N₁)³ | Power changes with speed cubed, which is why slowing a pump saves so much |
Cavitation and NPSH
The pump needs a minimum pressure at its inlet, expressed as the net positive suction head required, or NPSHr, which rises with flow. The system provides the NPSH available, set by the suction level, the pipe losses, and the vapour pressure of the liquid at its temperature. When the margin between them closes, vapour bubbles form at the impeller inlet and collapse further in, pitting the vanes and making a sound like gravel.
Cavitation is usually a system problem rather than a pump problem: a sump drawn too low, a blocked strainer, a long or narrow suction line, a hot liquid, or a pump running far to the right of its curve. The fix is in the system, and a replacement impeller without it simply cavitates again.
Running in the wrong place
Many repeat pump failures trace back to where the pump runs. A pump oversized for a duty that never arrived spends its life throttled to the left of its curve. A pump on a system that was changed, with a new branch or a lower discharge, runs out to the right. Both show up as seal, bearing and impeller failures that root cause analysis attributes to the part rather than the duty, unless someone plots the operating point.
| Problem | Cause | Signs |
|---|---|---|
| Cavitation | Suction pressure too low for the flow, so vapour bubbles form and collapse on the impeller | Gravel noise, falling head, pitted impeller inlet |
| Running far left of BEP | Too little flow: internal recirculation, heating, radial load on the shaft | Vibration, seal and bearing failures, damage at the impeller eye or tips |
| Running far right of BEP | Too much flow, often from an oversized pump or a lower system curve than designed | High power, cavitation risk as NPSHr rises, motor overload |
| Dead-heading | Running against a closed valve with no flow | Rapid heating, seal failure, possible casing damage |
| Air or gas entrainment | Vortexing at a low sump, leaking suction joints | Loss of prime, surging, noisy and erratic flow |
| Throttled control | Flow controlled by a partly closed valve instead of speed | Wasted energy and a pump pushed away from BEP |
Maintaining centrifugal pumps
Routine pump maintenance is mostly seals, bearings and alignment, with an overhaul when performance or condition says so. A performance check before an overhaul, comparing flow, head and power with the pump curve, shows how much has been lost to wear ring clearance and impeller wear, and whether the overhaul is worth doing yet. Recording seal and bearing changes against each pump in the maintenance log is what reveals the bad actors whose duty needs looking at.
| Task | What it involves | When |
|---|---|---|
| Mechanical seal | Leakage, flush or quench, seal support system | Rounds, with seal changes recorded against the pump |
| Bearings and lubrication | Temperature, vibration, oil level and condition | Rounds and condition monitoring |
| Alignment | Coupling alignment and soft foot after any work | After work, and when vibration says so |
| Wear ring clearance | Measured and restored, since a worn clearance lets flow recirculate | At overhaul |
| Impeller | Erosion, cavitation damage, balance | At overhaul |
| Performance | Flow, head and power against the pump curve | Trended, or tested before an overhaul to show what is lost |
Common mistakes
- Oversizing pumps for future duty and throttling them for years.
- Treating repeat seal failures as seal problems without checking the operating point.
- Controlling flow by throttling when a variable speed drive would save energy and wear.
- Running sumps low, so the pump draws air or cavitates.
- Replacing a cavitated impeller without fixing the suction side.
- No flow or pressure measurement, so nobody knows where the pump runs.
- Overhauling on a fixed interval without a performance check.
Common questions
How does a centrifugal pump work?
A rotating impeller throws liquid outward, adding velocity, and the casing converts that velocity into pressure as the liquid leaves through the discharge. Liquid is drawn into the eye of the impeller to replace it. The head a centrifugal pump develops falls as the flow rises, which is shown by its pump curve.
How do you read a pump curve?
The pump curve shows head against flow for a given impeller and speed, usually with efficiency, power and NPSH required plotted against the same flow. The operating point is where the pump curve crosses the system curve, which is the head the piping system needs at each flow. The best efficiency point marks the flow where the pump is most efficient, and the pump should run near it.
What is the best efficiency point of a pump?
The best efficiency point, or BEP, is the flow at which a centrifugal pump converts shaft power into hydraulic power most efficiently. Near it, the flow through the impeller and casing is smooth and the radial load on the shaft is lowest. Running well to either side increases vibration, heat, recirculation or cavitation, and shortens seal and bearing life.
What are the pump affinity laws?
For a change of speed, flow changes in proportion to speed, head with speed squared, and power with speed cubed. A pump slowed to 80 per cent speed delivers about 80 per cent of the flow at 64 per cent of the head, and draws about 51 per cent of the power. The same laws apply approximately to trimming the impeller diameter.
What causes cavitation in a pump?
Cavitation occurs when the pressure at the impeller inlet falls below the vapour pressure of the liquid, so vapour bubbles form and then collapse violently on the impeller. It happens when the net positive suction head available falls close to the NPSH the pump requires, for example from a low suction level, a blocked suction strainer, hot liquid, long or narrow suction pipe, or running at high flow.
Key terms
Plain-language definitions from our glossary for the concepts this article leans on.
Standards and further reading
- Hydraulic Institute standards for rotodynamic pumps (Hydraulic Institute)
- Pump systems, energy efficiency resources (US Department of Energy)
- ISO 14224:2016 Collection and exchange of reliability and maintenance data for equipment (ISO)
Related case studies and tools
- Life Cycle Cost Calculator (tool)
Related reading
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