Pumps, Pump Curves and NPSH
Pump types, head and efficiency curves, the operating point, affinity laws, cavitation and net positive suction head.
Pump types
| Type | How it works | Good for | Watch out for |
|---|---|---|---|
| Centrifugal | an impeller throws the liquid outward; head falls as flow rises | water, process and cooling, large flows at moderate pressure | poor with viscous fluids; never run dry or closed-head |
| Axial flow | a propeller pushes the liquid along the pipe axis | very large flows at low head: drainage, circulation | steep curve; stalls at low flow |
| Gear, lobe, vane | positive displacement: a fixed volume per revolution | oils, viscous liquids, metering, hydraulics | flow is nearly constant, so needs a relief valve |
| Piston, plunger, diaphragm | positive displacement with valves, pulsating | high pressure, dosing, abrasive or hazardous liquids | pulsation: add a damper; wear on valves |
| Screw, peristaltic | positive displacement, gentle | viscous, shear-sensitive or solids-laden liquids | cost; hose life in peristaltic pumps |
A centrifugal pump delivers a head, a positive-displacement pump delivers a flow. A centrifugal pump into a closed valve just heats the liquid; a positive-displacement pump into a closed valve builds pressure until something bursts, so it always needs a relief valve.
The pump curve
The maker publishes a pump curve for each pump and impeller diameter: the head the pump can produce at each flow, together with efficiency, power and the net positive suction head it needs. The curve is measured with cold clean water at a stated speed.
Choose a pump whose best efficiency point is near the flow you want to run at, preferably 70 to 110 percent of BEP. Well to the left, the liquid recirculates inside the pump, vibration rises and the bearings suffer; well to the right, the pump needs more NPSH and may cavitate and overload the motor.
The operating point
A pump on a system runs where the pump curve meets the system curve (see Pipe Systems: Series, Parallel and Networks): the head the pump supplies equals the head the pipework demands.
Worked example: pump and system
- The system curve is the DN80 line with 12 m lift, 100 m of pipe and ΣK = 8.45; the pump head is H = 42 − 0.0230 Q² (Q in L/s).
- They meet at Q = 17.7 L/s, H = 34.8 m, where the pump efficiency is 70 percent.
- Shaft power: P = ρ g Q H ÷ η = 998 × 9.807 × 0.0177 × 34.8 ÷ 0.70 = 8.7 kW. Size the motor with a margin of 10 to 25 percent.
- The pump delivers more than the 10 L/s that the pipe example needs: it will run at 18 L/s unless a valve adds resistance. See the next two sections for what to do about that.
The affinity laws
For the same pump at a different speed N (or the same family of pumps at a different impeller diameter), the performance scales in a simple way:
Q₂ ÷ Q₁ = N₂ ÷ N₁ H₂ ÷ H₁ = (N₂ ÷ N₁)² P₂ ÷ P₁ = (N₂ ÷ N₁)³
Worked example: raising the speed
- At 1 450 rpm the pump delivers 20 L/s at 32.8 m, using 6.4 kW hydraulic.
- At 1 750 rpm (ratio 1.207): Q = 24.1 L/s, H = 47.8 m, and the power rises to 11.3 kW: a 21 percent speed increase takes 76 percent more power.
- The same law in reverse is the reason variable-speed drives save so much energy on pumps and fans: a 20 percent cut in speed cuts the power by 49 percent.
Throttling or speed control?
To cut the flow you can throttle with a valve (the valve burns the extra head as heat) or slow the pump with a variable-frequency drive.
Worked example: running at 70 percent of the flow
- Target: 12.4 L/s (70 percent of 17.7). The pipework needs only 23.4 m at that flow.
- Throttled: the full-speed pump produces 38.5 m at 12.4 L/s, so the valve must absorb 15.1 m. Efficiency 59 percent, power = 7.91 kW.
- Slowed: the pump must run at 80 percent of speed (1 161 rpm) to make 23.4 m at 12.4 L/s, at an equivalent point with efficiency 66 percent: power = 4.31 kW.
- The drive saves 46 percent of the input power at that flow. How much that is worth depends on how many hours the system runs at reduced flow; a drive pays back fastest where the static head is small and the flow varies a lot (see the AC Motors and VFD page for the drive side).
Cavitation and NPSH
The pressure at the impeller inlet is the lowest in the system. If it falls to the vapor pressure of the liquid, vapor bubbles form, then collapse violently as they reach higher pressure: cavitation. It sounds like gravel in the pump, pits the impeller, cuts the head and shortens the life of seals and bearings.
The margin is the net positive suction head. NPSHA (available) is a property of the system and NPSHR (required) is a property of the pump, from its curve. You need NPSHA comfortably above NPSHR.
NPSHA = (ps − pv) ÷ (ρ g) + zs − hL,suction
Worked example: NPSH available
- Open tank, liquid surface 2 m above the pump, suction loss 0.8 m.
- Water at 20 °C: (101 325 − 2 339) ÷ (998 × 9.807) + 2 − 0.8 = 10.11 + 1.2 = 11.3 m. The pump needs 2.7 m at 18 L/s: a margin of 8.6 m: safe.
- The same water at 80 °C (vapor pressure 47.4 kPa): NPSHA = 6.9 m. The margin shrinks to 4.1 m, still positive but thin; at 95 °C it would be negative.
- A 3 m suction lift instead of a flooded suction (cold water): NPSHA = 10.11 − 3.0 − 0.8 = 6.3 m: feasible only for pumps needing under about 4 m, and the margin is lost at altitude (atmospheric pressure at 2 000 m is 79.5 kPa).
- Cures for cavitation: raise the tank or lower the pump, enlarge and shorten the suction pipe, use a rounded inlet and fewer fittings, cool the liquid, slow the pump, or choose a pump with a lower NPSHR. A margin of 0.5 to 1 m (or 20 percent) above NPSHR is the usual minimum.
Pumps in series and parallel
- Series: two identical pumps add their heads at the same flow. At 20 L/s each gives 32.8 m, together 65.6 m. Use for high-head systems.
- Parallel: two identical pumps add their flows at the same head: at 32.8 m together they give 40 L/s. But on a system with a lot of friction the combined flow is far less than double (the second pump pushes the operating point along the system curve), so check on the system curve, not at the pump alone.
- Do not mix unequal pumps in parallel without checking that the weaker one is not pushed back past its shut-off head.
Related
- Previous: Pipe Systems: Series, Parallel and Networks. Next: Flow Measurement.
- Pump Power and Sizing Calculator: power, motor size, energy cost and NPSHa.
- AC Motors and Variable-Frequency Drives.
- Pneumatic and Hydraulic Actuators for hydraulic pumps.
- Fluid Mechanics and Pipe Flow: the whole guide.
FAQ
Why is my pump delivering more flow than the datasheet duty?
Because the system resistance is lower than designed (a valve more open, a pipe shorter or bigger), so the operating point moved right on the curve. Check the motor current: it will be high.
What is shut-off head?
The head a centrifugal pump produces at zero flow with the discharge closed. It is the pump’s maximum pressure, and the pump should not be run there for more than a short time: all the power goes into heating the liquid.
How much margin should I leave on the motor?
Check the power over the whole operating range, not just the duty point: a pump that can run out to the right of its curve must have a motor that covers that point. 10 to 25 percent is typical.
Does the pump curve change with the liquid?
Water curves are corrected for viscous liquids: above about 10 mm²/s the head, flow and efficiency fall and the power rises. Ask the maker for a viscosity-corrected curve.
References
- Hydraulic Institute, ANSI/HI 9.6.1 Rotodynamic Pumps: Guideline for NPSH Margin and Centrifugal Pump Handbook.
- Karassik, I. J., et al., Pump Handbook, McGraw-Hill.
- Gülich, J. F., Centrifugal Pumps, Springer.
- US Department of Energy, Improving Pumping System Performance: A Sourcebook for Industry.
Last reviewed: October 2026. The pump curve here is a model for teaching; use the maker’s curve for a real pump.