Most pump problems start with the selection, not the pump. A unit that is too small never delivers the flow, and one that is too big wastes energy, runs off its curve and wears out early. The method for getting it right is straightforward. Work through it in order and keep your assumptions written down.
Start with flow rate and pipe size
Flow rate is what the system needs, not what the pump can give. Work out the peak demand: how many outlets, how much a process uses, or how fast a tank must fill. Then choose a pipe size. Pipe size and pump size are linked, because a smaller pipe is cheaper but costs you in friction loss and pumping energy every hour it runs.
A common starting point is to keep water velocity in the order of one to two metres per second in the pipe. [CHECK: confirm this velocity range suits the advice you normally give for general water transfer] Velocity is flow divided by the internal cross-sectional area, so always use the real internal diameter, not the nominal size.
Worked example. We need to move 5 L/s (0.005 m³/s) of clean water at about 20 °C from a ground tank to a header tank. The pipe run is 120 m, with a 14 m rise from the tank water level to the discharge point. We try a pipe with a 69 mm internal diameter. The area is about 0.00374 m², so velocity is about 1.34 m/s. That is reasonable.
Work out friction loss and static lift
Static lift is the vertical difference between the water surface at the supply and the discharge point. It does not depend on pipe size or flow. In our example it is 14 m.
Friction loss is the energy lost as water moves through pipe and fittings. It rises roughly with the square of velocity, so small changes in pipe size matter. For straight pipe, the Darcy-Weisbach equation is the usual approach. It uses a friction factor that depends on pipe roughness and Reynolds number.
In our example, the Reynolds number is about 92,000, and for smooth plastic pipe the friction factor comes out near 0.018. That gives about 0.024 m of head loss per metre of pipe, or roughly 2.9 m over 120 m.
Fittings add more. Bends, valves, tees and entries and exits are each given a loss coefficient (K), and the loss is K multiplied by v²/2g. If we allow a total K of 5 for the discharge side, that is about 0.46 m more. Use the manufacturer’s K values where you have them.
Add up total head and pump power
Total dynamic head is static lift plus friction losses (plus any required delivery pressure, which is zero here as the water discharges to atmosphere).
- Static lift: 14.0 m
- Pipe friction: about 2.9 m
- Fittings: about 0.46 m
- Total: about 17.4 m at 5 L/s
It is sensible to allow a margin for pipe ageing, uncertainty in your assumptions and future changes. Ten per cent is a common allowance, which takes us to a duty point of about 5 L/s at 19 m. [CHECK: confirm the margin you would normally recommend] Do not stack large margins on top of each other, because that is how pumps end up oversized.
Hydraulic power is density × gravity × flow × head. At 19 m that is 1000 × 9.81 × 0.005 × 19, or about 0.93 kW. Pumps are not 100% efficient, so divide by the pump efficiency from the curve. At an assumed 60%, shaft power is about 1.55 kW. Motors come in standard sizes, so you would then select the next size up that covers the shaft power across the whole operating range, which in this case is likely to be a 2.2 kW motor. [CHECK: confirm motor sizing approach and typical standard sizes you would quote]
Then compare your duty point with real pump curves. You want the duty point near the best efficiency point of the pump, not out at one end of the curve.
If you would rather not do this by hand, our free pipe flow and pump power calculator works through velocity, friction loss and power for you. It is also handy for testing different pipe sizes quickly.
Check NPSH before you buy
Total head tells you if the pump can push the water where you need it. NPSH tells you if the pump can draw water in without cavitating. Cavitation damages impellers, causes noise and vibration and reduces performance.
The rule is that the Net Positive Suction Head available (NPSHa) from your system must exceed the NPSH required (NPSHr) by the pump, which is on the manufacturer’s curve, with a safety margin. NPSHa is atmospheric pressure head, minus vapour pressure head, plus the static head on the suction side (negative if it is a suction lift), minus suction-side friction losses.
In our example, the tank water level is 1.5 m above the pump centreline, with 6 m of suction pipe and a few fittings. Assume sea level and 20 °C water.
- Atmospheric head: about 10.3 m
- Vapour pressure head: about 0.24 m [CHECK: vapour pressure of water at 20 °C and conversion to head]
- Static suction head: +1.5 m
- Suction friction: about 0.3 m
- NPSHa: about 11.3 m
That is comfortable for most small pumps. It gets much tighter with a suction lift, long suction lines, hot water or a high altitude site. Use our NPSH calculator to test those cases. A practical habit is to keep suction piping short, straight and one size larger than the pump inlet.
Where to start
Write down five things: required flow, fluid and temperature, pipe route and length, static lift, and the suction arrangement. Run the numbers using the two calculators above, then check your duty point against two or three manufacturer curves. If the answer is borderline, test it with a different pipe size before you test it with a bigger pump.
Be aware that this method suits clean water in simple systems. Variable-speed drives, multiple pumps, long or branching networks, dirty or hot fluids and anything with safety consequences need a closer look, and sometimes a qualified professional’s sign-off.
If you want a second opinion, Drafting Central can help with the mechanical design and drawings around the pump, including layouts, 3D models and safety-in-design reviews. You can also look at our services or request a quote with your duty figures and site details.
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