Pump calculator: total head and motor size
A pump calculation gives two numbers: the flow and the total head (TDH). The total head adds up the water level in the well, the height difference, the loss in the pipes and filters, and the pressure the field needs. From these two come the pump’s shaft power and a standard motor size. The tool covers submersible and surface pumps, and gives the duty point to look up on a pump curve.
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A worked example
A 25-decare soilless strawberry greenhouse in Serik. Each valve zone draws 36 tonnes an hour. With the pump running, the water in the well is 45 m down. The farthest valve is 8 m above the well, fed by 400 m of Ø110 PE100 pipe that loses 0.84 bar. The valve needs 1.5 bar at its inlet, and the filters and fertigation unit lose 0.5 bar.
- Total head 45 + 8 + 8.6 (pipe) + 5.1 (filters and fertigation) + 15.3 (pressure at the field) = 82 m
- Water power 36 × 82 ÷ 367 = 8.0 kW
- Shaft power at 65% pump efficiency, 8.0 ÷ 0.65 = 12.4 kW
- Motor 12.4 kW + 10% = 13.6 kW, so a standard 15 kW (20 HP) motor; it needs three-phase power
- Power drawn from the mains 15.5 kW at 80% motor efficiency
- Duty point 36 m³/h at 82 m. With a Ø125 main it drops to 78 m and the pump draws 0.75 kW less.
[The example’s numbers await Perge’s check.]
How it is worked out
The total head is the height the water is lifted plus the pressure spent on the way. For a submersible pump the lift is the pumping water level: how deep the water is below ground while the pump runs. For a surface pump it is the suction lift between the water surface and the pump. Add the height difference up to the highest point on the land. The pipe loss, the loss in the filters and fertigation unit, and the pressure the valve needs are in bar; each is multiplied by 10.2 to turn it into metres.
Water power (kW) = flow (m³/h) × total head (m) ÷ 367. A pump can’t give this power without loss, so the shaft power is the water power divided by the pump’s efficiency. The motor is the first standard IEC size that covers the shaft power plus 10%. The power drawn from the mains is the shaft power divided by the motor’s efficiency.
This tool names no brand or model. Look the duty point (flow and head) up on the pump curve of the make you choose. The point should fall where the curve is most efficient.
Questions
How do I work out a pump’s total head?
Add the pumping water level in the well, the height difference up to the highest point, and the loss in the pipes, filters and fertigation unit. Then add the pressure the field needs. Multiply any value in bar by 10.2 to get metres. The sum, in metres of water, is the total head.
What size motor does a submersible pump need?
Flow (m³/h) × head (m) ÷ 367 gives the water power. Divide it by the pump’s efficiency, add 10% and take the next standard motor size up. A pump lifting 36 tonnes an hour to 82 m needs 12.4 kW at the shaft and runs on a 15 kW (20 HP) motor.
How many metres of head is 1 bar?
1 bar is about 10.2 m of water. If the valve needs 1.5 bar, the pump must give 15.3 m for it. That is why the metres on a pump’s plate are about ten times its bar figure.
How big can a single-phase pump be?
Single-phase (220 V) farm pumps mostly go up to 3 kW (4 HP). Bigger motors need three-phase (380 V) power on the land. A three-phase motor draws less current for the same power and lasts longer.
What pump efficiency should I use?
Farm irrigation pumps mostly run at 60% to 75%; 65% is a safe figure for a first sizing. The right value is the efficiency at the duty point on the pump’s curve. The lower the efficiency, the bigger the motor and the more power the same work takes.