Water deeper than about 7 m needs the pump cylinder, or the whole pump, placed down in the well, because a suction pump cannot lift more than the atmosphere supports. The loads follow from the weight of the water column: a piston of 75 mm bore lifting water 30 m carries 1,300 N on its rod, and lifting 12.5 L/min through 30 m takes at least 61 W, so the bore, stroke and handle ratio are chosen from how hard people can work.
Part of Deep Wells
Deep-well pumps lift water from below the reach of a suction pump. They are a force pump with a submerged cylinder driven by a long rod, a rope pump, or a motor-driven submersible pump, and their design follows directly from the weight of the water column.
Below About Seven Metres The Pump Must Go Down The Well
A suction pump can lift water at most about 7 m in practice, 10.3 m in theory (Suction Pump). For deeper water the pump must push rather than draw: the cylinder sits at or below the water in the well and pushes the water up the rising main to the surface, so the limit is no longer the atmosphere but the strength of the parts and the power of the operator. The Phase 3 Hand Pumps guide treats the build; this page sizes the options.
| Option | How it works | Needs |
|---|---|---|
| Force pump, submerged cylinder | A piston in a cylinder under water, lifted by a long rod from the surface handle | Steel or hardwood rod, rising main, leather or rubber cups, check valves |
| Rope pump | Washers on a rope loop push water up a pipe | Rope, pipe, wheel (Rope Pump) |
| Bucket and windlass | Water lifted in a container | Hoist, rope; open to contamination (Pump Selection) |
| Motor-driven submersible | An electric pump lowered into the casing | Salvaged pump, cable, and a power source such as solar panels |
The Rod Carries The Weight Of The Whole Water Column
On the upstroke the piston lifts all the water above it, and the rod carries that load. The force is the pressure of the water column times the piston area:
F = ρ × g × A × H (ρ = 1,000 kg/m³, g = 9.81 m/s², A the bore area, H the height from the water in the well to the spout)
For three bores lifting from 30 m, with a stroke of 200 mm (an assumed value) and a handle lever of 4:1 (the Hand Pumps guide uses 3:1 or 4:1):
| Bore | Area | Water above piston | Rod force | Handle force (÷ 4) | Water per stroke |
|---|---|---|---|---|---|
| 50 mm | 1.96×10⁻³ m² | 59 kg | 578 N | 144 N | 0.39 L |
| 75 mm | 4.42×10⁻³ m² | 133 kg | 1,300 N | 325 N | 0.88 L |
| 100 mm | 7.85×10⁻³ m² | 236 kg | 2,310 N | 578 N | 1.57 L |
Force and flow per stroke both scale with bore area, so the work per litre is the same whatever the bore. The bore sets how that work is divided between hard, slow strokes and light, quick ones. At 30 m a handle force of 325 N, the weight of a 33 kg mass, is heavy to sustain; the 50 mm bore asks 144 N. With a 50 mm bore and 15 strokes a minute the output is 15 × 0.39 = 5.9 L/min, with the 75 mm bore 15 × 0.88 = 13 L/min.
Power. The power is P = ρ × g × Q × H, whatever the pump. For Q = 12.5 L/min = 2.08×10⁻⁴ m³/s and H = 30 m: 1,000 × 9.81 × 2.08×10⁻⁴ × 30 = 61 W. At 10 L/min it is 49 W and at 15 L/min 74 W. These are hydraulic minimums: friction and leakage add to them. If a person can sustain well under 100 W (an order of magnitude recalled, not measured here), the 10-15 L/min of the Deep Wells table at 30 m is near what one person can do, and deeper wells give less water or need two people on the handle. The same limit applies to the rope pump (Rope Pump).
The Rod Cannot Push, So The Downstroke Runs On Weight
A rod 30 m long is far too slender to push. For a steel rod of 20 mm diameter (the minimum in the Hand Pumps guide) I = π × 0.02⁴ ÷ 64 = 7.85×10⁻⁹ m⁴, and the Euler buckling load of an unsupported length of 30 m is π² × E × I ÷ L² = 9.87 × 200×10⁹ × 7.85×10⁻⁹ ÷ 900 = 17 N. That is about the weight of a 1.7 kg mass. In practice the rod is held in line by the rising main and the couplings, but it is the rod’s own weight, plus a counterweight on the handle, that drives the piston down against the friction of the cups and the water passing the valves. Design the rod for tension (the upstroke) and the weight for the downstroke.
Stress and stretch. A steel rod of 20 mm has an area of 3.14×10⁻⁴ m² and a mass of 7,850 × 3.14×10⁻⁴ = 2.47 kg per metre, so 74 kg or 726 N over 30 m. The upstroke load of 1,300 N (75 mm bore) plus the rod weight is 2,026 N, giving a tension stress of 2,026 ÷ 3.14×10⁻⁴ = 6.4 MPa, far below the yield of mild steel (about 235 MPa, recalled). The stretch is ΔL = F × L ÷ (A × E) = 2,026 × 30 ÷ (3.14×10⁻⁴ × 200×10⁹) = 0.97 mm, about 1 mm against a 200 mm stroke. The rod itself is not the limit; the couplings, pins and the effort of the handle are.
Motor-Driven Pumps: The Energy Budget
A submersible pump needs a power source. Lifting V litres a day through H metres takes at least m × g × H. For V = 5,000 L/day (an example) and H = 30 m: 5,000 × 9.81 × 30 = 1.47 MJ = 0.41 kWh. With an overall pump-and-motor efficiency of 0.3 (an assumed value; take the real figure from the pump’s data) the supply must deliver 0.41 ÷ 0.3 = 1.4 kWh/day. If solar panels give about 5 hours a day of full-power sun (assumed), the panel power is 1.4 ÷ 5 = 0.27 kW, about 270 W, before losses in the wiring and controls. A salvaged pump is useful only if its power source survives as well.
What You Need
- The dry-season water level, and the cylinder placed below it
- Casing wide enough for the cylinder and the rising main
- A rod of tested strength, with couplings of equal strength
- Cups, valves and spares — leather or rubber cups wear and must be replaced
- A hoist — to lift the whole assembly out for repair
Method: Size, Place, Test
Step 1: Fix the lift H from the dry-season water level to the spout.
Step 2: Choose the bore from the handle force you accept and the flow you need, using the table.
Step 3: Set the cylinder below the lowest water level so that it always has water and never loses its prime. A cylinder left above the water must stay within 6 m of it, the limit in the Hand Pumps guide.
Step 4: Test with a load. Test rods and couplings above ground at the load in the table before lowering.
Common Mistakes
- Counting rod force as handle force. The lever ratio changes the force at the hand, not the work.
- Ignoring rod weight. A long rod adds to the upstroke load and drives the downstroke.
- Cylinder above the dry-season water. It will need priming or fail to lift.
- Over-trusting a fixed flow figure. Flow at depth depends on who is pumping.
Safety
A long string over an open well, and electricity in water
A rod, rising main and cylinder full of water can weigh well over a hundred kilograms: lift them with a hoist and keep everyone clear of the opening while the string is hauled. A dropped string can injure and ruin the pump. A motor pump brings electricity into a wet well: use only intact insulation, switch off before touching the cable, and follow the controller’s grounding. Nobody enters the well to fetch a dropped part (Safety Protocol).
What’s Next
- Learn next: Well Protection — sealing the head around the pump
- Combine with: Pump Construction — building the cylinder and valves