A suction hand pump mounted on the slab of an underground cistern draws water up a sealed pipe, and works as long as the water is within about 6–7 m below the pump, which covers any cistern the size of the 2.5 m example. Atmospheric pressure is what lifts the water, giving a theoretical limit of 10.3 m at sea level that falls to about 8.1 m at 2,000 m altitude, and every air leak in the suction pipe costs lift or stops the pump entirely.
Part of Cisterns and Rainwater Storage
A hand pump on an underground cistern draws water out through a sealed slab opening so the hatch can stay shut. A suction pump works by lowering the pressure above the water so the air around the cistern pushes water up the pipe.
Atmospheric Pressure Pushes The Water Up, So The Lift Has A Hard Ceiling
The pump creates a partial vacuum above the water in the suction pipe. The weight of the atmosphere on the cistern’s surface then pushes water up into it. Even a perfect vacuum can only be filled to the height at which the water column’s weight equals atmospheric pressure:
- Sea level: 101,300 Pa ÷ (1,000 kg/m³ × 9.81 m/s²) = 10.3 m
- At 2,000 m altitude, where the pressure is about 79,500 Pa: 79,500 ÷ 9,810 = 8.1 m
In practice leakage and friction cut the usable lift to 6–7 m (Hand Pumps). For the 2.5 m cistern, with the pump head perhaps 0.8 m above the slab, the lift from the water surface to the pump is at most about 2.5 + 0.8 = 3.3 m, with the vacuum needed 1,000 × 9.81 × 3.3 = 32,400 Pa = 32 kPa. That is well inside the limit, and it leaves margin for a worn pump.
What You Need
- A suction pump with its base plate
- A suction pipe reaching to 10 cm above the cistern floor, with a foot valve and strainer at its end
- A sleeve through the slab, sealed watertight and airtight
- A 1 m by 1 m apron and a drain that carries spill away from the cistern (Hand Pumps gives this for wells)
- Water for priming, clean
Method: Seal Every Joint, Then Prime
Step 1: Cast the sleeve and base into the slab when it is poured, or core through carefully afterwards and seal.
Step 2: Hang the suction pipe from the pump so its end is 10 cm above the floor, clear of the sediment. Make every joint airtight.
Step 3: Bolt the pump to the base and seal the slab around the pipe.
Step 4: Prime. Pour clean water in to wet the seals and fill the cylinder, then work the handle. Priming normally takes 10–15 strokes; if it has not primed in 20, check for air leaks.
Step 5: Fit the apron and drain and test the pump, watching for water arriving at a steady rate.
Diagnosing A Pump That Won’t Lift
| Symptom | Likely cause |
|---|---|
| Handle moves easily, no water | Air leak in the suction pipe or a dry, worn cup seal |
| Water comes then stops | Foot valve leaks and the pipe drains back |
| Needs more strokes each morning | Slow leak at a joint or the foot valve |
| Hard to work, little water | Strainer clogged with sediment |
| Water cloudy | Suction end too close to the floor |
Common Mistakes
- A suction pipe joint that “mostly” seals. Air gets in where water would not escape.
- Priming with dirty water. It contaminates the cistern.
- Leaving the hatch open “for convenience”. That defeats the pump’s sealing of the tank.
Safety
Servicing the foot valve tempts someone to climb in
A roofed cistern can be short of oxygen, and carbon dioxide or hydrogen sulphide can pool at the bottom. The commonest fatality pattern is a second person climbing in after the first collapses. Pull the suction pipe up through the sleeve to service the foot valve, rather than entering. If someone must enter, ventilate at two openings, use a harness tied to a line held by an attendant, and keep a second person outside who pulls the line and never follows. Do not test the air with a flame. Never run an engine pump or generator near the hatch: its exhaust is carbon monoxide.
What’s Next
- Learn next: No Bucket Dipping — why the pump is better
- Combine with: Hand Pumps — pump construction in detail