How it works, drop by drop
Follow one drop of water from the pipe, through the impeller, and out of the pump.
⏱ 6 min
- Everyday comparisonStep 1 of 6
A ride at the fair
Imagine you are a drop of water about to ride the “Impeller” at a fair. You queue at the entrance, get pulled into the middle, spun faster and faster, and shot out of the side.
Then a long, widening slide slows you down gently at the exit. Let’s ride it step by step.
- ExplainedStep 2 of 6
Step 1 — pulled into the eye
As the vanes fling water outward, they leave a region of low pressure at the eye. The water waiting in the inlet pipe has higher pressure, so it is pushed in to fill the gap.
Remember: fluids always move from high pressure to low. The pump doesn’t really “suck” — the water behind you pushes you in.
- WatchStep 3 of 6
Steps 2 and 3 — grabbed and flung
A vane catches you and carries you round. The further out you go, the faster that part of the impeller is moving — remember the kids on the merry-go-round — so you speed up.
By the time you reach the edge, you are moving almost as fast as the vane tips. Watch the drops turn from blue to orange as they gain energy.
- Try itStep 4 of 6
Step 4 — the casing slows you down
You shoot out of the impeller into the , a spiral passage that keeps getting wider. You slow down, and your speed turns into pressure.
Look at the three gauges: speed is highest leaving the vanes, pressure is highest in the outlet.
- ExplainedStep 5 of 6
Why a pump must be full of water
Air is about 800 times lighter than water. An impeller spinning in air flings almost nothing, so it creates almost no low pressure at the eye — not enough to pull water up the pipe.
That is why most pumps must be filled with water before they start. This is called . Some pumps are built to be and clear the air themselves.
- The impeller connectionStep 6 of 6
The whole trip in one line
pipe → eye (low pressure) → vanes (speed up) → volute (slow down, pressure up) → pipe (high pressure).
Every centrifugal pump in the world follows these same steps. Only the sizes change.
📌 Remember this
Water is pushed into the low-pressure eye, sped up by the vanes, then slowed down by the casing so its speed becomes pressure. The pump must be full of liquid to work.