The casing: volute and diffuser
Why the pump housing is shaped like a snail, and how it turns speed into pressure.
⏱ 6 min
- Everyday comparisonStep 1 of 6
The skateboard ramp
A skateboarder rolls fast towards a ramp. Going up, they slow down — but they end up high. Their speed turned into height.
A pump casing does the same thing with water: it takes the fast water from the impeller and turns its speed into pressure (which is like height for water).
- Try itStep 2 of 6
Take the casing away
Switch between No casing, Volute and Diffuser and watch where the water goes.
Without a casing the water just sprays off in all directions. The energy is there, but nothing collects it into a pipe.
Challenge: Compare the gauges for the three casings. Which casing gives pressure in the outlet?
- ExplainedStep 3 of 6
Why a snail shape?
As the water goes round the , more and more of it joins from the impeller. So the passage must get wider to make room — that is the snail shape.
Near the outlet it widens even more, like a funnel in reverse. The water slows down and its pressure climbs (Bernoulli again).
- ExplainedStep 4 of 6
The cutwater
Where the spiral meets the outlet pipe there is a sharp tongue called the . It splits the flow: most water goes out, a little goes round again.
It sits very close to the impeller, so every vane that passes it makes a small pressure “tick”. That is part of the hum you hear from a pump.
- ExplainedStep 5 of 6
The diffuser: fixed vanes
Instead of a spiral, some pumps have a ring of fixed vanes around the impeller: a . The channels between the fixed vanes widen gently and slow the water.
Diffusers are compact and guide water evenly, so they are used in : several impellers in a row, each adding pressure — like deep well pumps and the pumps that feed water into power-plant boilers.
- The impeller connectionStep 6 of 6
Half the job
A good casing is worth a lot. The impeller builds some pressure directly, but a big share — often a third or more — comes from the casing slowing the water down. A badly shaped casing simply wastes that energy as swirl and heat.
That is why engineers design the impeller and the casing together, as one team.
📌 Remember this
The casing catches the fast water and slows it down so its speed becomes pressure. A volute is a widening spiral; a diffuser uses fixed vanes. Without a casing, the water just sprays away.