Slip, losses and recirculation
Why a real pump never reaches Euler’s head — and where the missing energy goes.
⏱ 7 min
- Everyday comparisonStep 1 of 6
Cornering on ice
Drive round a bend on an icy road. The car doesn’t quite follow the curve of the road — it drifts a little wide.
Water between the vanes does something similar: it doesn’t follow the vanes perfectly. Engineers call it .
- ExplainedStep 2 of 6
Why water slips
Only the water right next to the vanes is pushed properly. In the middle of a wide channel, the water lags behind and leaves with less swirl than the vane angle promises.
More vanes guide the water better, so there is less slip — but each vane adds friction and blocks some of the flow. Designers use a , usually 0.7–0.9, to predict it.
- Try itStep 3 of 6
See the head shrink
The dashed arrow is the ideal outflow; the solid arrow is the real one with slip. The readouts follow the head from Euler’s ideal, down through slip, to a realistic final value.
Change the number of vanes.
Challenge: Compare 2 vanes with 12 vanes. How much head does slip cost in each case?
- ExplainedStep 4 of 6
Where the rest goes
: friction on the walls, water hitting the vane edges at the wrong angle (away from the design flow), and fast and slow streams mixing in the casing.
Leakage: some water sneaks back to the eye through the wear-ring gap and gets pumped twice. [[disc-friction|Disc friction]]: the outsides of the shrouds stir the water trapped beside them. Mechanical: bearings and the shaft seal.
- ExplainedStep 5 of 6
Recirculation at low flow
Far below the design flow, the water can’t fill the channels properly. Parts of it turn round and flow back out of the eye or back into the impeller at the outlet: .
It makes a rumbling noise, shakes the shaft and can damage the vanes — a second reason not to run a pump far left on its curve.
- The impeller connectionStep 6 of 6
Designers fight for every percent
Smooth, polished channels, tight wear rings, well-shaped vane edges and a volute matched to the impeller each win back a little. A big water-supply pump can reach around 90% efficiency; a small cheap one may be below 50%.
📌 Remember this
Real head = Euler head − slip (water not following the vanes) − hydraulic losses (friction, bad entry angles, mixing). Leakage, disc friction and mechanical losses cost extra power. At very low flow, recirculation adds noise and damage.