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Impeller speed means the rotational speed of an impeller, usually measured in revolutions per minute (RPM).
One-line meaning: It's simply how fast the impeller is spinning inside a pump or mixer.
Think of an impeller like a fan inside water:
Bicycle example: Pedal faster → you go faster. But if the cycle starts wobbling at a certain speed, you get careful. That wobble idea is similar to critical speed (resonance) in machines.
You'll commonly see "impeller speed" discussed in:
| Term | Meaning | Unit | Example |
|---|---|---|---|
| RPM | Rotations per minute (impeller turns per minute) | rpm | 1450 rpm motor (typical 4-pole @ 50 Hz full load) |
| Tip speed (peripheral speed) | Linear speed at impeller outer edge | m/s | 18.85 m/s |
| Head | "Pressure as height of water" (pump performance) | m | 30 m |
| Flow rate | Quantity delivered per time | m³/s or m³/h | 50 m³/h |
| Specific speed (Ns) | Index combining speed, flow, head (impeller type hint) | (varies by convention) | Used for comparing impellers |
| Critical speed | Resonance speed(s) where vibration spikes | rpm | 1st / 2nd critical speeds |
Tip speed is the linear speed at the impeller's outside edge. Higher tip speed generally means:
Speed changes can also shift suction requirements and cavitation risk. When actual speed increases, power input and Net Positive Suction Head Required (NPSHR) can increase, and speed reduction can reduce discharge and reduce NPSHr in some cases.
Tip speed (SI, m/s):
This comes from circumference (πD) × revolutions per second.
Example: D = 0.30 m, N = 1200 rpm
→ Tip speed = (3.1416 × 0.30 × 1200) / 60
= 18.85 m/s (approx)
Example: Tip speed = 12 m/s, D = 0.25 m
→ N = (12 × 60) / (3.1416 × 0.25) ≈ 917 rpm
Specific speed (ns or Ns) is a comparison index derived from similarity conditions that helps compare impellers of different sizes and operating points—and it's used to classify the "optimum" impeller design and pump curves.
A common metric training formula:
Where:
In practical classification terms:
| Specific speed level | Typical impeller flow character | Typical use |
|---|---|---|
| Low | Radial flow | High head, low flow |
| Medium | Mixed flow | Medium head and flow |
| High | Axial flow | Low head, very high flow |
Critical speed is the rotational speed where dynamic forces excite a component's natural frequency, creating resonance and potentially damaging vibration. KSB's pump lexicon describes it this way and notes risk can be reduced by passing through such speeds quickly.
If a machine is forced to operate near a critical speed, vibration and deflection can spike—accelerating wear and raising failure risk. In many systems, the goal is to avoid running continuously at resonance speeds (design/controls help).
Impeller speed matters heavily in:
It's the impeller's rotational speed, typically measured in RPM.
Use: N = (Tip speed × 60) / (π × D).
It's the linear speed at the outer edge; it's used for mixing intensity and wear/erosion considerations.
Not always—flow increases, but head and power rise faster, which can overload motors.
Speed change shifts the operating point; efficiency can improve or worsen depending on how the operating point moves relative to the pump's best efficiency region (and losses).
A coefficient/index derived from similarity conditions that helps compare impellers and classify design/curves.
Common training material shows a metric form using N in rpm, Q in m³/s, H in m (IS-5120 reference).
They're resonance speeds; the 1st is the lowest resonance speed and the 2nd occurs at a higher speed/mode.
It's risky because resonance can amplify vibration; many guidelines aim to avoid continuous operation at critical speeds.