Specific speed helps compare centrifugal impeller duties and shapes. It must not be read as the actual shaft-speed setting.
Identify the Physical Basis
Specific speed is derived from speed, flow and head at the best-efficiency point, with a stated unit convention. KSB’s definition uses it to compare impellers of different sizes and explains the one-stage head and half-flow basis for relevant multistage or double-entry cases. Numeric values using different conventions cannot be compared simply because both are labelled specific speed.
Work Through an Example
Suppose a hypothetical pump sheet lists shaft speed n = 1,450 rpm and specific speed n_s = 30 under its stated metric convention. Changing the drive to 30 rpm because of that second label would confuse a similarity descriptor with an operating speed. A second supplier may express the same descriptor using another flow and head unit convention, producing a different number without implying a different physical pump.
Use the Result in the Review
Capture the defining equation and units when transferring the descriptor to a comparison sheet. Confirm that its flow and head belong to the best-efficiency point, not an arbitrary operating point. For a multistage arrangement, identify whether the calculation uses one stage or total head. For a double-entry impeller, identify the flow basis. Then use the descriptor to discuss the type of impeller and curve evidence, not to replace the actual duty requirement.
In a processing-tank transfer discussion, state required flow and head separately from any pump descriptor. The selected pump’s speed, fluid suitability, curve and permitted operation still need review. This guide resolves interpretation; it does not select an impeller solely from a numerical category.
What comparative pump geometry information does the specific-speed definition represent?
This blank worksheet is for your own project. It contains no H M machine trial result.
| Review field | Reference or result | Responsible person |
|---|---|---|
| Specific-speed equation and unit convention | To complete | To complete |
| BEP flow and head reference | To complete | To complete |
| Stage and entry-side basis | To complete | To complete |
| Actual shaft speed kept separate | To complete | To complete |
Customer Questions
Is specific speed the drive speed?
No. It is a descriptor formed from several duty quantities.
Can values using US and metric conventions be compared directly?
No. Normalize their definitions and units first.
Should total head always be used for a multistage pump?
Check the definition; KSB’s stated basis uses optimum head per stage.
Related Equipment References
Review the actual offered equipment and application separately from this educational example.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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