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Fluid Mechanics

Q1:

Specific speed for a centrifugal pump is

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ANS:A -

Specific speed (NsN_sNs​) for a centrifugal pump is a dimensionless parameter that helps in characterizing the performance of the pump. It is defined as: Ns=NQH3/4N_s = \frac{N \sqrt{Q}}{H^{3/4}}Ns​=H3/4NQ​​ where:

  • NNN is the rotational speed of the pump (in revolutions per minute, rpm),
  • QQQ is the flow rate or discharge (in cubic meters per second, m3^33/s),
  • HHH is the total head developed by the pump (in meters, m).

Interpretation:

  1. Purpose of Specific Speed:
    • Specific speed is used to classify centrifugal pumps based on their geometric similarity and performance characteristics.
    • It helps in selecting or designing pumps for specific applications based on their operating conditions.
  2. Relationships:
    • Higher specific speed (NsN_sNs​) indicates a pump that is better suited for higher flow rates and lower heads.
    • Lower specific speed indicates a pump that is better suited for lower flow rates and higher heads.
  3. Typical Ranges:
    • Low specific speed (less than 500): Pumps for high head applications (e.g., boiler feed pumps).
    • Medium specific speed (500-3000): Pumps for general-purpose applications (e.g., water supply pumps).
    • High specific speed (greater than 3000): Pumps for low head, high flow applications (e.g., sewage pumps).

Conclusion:

Specific speed (NsN_sNs​) provides a standardized way to compare centrifugal pumps based on their rotational speed, flow rate, and head characteristics, making it a valuable parameter in pump selection and design.



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