Specific speed (N_s) of a centrifugal pump helps compare geometrically similar pumps across sizes. Judge the statement: “N_s is the speed of a geometrically similar pump that would deliver one litre of water per second against a head of one metre.”

Difficulty: Easy

Correct Answer: False

Explanation:


Introduction:
Specific speed allows classification of pumps independent of size by tying speed, discharge, and head through similarity relationships. The correct reference discharge in the standard definition matters for dimensional consistency and practical comparison.


Given Data / Assumptions:

  • Metric/S.I. convention for centrifugal pumps.
  • N_s relates to a pump that would deliver a standard discharge at unit head (H = 1 m).


Concept / Approach:
The conventional definition (metric) states: specific speed N_s is the speed of a geometrically similar pump which would deliver 1 m^3/s against a head of 1 m. Using “one litre” (1 L/s = 0.001 m^3/s) is incorrect and would change the numerical value by a factor of 1000. Hence the given statement is false as written.


Step-by-Step Solution:
1) Standard: N_s = N * sqrt(Q) / (H)^(3/4) (with consistent units).2) Unit head condition: H = 1 m.3) Reference discharge: Q = 1 m^3/s, not 1 L/s.4) Therefore, the statement using “one litre” is incorrect.


Verification / Alternative check:
Textbook tables and standards consistently cite 1 m^3/s at 1 m head for pump specific speed in the metric convention.


Why Other Options Are Wrong:
“True” variants contradict the standard definition; the kW-based form pertains to turbine N_s or power-specific speeds, not this definition.


Common Pitfalls:
Mixing litre and cubic metre bases; using turbine-style specific speed definitions for pumps.


Final Answer:
False

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