The 'specific speed' of a pump is used to:
Choose the correct answer
Measure shaft rotational speed only
Characterise pump type (radial/mixed/axial flow) and select pump for given Q and H
Calculate motor power
Measure efficiency at one operating point
Correct Answer
B. Characterise pump type (radial/mixed/axial flow) and select pump for given Q and H
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Specific speed Ns = N√Q/H^(3/4) (in consistent units). It characterises pump type: low Ns (500–1000 rpm, SI): radial flow (centrifugal, high head, low flow); medium Ns: mixed flow; high Ns (>3000 rpm, SI): axial flow (propeller, low head, high flow). Same Ns → geometrically similar pump. Used for pump selection and design.
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