Centrifugal Pumps — Selection by Specific Speed (Ns = 20 rpm-based) For a very low specific speed of approximately 20 (rpm-based units), which type of centrifugal pump is generally preferred at the outlet in terms of flow direction and speed category?

Introduction:
Pump specific speed is a powerful index for choosing impeller geometry. Very low specific speed (around 20 in rpm-based form) points to pumps designed for high head and low discharge, which naturally aligns with radial-flow (centrifugal) impellers operating at relatively slow rotational speeds.

Given Data / Assumptions:

• Specific speed Ns ≈ 20 (rpm-based).
• Standard water service; geometric similarity assumptions.
• Goal: choose the suitable pump outlet-flow type and speed category.

Concept / Approach:
Specific speed for pumps correlates with outlet flow pattern: low Ns → radial-flow impellers (slow speed, high head); medium Ns → mixed-flow; high Ns → axial-flow (propeller) pumps. Thus, Ns ≈ 20 clearly indicates a radial-flow design at slow speed.

Step-by-Step Solution:
Interpret Ns banding: low Ns values favor radial-flow pumps.Relate hydraulic duty: high head, small Q → slow rotational speed for given impeller diameters.Conclude appropriate choice: slow speed, radial outlet flow.

Verification / Alternative check:
Manufacturer selection charts map Ns ≈ 20 squarely into the radial and slow-speed region.

Why Other Options Are Wrong:
medium speed radial: Ns is too low for this category.high speed radial/axial: correspond to higher Ns bands, not ~20.

Common Pitfalls:
Confusing pump Ns (uses discharge) with turbine Ns (uses power).

Final Answer:
slow speed pump with radial flow at outlet