Page 65 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

Pump efficiency
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Numerous factors affect the pump’s efficiency. The impeller is one of
the most important efficiency factors.
Affecting the impeller’s behavior are:
1. The impeller velocity.
2. The impeller diameter.

3. The number of blades on the impeller.
4. The diameter of the eye of the impeller.
5. The thickness of the impeller.
6. The pitch (angle) of the blades.

Factors that affect the efficiency

1. Surface finish of internal surfaces - Efficiency increases from better
surface finishes are mostly attributable to the specific speed Ns
(discussed in Chapter 6) of the pump. Generally, the improvements
in surface finishes are economically justifiable in pumps with low
specific speeds.
2. Wear ring tolerance - Close tolerances on the wear rings have a
tremendous effect on the pump’s efficiency, particularly for pumps
with a low specific speed (Ns < 1500).
3. Mechanical losses - Bearings, lip seals, mechanical seals, packings,
etc., all consume energy and reduce the pump’s efficiency. Small
pumps (less than 15 HP) are particularly susceptible.
4. Impeller diameter - There will be an efficiency reduction with a
reduction in the impeller diameter. For this reason, it’s not
recommended to reduce (trim) the impeller by more than 20%. For
example, if a pump takes a full sized 10-inch impeller, don’t trim the
impeller to less than 8-inches diameter. This would be a 20%
reduction.
5. Viscosity - Viscous liquids generally have a prejudicial effect on
efficiency. As the viscosity of the fluid goes up, generally the
efficiency of most pumps goes down. There are exceptions.
6. Size of solid particles - Low solids concentrations (less than 10%
average) classified by size and material, generally exhibit no adverse
affect to pump efficiency. However, the discharge configuration of
the pump must be sufficiently large to prevent obstructions. For
example, sanitary and wastewater pumps that handle high solids,
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