Page 92 - Know and Understand Centrifugal Pumps

Pump Classification

points. Pumps operating away fiom their best efficiency points tend to
develop hydraulic side loads that can stress the shaft, damaging the
bearings, wear bands, and mechanical seal (Figure 6-21).
There is more information on this in Chapter 9. Dual volute casings
tend to equalize the radial hydraulic forces around the pump impeller,
thus expanding the operating window of the pump. The Ns is a guide
in selecting the adequate volute design.
The Ns is useful in analyzing a problematic pump and in purchasing a
new pump. When the parameters of a new pump are determined, the
speed, flow, and head can be worked through the Ns formula to give a
value indicating a certain type impeller design. See Figure 6-22.
VALUES OF SPECIFIC SPEEDS
SINGLE SUCTION IMPELLER

b-' a-

- -
Axial
centwli"e
Csnteriine
Ns = 500 to 1500 Ns = 1500 to 7000 Ns = 7000 to 20000

Figure 6-22

Pumps should be considered when their impeller profile corresponds to
the calculated Ns value.

Radial vane impellers (Ns values between 500 and 1,500) generate head
with pure centrifugal action. In Francis and Mixed vane impellers (Ns
values between 1,500 and S,OOO), some head is developed by
centrifugal action and other head is developed by the impeller's design.
These impellers are popular in multi-stage vertical turbine pumps. Also
with these designs, the wider impellers vanes indicate that these pumps
are better with developing flow and not so much head. Axial flow
impellers (Ns above 8,000) are almost exclusively specified in high flow
applications with little head.