Page 259 - Centrifugal Pumps Design and Application

232 Centrifugal Pumps: Design and Application

Castable urethanes in the 90 shore A hardness range exhibit good tear
strength and elongation properties and in certain applications have out
performed both rubber and metal.
Ceramic materials in castable form have excellent resistance to cutting
erosion but because of their brittle nature are unsuitable for direct im-
pact. Silicone carbide refrax liners and impellers in the 9,5 original Moris
hardness range are commonly used for pumping fines where the impeller
tip velocity is limited to less than 100 ft/sec.

Slurry Pump Types

There is no specific demarcation point where one pump design ceases
to be effective and another takes over. Figure 13-3 shows a classification
of pumps and materials according to particle size. It is important to note
that the selection of the pump type and its materials of construction de-
pend also on the abrasivity of the slurry and the total head to be gener-
ated.
The abrasivity of slurries can be divided into five distinct classifica-
tions to which limits on pump selection can be applied, lable 13-3 shows
a pump selection guide for wear resistance.

Specific Speed and Wear

The majority of centrifugal pumps are conventionally designed to
achieve the desired hydraulic performance at the highest efficiency and
lowest cost when handling clear fluids in reasonably clean environments.
Manufacturing limitations are not imposed on the configuration of the
pump, since conventional materials such as cast iron, bronze, and stain-
less steel are used. Since wear is not a major consideration, the highest
possible specific speed is chosen.
When a centrifugal pump is designed for a slurry service, the factors
that predominantly influence the pump design are wear and materials of
construction; efficiency is of lesser importance. To achieve these objec-
tives the pump has to operate at a lower rotational speed and the impeller
is typically a radial-flow type. This suggests that the pump must be of a
low specific speed design in the range 600 to 1,800. Specific speed is
defined in Chapter 2.
Since wear is a function of velocity it can be shown that for a given
head and capacity, wear will increase with increased N s.