Page 264 - Centrifugal Pumps Design and Application

236 Centrifugal Pumps: Design and Application

volute back to the suction will also be more uniform resulting in a reduc-
tion in localized wear in the vicinity of the casing near the impeller eye.
The degree of casing concentricity must be reconciled with the pump
specific speed and efficiency. For example, a 12-in. slurry pump (N s =
1,350) with a conventional volute could have a 82% peak efficiency,
whereas the same pump with a semi-concentric casing will have an effi-
ciency of 80.5%. Concentric casing designs produce flat efficiency
curves that are sustained at a high level over a wide range of flows mak-
ing it more amenable to off-design point operation.
Up to 1,200 N s fully concentric casings could be adopted without too
much sacrifice in efficiency (Figure 13~4a).
From 1,200 to 1,800 N s the casings should be semi-concentric, pro-
gressing towards a spiral-volute configuration at the high end of specific-
speed. A compromise is therefore reached between efficiency and rate of
wear (Figure 13-4b).
Above 1,800 N s the pump should only be applied to mildly abrasive
services and a spiral-volute casing will be utilized in the interests of
higher peak efficiency (Figure 13-4c).

Impeller

Open impellers are used where the abrasion is not too severe as they
have good air handling capabilities and are cheaper to produce. However,
performance deteriorates when the front clearance opens up due to wear.
Closed impellers are preferred over open impellers for severe abrasive
slurries since those impellers are more robust and will last longer. Also
closed impellers are not nearly so sensitive to fall off in performance
when the front clearance increases.
The requirement for extra thick impeller vanes can cause restrictions at
the impeller eye and inlets. Three to five vanes are normal, depending on
the pump specific speed and solids handling capability.
Pump out vanes are normally provided on the rear shroud. These vanes
have the effect of minimizing the pressure at the pump stuffing box and
reducing the axial hydraulic unbalance. The power absorbed by these
vanes is not all wasted since it helps to generate head. A small drop in
efficiency can be expected.

Wear Plates

Suction-side wear plates should always be provided on metal slurry
pumps, and if the service is severe, the plate should extend into the suc-
tion nozzle. The suction-side wear plate is usually the part which needs
replacement most often.