Page 50 - Know and Understand Centrifugal Pumps

Cavitation

check valve installed backwards, or operating the pump at or close to
shut-off head. Maybe a variable speed motor could help under certain
circumstances.

The vane passing syndrome
The vane passing syndrome can exist when the blade tips at the OD of
the impeller are passing too close to the cutwater on the pump casing.
This can be caused by exchanging an impeller for a larger diameter
impeller, or from re-metalizing or coating the internal housing of the
pump. The free space between the impeller blade tips and the cutwater
should be 4% of the impeller diameter (Figure 3-3).
With the pump disassembled on the shop table, the damage is seen on
the blade tips at the OD of the impeller, and just behind the cutwater
on the internal volute wall.
For a 13“ impeller, the free space should be 4% of the impeller diameter
between the blade tips and the cutwater.

13” x 0.04 = .520”
On a 13” impeller, there should be at least a half-inch free space
between the blade tips as they spin past the pump cutwater.

Air aspiration
Air can be drawn into the piping and pump from diverse forms and
different points. Air can enter into the piping when the pump is in
vacuum. An example of this is a lift pump. Lift pumps tend to lose their
prime and aspirate air into the suction piping and pump.
1. The air can come into the pump through:
w The pump shaft packing.

w Valve stem packings on valves in the suction piping.

syndrome
Figure 3-3
~. .