Water-Lifting Devices – Pumps - Practices of Irrigation and On-farm Water Management - page 463

12.7 Cavitation in Pump
12.7.1 Cavitation in Radial Flow and Mixed Flow Pumps
In radial-flow and mixed-flow types of centrifugal pump, when the water enters the
eye of the impeller, an increase in velocity takes place. As a result of this velocity
increase, water pressure is reduced (as the water flows from the inlet of the pump to
the entrance to the impeller vane) resulting in cavitation.
A concentrated transfer of energy during cavitation creates local forces capable
of destroying metal surfaces. The more brittle the material which the impeller is
constructed of, the greater is the damage. In addition to causing severe mechanical
damage, cavitation causes a loss of head, reduces pump efficiency, and results in
noisy pump operation.
If cavitation is to be prevented, volute or diffuser pumps must be provided with
water under absolute pressure which exceeds the NPSHr. The following conditions
should be avoided in volute and diffuser pump installations:
(a) Heads much lower than head at peak efficiency of pump.
(b) Capacity much higher than capacity at peak efficiency of pump.
(c) Suction lift higher or submergence head lower than recommended by manufac-
turer of the pump.
(d) Water temperature higher than that for which the system was originally
designed.
(e) Speeds higher than manufacturer's recommendation.
12.7.2 Cavitation in Axial-Flow Pumps
In axial-flow pumps cavitation cannot be explained in the same way as for radial-
flow and mixed-flow pumps. The water enters an axial-flow pump in a large bell-
mouth inlet and is guided to the smallest section, called the throat, immediately
ahead of the propeller. The capacity at this point should be sufficient to fill the
ports between the propeller blades. When the head is increased beyond a safe limit,
the capacity is reduced to a quantity insufficient to fill up the space between the
propeller vanes, creating cavities of almost a perfect vacuum. When these cavities
collapse, the water hits the propeller vane with a force sufficient to pit the surface
of the vane. The first two cavitation prevention rules listed for volute and diffuser
pump are different for an axial-flow pump. Avoid:
- Heads much higher than head at peak efficiency of pump.
- Capacity much lower than capacity at peak efficiency of pump.
The last three rules are the same for all centrifugal pumps.