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.............................................. Suction Conditions

4.6 Confusing conditions

The reason that Cavitation continues to be a difficult problem to
correct on a consistent basis, is that the classic symptoms of Cavitation
are shared by three other conditions. This means that, when we
experience the unique noise and high vibration levels, they could also
be caused by Suction or Discharge Recirculation or by Air Entrainment,
all of which have little to do with Cavitation or Suction Pressure.

.6.1 Suction recirculation
This condition results from various types of instability such as
turbulence, backflow circulation and swirling actions that can occur in
the impeller when operating the pump at a low flow rate. Sometimes
referred to as 'separation' or 'hydrodynamic' cavitation, these flow
patterns tend to double back on themselves under low flows. Unfortun-
ately, the flow rate at which this occurs will vary from one impeller to
the next. Frequent occurrences at flows lower than 30% of the B.E.P.
have been identified, while others have it tagged as high as 80%.
While the petrochemical industries favor a model that identifies
recirculation taking place at the eye of the impeller, physical evidence in
other industries shows the pitting damage almost halfway along the
vane as shown in Figure 4.9. It would also appear as though the
impeller design contributes to a condition where that damage could be
on either the leading or the trailing edge of the vane.
In a nutshell, suction rccirculation happens when the pump is operating
at low flows, and the pitting damage normally takes place about halfway
along the vanes.

4.6.2 Discharge recirculation
Discharge Recirculation is a very similar occurrence that results in
pitting damage at the tip of the vanes and sometimes at the cut-water of
the casing. It too can be caused by operating the pump at low flow
rates. A similar type of damage can also be caused by recirculation
between the tip of the impeller vanes and the cut-water of the casing
when the radial clearance between these points is inappropriate.

4.6.3 Air entrainment
Air entrainment defines a variety of conditions where the vapor bubbles
are already in the liquid before it reaches the pump. When they arrive in
the eye of the impeller, exactly the same thing happens as if they were
created at that point. In other words, the vapor is subjected to the
increasing pressure at the start of the vanes and are then imploded,
causing the identical damage as cavitation, and at the same location.

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