In our last blog we discussed cavitation singling out suction cavitation to illustrate the concept. In this blog, as promised, we will discuss discharge recirculation, also sometimes referred to as discharge cavitation.
The term discharge recirculation is often used interchangeably with the term discharge cavitation. Although these two are clearly related they have a few distinct differences.
Let us first examine discharge recirculation. The term simply describes an operating condition within a centrifugal pump where significant portions of the design flow is not exiting out of the discharge. This occurs when the operating point is well to the left of the BEP, approaching or even reaching shut off conditions. The reference to recirculation actually applies to two distinct locations or types of recirculation within the pump.
One of the two locations is at the pumps splitter or cut water. With lower than design flow exiting out of the discharge, large volumes of liquid are forced to recirculate within the casing. A high flow rate passing through the small gap between the impeller and the volute’s cut water results in a high fluid velocity. The extreme velocity at this location causes a vacuum to develop at the housing wall (similar to what occurs in a venturi), which turns the liquid into a vapour.
The drawing below illustrates this issue:
The vapour bubbles forming and then collapsing as they move away from the low pressure zone is a process we described last blog as cavitation. Due to its location, this form of cavitation is labelled as “volute discharge cavitation”.
The second location for discharge recirculation to occur is in the impeller vane. Similar to cutwater recirculation, it becomes more of an issue as the operating point approaches shut off conditions. As illustrated, discharge recirculation is the reversal of flow at the discharge tips of the impeller blades.
The high shear rate between the inward and outward relative velocities produces vortices. The fluctuation in pressure that occurs within these vortices results in the formation and collapse of vapour bubbles. Again, this is the definition of cavitation and in this second location it is noted as “impeller discharge cavitation”.
On a separate note; a problem referred to as “vane passing syndrome” is almost identical to the discharge recirculation issue as described at the beginning of this blog. There is however a fine distinction. Vane passing syndrome is normally associated with pumps being designed with a closed impeller to split water clearances that can suffer from cavitation issues at close to BEP flow rates. This is of course different than plain recirculation that is normally associated at conditions from 10% of BEP flow to shut off.
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That’s all for this blog but as always, if you have any questions or would like to add any further knowledge, feel free to give us a call or email. In the next blog we will look at the effects of complete discharge recirculation and how to prevent pump damage once it has occurred.
Until Next Time,