The present invention relates to centrifugal pumps in general, and more particularly to improvements in means for reducing or eliminating the effects of cavitation in part-load region of operation of centrifugal pumps. Still more particularly, the invention relates to improvements in means for reducing or eliminating cavitation in centrifugal pumps of the type wherein the shaft which drives the impeller or impellers extends through the fluid-admitting inlet of the pump.
In a conventional centrifugal pump which is to operate within a wide range of capacities, and wherein the optimum rate of inflow of fluid takes place at the design duty point, the parts undergo extensive wear owing to cavitation whenever the pump is operated in part-load region. The undesirable effects of cavitation are more pronounced in larger centrifugal pumps, and the wear owing to cavitation increases with increasing rotational speed. Attempts to reduce the wear which is attributable to cavitation include limiting the interval of operation in the part-load region, utilizing highly wear-resistant materials for those parts which are most likely to be affected by cavitation and/or providing inserts which are installed in regions where the effects of cavitation are most likely to induce rapid wear or destruction and to provide for replacement of such inserts at regular intervals or when the need arises.
The reasons for pronounced cavitation during operation in the part-load region are manifold. They include the following: When the rate of flow (capacity) is less than 60-70 percent of the flow rate at the design duty point, a ring-shaped eddy develops in the outer zone of the intake of the impeller (such eddy is known as part load eddy). The flow of fluid which forms the eddy is in a direction away from the impeller and counter to the direction of inflowing fluid. The intensity of the eddy increases in response to decreasing rate of flow into the impeller, i.e., the eddy constricts the inlet at a rate which increases with decreasing capacity. The annular stream of inflowing fluid, whose outer radius decreases with decreasing rate of flow, enters the pump at the same speed as when the operating point matches the design duty point; however, the relative speed of fluid in the hub region of the impeller is lower.
The just discussed phenomenon entails a reduction of the NPSH (net positive suction head) for incipient cavitation (NPSH.sub.i) when the pump is operated in the part-load region, i.e., when the operating point is such as to permit for development of part load eddies. The formation of a part load eddy is affected by the presence of ribs, elbows or other structural elements at the inlet, and such influence upon the eddy results in an increase (rather than a decrease) of NPSH.sub.i to a multiple of the anticipated value. Thus, damage owing to cavitation is very pronounced especially since (and as already pointed out above) the optimum rate of fluid flow takes place only at the design duty point.