ABSTRACT
The impeller rear side (or hub) can also be subject to a large pressure gradient. Referring again to Fig. 2, we can see that the reference pressure on the outboard side of the hub is, again, the impeller discharge static pressure. The inboard side reference pressure is somewhat dependent on the precise anatomy of the pump in question. In some cases, the leakage flow is allowed to "drain" into the impeller inlet through holes in the hub. In this case, the hub pressure-area force is bounded on the inboard side by the impeller inlet pressure (plus the losses in flow through the hub holes). In other cases, the leakage flow is used other places in the machine (as a coolant for instance). In this event, the leakage flow rate can be controlled by a shaft seal, with the precise pressure at the base of the impeller hub controlled by a combination of the seal leakage characteristics and the pressure-flow characteristics of the downstream hardware. If further control of the static pressure on the hub is needed, a dynamic seal may be placed on the hub at any radius desired. The exact radius on which the seal is to be placed and the required dynamic seal gap can be determined by trial and error. Examples of this exercise are contained in our Case Studies. Another possibility involves the use of what are referred to as "antivortex ribs." These are simply a set of radial ribs on the impeller hub, which function as a small radial-vaned pump. Actually, the term "antivortex ribs" is something of a misnomer because these vanes simply substitute one type of vortex pattern for another. In cases where the axial clearance between the ribs and the pump housing can be kept small, these ribs can exert the greatest control of the hub side thrust force. If the rib-to-housing clearance is kept large, these ribs will be substantially less effective. Large internal clearances are sometimes designed into a machine intentionally. For example, in a pump that is moving a highly reactive chemical (such as an aggressive oxidizer}, rubbing contact between rotating and stationary parts must be prevented at all costs. If a pump uses a separate balance piston to react rotor axial thrust, changes in the pump operating point can result in significant rotor axial motion. In this case, clearances between the balance ribs and the pump housing can vary appreciably, resulting in inconsistent
performance of the balance ribs. (Note that it is possible to use both impeller balance ribs and a separate balance piston in the same turbomachine, although this is arguably unlikely.)
Axial-flow pump stages are subjected to forces generated by the same mechanisms as mentioned earlier. They are quite analogous to axial turbine thrust forces, and the previous section will provide insight into these. However, we should remember that because an axial-flow pump blade row is a "driving" member, not a "driven" member (as in the case of a turbine blade row), the directions of certain pressure differentials will be reversed from those in the turbine blade row. Also, because most axial pump blade rows accomplish diffusion (as opposed to the acceleration generated in most turbine rows), the direction of many axial momentum thrust forces will also be reversed from those in a typical turbine.
