ABSTRACT
We have seen that balance pistons are capable of reacting extremely high thrust loads under conditions that would cause rolling-element bearings to fail. In addition, the piston can be made capable of reacting to changes in the machine operating conditions over a fairly wide range. Despite these virtues, balance pistons have a number of fairly serious disadvantages, which bear discussion. Because balance pistons operate by generating a pressure-area force, system operating pressures must be high enough to provide the needed authority. The areas over which these pressures act must also be high enough to generate forces of the needed magnitude. For example, if we wished to use the front shroud surface of the pump stage impeller in Fig. 19 as a balance piston surface, the need for sufficient shroud area could place constraints on the impeller hydrodynamic design. An impeller of a large radius ratio (inlet radius to outlet radius) would have a narrow shroud that may not provide sufficient balance piston capacity. In this event, an increase in the impeller diameter may be necessary to increase the shroud area. This would decrease the pump-stage specific speed, adversely effect pump efficiency, and increase the pump bulk and weight.
