Condensate is a term that describes a liquid that has been condensed from vapor back to a liquid. Pumping condensate comes with the challenge that it has little margin to vapor. In pumping applications, this relates to a low net positive suction head available (NPSHa). The low NPSHa is an important pump design consideration because the pump will have to be designed with a relatively low net positive suction head required (NPSHr) so it can operate in the application with acceptable levels of cavitation. Rotodynamic pumps are commonly used in condensate applications. Some strategies included in their design to achieve a low NPSHr include low operating speed, double suction impellers and higher suction specific speed designs. Another strategy that may be employed is to use materials of construction that are better at withstanding erosion due to cavitation. Note that cavitation resistant materials do not limit cavitation and associated performance reduction, but they may result in longer life.
In power plants, condensate is the first pumping application in the steam cycle. Exhaust steam that has driven a steam turbine generator then flows through a condenser where it is condensed to liquid and collects in a hot well. Due to the low vapor pressure margin and the cost of elevating the hotwell, it is common to excavate deep enough to provide the required NPSH and use a vertically suspended can pump, as illustrated in Image 1. This design allows the vertical pump to take inlet from closed-suction piping that is under vacuum, and the depth of the can and length of the column and bowl assembly provide the required NPSH. Image 1 illustrates a bowl diffuser configuration, but a VS7 volute design could also be used, which enables the use of a double-suction, first-stage impeller to lower the NPSHr.
For more information on condensate pumping, refer to Pump Application Guidebook for Power Plant Pumps at
pumps.org.