Within a power generating plant, condensate pumps take suction from a condenser hot well and discharge either to a deaerating heater or directly to the suction of a boiler feed pump. The condensate that is in the hot well has condensed from steam after exiting the steam turbine generator and is at a very low absolute pressure with very little margin below its vapor pressure.
Because there is very little vapor pressure margin, the liquid can change to vapor with a small reduction in pressure, which occurs due to frictional loss in suction piping and acceleration of the liquid into the pump impeller eye. If the liquid vaporizes due to lower local pressure in the impeller eye, cavitation occurs, which is the formation of vapor bubbles that then collapse back to liquid when they reach a higher-pressure region in the impeller. This causes performance reduction and damage to the pump. Because the hot well is under vacuum, to achieve adequate net positive suction head (NPSH), vertically suspended (VS) pumps installed in a suction can are used. The suction can enables the VS pump’s first stage impeller to be installed below the floor level to achieve higher required pressure at the pump inlet and limit cavitation.
Image 1 illustrates a VS6 pump type that is typical of the types used for condensate in power generating plants. Additionally, a VS7 type could be used, which has a double suction first stage impeller that will have lower NPSH requirements for the same flow rate and rotational speed. It is possible for an end suction or between bearing pump to be used if the hot well level provides sufficient NPSH for the pump design.
For additional information on condensate pumps and other applications in power generating plants, refer to HI’s Pump Application Guidebook for Power Plant Pumps.
