13 Condenser extraction pumps - part 2
In recent CEGB stations, where surface tubular LP feedheaters are used, the required pump generated head has risen significantly to over 250 m. This increase in head produces design problems on large split-casing pumps, particularly in sealing the main joint and withstanding impeller radial loads. An alternative caisson type pump (Fig 4.54) has now been adopted.
The multi-stage vertical pump design has several attractions over the horizontal split casing pump:
- The NPSH available is increased by installing the pump in a pit, thereby reducing the risk of cavitation erosion.
- An increase in the NPSH available allows the speed to rise, resulting in a smaller pump for the same NPSH margin.
- There is no need to take static shaft deflections into account on the vertical pump. Radial stresses are reduced allowing a slimmer shaft to be used.
- Only a single mechanical seal is required which operates under pressure, providing less danger of the oxygen ingress that was sometimes encountered with the older traditional pumpsets. To prevent oxygen ingress while the pump is on standby, the inner seal chamber is filled at all times with pressurised water supplied either from the common discharge manifold or from a back-up reserve feed-water supply when both pumps are stopped.
- There is a facility for a cartridge-type repair which improves pumpset availability. The pumps are designed so that, by disconnecting the pipework at the discharge branch, the pump element can be readily removed from the caisson in one piece following removal of the electric motor.
- A more compact layout can be achieved with the vertical pumps which take less space than the horizontal designs.
To ensure that the extraction pumps have stable head/flow characteristics and are capable of operating in parallel over the complete operating range, the pumps are now designed so that the generated head rises by at least 1% for every 15% reduction in flowrate between the rated duty point and 25% rated flowrate.
Extraction pumps on CEGB stations have been operated on either free suction or recirculation control.
The latter is now preferred, as it ensures an adequate pump throughput even at low unit loads. This minimises the risk of the cavitation erosion problems, which can occur on the alternative arrangement if the pumps are operated significantly away from their duty points.
Pumps are specified with a requirement that cavitation erosion does not produce mechanical failure or loss of performance in less than 45 000 hours when operating at 80% of rated flowrate. To ensure this objective is achieved, the suction specific speed (nss) is limited to 2.4 (ISO).