13 Condenser extraction pumps - part 1
Two 100%-duty condenser extraction pumps are normally provided with the standby being ready for immediate operation, when required. The extraction pumps are designed to pump condensate from the condenser at a saturation temperature corresponding to the sub-atmospheric pressure within the condenser.
Under these conditions, the available positive suction head is very low and suction conditions are very stringent. The rotational speed of the pump and the flow of condensate into the first stage impeller should be kept as low as possible and the impeller eye must have the maximum capacity for handling water in order to provide the minimum NPSH requirements and thus avoid unnecessary depth of the extraction pumps below the condenser.
For many years, the CEGB relied on horizontal split-casing type extractions pumps (Fig 4.53) and these have generally given satisfactory service. The generated heads developed by these pumps were modest at around 100 m on 500 MW and early 660 MW stations with direct-contact heaters.
With horizontal extraction pump designs, one of the most important factors is the avoidance of oxygen ingress into the condensate through glands which may be required to seal against sub-atmospheric pressures. A secure supply of seal water is necessary and this is usually provided from a common manifold which is supplied by connections in each pump volute. This caters for both running and standby conditions. Water from the manifold can also be used as a supplementary supply to any interstage water-lubricated bearings.
To prevent the accumulation of vapour at the impeller inlet, the first stage impeller eye of a horizontal extraction pump should be vented back to the vapour space in the condenser, with the vent piping having an upward slope to the condenser.