10.3.1 Hydraulic air pumps
A typical hydraulic air pump arrangement is shown in Fig 4.39. The hydraulic pump unit is a water-operated ejector, which uses a motor-driven single-stage recirculating lift pump, and a covered water seal and air separation tank.
The water tank serves the dual function of acting as a heat sink for the air and vapour drawn from the condenser and providing a suction supply for the lift pump to recirculate the sealing water.
Make-up water is introduced to the tank at the bottom to replace any losses, and maintain the tank at a steady temperature. The tank overflow is controlled by a weir, and is discharged to waste.
Sealing water from the tank is circulated to the hydraulic air pump where it is fed into a series of nozzles. Streams of water leaving the nozzles pass over the blades of a spinner which is mounted on the end of a shaft, and is free to rotate. The blades are formed so that the sealing water propels the spinner and, in doing so, is broken into slugs. The water which discharges from the spinner seals pockets of air in the diffuser, pressurises them, and discharges the mixture into the tank. The tank is suitably vented, and the sealing water is recirculated to the lift pump.
The air suction line to the water-operated ejector incorporates an electrically-operated isolation valve and disc-type non-return valve to prevent backflow of air and water to the condenser in the event of pump failure. Some arrangements also include a barometric loop in the air suction line as a further safeguard.
As already mentioned, the requirement of a 660 MW turbine to reach full speed at no-load within 20 minutes of start of vacuum raising is beyond the capability of hydraulic air pumps alone; the installation of supplementary vacuum raising booster pumps overcomes this problem. On their own, hydraulic air pumps will take in excess of 40 minutes to achieve desired vacuum levels. In some cases the booster pumps are supplied on a unit basis, but it has become increasingly popular to adopt common booster equipment for two or more turbines, therefore reducing capital costs.
Separate electrical control systems are provided for the hydraulic air pumps and booster pumps. The operation of the pumps and their ancillary equipment is semi-automatic and sequence controlled to ensure that start-up and shutdown procedures are correctly implemented. When common booster pumps are installed, the system is interlocked to prevent the pumps being connected to a running turbine. Booster pumps are generally arranged to shut down automatically when the condenser vacuum reaches a predetermined level.