1.2 Functional needs of the system
To play its part in the boiler/turbine-generator cycle, the feedheating system has to perform the following functions:
- Provide the required amount of feedwater to the boiler at a temperature dictated by cycle design.
- Maintain a stable suction regime for the boiler feed pumps.
- Contain a buffer store for water to allow for a short term mismatch between boiler demand and available feedwater.
- In the longer term, to keep the water content constant.
- Supply water to the boiler with an acceptable oxygen content.
- Automatically cut off the supply of contaminated feedwater to the de-aerator, and hence to the boiler, in the event of ingress of chlorides due to inleakage of the condenser cooling water or chemicals from the polishing plant.
- In the event of a turbine trip or sudden reduction of unit load, the stored energy within the feed system must not overspeed the turbine-generator.
- Be so engineered as to protect the turbine from possible reflux of water or water/steam mixture from the feed system under all conditions of operation.
The above functions are the principal aims of feed system design: how they have been achieved by the systems installed in modern CEGB power stations is described in the following sections.