3 Historical development and layout
Since the mid-1940s the turbine manufacturing and electricity supply industries have benefited from a continuing policy of standardisation, which has resuited in consistent concepts of condensing plant layout.
The costs incurred in extensive design work, and the development of special manufacturing techniques, have been spread over a large number of units which have progressively benefited from minimised commissioning problems, and increases in service reliability and maintainability. During this period the size of generating plant has also progressively increased from 120 MW to units of 660 MW.
Standardisation of approximately fifty machines was achieved in two stages; first at a unit size of 120 MW, and later at a unit size of 500 MW. The latest machines of 660 MW size represent an evolutionary development: at present, there are some 23 machines at this rating either in service or under construction in the CEGB.
Certain features associated with 660 MW turbines have had a major influence on the development of condenser design and layout. In order to appreciate the significance of these features, a slight digression to highlight the development of their design is worthwhile.
Single shaft turbines running at speeds of 3000 r/min have become increasingly complex, as set ratings have increased by increasing the steam mass flow. Increasing the steam mass flow has demanded a need for multiple LP exhausts with last row turbine blades approximately 914 mm long, and an exhaust area of approximately 6.7 m2.
While the turbine manufacturers (each of which has developed its own blading design) have made improvements to details such as aerodynamic performance, erosion shield vibration characteristics and root fixings, the main dimensions have remained largely unchanged until recently when the development of new exhaust blading has increased the operating capability to very much higher specific steam loadings.
Economic considerations have also favoured substantially increased turbine exhaust specific loadings. This is reflected in the 660 MW machines at Littlebrook, where a four LP turbine exhaust design (of approximately 7.8 m2 annulus area per exhaust) has been adopted. Structural relationships between turbine and condenser, and the method of overall support, have also developed as unit ratings have increased.
Three main phases of design exist within a period of development which covers early 120 MW plant right through to the present 660 MW plant design. Justification for these various changes are now discussed.