3.5.3 Full-speed or half-speed machines
The final issue dealt with in this section involves a comparison of half-speed (1500 r/min) and full-speed (3000 r/min) machines. The study is limited to the steam turbine rather than the generator, as the turbine is more affected by the wet steam cycle.
It is, however, pertinent to observe that generators at 3000 r/min and 1500 r/min are technically feasible up to the largest required outputs. There are substantial differences in the weights and manufacturing costs of 2-pole and 4-pole generators favouring the full-speed machine. Whilst small differences in efficiency exist, they are much smaller than those arising in the steam turbine area.
- The principal factors involved in a comparison of half and full-speed turbines are:
- Relative sizes and weights of the turbines and differences in their constructional features.
- Relative expectations of reliability, operational flexibility, and ease of maintenance.
- Relative thermal efficiencies.
- Economic comparisons.
Size, weight and construction
For equal stress levels in geometrically-similar turbine rotating components, all linear dimensions should be inversely proportional to the speed. Theoretically, the last-stage blades of a half-speed turbine could be made twice as long as those of a full-speed turbine and could be carried on a rotor of twice the diameter, leading to an exhaust area of four times the size. However, practical half-speed turbines are limited by weight and physical size to be about 40% larger than full speed turbines: Table 1.2 gives a comparison between realistic half and full-speed machines in terms of relative sizes and weights.
Note that the total relative volume and weight of the rotors and casings per unit exhaust area for the half-speed turbine is 2.85 times the full-speed turbine. The manufacturing costs associated with the larger half-speed machine are higher as more materials are used and a greater number of turbine stages have to be built. The constructional differences between the two types are largely associated with the increased size and weight of the half-speed turbine. The dimensional limitations to the forgings of HP rotors of the half-speed turbine have resulted in a crossover pressure higher than optimum. These higher crossover pressures require an additional inner casing in the LP cylinder to avoid excessive thermal gradients. In summary, the large size of the individual components of the half-speed turbine, forces the designer to 'over design' some constructional features (see Fig 1.55).
Reliability, operational flexibility and maintenance
The reliability of full and half-speed turbines are just as important in the choice of machine as efficiency comparisons, and need to be considered.
It is important to observe that there is no valid statistical evaluation of experience to support any argument for one design being more reliable than the other. Reliance must be placed upon objective assessments of technical (including engineering) differences and potential sources of difficulty in various areas of turbine design and operation.
There are no systematic differences between the two turbines with regard to the HP blade and rotor erosion. The stationary components of the HP cylinder are subjected to pressure differences with wet steam at their joint faces; if leakage flows are generated, there is a danger of wire-drawing erosion. The increased radial dimensions of the half-speed turbine result in pressure loadings at the joints as great as twice that for the full-speed machine. Therefore the achievement of zero erosion is likely to be more difficult with a half-speed machine.
The LP cylinder for the full-speed machine is similar to that of a fossil-fired plant. Consequently, though there are few differences in blading erosion problems, the experience of UK utilities is in the full-speed area. The design of half-speed LP rotors involves a shaft with various discs, shrink fitted; this type of rotor presents poorer dynamic behaviour and is more susceptible to stress corrosion cracking problems.
There are two disadvantages associated with the half-speed machine which are related to plant operation. The larger diameter of the HP rotor leads to increased thermal stress. In addition, the rate of increase of steam temperature at the inlet to the LP cylinder may have to be restricted during start-up, to avoid losing the shrink fit rotor.
There is no reason to suggest that the frequency of maintenance of the half-speed machine is significantly different from that of the full-speed machine. However, there will be increased problems in lifting, handling, transporting and machining components of the half-speed turbines, due to their weight and size.
Relative thermal efficiencies
Marginal differences exist between the two designs with regard to the internal cylinder efficiences. They can be summarised as:
- The enforced high crossover pressure of the half-speed machine results in higher leakage loss in the last few stages of the LP cylinder.
- The half-speed LP cylinder experiences higher tip losses due to the increased radial clearances associated with a more flexible rotor.
- The full-speed machine has marginally higher tip losses in the last stage LP blade due to aerodynamic effects.
It has already been shown that the half-speed turbine is more expensive than the full-speed turbine of equal output and exhaust area, with no advantage in efficiency or reliability. Hence the full-speed turbine will be advantageous whenever its exhaust area can match the value which is desirable for an economically achievable condenser pressure.
In order to establish the regions of potential economic application of the two types of turbine, it is necessary to consider:
- The relationship of desirable exhaust areas to different combinations of outputs and condenser pressures.
- The economically achievable condenser pressures available.
- Available total turbine blade exhaust areas for full-speed turbines and the regions of outputs and condenser pressures where they are economically advantageous.
Half-speed turbines are economical only in circumstances where the largest outputs are combined with very low condenser pressures, favouring the adoption of larger exhaust areas than those which could be provided by currently available full-speed turbines (see Fig 1.56). Even this region of economic application of the half-speed turbine is under threat from the impending availability of full-speed machines with greater exhaust areas.
Turbine-generator for the CEGB PWR
The proposed station at Sizewell В will incorporate a single 3425 MW (thermal) PWR of a four-loop design. The NSSS will provide steam to two full-speed turbine-generators, each with a gross output of 622.5 MW.
The current turbine design incorporates a means of isolation of the steam supply to enable maintenance work to be performed on one machine whilst continuing to operate the other. This means of isolation will improve generating availability.
The turbine comprises one HP cylinder and three LP cylinders on a single shaft. The six-flow exhaust provides an area of 47 m2, which is sufficient for the rated output, whilst using seawater cooling. However, modern blading developments will enable the design of high speed machines with much larger exhaust areas, suitable for outputs in excess of 1000 MW.