3.5.2 Cycle considerations
Consider now the effects of the various processes upon the thermodynamics and economics of the wet steam cycle. Earlier it was concluded that the inclusion of a superheating surface within the steam generator was economically impractical.
However, it is desirable to reduce the steam wetness as far as possible with the moisture separating equipment within the steam generator. The Westinghouse four-loop 3425 MW (thermal) design, which has been adapted for the UK's first PWR station at Sizewell, provides steam to the turbine whose full load conditions are nominally:
- Pressure = 66.6 bar; Temperature = 282°C
- Maximum wetness factor = 0.25%
The exhaust pressure in the condenser is of the order of 50 mbar. The expansion of the steam from the initial conditions to this pressure would result in an intolerable wetness factor in the final stages. The expansion is shown on a Mollier diagram (Fig 1.49) for a turbine with 85% efficiency.
This expansion results in steam at the later stages with a moisture content of 25%. A high moisture content is intolerable as it results in blade erosion and stage efficiency losses. The undesirability of blade erosion was discussed earlier in this chapter. As steam is expanded through the turbine it starts to condense in the wet region along the 'Wilson line' which corresponds to a wetness factor of 3-4% (see Fig 1.50). Above the Wilson line, the steam behaves as dry steam; below the line, wetness becomes apparent in the turbine expansion. Water droplets entrained in the steam flow only accelerate to a small proportion of the dry steam velocity as they pass through the fixed blades. The resultant mist has a velocity which does not suit the inlet velocity triangle of the moving blade (Section 1.2.2 of this chapter), neither in magnitude nor direction. As the mist progresses through the turbine, the droplets migrate toward the blade tips by virtue of centripetal inertia. The resultant effect of the wet steam is a mechanical drag corresponding to a 1% cylinder loss for every 1% wetness.
The effects of a high moisture content are unattractive from the viewpoints of turbine efficiency and operational economics. The cycle described earlier included a moisture separator reheater (MSR), which takes partially expanded steam, removes the moisture and reheats to a superheat temperature. The choice of reheat or crossover pressure is affected by various parameters for a machine rotating at 3000 r/min; these are:
- The maximum wetness in the LP turbine exhaust (10-12%).
- The maximum wetness in the HP turbine exhaust (14-15%).
- The temperature to which reheat steam can be raised, limited by the maximum steam temperature available at the turbine stop valve (TSV).
- Overall cycle efficiency.
- Plant capital costs.
The wetness in the HP turbine can be a little higher than that for the LP turbine as the blades are shorter and the tip velocities are considerably less.
Considering these parameters on the Mollier chart (Fig 1.51) presents a 'window' within which the crossover pressure can vary. The resultant LP condition line is very similar to that of a conventional LP cylinder on a fossil-fired or AGR plant. The option to replicate the LP cylinder is attractive, as it presents savings in terms of design and manufacture and offers a utility, such as the CEGB, the option to rationalise its national spares holding. The replication of the LP cylinder would, however, restrict the variation in crossover pressure still further.
The inclusion of steam reheating following moisture separation has obvious advantages in terms of improving the LP exhaust wetness. The more effective the moisture separator, the less reheating is required; modern separators can produce steam with wetness factors of 0.25% or better.
The need for steam reheating has been addressed, but the means of reheating has not been discussed in detail. The objective of the reheating process is to produce the best steam conditions for the minimum cycle and plant capital cost. The factors to be considered are:
- The choice of live steam (single-stage reheating) or live and bled-steam (double stage reheating).
- The final terminal temperature difference achieved between reheated steam and live steam.
- The pressure loss across the reheater.
Although all these factors are considered simultaneously in the design, consider the choice between single and double-stage reheating. The plant arrangements for single and double-stage reheating are shown in Fig 1.52. The variation in net cycle output against total reheater surface area (Fig 1.53), shows a consistent advantage over the single stage reheating.
The thermodynamic benefit derives from using a higher proportion of the high quality, dry steam passing through the turbine to do work. Hence the HP turbine efficiency is improved, since wetness reduces stage and cylinder efficiency. The other benefit of two stages over one concerns the efficiency improvement associated with dividing the temperature rise across an increased number of heaters (in a similar way, the increase in the number of feedheaters increases overall cycle efficiency). The benefits to operating efficiency must be judged against the increased capital cost of the additional plant. The increased capital cost includes extra heating surface, a HP cylinder bled-steam tapping, extra pipework and increased pressure loss in the reheater.
The effect of HP exhaust pressure upon the choice between single and double-stage reheating, is shown in Fig 1.54.
The optimum pressure is shown, but there is little penalty associated with deviating from the optimum. However, this curve shows the general benefit of double-stage reheating over a range of exhaust pressures.