3.3.1 Effect of steam inlet conditions
Firstly, consider the effect of steam inlet conditions to the turbine. The temperature of the steam supplied to the turbine stop valve is a major factor in the cycle efficiency and the efficiency of the turbine in converting available energy into work. The effect of increasing the steam inlet temperature for a turbine expanding between set pressures is shown on the Mollier diagram (Fig 1.36).
The expansion process shown is for a single-cylinder turbine with no increase in entropy. As the temperature is increased from T1 to T3, the available energy increases, since A3B3 > A1B1.
Now consider the effect of increasing the inlet steam pressure at a constant temperature. The additional available energy is shown on Fig 1.37. The increase in available energy is less than that for temperature for the same entropy change. The significant advantage of increasing both temperature and pressure is illustrated on Fig 1.38.
There are limitations to the selection of inlet steam conditions. As mentioned in the previous section there are criteria which affect the choice of inlet conditions:
- Maximum turbine exhaust wetness 12%.
- Metallurgical constraints on the choice of turbine and boiler materials.
The metallurgical limit on temperature is 565°C for coal-fired plant brought into service with the CEGB over the years 1960 to 1986. This limit reflects a compromise between ease of manufacture, creep strength and cost. The practice on CEGB 500 MW and 660 MW sub-critical units has been to select a pressure of 159 bar at the turbine stop valve.