Most nuclear power plants operate a single-shaft turbine-generator that consists of one multi-stage HP turbine and three parallel multi-stage LP turbines, the main generator and an exciter.
Each LP Turbine (low-pressure turbine) is usually a double-flow reaction turbine with about 5-8 stages (with shrouded blades and with free-standing blades of the last 3 stages). LP turbines produce approximately 60-70% of the gross power output of the power plant unit. Each turbine rotor is mounted on two bearings, i.e., there are double bearings between each turbine element.
LP turbine is equipped usually with 3 or 4 self-regulating extraction lines, which are used to provide steam for:
- the low-pressure feedwater heaters
In the LP turbine, the low-pressure stage receives steam (this steam is usually superheated steam – point E at the figure; ~1.15 MPa; 250°C) from a moisture separator-reheater (MSR). The steam from the HP turbine must be reheated to avoid damages that could be caused to the blades of the steam turbine by low-quality steam. High content of water droplets can cause rapid impingement and erosion of the blades, which occurs when condensed water is blasted onto the blades. The moisture-free steam is superheated by extraction steam from the high-pressure stage of the turbine and by steam directly from the main steam lines.
In the low-pressure turbine the steam continuously expands from point E to F. The exhausted steam then condenses in the condenser and it is at a pressure well below atmospheric (absolute pressure of 0.008 MPa). The steam is in a partially condensed state (point F), typically of a quality near 90%. High pressure and low pressure stages of the turbine are usually on the same shaft to drive a common generator, but they have separate cases. The main generator produces electrical power, which is supplied to the electrical grid.