HP Turbine - High-pressure Steam Turbine

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.

HP Turbine (high-pressure turbine) is usually a double-flow turbine element with an impulse control stage followed by reaction blading at each end of the element. There are about 10 stages with shrouded blades in the HP turbine. It produces about 30-40% of the gross power output of the power plant unit.

HP turbine is equipped usually with 3 or 4 self-regulating extraction lines, which are used to provide steam for:

the deaerator
the high-pressure feedwater heaters
the feedwater pumps (when driven by steam turbines)

In HP turbine, the high-pressure stage receives steam (this steam is nearly saturated steam – x = 0.995 – point C at the figure; 6 MPa; 275.6°C) from a steam generator and exhaust it to moisture separator-reheater (MSR – point D – ~1.15 MPa; ~186°C; x ≈ 0.9). The steam must be reheated to avoid damages that could be caused to the steam turbine blades 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 reheater heats the steam (point D), and then the steam is directed to the low-pressure stage of the steam turbine, where it expands (point E to F).

Rankine Cycle - Ts diagram — Rankine cycle – Ts diagram

Advantages and Disadvantages of Steam Turbines

Advantages

Since the steam turbine is a rotary heat engine, it is particularly suited to be used to drive an electrical generator.
The thermal efficiency of a steam turbine is usually higher than that of a reciprocating engine.
The very high power-to-weight ratio, compared to reciprocating engines.
Fewer moving parts than reciprocating engines.
Steam turbines are suitable for large thermal power plants. They are made in various sizes up to 1.5 GW (2,000,000 hp) turbines used to generate electricity.
In general, steam contains a high amount of enthalpy (especially in the form of heat of vaporization). This implies lower mass flow rates compared to gas turbines.
In general, the turbine moves in one direction only, with far less vibration than a reciprocating engine.
Steam turbines have greater reliability, particularly in applications where sustained high power output is required.

Disadvantages

Although approximately 90% of all electricity generation in the world uses steam turbines, they also have some disadvantages.

Relatively high overnight cost.
Steam turbines are less efficient than reciprocating engines at part load operation.
They have longer startup than gas turbines and surely than reciprocating engines.
Less responsive to changes in power demand compared with gas turbines and with reciprocating engines.

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Expansion in the high-pressure stage of the steam turbine

A high-pressure stage of steam turbine operates at steady state with inlet conditions of 6 MPa, t = 275.6°C, x = 1 (point C). Steam leaves this stage of turbine at a pressure of 1.15 MPa, 186°C and x = 0.87 (point D). Calculate the enthalpy difference between these two states.

The enthalpy for the state C can be picked directly from steam tables, whereas the enthalpy for the state D must be calculated using vapor quality:

h_{1, wet} = 2785 kJ/kg

h_{2, wet} = h_2,s x + (1 – x ) h_2,l = 2782 . 0.87 + (1 – 0.87) . 790 = 2420 + 103 = 2523 kJ/kg

Δh = 262 kJ/kg

Steam turbine of typical 3000MWth PWR — Schema of a steam turbine of a typical 3000MWth PWR.

References:

Nuclear and Reactor Physics:

J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467
Kenneth S. Krane. Introductory Nuclear Physics, 3rd Edition, Wiley, 1987, ISBN: 978-0471805533
G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.

Advanced Reactor Physics:

K. O. Ott, W. A. Bezella, Introductory Nuclear Reactor Statics, American Nuclear Society, Revised edition (1989), 1989, ISBN: 0-894-48033-2.
K. O. Ott, R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 0-894-48029-4.
D. L. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993, ISBN: 0-894-48453-2.
E. E. Lewis, W. F. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 0-894-48452-4.

See above:

Steam Turbine