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wet steam turbines for nuclear power plants pdf

Preface xxxv
Chinese, 1985) and Technological Fundamentals of Power Steam
Turbine Start-Up Automation (1985), as well as in many papers
published in various power engineering periodicals, in both Russian
and English.
I would like to express my deep gratitude to E. R. Plotkin,
N. S. Tchernetsky, V. B. Kirillov, A. D. Melamed, N. A. Rusanova,
E. N. Sergiyevskaya, V. A. Panfilov, N. I. Davydov, B. N. Lyudomirsky, and
others of my collaborators at VTI, as well as many of my colleagues
from the turbine manufacturers and nuclear power plants with whom
I was working in close contact during those years.

I would also like to
thank all of the colleagues from various countries who have helped
me in gathering materials for this book.

1 The Nuclear Power
Industry at the Turn of
the 21st Century

 The Early History of Nuclear
Wet-Steam Turbines
The fi rst kilowatt-hours of electricity from nuclear energy were
produced on December 20, 1951, in the United States by a steam
turbine generator fed with steam from Experimental Breeder Reactor-I (EBR-I). The turbine had a rated output of 200 kW and initial
steam conditions of 2.8 MPa and 220ºC (405 psi, 429ºF). In 1953,
tests began at the shore-based prototype of a steam-turbine unit for
the first nuclear-powered U.S.

submarine, the Nautilus, and in 1954,
the Soviet Union launched the fi rst experimental nuclear power
installation, with a rated output of 5 MW.

The first commercial power
generating unit with a nuclear reactor as a steam supply source was
placed in service in 1957 at Shippingport.1 The Westinghouse turbine
of this unit was designed for a rotation speed of 1,800 rpm and a
maximum capability rating of 100 MW.

It was fed with saturated dry
steam, with the inlet steam pressure varying from 3.8 MPa (545 psi)
at the maximum load to 5.9 MPa (850 psi) when the reactor was at
idle.

As shown in Figure 1–1, the turbine was a single-cylinder, singleexhaust machine with 40-inch last stage blades.

Operating Performances in the
Nuclear Power Industry
By the end of 2000—about 40 years after the first experience
in deriving electricity from nuclear energy—according to the International Atomic Energy Agency (IAEA), there were 438 nuclear
power reactors operating in the world, with a total net electric power
generation capacity of 351,327 MW.

These values seem somewhat
understated, not taking into account for various reasons a few nuclear
power units under operation in several countries, that may or may
not be IAEA members, as well as some nuclear power units that were

Main Types of Reactors Used
for Power Production
The vast majority of the nuclear power units in service are
equipped with light-water reactors (LWRs), which use light water
as both coolant and moderator.
21 There are two main types of
LWRs: pressurized water reactors (PWRs) and boiling water reactors
(BWRs). Pressurized-water reactor versions developed in the former
Soviet Union (FSU) are commonly designated as VVER, or WWER. In
PWRs, the primary circuit’s water is pumped at relatively high pressure into the reactor vessel where it is heated and then passed to heat
exchangers (steam generators), where it boils the secondary circuit’s
water, which in turn evaporates and passes to the turbine (Fig. 1–3).
In BWRs, the water in the reactor core is allowed to boil, and the
produced steam passes directly to the turbine (Fig. 1–4). The nuclear
power units with PWRs represent indirect, or two-circuit, cycles,
whereas power units with BWRs employ a direct, or single-circuit,
cycle.

The fuel for both of these reactor types is enriched uranium
dioxide, which is clad in zirconium alloy tube assemblies.

The Nearest Prospects for
Wet-Steam Turbine–Based
Nuclear Power Plants
In view of the worldwide energy situation, there is no doubt
that the importance of nuclear energy will continue to grow in the
foreseeable future.

Nuclear power plants currently make, and will
have to continue to make, a vital contribution toward reducing CO2
emissions into the atmosphere and conserving fossil fuel energy
resources.

Because of this, a new generation of power reactors is currently being implemented in newly constructed nuclear power units.
Using the terminology of the U.S. Department of Energy, as proposed
by the NEI, they belong to a Generation III reactor technology, embracing new design concepts developed since 2000 and certified by
the NRC.

Some experts have also coined a term for an intermediate
Generation III+, intended for newly developed designs not yet certified by the NRC.
30
The next generation of technology, Generation IV,

2 The Thermal Process
in Wet-steam Turbines

Initial, Partition,
and End Steam Conditions
A characteristic schematic diagram for a turboset of a typical modern
nuclear power unit (for example, with a PWR), with the working
fluid’s operating conditions corresponding to its 100% maximum
continuous rating (MCR), is shown in Figure 2–1.

The main (live)
steam leaves the reactor’s steam generator(s) with a steam pressure
of 6.68 MPa (969 psi) and moisture content of 0.25% and enters the
double-flow HP cylinder.

After this, it passes through the moisture
separators (MS) and two-stage reheaters (R) and reaches two or
three double-flow, double-exhaust LP cylinders, being superheated
to 273°C (523ºF) by steam extracted from the HP cylinder (the first
reheat stage) and a portion of main steam (the second reheat stage).
After the LP cylinders, the working steam passes to the condensers.
The resultant steam condensate is pumped through the LP regenerative heaters to the deaerator, from which the feed water is directed
by the feed pump(s) back into the steam generator(s) through the
HP heaters.

The LP and HP regenerative heaters and deaerator are fed
with steam from the turbine’s steam extractions (bleedings).

The turbine’s regenerative system also comprises the gland steam condenser
and drain coolers.

Steam admission elements
Huge steam flow amounts for large wet-steam turbines require
special attention to their steam admission elements.

With raising the
turbine output, the size of the turbine valves and the energy amounts
dissipated in them also increase.

Simultaneously, it becomes more difficult to provide their tightness.

Increased valve size results in lower
natural frequencies of the moving elements (the valve itself and the
valve stem).

This also increases a risk of high-amplitude vibrations of
the valves.

There is a rule of thumb that calls for limiting the steam
velocity in HP valves and their inlet pipes to 75 m/s (≈ 250 ft/s). This
standard has stood the test of time, but designers of large wet-steam
turbines are often forced to ignore it.

Bearings
Large wet-steam turbines feature large-diameter journal necks—
up to 800–900 mm (30–35 in) for the largest low-speed turbines and
up to 520–560 mm (20–22 in) for high-speed turbines.

Due to a large
circular speed on the neck surface, the lubricating oil in the clearance
between the rotor neck and the journal bearing shell is rapidly turbulized, and the energy losses caused by friction sharply rise.

That is why the journal bearings for large high-speed turbines are frequently
made with multi-wedge, or segment, shells.

In this case, the lubricant
is delivered to each segment, forming separate oil wedges, and has no
time for turbulization because of a small distance between segments.
A sketch of such a four-segment journal bearing employed by Turboatom is given in Figure 3–36.This type of journal bearing is used in
their high-speed wet-steam turbines, with the single capacities of 500
MW and 750 MW.

For low-speed wet-steam turbines, despite greater
diameters of their journal necks, the circular speed on the neck surface is considerably less than that for high-speed turbines and, as a
rule, their journal bearings can be built with single-wedge shells.