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Charles Parsons, among others, realized the need for a rotating
machine or turbine to convert the power of steam directly into
electricity. He built his first multi-stage reaction turbine in
1884. The idea of the steam engine, which Charles Parsons patented
in 1884, was not a new one. Hero of Alexandria had demonstrated a
crude form of steam turbine around 130 BC. The modern day steam
turbine design is essentially the same as it was in the late 19th
century, although improvements have been made for efficiency.
In a power plant, the steam turbine is attached to a generator to
produce electrical power. The turbine acts as the more
mechanical side of the system by providing the rotary motion for the
generator, while the generator acts as the electrical side by
employing the laws of electricity and magnetism to produce
electrical power. The rotor is the spinning component that has
wheels and blades attached to it. The blade is the component that
extracts energy from the steam. A single-flow turbine design has
steam entering at one end. The steam then travels in one direction
toward the other end of the section and exits the casing to be
reheated, or passes on to the next section. A double-flow section,
however, has steam entering in the middle and flowing in both
directions toward the ends of the section.
The bulk of electric power generated today is furnished by
generators driven by steam turbines. The
turbines are supplied
with energy in the form of heat
energy
in the steam, and they convert this into useful
mechanical energy.
Because of this they are called
"prime
movers." Any engine which, in the sequence
of energy
transformations in the generation and use
of power, first converts
any form of energy into mechanical energy is called a prime mover.
Of all prime movers the turbine is the
most flexible.
This is
due basically to the fact that it converts heat
directly
into rotary motion without any intermediate
steps. Many other
prime movers convert energy first
into
reciprocating motion, and this in turn into rotating motion. It is
this intermediate step, the reciprocating motion that inherently
limits the size of such
machines.
All parts of the turbine rotor move
constantly and
continuously
without reversals of direction, thus
avoiding the large alternating stresses inherent with
the reciprocating
masses involved in a diesel or any
other
reciprocating type of prime mover. This direct
conversion of heat
energy into rotating motion without any intermediate motions is the
primary advantage of a turbine.
A second great advantage of a turbine
relative to other prime movers is the fact that it delivers a
constant
and
uniform turning force or torque to the shaft. This
is
important whether driving an electrical generator,
centrifugal
pump, fan, or axial flow compressor, all
of
which require constant power input if the speed is
to remain
constant.
There are two fundamental forms of
steam turbines.
One form
is the impulse turbine, deriving its name
from
the fact that the rotating member is pushed
around by the force
of steam impinging on blades or
buckets.
The second form is the reaction turbine, so
called because it is the reactive kick from steam in
the rotating element
which causes it to rotate.
Commercial turbines do not look like
these elemental models, but operate on the same basic principles.
If we understand that
in an impulse turbine the rotor
is
maintained in motion by steam striking rotating
buckets, that in the
reaction turbine the rotating
member
derives its rotational force from the steam
leaving the blades,
and that all commercial turbines
make
use of one or the other of these principles, or a
combination of them,
we have sufficient foundation
to
proceed with a further examination of the turbine.
Note the few parts of a turbine.
Basically all that is
needed is
an orifice, or nozzle, through which steam
issues,
and buckets mounted on the rim of the wheel.
Fundamentally,
nothing else is needed for a workable power-producing turbine. A
casing is added to
confine
the steam, and valves are added to control
the
admission of steam to the nozzles. These valves
are in turn
controlled by a governor and more stages
may
be added to aid in efficiently utilizing the energy in the steam.
For various reasons other modifications may be made, but basically,
a turbine consists
of only
two elements: first, the nozzles, and second,
the
rotor.
Multi-staging does not change the
principle of operation. The only reason for adding stages is to
increase
the efficiency of the
turbine at any given speed, and,
as
any stage has its best efficiency under certain
conditions of speed
and pressures, it is usually necessary to multistage the turbine to
obtain the high efficiencies required today. Why a single turbine
wheel has its best
efficiency under one set of operating conditions may be understood
from a study of
the
elemental turbine (Figure 1).
Fig.
1 Impulse
Turbine Fig.
2 Reaction Turbine
If it is assumed that this simple
turbine has a fixed
pressure
in its boiler, it follows that a constant flow
of
steam will issue from its nozzle and this steam
will be traveling at
a constant velocity. When the
paddle
wheel (or turbine rotor) is held stationary, the
steam issuing from
the nozzle strikes stationary
buckets.
But under this condition the rotor is not
moving and hence no
work can be done. It is the condition of maximum torque, zero speed,
and zero
work.
At the other extreme consider the case
where the
speed of
the rotor is the same as the speed of the
steam.
With equal bucket and steam speeds, the
steam has no velocity
relative to the bucket and can
exert
no turning effort. This condition, then, is one of
maximum
speed, zero torque, and zero work.
In between these two extremes work can
be done, for
there
will always be force exerted by the steam and
the
rotor will always be in motion. But, as the speed
is increased from
zero to the maximum, there will be
a
point where the product of turning effort and speed
will
result in the greatest work being done. This will
be
the point of best efficiency for that stage.
In actual practice turbines are seldom
applied to
loads
where the turbine can seek its most efficient
speed.
Usually the turbine speed must be held
constant, and this is
done by a speed governor which
adjusts
the steam flow to the load to be carried.
Structural
limitations prevent turbines being built
for
usual commercial speeds with a single wheel
large enough and
efficient enough to use the energy
available
from most conditions of steam pressures,
so another turbine is
placed in series somewhat in
the
fashion shown below in figure 3.
This is a true multistage turbine.
Steam is generated
in the
boiler at a high pressure, issues from the first-
stage nozzle, and
gives up a portion of its energy to
the
first-stage wheel. The steam in the first stage
shell is at a
pressure less than boiler pressure, and
this
pressure will be reduced in each succeeding
stage until finally
the steam is exhausted. Note, however, that this turbine would be
neither practical to build nor to operate, with its integral boiler
and separate shaft for each stage. Figure 4 shows how the
turbine would look, redesigned with all wheels on a
single
shaft, and the boiler divorced from the turbine.
Fig. 3 Elementary Multistage Turbine and Boiler
Fig. 4 Elementary Multistage Impulse
Turbine
and Boiler
The final
step in making the elemental turbine into a
commercial
turbine requires simply multiple
nozzles of proper
design and a change in the shape of
the
inefficient paddles to efficient buckets having
curved entrances and
exits. The resultant steam path
may
appear as shown in figures 5 and 6.
Fig.5 Cutaway of Nozzles and
Buckets of an
Impulse Turbine
Compare the simple two-element turbine
(Figure
1),
with its single
moving part, to an elemental type
of
steam engine (Figure 7). For such a reciprocating
engine
there must be cylinder, piston, connecting
rod, crank, and
flywheel: five basic elements, four of
which
are moving. Such an engine, or other familiar
prime movers
employing a reciprocating principle,
develop
power that is pulsating. The flow of power
is not uniform
through even a single power stroke of
the
piston, but increases and diminishes in force.
These pulsations are inherent in the
reciprocating
type of
engine, and, in former years, were apparent
in
every revolution of the then familiar Ford automobile model T
engine. Every explosion in the cylinders could be felt as a distinct
vibration, and these
vibrations
could be effectually minimized only by
adding
cylinders or a flywheel. More cylinders increase the number of
impulses received by the
crankshaft
in each revolution, and heavy flywheels
store the energy of
each impulse, spreading this energy over an entire revolution.
Additional cylinders
complicate
the mechanism and flywheels add
weight,
both of which increase the difficulty of the
problem of the
automotive designer in approaching
the
same smooth flow of power that could be had
from a turbine.
In a simple turbine, such as is shown
in Figure 1, the
shaft
would not rotate exactly smoothly but would
receive
four power impulses in each revolution. Offhand this would seem
jerky, but, with its four
paddles,
it is receiving the same number of impulses
in a revolution as a
modern eight-cylinder automobile. One thinks of the "V type-eight"
as giving a
continuous
smooth flow of power to the wheels; but
consider the turbine
in comparison. Simple commercial turbines, with more than a hundred
buckets
on each
of several stages, and these buckets fed by
steam
from multiple nozzles, are the equivalent of an
automobile engine
with more than 100,000 cylinders. Flow of power from such a turbine
is so smooth
that a
flywheel is not needed to stabilize it.
How little flywheel effect there is in
a steam turbine
may be
illustrated by a typical modern steam turbine
rated
5000 kw. If full load were suddenly dumped
from its generator
and the turbine had no operating
or
Fig. 7
Elementary Steam Engine
safety governor, the turbine would
accelerate at a
rate fast
enough to reach double speed in less than 6
seconds.
This would be alarming if it were not for
the fact the modern
governors are so rapid in their
action
that even under such severe conditions the
speed would rise only
a few percent.
Omission of the flywheel and the use
of high rotative
speeds
make it possible to build powerful turbines
occupying
relatively small spaces with no sacrifice
in factors of safety.
For the higher ratings this reduction in size coupled with
simplicity of design, reduces the cost of turbines relative to the
large (in
physical size) and
more complicated prime movers,
and,
with the freedom from vibration, lighter
foundations are used.
Because space is usually at a
premium
in crowded metropolitan areas, industrial
plants, or aboard
ship, this factor of small size alone
is
sometimes important enough to determine the
choice of a turbine
rather than some other prime
mover.
It might seem, therefore, that the
turbine could have
no
logical competitor. This is true insofar as the large
turbines
are concerned, but far from the fact when
the small turbines
are considered. In present stage of
development
of small turbines there are many applications where a turbine would
be far from the best
source
of power. For example, it might be troublesome to have to carry
a boiler plant in an automobile
when
the internal-combustion engine combines
boiler plant and
prime mover within a single engine.
And,
again, for slow-speed applications the turbine's small size is
offset to a certain extent by the reduction gear, which may not be
necessary with a
slow-speed
prime mover. Nevertheless, if the auto¬
mobile
engine could be replaced with a "gas turbine," and if the
slow-speed prime mover could be
replaced
with a slow-speed turbine, maintenance
would be less and
operation would be smooth be¬
yond
anything yet developed for these and kindred
applications.
Assuming that an automobile with
direct turbine
drive
could be purchased, and driven as far each year
as
the average car travels, its owner within his life¬
time
would never have to make a major repair or re¬
placement.
The average modern turbine, compared
to an automobile
running at 30 miles an hour, would
cover
a distance of 250,000 miles the first year and
even then would not
be ready for its first inspection.
Given
reasonable care and minor replacements,
many turbines have
operating records, which, again
on
the basis of the automobile, amount of several
million miles of
travel with never an outage caused
by
failure of any part.
So much for the fundamentals necessary
to an understanding of why turbines today are paramount in
the field of prime
movers. A generation ago it was
the
mighty Corliss engines which delivered power
impressively. Their
places were taken by silent
steam
turbines; smooth in operation, not nearly so
impressive to see,
but far more powerful. Of present day prime movers the turbine was
the first to be
invented
and the last to be perfected; but with its
continued
development its field of usefulness will
be increasingly
extended.
Most turbines are fundamentally impulse
turbines.
The distinguishing characteristic of the impulse turbine is that
expansion and pressure drop occurs in stationary parts only, in
contrast with the
reaction turbine in
which a substantial part of the
steam
expansion takes place in moving parts. Impulse turbines are further
characterized by diaphragm and wheel-type construction as
illustrated
in
Figures 5 and 6 in contrast to the typical drum-
type
rotor construction of a reaction machine.
Impulse turbines may be compounded in
two basic
ways. In
Figure 6, the first stage shows a row of nozzles followed by two
rows of buckets, with a set
of
stationary buckets between them. The first row of
moving
buckets absorbs about half the jet velocity;
the stationary row
redirects the jet into the second
moving
row, which absorbs most of the remaining
steam velocity. This
is called velocity compounding
or
Curtis staging. The remaining stages, known as
"group stages" are
pressure compounded stages
where
the pressure drop is divided among a sequence of nozzles, each
followed by its row of buckets. One row of nozzles and the row of
buckets
associated
with it is considered a pressure stage.
This
total arrangement is typical of many straight
impulse machines, a
velocity compounded first
stage
followed by a number of pressure or diaphragm stages. The
type and number of stages and
blade
proportions of commercial turbines depend among other things, on
inlet steam pressure and temperature, exhaust pressure, the speed
and the output.
Thus far mainly the nozzles and
buckets have been
considered.
These are the heart of any turbine but a
number
of additional elements are needed to make a
complete unit ready
for power-plant application, as
shown
in Figure 6. There is a rotor and wheels to
carry the buckets,
and a casing or shell to confine the
steam
support the stationary diaphragms, and pro¬
vide a structural
frame.
The casing supports the main bearings
and the thrust
bearing
which maintains the shaft's axial position.
To
minimize and control steam leakage, various
seals or glands are
needed, at the diaphragm bores
and
at the ends of the casing. A lubrication system
must be provided for
the moving parts. To control
steam
admission a stop valve or throttle valve, a
steam chest, steam
admission valves, and valve gear,
and
a governor must be provided. For protection
against excessive
overspeed, an overspeed governor
and
trip mechanism is provided.
Various types of turbines have
developed to fit the
many
desired applications. Figure 8 illustrates these
various
types.
Basically, all turbines may be divided
into two broad
classes:
condensing units, which operate at back
pressures
less than atmospheric, and non-condensing units, with back pressures
above atmospheric.
This
division relates only to pressure at exhaust
flange and not to
what happens to the steam after it
leaves
the machine.
Each class may be subdivided according
to whether
full
throttle flow continues through the machine to
exhaust
or whether part of the steam is withdrawn
from the unit after
some expansion. Units of the latter types are referred to as
extraction turbines. These
may be
further classified as simple extraction turbines or
automatic-extraction units. In the latter, a
regulating
valve gear is introduced to control the
pressure
at the extraction flange. In the simple ex¬
traction turbine,
sometimes known as a bleeder turbine, one or more stages have
openings of fixed size
through
which steam may be withdrawn. Pressure of
this
extracted steam varies directly with throttle
flow. Each successive
stage is separated from the
next
by nozzles, which, in effect, constitute a series
of fixed orifices. At
any given steam flow, a definite
pressure
exists in each stage. At higher flows stage
pressures are higher
and at lower flows lower.
Introduction
of an extraction connection at any stage
adds another orifice
so that some of the steam entering the stage continues through the
turbine and some
discharges
through the extraction opening. Thus
pressure
at the extraction opening is essentially the
same as stage
pressure, and since stage pressure depends on inlet flow, extraction
pressure also varies
with
inlet or throttle flow (turbine load). For many
applications,
such as feedwater heating, extraction
steam pressure
variations can be tolerated. Where
constant
pressure of extracted steam is important, as
in process work, some
form of automatic pressure
control
is required. Machines so equipped are called
automatic-extraction
turbines in contrast to the simple extraction types.
In an automatic-extraction machine,
the section following the extraction opening is separated from the
section ahead of it
and the steam flow between them
is
regulated by a valve under automatic control. At
the extraction stage
a condition exists similar to that
in
the simple extraction turbine, with total steam
flow entering through
what amounts to a fixed orifice and leaving through two openings. In
this case, however, only one of the openings is fixed in size
and the other is
variable. The valves controlling
steam
admission to the stages following the extraction point, being
regulated by stage pressure, holds
stage
and extraction pressure constant over a wide
range of throttle
flows.
In the case of simple extraction
units, openings ranging in number from one to four is determined by
economic analysis of the overall cycle.
Automatic-extraction
turbines may be obtained
with
either one, two, or three controlled openings,
the number and
pressure being set by process needs.
Under some conditions, it may prove
desirable to
supply
excess low-pressure steam to a turbine in
addition
to the throttle steam. Turbines designed for this service are known
as mixed-pressure machines.
If
it is also desired to withdraw low-pressure steam
at times, an
extraction unit is used.
A number of other special designs have
been developed. Where extremely high-throttle pressures are
employed for
high-cycle efficiency, considerable
moisture
may be present in the last stages unless
throttle temperature
is also extremely high. Under
such
conditions it may be advantageous to divide the
turbine into two
sections, passing steam exhausted
from
the high-pressure section through a reheater to
restore the initial
temperature before expansion
through
the low-pressure section. This is known as a
reheat turbine.
Fig. 8
Various Types of Turbines
Typical Steam Turbine
 
Off-shell control valve(s), single-shell high-pressure
section with diaphragm first stage, generator on high-
pressure end, sliding support of shell on front standard
Tip Leakage for Impulse and Reaction Stages
Root Leakage for Impulse and Reaction Stages
Typical Interstage Diaphragm
Typical HP Bucket
Turbine Wheel Assembly |
