View
58
Download
1
Category
Preview:
Citation preview
Main components of steam turbine:
Stationary components:
Almost all stationary parts are two halves.
Diaphragm: Partitions between pressure stages in a
turbine casing are called diaphragms. They hold the vane-
shaped nozzles and seals between the stages. Usually
labyrinth-type seals are used. One-half of a diaphragm is
fitted into the top of the casing, the other half into the
bottom as in Fig. and Fig. . The interstage
diaphragms are located in grooves in the casing accurately
Fig. Top Diaphragm and bottom diaphragm from Michigan State
University power plant [9]
Steam nozzles: Steam nozzles are installed on the
peripheral of the diaphragms. There are admission Nozzles and
interstage Nozzles their function is to accelerate the steam flow to
high velocity by expanding it to low pressure. Located in the
casing are the steam-admission nozzles which are cut into a solid
block of bronze or alloy steel, depending on steam conditions.
Nozzles are so proportioned as to be contributory to efficient
operation and are made of corrosion- and erosion-resistant
materials. This nozzle block is bolted to the steam chest, which in
turn is bolted to the base of the turbine casing. The entire assembly
of nozzles for one stage is called a diaphragm. The casing
Fig. Two Diaphragm halves
assembly with the stationary blading or nozzles is referred to as the
turbine cylinder. The cylinder of an impulse turbine is frequently
referred to as the wheel casing. (See Fig. )
Rotating parts:
Rotors for small turbines consist of a machined-steel disk
shrunk and keyed onto a heavy steel shaft. The shaft is rust
protected at the gland zones by a sprayed coating of stainless
steel. The rotor is statically and dynamically balanced to
ensure smooth operation throughout its operating range.
Fig. Nozzles, buckets, diaphragm, wheel and Shrouds [ ]
Rotors for large turbines are formed from a single piece
forging, including both the journals and the coupling flange.
Thrust-bearing collar and oil impeller may be carried on a
stub shaft bolted to the end of the rotor. Forgings of this type
are carefully heat treated and must conform to specifications.
Rotors are machined, and after the blades are in place, they
are dynamically balanced and tested. (Fig. )
1.4.2.1 Wheel: A simple turbine consists of a shaft on which is
mounted one or more wheels (discs). On the circumference of the
wheels are located blades or buckets to receive the steam and
convert it into useful work. The rims of the wheels have dovetail
channels for receiving the blades. The ends of the blades are made
to fit these dovetail channels. (See Fig. )
Fig. Rotor of a turbine in Michigan State University power plant
]
Fig. Rotors for various types of turbines (a) rotor for condensing
turbine; (b) rotor for non-condensing turbine; (c) rotor for non-
condensing single-extraction turbine; (d) rotor for condensing double-
extraction turbine. (Siemens Westinghouse Power Corp.)
Turbine blades: On the outer portion, or circumference, of
each wheel located on the shaft are blades where steam is directed
and converted into work by rotation of the shaft (Fig.1.29). There
are many blades in each turbine stage, and larger turbines have
more stages. (Fig.1. ). as the steam flows through the turbine, it
expands and its volume increases. This increased volume is
handled by having longer blades and thus a larger casing for each
stage of the turbine. Figure 1.31 and Figure 1. are a schematic
showing how the blade size varies as the steam flows through the
turbine. The turbine efficiency, as well as its reliable performance,
depends on the design and construction of the blades. Blades not
only must handle the steam velocity and temperature but also must
be able to handle the centrifugal force caused by the high speed of
the turbine. Any vibration in a turbine is significant because there
is little clearance between the moving blades and the stationary
portions on the casing. A vibration of the moving blades could
cause contact with the stationary components, which would result
in severe damage to the turbine. Vibration has to be monitored
continuously and corrected immediately when required.
Fig. Turbine blades of a turbine in Michigan State University
power plant ]
Shroud ring: It is placed around the blades outer ends (Fig.1.26).
The tips of the blades pass through holes in the shroud ring. The
Fig A Turbine blade
ends are then welded so that they are held securely by the ring.
When the blades are very long, extra lacing is sometimes used.
The function of shrouds:
a. Stiffen the blades against vibration
b. Confine the steam to the blade path and prevent steam
axial flow.
Fig. Double-flow low-pressure turbine showing variation in
blade size. (Power Magazine, a McGraw-Hill publication.)
Packing: (steam Sealing)
The shaft at the high-pressure end of the turbine must be packed to
prevent leakage of steam from the turbine. The one at the low-
pressure end of a condensing turbine must be packed to prevent the
leakage of air into the condenser.
There are external steam sealing (high pressure sealing at the
boiler side and low pressure sealing at the condenser side
Fig.1.3 ) and Interstage steam sealing Fig.1.3 .
Types of packing:
Fig. Turbine components
1: Shrouds
2: Diaphragm
3: Nozzles
4: wheel
5: Blades
6: Shaft
Labyrinth packing.
Water seals.
Carbon packing.
Flexible metallic packing.
Fig. external steam sealing [9]
Labyrinth packing
Labyrinth packing is used widely in steam turbine practice. It gets
its name from the fact that it is so constructed that steam in leaking
must follow a winding path and change its direction many times.
This device consists of a drum that turns with the shaft and is
grooved on the outside. The drum turns inside a stationary cylinder
Fig. Interstage steam sealing
that is grooved on the inside (Fig. Fig1.3 ). There are many
different types of labyrinth packing, but the general principle
involved is the same for all. Steam in leaking past the packing is
subjected to a throttling action. This action produces a reduction in
pressure with each groove that the steam passes. The amount of
leakage past the packing depends on the clearance between the
stationary and the rotating elements. The amount of clearance
necessary depends on the type of equipment, steam temperatures,
and general service conditions. The steam that leaks past the
labyrinth packing is piped to some low-pressure system or to a low
stage on the turbine.
Fig. (a) Water-sealed glands and labyrinth seals as used on
the high-pressure end of condensing turbines. (b) Labyrinth-type
gland as used on no condensing turbines. (Siemens Westinghouse
Power Corp.)
Water seals: A water-packed gland consists of a centrifugal-
pump runner attached to the turbine shaft. The runner rotates in a
chamber in the gland casing. In some designs, water is supplied to
the chamber at a pressure of 3 to 8 psi and is thrown out against the
sides by the runner, forming a seal. Water seals are used in
connection with labyrinth packing to prevent the steam that passes
the packing from leaking into the turbine room. Such a seal is also
used on the low-pressure end of condensing turbines. In this case
the leakage to the condenser is water instead of air.
Figure1.3 shows water-sealed glands and labyrinth seals as used
on the high-pressure end of condensing turbines. They are used
singly or in combination, depending on the service required. Each
labyrinth consists of a multiplicity of seals to minimize steam
leakage. The seal rings are spring backed and made of material that
permits close running clearances with safety. The glands are
usually supplied with condensate water for sealing to prevent
contamination of the condensate water. Seal designs are
continuously being improved to minimize steam leakage and thus
improve turbine performance. The illustrated designs are typical of
those found on operating turbines.
Carbon packing: Carbon packing is composed of rings of
carbon held against the shaft by means of springs. Each ring
fits into a separate groove in the gland casing. When
adjustments are made while the turbine is cold, carbon
packing should have from 0.001 to 0.002 in of clearance per
inch of shaft diameter. The width of the groove in the
packing casing should exceed the axial thickness of the
packing ring by about 0.005 in. Carbon packing is sometimes
used to pack the diaphragms of impulse turbines. Steam seals
are used in connection with carbon packing. This is essential
when carbon packing is used on the low-pressure end of
condensing turbines, because if there is a slight packing leak,
steam instead of air will leak into the condenser. In operating
a turbine equipped with carbon packing, a slight leak is
desirable because a small amount of steam keeps the packing
lubricated.
Flexible metallic packing: It is used to pack small single-
stage turbines operating at low backpressure. In most cases
the pressure in the casing of these turbines is only slightly
above atmospheric pressure. The application is the same as
when this packing is used for other purposes, except that care
must be exercised in adjusting. Due to the high speed at
which the shaft operates, even a small amount of friction will
cause overheating.
Bearings: Bearings support and/or properly position the
turbine rotor with respect to the stationary turbine parts.
Types of bearings:
Journal Bearings.
Thrust Bearings.
.1 Journal Bearing: Their main function is to the journal or
radial bearings support the weight of the rotor and position it
radially.
Utility turbines use journal bearing instead of ball or roller
bearings. Journal bearings have a smooth surface of a soft material
called Babbitt. The bearings are fed with oil as the rotor turns; it
produces a pumping action that builds up pressure and a film of oil
between the journal surface and the Babbitt so that in normal
operation the surfaces never touch. Figure 1.36 shows the pressure
distribution of the oil in the bearing.
Thrust Bearing: The thrust bearing absorbs axial forces on
the rotor and positions it axially with respect to the stationary
turbine parts.
The thrust bearing (see Figs. and ) consists of a collar
rigidly attached to the turbine shaft rotating between two Babbitt-
lined shoes. The clearance between the collar and the shoes is
small. The piston is attached to the spindle, and steam pressure is
Fig. Formation of Oil Film in Journal Bearing
exerted on one side and atmospheric pressure is exerted on the
other side. The difference in pressure produces a force that
balances the thrust exerted on the rotating blades. If the shaft starts
to move in either direction, the collar comes into contact with the
shoes, and the shaft is held in proper position. Larger thrust
bearings have several collars on the shaft and a corresponding
number of stationary shoes.
The Kingsbury thrust bearing (Fig. ) is used when a large
thrust load must be carried to maintain the proper axial position in
the turbine cylinder. (The one shown in Fig. 1.37 is a combination
of the Kingsbury and collar types.) The thrust collar is the same as
that used in the common type of thrust bearing. The thrust shoes
are made up of segments that are individually pivoted. With this
arrangement, the pressure is distributed equally not only between
the different segments but also on the individual segments. The
openings between the segments permit the oil to enter the bearing
surfaces. Almost 10 times as much pressure per square inch can be
carried on the Kingsbury-type bearing as on the ordinary thrust
bearing. Axial position of the bearing and turbine rotor may be
adjusted by liners, located at the retainer rings, on each end of the
bearing. The bearing is lubricated by circulating oil to all its
moving parts. The impulse turbine does not require as large a
thrust bearing as the reaction turbine because there is little or no
pressure drop through the rotating blades. However, the thrust
bearing must be used to ensure proper clearance between the
stationary and rotating elements. Reaction turbines that do not have
some method of balancing the force caused by the drop in pressure
in the rotating blades must be equipped with large thrust bearings.
Turbine bearings are subjected to very severe service and require
careful attention on the part of the operator. Most turbines operate
at high speed (3600 rpm) and are subjected to the heat generated in
the bearing itself as well as that received from the high-
temperature steam. These conditions make necessary some method
of cooling. In some cases the bearings are cooled by water
jacketing; in others the oil is circulated through a cooler.
Fig. Main and thrust bearings: (a) main bearing; (b) section of
thrust bearing and housing; (c) thrust bearing cage in place. (Siemens
Westinghouse Power Corp.)
Casing: Casings are steel castings whose purpose is to
support the rotor bearings and to have internal surfaces that
will efficiently assist in the flow of steam through the turbine. The
casing also supports the stationary blades and nozzles for all stages
and also it keeps the steam in the turbine and the air out. The
casing is divided into two halves upper casing and lower casing
Fig.1.39.
The HP/IP turbine always has shells or castings. When steam
pressures and temperatures are high enough, there are two shells
used to split up the pressure and temperature change. The inner
shells are supported and positioned within the outer shell. The
inner shells in turn support and position the other internals,
Fig. Turbine thrust end showing balance piston and thrust bearing.
diaphragms and labyrinth seals. The shells have bolted joints at
the horizontal centerline to permit assembly of the internals. In
operation, the shells are covered with insulation to prevent heat
loss.
The low pressure turbine always has inner and outer shells or
casings. Shells are most common in smaller and older units and
casings on larger newer units. The outer shell or casing prevents
air from entering the turbine exhaust and condenser and directs the
steam from the turbine exhaust to the condenser.
The exhaust hood is connected directly to the condenser, usually at
the end of turbine, and so is under a partial vacuum in operation.
There is a safety device (rapture disc) in the exhaust hood to
prevent excessive pressure buildup if the condenser loses its
vacuum.
1.8 Oil pumps: To pump the lubricating oil to the bearings.
Main oil pump.
AC auxiliary pump.
DC auxiliary pump.
Steam driven pump.
Hydraulically driven pump.
Fig.1.39 Single-casing condensing turbine for approximately35-MW
output. (Siemens Westinghouse Power Corp.).
Front standard: It is an extension to the turbine connected
to it through a key. Also it is not insulated.
The function of the front standard :
Support all control systems (Main oil pump, speed
governor, over speed trip and thrust wear detector).
Support all measuring equipments (Pressure indicators,
Temperature indicators, Speed indicators...).
Steam chest (Nozzle box): In high-temperature turbines
these components are separate from the main turbine structure. In
smaller units, the steam chest is usually mounted directly on the
casing.
It contains the inlet control valves and the admission nozzles
Fig.1. .
The steam chest and valve assembly shown in Fig.1.4 shows
another design, and the illustration identifies the major
components, including the steam inlet, the throttle valve, governor
valve, and valve actuators.
Fig.1 Turbine steam admission section. (Siemens Westinghouse Power
Corp.) . ]
The steam chest is bolted to the base and is made of iron or steel. It
contains a governor valve, a strainer, and an operating hand valve
that is used for manual adjustment to obtain maximum efficiency.
Regardless of whether or not a hand valve is provided, the steam is
made to pass through the governor-controlled admission valve
contained in the steam chest. These are only typical illustrations of
a small turbine design, since there are numerous designs with
different features that vary between manufacturers.
The multi valve steam chest (Fig.1.4 ) is cast integrally with the
cylinder cover with a cored passage from each valve to a nozzle
group. Single-seated valves are used, arranged in parallel within
the steam chest and surrounded by steam at throttle pressure. The
Fig.1. Turbine steam chest and valve assembly. (Siemens
Westinghouse Power Corp.). [1]
governor mechanism raises and lowers the valve-lift bar in a
horizontal plane, opening the valves in sequence, with an
unbalanced force tending to close the valves.
Turning gear:
If a turbine is shut down and the rotor was allowed to rest in one position then due to unequal heating the spindle bends. If the
turbine is a large one, vibration may occur when the turbine is
started again. For these reasons most large steam turbines are provided with motor driven gear to turn the rotor slowly while the
unit is out of service Fig.1.4 . and Fig. 1.44
Fig.1. Simplified steam chest with multiple valves. (Siemens
Westinghouse Power Corp.)
The function of turning gear:
Rotates the turbine rotor after shutdown at low speed.
Rotates the turbine rotor at low speed (20-30 rpm) before
starting up.
Decreases the starting torque.
Fig.1. Turning gear of a steam turbine at Michigan State University
power plant ]
Fig.1. Simplified Turning Gear
Recommended