CHAPTER-1 INTRODUCTION 1. INTRODUCTION The most critical aspect of steam turbine reliability centers on the bucket design. Since buckets, or rotating blades, are subjected to unsteady steam forces during operation, the phenomenon of vibration resonance must be considered. Resonance occurs when a stimulating frequency coincides with a natural frequency of the system. At resonance conditions, the amplitude of vibration is related primarily to the amount of stimulus and damping present in the system. High bucket reliability requires designs with minimum resonant vibration. The design process starts with accurate calculation of bucket natural frequencies in the tangential, axial, torsional, and complex modes, which are verified by test data. In addition, improved aerodynamic nozzle shapes and generous stage axial clearances are used to reduce bucket stimulus. Bucket covers are used on some or all stages to attenuate induced vibration. These design practices, together with advanced precision manufacturing techniques, ensure the necessary bucket reliability. Almost all of the blading used in modern mechanical drive steam turbines is either of drawn or milled type construction. Drawn blades are machined from extruded airfoil shaped pieces of material stock. Milled blades are machined from a rectangular piece of bar stock. As will be seen later, a certain percentage of steam turbine blades are neither drawn nor milled type construction. These blades are usually large, last-stage blades of steam turbines or jet gas expanders. They are either made by forging or a precision cast process. To keep the lowest natural frequency of the blades principally above the sixth harmonic frequency of the turbine speed, the aspect ratio, i.e., the ratio of blade length to 1
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39538340 Study of Manufacturing of Steam Turbine Blade
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CHAPTER-1
INTRODUCTION
1. INTRODUCTION
The most critical aspect of steam turbine reliability centers on the bucket design.
Since buckets, or rotating blades, are subjected to unsteady steam forces during
operation, the phenomenon of vibration resonance must be considered. Resonance occurs
when a stimulating frequency coincides with a natural frequency of the system. At
resonance conditions, the amplitude of vibration is related primarily to the amount of
stimulus and damping present in the system. High bucket reliability requires designs with
minimum resonant vibration. The design process starts with accurate calculation of
bucket natural frequencies in the tangential, axial, torsional, and complex modes, which
are verified by test data. In addition, improved aerodynamic nozzle shapes and generous
stage axial clearances are used to reduce bucket stimulus. Bucket covers are used on
some or all stages to attenuate induced vibration.
These design practices, together with advanced precision manufacturing
techniques, ensure the necessary bucket reliability. Almost all of the blading used in
modern mechanical drive steam turbines is either of drawn or milled type construction.
Drawn blades are machined from extruded airfoil shaped pieces of material stock. Milled
blades are machined from a rectangular piece of bar stock.
As will be seen later, a certain percentage of steam turbine blades are neither
drawn nor milled type construction. These blades are usually large, last-stage blades of
steam turbines or jet gas expanders. They are either made by forging or a precision cast
process.
To keep the lowest natural frequency of the blades principally above the sixth
harmonic frequency of the turbine speed, the aspect ratio, i.e., the ratio of blade length to
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profile chord length, is limited to a value below 5. In the transition zone, which is
particularly endangered by vibration failures, this ratio is further reduced. Transition zone
means the range of the turbine blading, which depending on the turbine operating point,
alternately admits superheated steam or wet steam. The operating point is determined by
the power generated by the turbine and the live steam conditions. As a general rule the
width of the axial gap between guide blades and moving blades is made at least 20
percent of the profile chord length.
The actual value may be larger and is determined by the expected relative
expansion between guide blades and moving blades. Manufacturers usually standardize
shroud dimensions for each profile chord length. The clearance between moving blade
shrouds and guide blade carrier, as well as between guide blade shrouds and rotor is
several millimeters. Sealing is effected by caulked-in sealing strips a few tenths of a
millimeter thick. The moving blades are held in the shaft groove by T-roots. Axial root
dimensions typically equal the profile chord length. All sizes of T-roots produced by a
given manufacturer are geometrically similar. For all the reaction blading only a single
profile shape and a single root shape is necessary.
Blade roots and shrouds are sometimes designed in rhomboid shape. The
rhomboid faces are ground and thus provide an optimal fit for the blade roots and blade
shrouds. Some notes on the stresses acting on the turbine blading will be of interest. The
turbine blading is subject to dynamic forces because the steam flow entering the rotor
blades in the circumferential direction is not homogeneous. Blades alternate with flow
passages so that the rotating blades pass areas of differing flow velocities and directions.
Since the forces affecting the rotor blading are generated by this flow, the blade stresses
also vary. The magnitude of the stress variation depends very much on the quality of the
blading. Poorly designed blading will often experience flow separation. This induces
particularly high bending stresses on the blades. Dynamic blade stresses are also
produced by ribs or other asymmetries in the flow area.
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If the steam turbine is driving a compressor, surge events can induce high
dynamic stresses in the rotor blades. These surges excite torsional vibrations of the
turbine rotor which in turn excite bending oscillations in the blades. The severity of the
alternating bending load in the blade due to the dynamic blade stresses depends on such
parameters as magnitude of the dynamic blade force, frequency level of the blade, and the
damping properties of the blade. The frequency level is determined by the ratio of natural
frequency to exciting frequency. With constant dynamic blade force the vibrational
amplitude and thus the bending load increase with the decreasing difference of these two
frequencies (resonance conditions). With a given dynamic blade force and a given
resonance condition the alternating bending stress is determined by the damping. Large
excitation forces and resonance conditions are not dangerous as long as the damping is
high. So much of the vibration energy is transformed into heat that the vibration
amplitude remains small.
The vibration of a blade is damped by the material-damping capacity, by the
damping at the blade root and by the steam surrounding the blade. All cylindrical blades
on drum rotors from such notable manufacturers as Siemens are machined with integral
shrouds. When the blades of a row are assembled, these shrouds are pressed against each
other and form a closed shroud ring. The complete shroudband links all blades of the
stage to a coupled vibration system whose natural frequencies are substantially higher
than those experienced by individual freestanding blades. The transmitted energy of a
vibration excitation into the linked blade system will be equally distributed to all blades
within a row; the entire blade row has to be excited. For comparison, in an unlinked
system (freestanding blades) the excitation energy will mainly be absorbed by the blade
that has a natural frequency equal to the excitation frequency. This blade is then
susceptible to breakage. Some considerations of the effect of narrow gaps, which may
form between the shrouds during operation, are given as follows (Fig. 6.12).
Gaps could occur by:
1. Insertion of blades made from martensitic material (chrome steel) into a shaft
made from ferritic material. The ferritic shaft material has a higher thermal
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expansion coefficient than the martensitic material. As shaft and blading heat up,
there will be a proportionally larger expansion of the shroud in the radial direction
than in the circumferential direction.
2. Expansion of shaft and lengthening of blades due to the centrifugal force at
operating speed. Gap formation will be eliminated through selection of suitable
root and shroud geometry. Assembly-related forces on blade roots in the
circumferential direction cause a small angular deflection in the blade
profile/shroud section. In a completed blade row the counteracting torsional
moment from each blade to its respective shroud prevents the formation of gaps
as described by effects 1 and 2.
If the prestress in the shroud area is still not sufficient and gaps form because of
extreme changes of the steam temperature in the blading, the vibration behavior of the
circumferential unlinked shroudband is still substantially different from that of a row of
freestanding blades. All drum rotor blades have manufacturing and assembling
tolerances, which cause the natural frequencies of the blades of a rotating row to be
spread over a wide range. Therefore it is statistically impossible for all blades to get into
resonance simultaneously. The blade that is exactly in resonance is prevented from
developing its maximum resonance amplitude by the neighboring blade, which is not in
resonance. The shrouds of the neighboring blades act as amplitude limiters, and the
vibration energy is transformed into heat by impact forces.
Energy is also dissipated from vibration amplitude by the following effect:
Because of machining tolerances the existing gaps are not of uniform width, but wedge-
shaped, crowned or another shape. This, for instance, causes the energy of vibration
about the axis of minimum inertia to be partly converted into a torsional vibration by
impact against the neighboring shroud. The available vibration energy is thus distributed
over several forms of vibration so that the maximum possible amplitude is decreased.
With existing gaps the shrouds act as amplitude limiters and vibration converters. The
shrouds add further to the operational safety because there is a wide radial gap between
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shrouding and guide blade carrier. If because of a drop of the steam temperature the rotor
or the casing should suffer distortion, the thin sealing strips are damaged without
generation of excessive friction heat, but the radial clearance is never taken up so that the
rotor cannot touch the casing.
1.1 TURBINE
A turbine is a device that converts chemical energy into mechanical energy,
specifically when a rotor of multiple blades or vanes is driven by the movement of a fluid
or gas. In the case of a steam turbine, the pressure and flow of newly condensed steam
rapidly turns the rotor. This movement is possible because the water to steam conversion
results in a rapidly expanding gas. As the turbine’s rotor turns, the rotating shaft can work
to accomplish numerous applications, often electricity generation.
Fig 1 Sectional View of a Steam turbine
In a steam turbine, the steam’s energy is extracted through the turbine and the
steam leaves the turbine at a lower energy state. High pressure and temperature fluid at
the inlet of the turbine exit as lower pressure and temperature fluid. The difference is
energy converted by the turbine to mechanical rotational energy, less any aerodynamic
and mechanical inefficiencies incurred in the process. Since the fluid is at a lower
pressure at the exit of the turbine than at the inlet, it is common to say the fluid has been
“expanded” across the turbine. Because of the expanding flow, higher volumetric flow
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occurs at the turbine exit (at least for compressible fluids) leading to the need for larger
turbine exit areas than at the inlet.
The generic symbol for a turbine used in a flow diagram is shown in Figure
below. The symbol diverges with a larger area at the exit than at the inlet. This is how
one can tell a turbine symbol from a compressor symbol. In Figure , the graphic is
colored to indicate the general trend of temperature drop through a turbine. In a turbine
with a high inlet pressure, the turbine blades convert this pressure energy into velocity or
kinetic energy, which causes the blades to rotate. Many green cycles use a turbine in this
fashion, although the inlet conditions may not be the same as for a conventional high
pressure and temperature steam turbine. Bottoming cycles, for instance, extract fluid
energy that is at a lower pressure and temperature than a turbine in a conventional power
plant. A bottoming cycle might be used to extract energy from the exhaust gases of a
large diesel engine, but the fluid in a bottoming cycle still has sufficient energy to be
extracted across a turbine, with the energy converted into rotational energy.
Fig 2 Flow diagram of a steam turbine
Turbines also extract energy in fluid flow where the pressure is not high but
where the fluid has sufficient fluid kinetic energy. The classic example is a wind turbine,
which converts the wind’s kinetic energy to rotational energy. This type of kinetic energy
conversion is common in green energy cycles for applications ranging from larger wind
turbines to smaller hydrokinetic turbines currently being designed for and demonstrated
in river and tidal applications. Turbines can be designed to work well in a variety of
fluids, including gases and liquids, where they are used not only to drive generators, but
also to drive compressors or pumps.
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One common (and somewhat misleading) use of the word “turbine” is “gas
turbine,” as in a gas turbine engine. A gas turbine engine is more than just a turbine and
typically includes a compressor, combustor and turbine combined to be a self-contained
unit used to provide shaft or thrust power. The turbine component inside the gas turbine
still provides power, but a compressor and combustor are required to make a self-
contained system that needs only the fuel to burn in the combustor.
An additional use for turbines in industrial applications that may also be
applicable in some green energy systems is to cool a fluid. As previously mentioned,
when a turbine extracts energy from a fluid, the fluid temperature is reduced. Some
industries, such as the gas processing industry, use turbines as sources of refrigeration,
dropping the temperature of the gas going through the turbine. In other words, the
primary purpose of the turbine is to reduce the temperature of the working fluid as
opposed to providing power. Generally speaking, the higher the pressure ratio across a
turbine, the greater the expansion and the greater the temperature drop. Even where
turbines are used to cool fluids, the turbines still produce power and must be connected to
a power absorbing device that is part of an overall system.
Also note that turbines in high inlet-pressure applications are sometimes called
expanders. The terms “turbine” and “expander” can be used interchangeably for most
applications, but expander is not used when referring to kinetic energy applications, as
the fluid does not go through significant expansion.
1.2 TYPES OF STEAM TURBINES
There are complicated methods to properly harness steam power that give rise to
the two primary turbine designs: impulse and reaction turbines. These different designs
engage the steam in a different method so as to turn the rotor. As water converts into
steam, the molecules grow further apart.
While steam can exert pressure, it cannot exert the correct pressure needed to spin
the rotor quickly enough to generate electricity. Thus, a special design of rotor is required
to properly harness the steam and spin.
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In an impulse turbine, nozzles direct the steam towards the rotors, which are
equipped with concave panels called buckets. The nozzles are able to project a jet of
steam that spins the rotor at a loss of roughly 10 percent energy. As the jets change their
position, they can increase or decrease the rate of rotor spin.
A reaction turbine works opposite the impulse turbine. The steam nozzles are
attached to the rotor blades on opposite sides. The nozzles are so positioned that when
they release jets of stream, they propel the rotor in a spinning motion that keeps it
rotating as long as steam is being expelled.
It can reach high speeds because the nozzle designs focus the steam into a thin
stream, although the initial warm up period may take several moments.
Fig 3 High inlet pressure reaction turbine, back-pressure type
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1.3 TURBINE FUNCTION
There are different methods for producing steam to propel a steam turbine.
Condensing steam turbines typically employ low-pressure steam that is not fully
condensed—it is usually approximately 90 percent steam. When steam has lower
pressure than the atmospheric pressure surrounding it, it can be expanded to a greater
degree for turning standard piston engines. Non-condensing steam turbines also work
with low pressure steam, usually at refineries or pulping plants, where low pressure steam
is typically available. These turbines take advantage of exhaust steam, a product of other
applications.
Turbines also require a governor, or speed limiter, which controls the speed of the
rotor rotation. Turbines require a slow warm up period to prevent accidents or damage.
The governor can control the pressure and amount of steam emitted so as to properly
monitor and control the speed of the spinning rotors. This is necessary in applications like
electrical generation. The electrical grid in the United States and in other countries
utilizes droop speed control. When a plant is functioning in a full-load output capacity, it
runs at 100 percent speed, while it runs at 105 percent speed when at no-load. The speed
variance is required because of the myriad power plants operating simultaneously, which
need to provide dependable frequency despite constant changes, drop offs and
capabilities of power.
1.4 PARTS OF STEAM TURBINE
Steam turbines are the most common and versatile prime movers used today. The
capabilities and flexibility of operation, as well as the range of power provided is
unparalleled in today's power generation and process markets. The components of Steam
Turbine are:
(1) The rotor that carries the blading to convert the thermal energy of the steam