14-07-2012, 02:50 PM
critical aspect of steam turbine
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. 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.
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 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. 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.
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 shroud band 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.
Gaps could occur by:
a) Insertion of blades made from martensitic material (chrome steel) into a shaft made from ferritic material. The ferritic shaft material has a higher thermal 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.
b) 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.
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.
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 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.
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.
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.
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.
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.