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
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 3
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 wedgeshaped, 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 4
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 5
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 selfcontained 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 into the rotary motion of the shaft.
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(2) The casing, inside of which the rotor turns, that serves as a pressure vessel for containing the steam (it also accommodates fixed nozzle passages or stator vanes through which the steam is accelerated before being directed against and through the rotor blading) (3) The speed-regulating mechanism and (4) The support system, which includes the lubrication system for the bearings that support the rotor and also absorb any end thrust developed.
Steam turbines consist of circularly distributed stationary blades called nozzles which direct steam on to rotating blades or buckets mounted radially on a rotating wheel. In a steam turbine nozzles apply supersonic steam to a curved blade. The blade whips the steam back in the opposite direction, simultaneously allowing the steam to expand a bit. A stationary blade then redirects the steam towards the next blade. The process repeats until the steam is completely expanded. The moving blades are mounted radially on the rotor. The stationary blades are mounted to the case of the turbine. A compact machine can be built economically with ten or more stages for optimum use of high pressure steam and vacuum exhaust by mounting the wheels of a number of stages on a single shaft, and supporting the nozzles of all stages from a continuous housing. Large axial turbines must be operated under such conditions that the exhaust steam does not contain more than 10 to 13% of liquid since condensate droplets could seriously erode the high velocity nozzles and blades. The moisture content of the exhaust is dependent upon the inlet steam pressure/temperature combination. Special moisture removal stages may be incorporated in the design when the steam superheat temperature is limited.
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CHAPTER-2 LITERATURE SURVEY 2.1 STEAM TURBINE BLADE Blades are the heart of a steam turbine, as they are the principal elements that convert the thermal energy into kinetic energy. The efficiency and reliability of a turbine depend on the proper design of the blades. It is therefore necessary for all engineers involved in the steam turbines engineering to have an overview of the importance and the basic design aspects of the steam turbine blades.
Fig 4 View of blades on a rotor
Blade design is a multi-disciplinary task. It involves the thermodynamic, aerodynamic, mechanical and material science disciplines. A total development of a new blade is therefore possible only when experts of all these fields come together as a team. 11
The development process of a new profile took years of development and testing in the earlier years. But with the advent of CFD and FEM packages, there is a significant reduction in design and testing times. The feasibility of 3-D designs also has improved because of the advances in these software packages. This paper deals mainly with the mechanical aspects of the blade design. It aims mainly at understanding the principles of design of the existing blades, and giving an overview of other related issues to blades which a designer should be aware of.
2.2 CONSTRUCTIONAL FEATURES OF A BLADE The blade can be divided into 3 parts:
The profile, which converts the thermal energy of steam into kinetic energy, with a certain efficiency depending upon the profile shape.
The root, which fixes the blade to the turbine rotor, giving a proper anchor to the blade, and transmitting the kinetic energy of the blade to the rotor.
The damping element, which reduces the vibrations which necessarily occur in the blades due to the steam flowing through the blades. These damping elements may be integral with blades, or they may be separate elements mounted between the blades.
Each of these elements will be separately dealt with in the following sections.
2.2.1 H.P. BLADE PROFILES In order to understand the further explanation, a familiarity of the terminology used is required. The following terminology is used in the subsequent sections.
Fig 5 High Pressure Blade Profile 12
If circles are drawn tangential to the suction side and pressure side profiles of a blade, and their centers are joined by a curve, this curve is called the camber line. This camber line intersects the profile at two points A and B. The line joining these points is called chord, and the length of this line is called the chord length. A line which is tangential to the inlet and outlet edges is called the bitangent line. The angle which this line makes with the circumferential direction is called the setting angle. Pitch of a blade is the circumferential distance between any point on the profile and an identical point on the next blade.
2.2.2 CLASSIFICATION OF PROFILES There are two basic types of profiles - Impulse and Reaction. In the impulse type of profiles, the entire heat drop of the stage occurs only in the stationary blades. In the reaction type of blades, the heat drop of the stage is distributed almost equally between the guide and moving blades. Though the theoretical impulse blades have zero pressure drop in the moving blades, practically, for the flow to take place across the moving blades, there must be a small pressure drop across the moving blades also. Therefore, the impulse stages in practice have a small degree of reaction. These stages are therefore more accurately, though less widely, described as low-reaction stages. The typical impulse and reaction stages are plotted in the following figure.
Impulse stage
Reaction stage
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The presently used reaction profiles are more efficient than the impulse profiles at part loads. This is because of the more rounded inlet edge for reaction profiles. Due to this, even if the inlet angle of the steam is not tangential to the pressure-side profile of the blade, the losses are low. However, the impulse profiles have one advantage. The impulse profiles can take a large heat drop across a single stage, and the same heat drop would require a greater number of stages if reaction profiles are used, thereby increasing the turbine length. The Steam turbines use the impulse profiles for the control stage (1st stage), and the reaction profiles for subsequent stages. There are three reasons for using impulse profile for the first stage. a)
Most of the turbines are partial arc admission turbines. If the first stage is a reaction stage, the lower half of the moving blades do not have any inlet steam, and would ventilate. Therefore, most of the stage heat drop should occur in the guide blades.
b)
The heat drop across the first stage should be high, so that the wheel chamber of the outer casing is not exposed to the high inlet parameters. In case of -4 turbines, the inner casing parting plane strength becomes the limitation, and therefore requires a large heat drop across the 1st stage.
c) Nozzle control gives better efficiency at part loads than throttle control. d) The number of stages in the turbine should not be too high, as this will increase the length of the turbine. There are exceptions to the rule. Turbines used for CCPs, and BFP drive turbines do not have a control stage. They are throttle-governed machines. Such designs are used when the inlet pressure slides. Such machines only have reaction stages. However, the inlet passages of such turbines must be so designed that the inlet steam to the first reaction stage is properly mixed, and occupies the entire 360 degrees.
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There are also cases of controlled extraction turbines where the L.P. control stage is an impulse stage. This is either to reduce the number of stages to make the turbine short, or to increase the part load efficiency by using nozzle control, which minimises throttle losses.
2.2.3 H.P. BLADE ROOTS The root is a part of the blade that fixes the blade to the rotor or stator. Its design depends upon the centrifugal and steam bending forces of the blade. It should be designed such that the material in the blade root as well as the rotor / stator claw and any fixing element are in the safe limits to avoid failure. The roots are T-root and Fork-root. The fork root has a higher load-carrying capacity than the T-root.It was found that machining this T-root with side grip is more of a problem. It has to be machined by broaching, and the broaching machine available could not handle the sizes of the root. The typical roots used for the HP moving blades for various steam turbine applications are shown in the following figure:
T-root
Fork-root T-root with side-grip Fig 6 High pressure blade roots
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2.2.4 L.P. BLADE PROFILES The LP blade profiles of moving blades are twisted and tapered. These blades are used when blade height-to-mean stage diameter ratio (h/Dm) exceeds 0.2.
2.2.5 LP BLADE ROOTS The roots of LP blades are as follows: 1) –2 blading: The roots of both the LP stages in –2 type of LP blading are T-roots. 2) –3 blading: The last stage LP blade of HK, SK and LK blades have a fork-root. SK blades have 4-fork roots for al sizes. HK blades have 4-fork roots upto 56 size, where modified profiles are used. Beyond this size, HK blades have 3 fork roots. LK blades have 3-fork roots for all sizes. The roots of the LP blades of preceding stages are of T-roots.
2.3 DYNAMICS IN BLADES The excitation of any blade comes from different sources. They are: a) nozzle-passing excitation: As the blades pass the nozzles of the stage, they encounter flow disturbances due to the pressure variations across the guide blade passage. They also encounter disturbances due to the wakes and eddies in the flow path. These are sufficient to cause excitation in the moving blades. The excitation gets repeated at every pitch of the blade. This is called nozzle-passing frequency excitation. The order of this frequency = no. of guide blades x speed of the machine. Multiples of this frequency are considered for checking for resonance. b) excitation due to non-uniformities in guide-blades around the periphery. These can occur due to manufacturing inaccuracies, like pitch errors, setting angle variations, inlet and outlet edge variations, etc. 16
For HP blades, due to the thick and cylindrical cross-sections and short blade heights, the natural frequencies are very high. Nozzle-passing frequencies are therefore necessarily considered, since resonance with the lower natural frequencies occurs only with these orders of excitation. In LP blades, since the blades are thin and long, the natural frequencies are low. The excitation frequencies to be considered are therefore the first few multiples of speed, since the nozzle-passing frequencies only give resonance with very high modes, where the vibration stresses are low. The HP moving blades experience relatively low vibration amplitudes due to their thicker sections and shorter heights. They also have integral shrouds. These shrouds of adjacent blades butt against each other forming a continuous ring. This ring serves two purposes – it acts as a steam seal, and it acts as a damper for the vibrations. When vibrations occur, the vibration energy is dissipated as friction between shrouds of adjacent blades. For HP guide blades of Wesel design, the shroud is not integral, but a shroud band is riveted to a number of guide blades together. The function of this shroud band is mainly to seat the steam. In some designs HP guide blades may have integral shrouds like moving blades. The primary function remains steam sealing. In industrial turbines, in LP blades, the resonant vibrations have high amplitudes due to the thin sections of the blades, and the large lengths. It may also not always be possible to avoid resonance at all operating conditions. This is because of two reasons. Firstly, the LP blades are standardised for certain ranges of speeds, and turbines may be selected to operate anywhere in the speed range. The entire design range of operating speed of the LP blades cannot be outside the resonance range. It is, of course, possible to design a new LP blade for each application, but this involves a lot of design efforts and manufacturing cycle time. However, with the present-day computer packages and manufacturing methods, it has become feasible to do so. 17
Secondly, the driven machine may be a variable speed machine like a compressor or a boiler-feed-pump. In this case also, it is not possible to avoid resonance. In such cases, where it is not possible to avoid resonance, a damping element is to be used in the LP blades to reduce the dynamic stresses, so that the blades can operate continuously under resonance also. There may be blades which are not adequately damped due to manufacturing inaccuracies. The need for a damping element is therefore eliminated. In case the frequencies of the blades tend towards resonance due to manufacturing inaccuracies, tuning is to be done on the blades to correct the frequency. This tuning is done by grinding off material at the tip (which reduces the inertia more than the stiffness) to increase the frequency, and by grinding off material at the base of the profile (which reduces the stiffness more than the inertia) to reduce the natural frequency. The damping in any blade can be of any of the following types: a) Material damping: This type of damping is because of the inherent damping properties of the material which makes up the component. b) Aerodynamic damping: This is due to the damping of the fluid which surrounds the component in operation. c) Friction damping: This is due to the rubbing friction between the component under consideration with any other object. Out of these damping mechanisms, the material and aerodynamic types of damping are very small in magnitude. Friction damping is enormous as compared to the other two types of damping. Because of this reason, the damping elements in blades generally incorporate a feature by which the vibrational energy is dissipated as frictional heat. The frictional damping has a particular characteristic. When the frictional force between the rubbing surfaces is very small as compared to the excitation force, the surfaces slip, resulting in friction damping. However, when the excitation force is small when compared to the frictional force, the surfaces do not slip, resulting in locking of the 18
surfaces. This condition gives zero friction damping, and only the material and aerodynamic damping exists. In a periodically varying excitation force, it may frequently happen that the force is less than the friction force. During this phase, the damping is very less. At the same time, due to the locking of the rubbing surfaces, the overall stiffness increases and the natural frequency shifts drastically away from the individual value. The response therefore also changes in the locked condition. The resonant response of a system therefore depends upon the amount of damping in the system (which is determined by the relative duration of slip and stick in the system, i.e., the relative magnitude of excitation and friction forces) and the natural frequency of the system (which alters between the individual values and the locked condition value, depending upon the slip or stick condition).
2.4 BLADING MATERIALS Among the different materials typically used for blading are 403 stainless steel, 422 stainless steel, A-286, and Haynes Stellite Alloy Number 31 and titanium alloy. The 403 stainless steel is essentially the industry’s standard blade material and, on impulse steam turbines, it is probably found on over 90 percent of all the stages. It is used because of its high yield strength, endurance limit, ductility, toughness, erosion and corrosion resistance, and damping. It is used within a Brinell hardness range of 207 to 248 to maximize its damping and corrosion resistance. The 422 stainless steel material is applied only on high temperature stages (between 700 and 900°F or 371 and 482°C), where its higher yield, endurance, creep and rupture strengths are needed. The A-286 material is a nickel-based super alloy that is generally used in hot gas expanders with stage temperatures between 900 and 1150°F (482 and 621°C). The Haynes Stellite Alloy Number 31 is a cobalt-based super alloy and is used on jet expanders when precision cast blades are needed. The Haynes Stellite Number 31 is used at stage temperatures between 900 and 1200°F (482 and 649°C). Another blade material is titanium. Its high strength, low density, and good erosion resistance make it a good candidate for highspeed or long-last stage blading. 19
CHAPTER-3 MANUFACTURING PROCESS 3.1 INTRODUCTION Manufacturing process is that part of the production process which is directly concerned with the change of form or dimensions of the part being produced. It does not include the transportation, handling or storage of parts, as they are not directly concerned with the changes into the form or dimensions of the part produced. Manufacturing is the backbone of any industrialized nation. Manufacturing and technical staff in industry must know the various manufacturing processes, materials being processed, tools and equipments for manufacturing different components or products with optimal process plan using proper precautions and specified safety rules to avoid accidents. Beside above, all kinds of the future engineers must know the basic requirements of workshop activities in term of man, machine, material, methods, money and other infrastructure facilities needed to be positioned properly for optimal shop layouts or plant layout and other support services effectively adjusted or located in the industry or plant within a well planned manufacturing organization. Today’s competitive manufacturing era of high industrial development and research, is being called the age of mechanization, automation and computer integrated manufacturing. Due to new researches in the manufacturing field, the advancement has come to this extent that every different aspect of this technology has become a fullfledged fundamental and advanced study in itself. This has led to introduction of optimized design and manufacturing of new products. New developments in manufacturing areas are deciding to transfer more skill to the machines for considerably reduction of manual labor. 20
3.2 CLASSIFICATION OF MANUFACTURING PROCESSES For producing of products materials are needed. It is therefore important to know the characteristics of the available engineering materials. Raw materials used manufacturing of products, tools, machines and equipments in factories or industries are extracted from ores. The ores are suitably converted the metal into a molten form by reducing or refining processes in foundries. This molten metal is poured into moulds for providing commercial castings, called ingots. Such ingots are then processed in rolling mills to obtain market form of material supply in form of bloom, billets, slabs and rods. These forms of material supply are further subjected to various manufacturing processes for getting usable metal products of different shapes and sizes in various manufacturing shops. All these processes used in manufacturing concern for changing the ingots into usable products may be classified into six major groups as
primary shaping processes
secondary machining processes
metal forming processes
joining processes
surface finishing processes and
processes effecting change in properties.
3.2.1 PRIMARY SHAPING PROCESSES Primary shaping processes are manufacturing of a product from an amorphous material. Some processes produces finish products or articles into its usual form whereas others do not, and require further working to finish component to the desired shape and size. The parts produced through these processes may or may not require to under go further operations. Some of the important primary shaping processes are: (1) Casting (2) Powder metallurgy 21
(3) Plastic technology (4) Gas cutting (5) Bending and (6) Forging.
3.2.2 SECONDARY OR MACHINING PROCESSES As large number of components require further processing after the primary processes. These components are subjected to one or more number of machining operations in machine shops, to obtain the desired shape and dimensional accuracy on flat and cylindrical jobs. Thus, the jobs undergoing these operations are the roughly finished products received through primary shaping processes. The process of removing the undesired or unwanted material from the work-piece or job or component to produce a required shape using a cutting tool is known as machining. This can be done by a manual process or by using a machine called machine tool (traditional machines namely lathe, milling machine, drilling, shaper, planner, slotter). In many cases these operations are performed on rods, bars and flat surfaces in machine shops. These secondary processes are mainly required for achieving dimensional accuracy and a very high degree of surface finish. The secondary processes require the use of one or more machine tools, various single or multi-point cutting tools (cutters), job holding devices, marking and measuring instruments, testing devices and gauges etc. for getting desired dimensional control and required degree of surface finish on the workpieces. The example of parts produced by machining processes includes hand tools machine tools instruments, automobile parts, nuts, bolts and gears etc. Lot of material is wasted as scrap in the secondary or machining process. Some of the common secondary or machining processes are: a. Turning b. Threading c. Knurling 22
d. Milling e. Drilling f. Boring g. Planning h. Shaping i. Slotting j. Sawing k. Broaching l. Hobbing m. Grinding n.
Gear Cutting
o. Thread cutting and p. Unconventional machining processes namely machining with Numerical Control (NC) machines tools or Computer Numerical Control (CNC) machine tools using ECM, LBM, AJM, USM setups etc.
3.3 MILLING 3.3.1 INTRODUCTION A milling machine is a machine tool that removes metal as the work is fed against a rotating multipoint cutter. The milling cutter rotates at high speed and it removes metal at a very fast rate with the help of multiple cutting edges. One or more number of cutters can be mounted simultaneously on the arbor of milling machine. This is the reason that a milling machine finds wide application in production work. Milling machine is used for machining flat surfaces, contoured surfaces, surfaces of revolution, external and internal threads, and helical surfaces of various cross-sections. In many applications, due to its higher production rate and accuracy, milling machine has even replaced shapers and slotters.
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Milling can be used to produce a practically infinite variety of workpiece surfaces. A distinguishing feature of a process is the cutting edge (major or minor) that produces the workpiece surface in face milling the minor cutting edge is located at the face of the milling cutter, while in peripheral milling the major cutting edge is located on the circumference of the milling cutter. A distinction can be made on the basis of the feed direction angle ϕ in down-milling the feed direction angle ϕ is > 90◦, thus the cutting edge of the milling cutter enters the workpiece at the maximum undeformed chip thickness, while in up-milling the feed direction angle ϕ is < 90◦, thus the cutting edge enters at the theoretical undeformed chip thickness h = 0. This initially results in pinching and rubbing.
Fig 7 Milling machine
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Fig 8 Job surfaces generated by milling machine
3.3.2 PRINCIPLE OF MILLING In milling machine, the metal is cut by means of a rotating cutter having multiple cutting edges. For cutting operation, the work piece is fed against the rotary cutter. As the work piece moves against the cutting edges of milling cutter, metal is removed in form chips of trochoid shape. Machined surface is formed in one or more passes of the work. The work to be machined is held in a vice, a rotary table, a three jaw chuck, an index head, between centers, in a special fixture or bolted to machine table. The rotator speed of the cutting tool and the feed rate of the workpiece depend upon the type of material being machined.
3.3.3 MILLING METHODS The milling process is broadly classified into peripheral milling and face milling. In peripheral milling, the cutting edges are primarily on the circumference or periphery of the milling cutter and the milled surface is generally parallel to cutter axis. In face milling, although the cutting edges are provided on the face as well as the periphery of
25
the cutter, the surface generated is parallel to the face of the cutter and is perpendicular to the cutter axis.
Fig 9 Different Milling methods
3.3.4 TOOLS USED FOR MILLING
Fig 10 Milling cutters
26
Fig 11 Different types of Milling cuts at a glance
3.3.5 MILLING CUTTER TERMS
Fig 12 Milling cutter terms 27
3.3.6 Milling Calculations
The following calculation methods and procedures for milling operations are intended to be guidelines and not absolute because of the many variables encountered in actual practice. Metal-Removal RatesThe metal-removal rate R (sometimes indicated as mrr) for all types of milling is equal to the volume of metal removed by the cutting process in a given time, usually expressed as cubic inches per minute (in3/min).Thus,
R = WHf where R = metal-removal rate, in3/min. W = width of cut, in H = depth of cut, in f = feed rate, inches per minute (ipm) In peripheral or slab milling, W is measured parallel to the cutter axis and H perpendicular to the axis. In face milling,W is measured perpendicular to the axis and H parallel to the axis. Feed RateThe speed or rate at which the workpiece moves past the cutter is the feed rate f, which is measured in inches per minute (ipm).Thus,
f = Ft NC
rpm
Where, f = feed rate, ipm Ft = feed per tooth (chip thickness), in or cpt 28
N = number of cutter teeth Crpm = rotation of the cutter, rpm Feed per ToothProduction rates of milled parts are directly related to the feed rate that can be used. The feed rate should be as high as possible, considering machine rigidity and power available at the cutter. To prevent overloading the machine drive motor, the feed per tooth allowable Ft may be calculated from
Ft
=
Khp c NC rpm WH
where hpc = horsepower available at the cutter (80 to 90 percent of motor rating), i.e., if motor nameplate states 15 hp, then hp available at the cutter is 0.8 to 0.9 × 15 (80 to 90 percent represents motor efficiency) K = machinability factor Cutting SpeedThe cutting speed of a milling cutter is the peripheral linear speed resulting from the rotation of the cutter.The cutting speed is expressed in feet per minute (fpm or ft/min) or surface feet per minute (sfpm or sfm) and is determined from
S=
πD (rpm ) 12
where S = cutting speed, fpm or sfpm (sfpm is also termed spm) D = outside diameter of the cutter, in rpm = rotational speed of cutter, rpm
29
The required rotational speed of the cutter may be found from the following simple equation:
rpm =
S S or (D / 12 )π 0.26 D
When it is necessary to increase the production rate, it is better to change the cutter material rather than to increase the cutting speed. Increasing the cutting speed alone may shorten the life of the cutter, since the cutter is usually being operated at its maximum speed for optimal productivity. General Rules for Selection of the Cutting Speed _ Use lower cutting speeds for longer tool life. _ Take into account the Brinell hardness of the material. _ Use the lower range of recommended cutting speeds when starting a job. _ For a fine finish, use a lower feed rate in preference to a higher cutting speed. Number of Teeth: CutterThe number of cutter teeth N required for a particular application may be found from the simple expression (not applicable to carbide or other high-speed cutters)
N =
f Ft C rpm
where f = feed rate, ipm Ft = feed per tooth (chip thickness), in Crpm = rotational speed of cutter, rpm N = number of cutter teeth. An industry-recommended equation for calculating the number of cutter teeth required for a particular operation is
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N =19 .5 R −5.8
where N = number of cutter teeth R = radius of cutter, in This simple equation is suitable for HSS cutters only and is not valid for carbide, cobalt cast alloy, or other high-speed cutting tool materials. Milling HorsepowerRatios for metal removal per horsepower (cubic inches per minute per horsepower at the milling cutter) have been given for various materials. The general equation is
K=
in 3 / min WHf = hp c hp c
where K = metal removal factor, in3/min/hpc hpc = horsepower at the cutter W = width of cut, in H = depth of cut, in f = feed rate, ipm. The total horsepower required at the cutter may then be expressed as
hp c =
in 3 / min WHf = K K
The K factor varies with type and hardness of material, and for the same material varies with the feed per tooth, increasing as the chip thickness increases.The K factor represents a particular rate of metal removal and not a general or average rate.
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3.4 POLISHING
Polishing is usually a multistage process. The first stage starts with a rough abrasive and each subsequent stage uses a finer abrasive until the desired finish is achieved. The rough pass removes surface defects like pits, nicks, lines and scratches. The finer abrasives leave very thin lines that are not visible to the naked eye. Lubricants like wax and kerosene are used as lubricating and cooling media during these operations. Polishing operations for items such as chisels, hammers, screwdrivers, wrenches, etc., are given a fine finish but not plated. In order to achieve this finish four operations are required: roughing, dry fining, greasing, and coloring. For an extra fine polish the greasing operation may be broken up into two operations: rough greasing and fine greasing.
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CHAPTER-4 EXPERIMENTAL WORK 4.1 MANUFACTURING OF A STEAM TURBINE BLADE
Fig 13 Machining steps in the manufacturing of milled blades
The following steps are involved in the machining of a Steam Turbine Blade: 1.
Size Milling 33
2.
Size Grinding
3.
Facing
4.
Root Bottom Width Milling
5.
Neck Milling
6.
Total Length Milling
7.
Convex Profile
8.
Concave Profile
9.
Pitch Milling
10.
Pitch Grinding
11.
Finishing
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Fig 14 Size Milling of the raw material work piece
The fixturing elements at the head and root of the blade structure are ultimately removed to leave the final shaped item, but during the machining process itself their accuracy and form have a crucial impact on the success of the overall operation. •
For Root Rectangle, It is done by standard endmill- only roughing necessary.
•
Now for Root Trapezoid
or dovetail, special roughing and finishing
necessary. •
The Head Counter sinking is carried out by counter-bore tool.
•
The Head rectangle Standard is done by standard end-mill and only roughing is required.
•
Now the Head Cylindrical shape is done by the standard end-mill followed by roughing and finishing.
Whichever processing methods are employed, the first step is to machine the reference surfaces by which the workpiece will be clamped during the subsequent machining. Several Coromant tools are suitable for this operation, and the CoroMill 390 long edge cutter is particularly recommended. CoroMill 200, 300 and 390 are also good alternatives. It may also be possible in this operation to also machine the clearances necessary for subsequent processes, if the machining strategy would benefit from this.
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Fig 15 Bending of work piece
It is possible that the blade workpiece may deform or bend during subsequent stages of the machining process, the result of machining away 80% of the original rolled or annealed raw material and the residual stresses thus created. This is particularly possible for large blades, 400–600 mm long, which may bend by as much as 2 mm. Reworking the fixturing elements during the machining process, so that the position of the workpiece in the machining centres is modified to account for the deformation, can counteract this phenomenon. The recommended procedure for such reworking on a 5-axis machine is: _ opening the fixturing system on the blade head and moving it back, so that the blade is now secured only by the root. _ creating a new centre line for the workpiece, by counterboring or turnmilling. _ fixing the blade by the new element. An alternative is to modify the adaptor itself, so that the position of the workpiece is suitably adjusted when the modified adaptor is held in the machine, without any changes to the fixturing elements.
Machining the root of the blade The machining process to shape the root of the blade will depend on several factors, notably the dimensions of the finished item. Small blades are often machined directly from round bar stock, which is then is milled to a square shape. 160 mm Larger 36
blades are often made from rectangular bar stock or forging. Normally these blades are first machined with cutting tools, and then broached or ground. Turbine blades can be divided into two classes, stator and rotor blades, and in normal practice these two designs have different mounting systems and different styles of root, to accommodate the different loadings they receive in use. Stator blades normally have one small slot in one side of the root, which is relatively easy to machine with solid carbide or indexable insert endmills. Rotor blades may have different mounting systems, such as a “Christmas tree” profile, or deep slots machined in a trapezoidal cross-section. These variations in the profile and geometry of the blade’s root will require different machining strategies.
Machining a ‘Christmas tree’ profile For machining the Christmas tree profile on a blade, it can be helpful to change the fixturing arrangement, and make the tool axis parallel to the blade length. It may also then be possible to use a special adaptor on the Christmas tree profile to hold the blade during subsequent roughing operations, and so avoid the need for machining (and later removing) separate fixturing elements onto the workpiece. A milling strategy using CoroMill 390 long edge milling cutters, applying down milling for each side of the profile, will allow maximized metal removal rates and tool life.
1. Roughing with the long edge cutter in different ap-steps, using down milling Calculate a suitable ae/Dc ratio so as to bring more than one effective tooth into cut during the cutting cycle. 2. Roughing completed. 3. Machining the christmas tree profile, with special HSS tooling.
37
Roughing the christmas tree profile may also be performed by CoroMill 331 side and face milling cutters in different diameters, to achieve the stair-like shape on the component. However, using a set of different diameter cutters mounted in this manner results in large differences in effective cutting speed between the largest and the smallest cutter. An alternative is to employ solid tools, particularly if there are difficulties with accessibility or the complexity of the shapes being produced.
(a) Roughing with the long edge cutter
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(b)Roughing completed
Fig 16 Christmas tree profile machined
Machining a deep slot in the blade root by end milling The type of workpiece material will have a large influence over the machining parameters when machining slots into the blade roots. In many cases it will be stainless steel, and thus problems of chip adhesion to the cutting tool will occur. However, carefully selected tooling and the correct machining methods will counteract these difficulties. The blade’s size and material, and the slot’s position and form, will determine the machining strategy. In most cases it will be better to leave the machining of the slots, along with their roughing and finishing, until after the other machining operations are complete. That way the machining of the blade profile itself can be carried out without any slots in the blade root which might conceivably affect the clamping and stability of the workpiece. In addition any bending or deformation in the workpiece that occurs during profiling, due to the release of internal stresses, can be compensated for when the item is remounted prior to the finishing operations, an approach which should also help to maximise the quality of the final blade. In general, machining deep slots in the blade root can be divided into: •
slot milling (L-style with endmill). 39
•
plunge milling (with endmill).
•
trochoidal milling (with endmill).
Machining the blade body Machining the blade rhombus is a critical step in blade manufacture, and a wide variety of potential machining solutions are available depending on the design of the blade and the types of cutting machinery available. A comprehensive description of all these different methods is beyond the scope of this book, but the basic principles can be outlined, emphasising the machining principles which underlie them: optimizing the cutting tool engagement, reducing vibrations, using the tooling as effectively as possible, and maximising productivity.
Fig 17 Machining blade body
Roughing the rhombus – parallel to the blade axis, using one tool This is a very common machining approach, using two separate cutting steps to reach the full depth of cut. In most cases this method allows the cutting force to be
40
reduced more effectively than by reducing the feed per tooth, as it allows the chip thickness to be modified towards the recommended target values.
Fig 18 Roughing the rhombus – parallel to the blade axis, using one tool
Material – CMC 5.2 Tool – R200-L, Dc 63 mm, zn 6 Insert – RCKT 1204M0-MM 2040 vc 220 m/min, fz 0,21 mm, ap 2–4 mm, ae 30–63 mm.
41
To achieve the full benefits of this approach, the milling strategy must use down milling, and a 45° angle of cutting entry into the work piece. The tool path must not change through 90° angles. Instead, change the feed direction incrementally through small changes of radii. Ensure a tool engagement of 60–80%, if necessary by changing the tool diameter or cutting path. Employ a different depth of cut in each of the two passes, to minimize notch wear on the cutting insert. Maximize the larger depth of cut as much as possible.
42
Fig 19 Machining the rhombus profile with the end mill cutter
Vibrations and heavy axial pressure on the inserts will occur if the feed forces cause any movement or deflection of the work piece. If this occurs the feed direction should be modified so the forces act in directions where the blade fixturing arrangement supports the work piece most effectively.
Vibrations can also be reduced by adopting cutting paths which machine the metal in small triangular steps, in both the longitudinal and lateral directions. This approach requires modifications to the cutting speed and feed, along with no more than 60% of the usual maximum depth of cut, and the modified cutting forces will also produce changes in the wear patterns seen on the cutting inserts.
Fig 20 End milling the profile in two dirctions
43
Roughing the rhombus – parallel to the blade axis, using two tools of different diameter The use of two different tools to machine the rhombus is an effective strategy in many situations. A first cut, producing a slot perpendicular to the blade axis, can be made with an endmill such as CoroMill 390 (using L-milling or plunge milling) or a slot milling cutter such as CoroMill 331. This slot then provides clearance for a subsequent cutting tool of different diameter, which should experience a less severe cutting environment and generate lower vibrations while it machines along the blade’s longitudinal axis.
44
Fig 21 Roughing the rhombus – parallel to the blade axis (using two tools of different diameter)
Roughing the rhombus – machining the roof slopes This penultimate operation in roughing the blade’s contour uses a roughing tool whose size will depend on the design of the blade, and on the radius between the roof slope and the blade’s root.
45
Fig 22 Roughing the rhombus – machining the roof slopes
Roughing the pressure side – peripheral milling Roughing the pressure side of the blade – the concave side – is usually the last stage of the roughing process, and also one of the most complex. Modern designs of turbine blades maximize their efficiency through complicated surface geometries, and machining these surfaces requires a careful machining strategy to account for both the profile of the blade, and changes in the effective stiffness of the work piece as the 46
machining operation proceeds. Peripheral milling is an effective way to carry out this operation, with a depth of cut between 1–5 mm.
Fig 23 Roughing the pressure side – peripheral milling
Roughing the pressure side – waterline milling, parallel to the blade axis An alternative strategy to machine the pressure side is “waterline milling”, an approach originally derived from 3-axis milling in the die and mould industry, now adapted to 5-axis milling machines. In this technique, the cutting operation consists of a sequence of 2-dimensional layers, each completed before the tool moves down to the next. Transitions between the layers are carried out by helical ramping or circular interpolation, with the initial feed direction always away from the solid fixturing at the root of the blade.
47
Fig 24 Roughing the pressure side – waterline milling, parallel to the blade axis
Semi-finishing the blade
Fig 25 Semi-Finishing the Blade by turn milling
The semi finishing operation requires a 5-axis milling operation, and will directly influence the surface quality of the final finished blade. Therefore the aim should always be to achieve a very regular, uniform level of residual material – if necessary, through two separate semi finishing operations. Normally this operation is done by turn milling.
48
A variety of tool paths can be employed. One common technique, especially when machining large cast blades, is to use a feed direction along the blade length, but other possibilities are shown in the diagram. For example, the blade can be shaped by milling across the blade, either using several passes in one direction with a rapid return movement between passes, or in a single continuous helical cut around the blade.
Convex and Concave Profiling
Fig 26 Convex and Concave Profiling
49
Finishing the Blade:
Fig 27 Finishing the Blade
Finishing the blade is probably the most difficult 5-axis machining operation, but its success will greatly depend on the quality of the other machining steps carried out previously. The most suitable tool depends on the type and size of the blade, and also on the spindle speed and the feed available in the machining centre. The principal problems when finishing are vibrations, and the quality of the pre-finished surfaces. Using tools with a smaller radius, or using a different number of inserts in the cutting head can help
50
combat vibrations. During the cutting process the tool follows a helical path around the blade, a path controlled by a specialised CAD-CAM system. To achieve the best surface quality and structure, the tool has to maintain a constant norm angle at each point on the surface, and always in a down milling manner. In this way, and combined with an oil mist coolant, the resulting surface can be highly polished. With suitable optimised equipment it is possible to achieve a surface roughness, although the final surface quality will strongly depend on the combination of normangle, feed and cutting engagement.
51
CONCLUSION •
This project is made on the basic manufacturing technique of steam turbine blades.
•
The procedure involved in this manufacturing leads to
achieve the best
surface quality and structure. •
This method uses two separate cutting steps to reach the full depth of cut, which allows the cutting force to be reduced more effectively than by reducing the feed per tooth, as it allows the chip thickness to be modified towards the recommended target values.
52
REFERENCES
Principles of Metal Manufacturing Processes – J. Beddodes & M.J.Bibby- Carleton University Canada. McGraw-Hill machining and metalworking handbook / Ronald A. Walsh. and Denis R. Cormier—3rd ed. Handbook Of Machining And Metalworking Calculations- Ronald A. Walsh- McGRAWHILL. Cutting Tool Technology Industrial Handbook- Graham T. Smith, MPhil (Brunel), PhD Birmingham), CEng, FIMechE, FIEE Formerly Professor of Industrial Engineering Southampton Solent University Southampton U. K.
Machinery’s Handbook 28th Edition - Erik Oberg, Franklin D. Jones, Holbrook L. Horton, And Henry H. Ryffel Christopher J. Mccauley, Senior Editor Riccardo M. Heald, Associate Editor Muhammed Iqbal Hussain, Associate Editor 2008 Industrial Press New York. http://en.wikipedia.org/wiki/Steam_turbine http://www.bechtel.com/assets/files/TechPapers/steam-turbine.doc. Steam Turbines: A Book of Instruction for the Adjustment and Operation of the Principal Types of this Class of Prime Movers by Hubert E. Collins. Power Plant Engineering - A.K. Raja, Amit Prakash Srivastava.
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Protective coatings for turbine blades- Y. Tamarin, ASM International TURBO MACHINERY ENGINEERING INDUSTRIES LTD., AN ISO 9001:2000 CERTIFIED COMPANY
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