pdf. DESIGN AND ANALYSIS OF STEAM TURBINE BLADE AND SHAFT Using analysis results the best material for both shaft and blade is suggested. Engineering Books Pdf > Mechanical Engineering > Steam Turbine > Blade Design and Analysis for Steam Turbines by Murari P. Singh. Blade Design and Analysis for Steam Turbines. by: Dr. Murari P. Singh, Dr. George M. Lucas, PE. Abstract: A concise reference for practicing engineers involved.
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Short Desciption: This "Blade Design and Analysis for Steam Turbines by Murari P Singh" book is available in PDF Formate. Downlod free this book, Learn from. Request PDF on ResearchGate | Design and Analysis of Steam Turbine Blade | Steam turbine changes over the warmth vitality of steam into helpful work. Steam . Design and Analysis of Micro Steam Turbine Using Catia and. Ansys. 1. The blade model is generated by using CATIA AND ANSYS software. Blade profile.
Reaction 3. Single stage 2. Axial 2. Mixed 4. Condensing 2. Extraction 4. This was the irst oil reinery on the variation if the blade speeds across the height of the the east coast and the irst major industry in the city of blade. The delection in the blade passage also reduces Visakhapatnam. Hindustan Petroleum Corporation came from hub to tip to vary the loading on each section. Thus into being in mid after take over and merging of the pressure distribution on the suction and pressure Erstwhile Esso and Lube India in and was subse- surface of the blade changes considerably from hub to tip quently merged with HPCL Kosan Gas Company in It is known fact that HPCL thus came into being after merging four different the area of pressure distribution curve representing the organizations at different parts of time.
Hence it has been decided to generate the Initial installed capacity of 0. The crude processing capacity is raised to 8. But, various modiications and BladeGen plus package. Reinery ated as a loop with the following notations. Proile curve is generated with above coordinates of all to meet Environment norms. The enhancing the quality of Petrol product.
VR has its additional storage facilities at the North of the 3. The ate heights with —Y negative meridional axis corresponded ATP storage tanks are spread over area of acres. And positive distance from me- ridional distance from TE Tailing Edge. Hub Curve ile is generated as follows: X, Y, Z X, Y, Z A proile contains total 60 points for all sections. Proile Curve ile is generated as follows: The structural, vibrational and ther- mal analysis is carried out with three different materials as mentioned.
The temperature changes in the turbine are monitored periodically using the thermal images taken by the thermal camera. When a image taken by the thermal camera the image will show the temperature variations directly Generated part of Blade Image showing blade assembly. Courant, who utilized the Ritz method of numerical analysis and minimization of variational calculus to ob- tain approximate solutions to vibration systems.
Shortly thereafter, a paper published in by M. Turner, R. Clough, H. Martin, and L. Topp established a broader deinition of numerical analysis. At irst the static analy- sis is done for three different types of materials. It gives stresses developed in the turbine blade and shaft assem- bly, strain values and displacement. Now all the stress, strain and displacement values are shown in igures below.
Strain for EN24 Stainless Steel. After the loads are applied to the imported model, the Result summary for vibrational analysis for EN24 Stain- meshing is done successfully and the results are com- less Steel piled.
Graph showing different mode shape curves for all the three ma- terials. Now the thermal analysis is carried out by importing the material and meshing is done with solid mesh.
The con- straints are applied to blades with an inlet temperature of C. Result summary table for thermal analysis S. In thermal analysis, ZAMAK is having high thermal gradi- ent and thermal lux and the thermal gradient is 0. For 50 percent reaction stages, symmetric blading may be used, which in Fig. Ignoring friction and other losses, the energy balance across the moving row of blades is given by. The rate of work output in the moving row again, only the tangential velocities count in this calculation is given by.
As noted previously, Fig. The reader must note that these efficiency values are based on some rather unrealistic assumptions about blade path losses i. So our simplifying assump- tion that the energy represented by V3 is wasted adds a little pessi- mism in our efficiency relations for both reaction and impulse stages. However, this pessimistic assumption only slightly offsets the far more optimistic assumptions that friction and other flow losses equal zero.
There are a few important conclusions the reader should draw in comparing the efficiency curves for impulse and reaction stages. This implies that for identical tangential blade speeds, the energy converted per stage by a reaction turbine is one-half that converted per stage by an impulse turbine. In practical terms, this means that a reaction turbine requires twice as many stages to convert the same available enthalpy as an impulse turbine. However, the peak efficiency for a reaction turbine is slightly higher than that for an impulse turbine doing the same work.
In general. Since the turbine con- verts heat and kinetic energy in the working fluid steam in this case to shaft torque and power. Blade Loads 25 2. The designer must determine. The most basic application requirements in this case include the power required by the driven machine. For a typical process drive steam turbine. Since this com- bination of conditions results in both the least energy available per pound of steam and the lowest fluid density at the turbine inlet.
Rated output at minimum speed. The imposed loads due to rotational speed i. With a basic understanding of turbine stage design and an under- standing of the fundamental thermodynamics involved. For multistage turbines. When specified.. The purpose of identifying the range of service con- ditions for the turbine is to ensure that the turbine designer knows.
The engineer must exercise judgment before applying such a conservative assumption. Maximum flow at maximum inlet steam pressure and tem- perature and minimum exhaust pressure. If the turbine is designed to produce rated power at minimum steam conditions that are signifi- cantly lower than normal steam conditions. General-Purpose Steam Turbines. Control stages are also often evaluated at the first valve wide- open point while operating at minimum continuous speed as well.
Control stages are a common exception to item 2 above. Special Purpose Steam Turbines.
This usually results in the highest combined loading on the control stage blades. This set of concurrent condi- tions almost always results in the highest combined loading on the rotating blades. This set of conditions may be more likely to occur in real-life service conditions than some other common condi- tions at which blade loading is evaluated.
Many designers. This potential operating point usually results in the highest potential steam bending forces on the control stage blades. For most control stages. When the driven machine is a process compressor. These specifications are very useful as they define a common. By selecting an appropriate combination of number of stages.
The first step in this process is to determine the number of stages required. A multistage turbine breaks up the available pressure and enthalpy drop across a number of stages. A key step in this process is the decision regarding reaction levels and number of stages. These definitions. They specify in some detail how to determine the range of conditions for which process drive steam turbines must be designed including. Blade Loads 27 set of terms and definitions that are used by end-users. Steam turbine manufacturers self-classify themselves as either impulse turbine builders or reaction turbine builders.
To determine the number of stages required. Based on these data. Impulse Turbine—Number of Stages For a known available enthalpy that is. At this point in the design process. In real-world practice.
The turbine designer wants to determine the optimum number of stages to compare different turbines using On the other hand. For impulse turbines. From the H—S diagram. For the turbine with mm wheels. An appro- priate blade or wheel speed W is usually chosen by the designer based on some knowledge of the application.
Blade Loads 29 Reaction Turbine For the same set of conditions. For a similar mm mean blade diameter. For this reason. As these examples show. For this example. Because of the lower overall velocities. Turbine designers use the velocity diagram for a stage to determine the best shape for the airfoils. Designers either choose or design an airfoil with entrance and exit angles that match the calculated flow angles.
The angular difference between the steam entering the moving blades V1 and the steam leaving the moving row V2 is referred to as turning. For 50 percent reaction stages.
As one would logically expect. Referring to Fig. But since this is rarely possible. That is. In turn this tends to produce smaller entrance and exit angles as well. At the entrance to the moving row. Twisted Blades In discussions so far. In the case of tall blades.
Since blade velocities are smallest at the root and highest at the blade tips. For more comprehensive discussions on the design of twisted airfoils.
At the base. To accommodate this. Besides the relationship between airfoil section shapes and steam flow velocities. To avoid significant flow disturbances that can affect reliability as well as stage efficiency. Blade Loads 31 shown in Fig. In some applications. In condensing stages of both industrial turbine generators and process compressor drives.
As one may imagine. Note that both the axial and tangential directions must be scaled by the same factor. In this way. Most turbine manufacturers rely on a library of standard airfoil section shapes that are selected to match the desired flow angles determined by the velocity diagram for each stage.
Once the basic airfoil shape has been selected. Once the optimum solidity is deter- mined for a selected airfoil. The effect of solidity can be seen intuitively by considering a stage with very low solidity. Solidity is defined as the ratio of blade chord to spac- ing or pitch. The turbine designer must juggle several factors when determin- ing the minimum chord and scale factor for a given airfoil. Depending on the application requirements. It is almost always desirable to minimize the rotor length to help achieve a stiff and unresponsive rotor-bearing system.
Too low solidity results in insufficient guidance for the flow. Since the number of blades is also inversely proportional to the scale factor. The optimum solidity for a stage using a specific airfoil shape depends primarily on the inlet and exit angles of the airfoil. Blade Loads 33 While the turbine designer is selecting or designing the airfoil shape.
The airfoil must have sufficient section modulus to keep peak bending stresses within appropriate design limits. Optimum blade spac- ing sometimes called the blade pitch is defined by a parameter called solidity. Another important consideration is the shroud attachment. For stages operating at moderate to high speed.
Regardless of their origin. To ensure that the blades are designed to resist these forces. To show how stage energy relates to power developed by a turbine stage and ultimately to the loads imposed on the turbine.
Espe- cially for blade rows with riveted shroud attachments. Through the mechanism of momentum change as it passes through the flow path passages bounded by the rotating blades. Blade Loads 35 blades. In USCS units lb. Turbine torque T is calculated from w and speed N: To produce torque T. Streamline codes are now being supplanted by computa- tional fluid dynamics CFD routines for the same purpose. For more critical stages. Courtesy of Advanced Turbomachine.
Streamline calculations have an advantage in that they have been in use for many years and conse- quently have been validated by both field experience and dedicated test data. This simple calculation carried out at the mean blade path diam- eter is often sufficient for short blades in light to moderately loaded stages. Turbine design engineers are. The flow downstream from the stationary nozzles contains wakes from the trailing edges.
In turn. Sufficient repetitions of even relatively small loads can lead to failure of most metals. In most cases. These include minimizing nozzle trailing edge thickness to reduce the size and effect of trailing edge blockage and wakes. The ability to identify the entire range of unsteady loads on a particular stage is critical to the design of reliable turbines.
Nozzle Passing Unsteady forces due to effects from the rotating blades passing the stationary nozzle partitions are possibly the most important. Turbine blade reliability is similarly highly dependent on the magnitude and frequency of the unsteady loads to which it is subjected. Designers have several design options available that they use to mini- mize the effect of nozzle disturbances on the adjacent rotating blades.
Blade Loads 37 2. Turbine designers also do their best to avoid transonic. Variable loads arise from varia- tions in imposed load during normal and abnormal operation of the driven machine. Trailing edge thickness depends on several factors. These same disturbances that influence the moving blades may also have a negative impact on stage losses and efficiency.
In the end.
Basic strength consider- ations lead to thicker trailing edges as the nozzle height increases. Regardless of the source of the flow field nonuniformi- ties.
In addition to nozzle strength considerations. If the nozzle assemblies were perfect. Blade Loads 39 The effects of these disturbances usually die out quickly with dis- tance. The effects of variations in nozzle exit area. One common measure of consistency in the measured param- eters that is used in many statistical evaluations looks at the standard deviation of area. The specified tolerance limits are based on field experience with a great number of turbine stages.
Virtually all turbine manufacturers and major turbine service organizations have developed procedures to evaluate and deal with the effect of manufacturing variations on the vibratory behavior of the rotating blades.
Dimensional and area tolerance criteria. Statistical criteria are often used in conjunction with simple tolerance criteria for area. Statistical criteria.
The application of automated inspection and data collection to document the variations in nozzle con- struction was followed closely by the application of statistical methods to evaluate whether these variations are acceptable. This is the simplest approach. Since a nozzle or diaphragm assembly has a finite number of nozzles.
As the name implies. One common cause of unsteady loading arises from the use of partial admission stages. There are cases. Where and why are partial admission stages used? By far. Partial Admission Several special cases arise in the design of turbine stages that affect blade loading. A number of more sophisticated methods have been developed to evaluate the effect of nozzle variations. Since the turbine is part of a larger system.
Based on knowledge of the design or the manufacturing processes involved. Blade Loads 41 deviation of each parameter. In other cases partial admission stages may be used to achieve lower costs. While there are differences in the details of the inspection routines and analytical processes for each manufacturer. Turbine manufacturers and most major turbine service organizations have developed pro- prietary analytical routines to carry out this task.
Examples of such features could include the nozzle passages immediately adjacent to the split line on horizontally split diaphragms. Forcing function criteria. The same calculation usually provides guidance to specify appropriate remedial operations for each specific nozzle passage within the assembly. Photograph courtesy of Dresser-Rand Company. Depending on the application. Valves are opened in a specific order by a mechanical or hydraulic mechanism. For partial admission stages.
Individual blade loading may be arrived at in one of two ways: In the previous section. The primary difference for partial admission is that the stage power is produced in only the working blades—those on which steam is acting instantaneously. Blade Loads 43 Partial arc stages sometimes pose difficult design challenges for two main reasons.
Critical loading usually does not occur at maximum flow or maximum stage horsepower. For multivalve control stages. In this approach. By nozzle count. In this case. While not as precise as the actual nozzle count. At this operating point. By arc of admission. To ensure that it truly represents the worst case loading. The blade thus sees the maximum stress range possible. In fact. This set of conditions makes the maximum energy available to the control stage. Variations in stage backpressure can also be caused by nonunifor- mities in the shape and size of casing passages in extraction and exhaust casings.
Turbine Structure Other potential sources of unsteady blade forces arise from various turbine structures that are necessary to support and align various internal components. Depending on the number and location of structural supports. Extraction openings in casings are an example of a structure that can potentially cause variations in stage backpressure around the periphery of the moving blade row immediately upstream of the extraction opening.
Struts such as these present obstructions to flow and cause flow disturbances that may propagate upstream to affect the last-stage rotating blades. Examples of such structural components include structural supports and struts in extraction and exhaust passages.
Steam is transferred from this stage to the double-flow section via one large or several smaller crossover passages that bring steam out from the main turbine casing and back into the multiflow section see Fig. Blade Loads 45 small enough to present no danger.
With modern blade analysis. Pressure variations around turbine stage exit P 0 Turbine stage exit Irregular exit duct distribution This happens where a single-flow turbine exits into a double-flow section in the same turbine case Figure 2.
An example of this may be found in many older process drives pres. A potential problem with multiflow exhausts occurs in the stage immediately ahead of the multiflow section. By dividing the flow into two streams. Although far from exhaustive. The turbine design engineer must recognize the potential impact of double-flow passages and must either design the blades in the stage ahead of the multiflow section to withstand high unsteady forces or take steps to reduce the magnitude of these forces.
A number of different construc- tion features have been developed by turbine manufacturers to atten- uate these forces. As readers consider how these structural features give rise to potentially destructive unsteady forces on the moving blade rows. For indus- trial steam turbines. The varied demands of such a vast application range have given rise to an amazing number of mechanical variations in the design of all turbine components.
They are usually char- acterized by variable-speed operation as distinct from the constant synchronous speed of generator drive turbines. Process drive steam turbines represent a specialized subset of industrial steam turbines that are mainly used by the refinery and petrochemical industries to drive compressors in critical services. Industrial steam turbines are still used in a wide variety of processes. In the intervening years. Chapter 3 Turbine Blade Construction. These machines range in size from a few kilowatts at the small end to perhaps MW at the large end of the range.
A brief overview such as this cannot adequately cover every one of the materials. Survey- ing the wide variety of designs that are found in successful operation around the globe makes it obvious that many equally good solutions have been developed to meet the demands of any steam turbine application imaginable.
While geometric detail dimensions may vary widely. In addition to these basic elements. These elements may appear as part of monolithic individual blades or as separate component parts that are assembled to form a complete blade system. Critical attributes of the airfoil include the profile accuracy of the airfoil surface contours. The final choice of blade design details is often influenced by available machine tools and manufacturing processes.
The airfoils of the rotating blades represent the only components in the entire turbine that convert energy in the steam to shaft torque and power. As we will see. Figure 3. To review. The momentum change in the moving blades is therefore contributed by conversion of the kinetic energy at the nozzle exit. This acceleration occurs as the remainder of the available stage energy is converted to kinetic energy in the moving blade row. That kinetic energy is then transferred to the shaft purely by momentum change i.
As you might expect.. In contrast. Once an airfoil section is selected with the desired total turning and appropriate inlet and exit angles. In contrast to the case of an impulse turbine. In counterpoint to impulse stages. Airfoil sec- tions for impulse stages are characterized by greater turning.. A few additional key features of each turbine type stand out: Such airfoils are often referred to as twisted or twisted- tapered airfoils.
Photograph courtesy of Dresser- Rand Company. The reac- tion turbine may be slightly longer than the impulse cascade. The ability to turn the form in the rotor means that the rotor machining is Figure 3. The root is the primary mechanical attachment between the rotating blade and the turbine disk.
Roots in common use can be divided into three broad classifications— dovetail or circumferential roots. The turbine design engineer must under- stand the advantages. From Figure 3. They are the most widely used root form in indus- trial steam turbines and process drives because they are easily machined on the rotor by turning in a lathe or machining center.
The root must restrain the blade against centrifugal forces to keep it attached to the periphery of the disk and at the same time transmit steam forces from the blade to the disk to provide shaft power. A single-tooth root form is always less expensive than multiple-tooth root forms. Without multiple contact points on each side of the root.
A single tooth on each side of the root engages a matching sin- gle tooth in the wheel form. If the load is shared between additional contact points. At the same time. The sim- plest root form is that shown in Fig. Multiple-tooth forms refer again to Fig. The internal dovetail root style and the straddle root style are typical of tangential roots in wide use today.
The milling cutters are intended to be used in a milling machine or flexible machining Figure 3. To ensure load sharing among multiple contact surfaces. It is common practice to produce one of the components. In addition to the con- tact stresses and tensile stresses in the root. Even with matched cutters. The disk stress Centrifugal load Rim deflection Figure 3.
Wheel rims and blade platforms can be made wider to resist this bending and reduce deflection. The wheel root form is machined using a matching form cutter that is procured as a set with the blade root milling cutter to ensure that the tolerances between parts are very closely held. While circumferential roots are perhaps the most widely used type of blade attachment.
Even with inter- locking geometry. For modern industrial steam turbines. Root designs have been created to resist this bending deflection using interlocking forms between the rim and the blade e. This is a consequence of the nearly universal applica- tion of integral rotors—i.
Blade root Symmetrical rim Contact surfaces and blade root forms Wheel rim-slots Axial fir tree root features Figure 3. The accuracy of the root profile as well as the angular indexing of each slot to the next must be held to a very close tolerance level. Axial entry roots are more expensive when compared to cir- cumferential roots. If the blade and disk materials have significant differences in strength. Inherent in their design. Fir tree roots are a good choice for high-speed stages with high centrifugal loads because the blade root and the disk root forms are essentially mirror images of each other.
By taking rim bending out of the total and equivalent stress calculations. Titanium blades coupled with axial fir tree roots are the ultimate choice in last- stage blades for process drives.
Note the narrow space between adjacent disks which limits the diameter of cutters and therefore the width of the root form. Typical applica- tions in high-speed process compressor drives include condensing last-stage blades and highly loaded first-stage blades.
In effect. While the wheel and blade root must still be produced to tolerances measured in thousandths of an inch 0. Pinned roots have been used extensively to retain tall low- pressure blades in medium and large utility condensing turbine generator sets. Pinned roots. Pinned roots derive their strength from multiple load-bearing points. Proper function of axial and circumferential roots depends upon the precision with which the mating surfaces are produced.
The area in con- tact with the pins is roughly equal for both blade and wheel rim. A common means to retain the pins involves staking or upsetting the material adjacent to the end of the pins in several locations at each end.
Because these root forms do not automatically determine the blade location. In either case. A close fit between the pin and its board minimizes this bending stress and helps to ensure that the cen- trifugal and steam bending loads are shared between multiple pins. To ensure close fit. Each pin shares the load on several shear sections.
Thermal tran- sients during startup or large load changes may result in looseness even in interference-fit assemblies. Depending on the tools available and the preference of the pro- cess engineer. As with the retaining pins for locking buckets. After align-reaming.
Pins must also be positively retained in their bores. Elastic deformation of the blade and disk materials inevitably results in bending stress in the pin. Multiple pins are often used. Coupled with vibration and pres- sure loads on the wheels.
The assembly process is therefore critical to successful pinned root designs. The designer may adjust the ratio of contact and tensile areas of the blade and wheel rim to make best use of the relative strength of the materials. Shrouds take many forms. To the extent that the stage depends on the shroud to reduce the amplitude of vibration or to change the frequency of vibra- tion. Shrouds are usually installed to create dis- crete packets of blades that are tied together by the shroud.
The length and number of packets are chosen by the stage designer to modify the vibratory behavior of the bladed disk and to avoid potentially danger- ous specific mode shapes and frequencies of vibration for the bladed disk assembly.
Longer shroud segments so-called long arc shrouds are difficult to produce. The complete shroud is therefore formed from the segments of the shroud that are attached to each blade tip. If long arc shrouds or even a completely shrouded stage is specified by the designer. Z-locks are almost always used on twisted airfoil sections because they are intrinsically activated by the twisting of the airfoil that occurs with increasing speed. The Z-lock Figure 3. The Z-lock shrouds are almost always used with axial fir tree root designs because the axial root design locates the blades and prevents relative twist between adja- cent blades that could lead to variations in contact force.
To be effective. A typical surface treatment consists of a Stellite 6 weld overlay. In these cases the blade designer must choose. For tall blades. At speed. The abutment surfaces may move relative to one another in service. One common form is a wire inserted in a slot in integral shrouds and retained either by the form of the wire and the slot themselves or. A variety of coatings have been used to com- bat this wear. The abutment surfaces are often treated with some type of surface modification to prevent wear and fretting on the abutment surfaces.
This process is so critical to successful long-term reliability that turbine design engineers are willing to sacrifice some degree of tensile strength in return for improved ductility.
Designers take advantage of this directional- ity by aligning the long axis of the grains with the direction of the maximum alternating bending stress. Force is applied by various forming processes such as forging. Stocks of blade materials are available in a variety of forms.
This last approach can significantly. On initial start- up therefore small zones of plastic deformation occur that result in improved contact between mating surfaces. The high rotational speed of modern turbines means the blades are subject to large centrifugal forces. In addition an ideal blade material is corrosion-resistant and erosion-resistant. An ideal turbine blade material has both high tensile strength and high fatigue strength.
One would therefore think that high tensile and fatigue strengths are the para- mount requirements for turbine blades. Wrought materials usually exhibit some degree of directionality in properties such as tensile and fatigue strength. In spite of closely held dimen- sional tolerances and careful assembly. Most forming processes also create directional variations in material properties as a result of the process.
In the process. Nearly all steam turbine blade materials in common use are wrought materials. While strength is a critical attribute of turbine blades. Both are martensitic stainless steel alloys that can. As used in blade applications. Type SS is often specified for high-temperature.
Type is not commonly used for low. This same property gives turbine blades the ability to absorb minor damage in service from erosion. This combination is ideal for turbine blades that are subject to high cen- trifugal and bending loads. Microscopic material defects and inclusions well known to reduce fatigue strength act as starting points for cracks in steel materials. To avoid these problems. The ductility is a very important charac- teristic as it allows.
Type stainless steel exhibits a significant decrease in usable material strength at elevated temperatures.
For most forgings and bar stock. While single vacuum melt materials are widely used. Because of this. For some forms. These alloys are very similar in chemical composition. Smaller titanium bars are often stocked by metals suppliers.
Ti-6Al-4V is the most commonly used alloy for blades. Taller blades. This produces fundamental blade frequencies and mode shapes that are nearly identical when the same blade geometry is produced in either titanium or series stainless steel. Coupled with an elastic modulus of approximately This characteristic provides an opportunity. For LP stages. In many applications.
Titanium is. As noted above. As one would intuitively expect. It is available in a range of forms and sizes. When Ti is substituted for SS. In many cases therefore tita- nium can be treated as a direct substitution for stainless steel in highly stressed stages.
Table 3. The usable strength of titanium falls in a similar range to that of and SS. With reduced stress level from centrifugal load. There are also some alloys that see limited use in low-pressure turbine stages because they exhibit good erosion resistance without additional surface treatment or overlays. Vasco M Other superalloys have also seen very limited application in steam turbines. It has the advantage of higher inherent strength than either titanium or series stainless steel.
Austenitic series SS materials have been used in very limited numbers of specialty turbines. IN is an excellent material for high-temperature applications. The reader is cautioned that the values in this table are average handbook values.
IN can be used as an alternate to SS in very highly loaded. The strength of most austenitic stainless steels cannot be increased by normal heat treatment processes.
It is now sometimes used for last-stage blades in erosive applications. It has been used suc- cessfully in last-stage blade applications without additional surface. A alloy is special case of an austenitic superalloy that can be strengthened by precipitation hardening. Since many of the desirable properties inherent in wrought material depend on the directionality of the final forming process.
The vast majority of blades for process compressor drives are fully machined from bar stock. Forged Bars The center of large greater than 3 in bar stock may not experience the degree of deformation necessary to impart the material enhancements expected from wrought materials. Additional material is often added to provide temporary fixturing points or lugs to facilitate stock holding on the machine tools. Envelope Forgings Envelope forgings are rough forgings of the airfoil with 1 mm to 3 mm stock allowance over the entire surface.
Due to the relatively relaxed tolerances required in the forging. Bar stock shapes are created by rerolling larger shapes or bil- lets to smaller size. The Some variations on this process include the following: Bar Stock The most common material form used in blade manufacture is bar stock. Forgings are often used for medium to large blade. In practice. Blanks for blade machining are often produced by cutting or slitting larger material forms to produce smaller blanks of appropriate size.
A material form known as forged bar is often used to provide improved control of properties in completely machined blades requiring stock with thickness greater than about 3 in. Strategically placed blocks of material is provided in the forged shapes from which roots. These auxiliary fixturing points are removed once they have served their purpose.
Envelope forgings that are later fully machined have largely replaced the use of net shape airfoil forgings in small and medium steam turbines. Almost any square or rectangular shape up to an approximate 3 in width can be procured as bar stock. Blades produced from envelope forgings. Lead times for envelope forgings are typically about one-half those for final net shape forgings.
Envelope forgings require less expensive tooling than net shape forgings and can be produced on a wider range of forging presses. Although envelope forgings must be completely machined. The blade is then cut to length and the tenon round or kidney is pro- duced on the blade tip by milling.
The airfoil surface is produced by a milling process in which the airfoil contour is formed by a milling cutter that traces the airfoil contour on a CNC machining center. For maximum accuracy and speed. Figures 3. For blades with flat-top integral shrouds. For blades with riveted tenons. When a slant-top integral shroud cannot be avoided. This may be a manual process using belt sanders and polishing wheels. Machining stock is provided for root and tip machining operations.
These ridges are removed by a subsequent grinding and polish- ing process to reach the final airfoil shape. Enve- lope forgings are formed to the airfoil contours with several millime- ters of excess stock.
The airfoil section varies continuously from the platform to the blade shroud or tip. This stock is removed by the same machining and polishing process used for bar stock.
The airfoil is numerically modeled by defining a series of sec- tions at regular intervals along the stacking axis from platform to shroud.
The same solid model created by these boundaries is used for engineering analysis e.. The points that define these sections are used to create a three-dimensional spline representation of the airfoil surfaces. A typical machining process uses a four-axis or five-axis milling machine or machining center equipped with a ball end mill to trace the three-dimensional surface of the airfoil.
The airfoil surface is produced by a milling process in which the airfoil contour is by a milling cutter that traces the airfoil contour under CNC. The choice of manual vs. Airfoil surfaces may also be produced by forging processes. For small a relative term blades the airfoil may be produced completely by machining from bar stock. The forging process can also produce an airfoil forged to net shape. Although as few as four sections can be used.
This machining process produces a high-accuracy sur- face. This process is capable of producing blades with min- imal dimensional tolerances. This is a time-consuming and expensive process. Twisted airfoil sections are produced by three main methods. Ten sections. Blades with Taper-Twisted Airfoils Twisted and tapered airfoil sections are used in reaction turbine stages.
The liquid condensate is then reused as feedwater. Low-pressure stages in condensing turbines. The minimal losses of water for this closed system are replaced by makeup water that is demineralized before entering the system.
The term condensing is used for turbines with subatmospheric exhaust pressure. In a closed system. Dissolved oxygen. So why is this important to the discussion? For condensing turbines.. Other potential contaminants include minerals such as silica that may accumulate on turbine blades and of nozzles. The liquid droplets move at much lower velocity than the vapor phase. These contaminants may be corrosive or erosive.
One of the simplest approaches is flame hardening of the lead- ing edge. The process is usually automated to ensure repeatability in heating and quenching for all the blades in a stage set. To combat erosion. Since velocities are greatest near the tips. Virtually every turbine manufacturer has developed their own empirical erosion protection guidelines based on field experience with their particular stage designs. Untreated or stainless steel is quite susceptible to liquid droplet erosion and should only be used in low-speed stages with rela- tively small amounts of liquid condensate present.
For a given unit. As a minimum. Water-soluble minerals. In general therefore large tip diameters and higher rotational speed increase susceptibility to erosion. As steam starts to condense in the low-pressure stages. In this process. As the water condenses on the surfaces of the stationary flow path. The objective in the flame hardening pro- cess is rapid heating of the surface area to be protected. This is an important subject in turbine design.
This can be highly erosive to untreated blade materials. This results in a thin layer of stronger, harder material on the surface with a core of ductile material underneath.
Flame hardening is quite effective for stages with moderate erosion potential. Where it is appropriate, it has significant advantages over more aggressive treatments. Since the ductile core is main- tained in all but the thinnest blades, flame hardening is a low-risk process.
Flame hardening is inexpensive and requires short pro- duction cycle times, and the retention of the ductile core makes it a very low-risk process. If erosion in service proves to be a problem, there are repair techniques available to salvage blades with moder- ate erosion damage and to upgrade erosion protection in future operations.
However, flame hardening is not applicable to all blades. Very thin blades, are susceptible to through-hardening, which completely eliminates the ductile core. And conventional flame hardening may not provide adequate erosion protection in stages with high erosion potential. One alternative in such cases is laser hardening. Laser hardening relies on the same concept as flame hardening, except that it uses a high-powered laser to heat a very thin surface layer very rapidly, and it quenches the hot metal just as quickly, resulting in a harder but thinner layer at the surface for conventional flame harden- ing, and the harder surface layer protection provides in stages with high erosive potential.
One of the earliest approaches to prevent leading-edge erosion was to undercut a portion of the leading-edge contour and insert a thin layer of hard, erosion-resistant material such as Stellite or similar alloys. In this process a thin layer of Stellite strip or bar stock is attached to the undercut by brazing or welding. The Stellite bar is then blended to using abrasive belts or grinders to restore the desired airfoil surface contours.
Stellite is extremely erosion-resistant, but it is common to observe blades with long service where the untreated blade material adjacent to the Stellite inserts has eroded away around and even underneath the insert.
Stellite bars are still widely used today, but more common today is a weld overlay process to deposit a layer of Stellite that more closely conforms to the surface of the blade. This reduces the total amount of material required and reduces the time and cost of blend- ing the Stellite overlay to the final airfoil profile. This process permits its use on many blades that are either too thin or have contour changes too sharp to use a flat insert.
Incidentally, the same process can also be used to build up and restore eroded blade surfaces on blades exhibiting erosion damage after service. Some available blade materials in service today are also natu- rally resistant, to a greater or lesser extent, to water droplet erosion. Titanium blades, as we have already noted, are used in many high- speed process drives, since the material has strength similar to that.
Titanium is also significantly more erosion-resistant than series stainless steel and is therefore used without added surface treatment in many industrial process drive turbines.
Recent experience shows that untreated titanium may still exhibit significant erosion in some con- ditions, and titanium stages with high erosion potential may still require additional surface treatment. As we noted in Sec. To reiterate, Vasco M has the advantage of higher inherent strength than either titanium or series stainless steel. It exhibits a high degree of inherent erosion resistance and has been used successfully in some last-stage blade applications without additional surface treatments.
In this section, we are going to review some of the common processes used in the assembly of the bladed disks. Among these are the installation of the blades in or on the disk to produce the bladed disk assembly. Also covered is the installation of shrouds and auxiliary dampers, with particular emphasis on the installa- tion and assembly of riveted shrouds, i.
As these techniques are dis- cussed, the most common forms of in-process inspection and typi- cal acceptance criteria for acceptable assemblies will be reviewed. The processes to be covered are widely used and are common to most makes and models of industrial steam turbines and process drives.
Dovetail roots are very simple features that involve contact between interlocking shapes on the blade root and the wheel groove. There are two main forms of dovetail roots previously discussed, the internal dovetail form with the wheel hooks on the inside of the wheel groove and the external or straddle root form where the wheel hooks are formed on the exposed lateral sides of the wheel.
To install the blades in or on the wheel, there must be a discontinuity in the wheel grooves that allows installation of the blades. This discontinuity is commonly called the filling slot, which takes the form of a gap in the wheel grooves through which the blades are installed. The assembler simply inserts the blades at the filling slot and then moves the blades around the wheel until the entire complement of blades has been installed.
Assembly of Dovetail Roots Turbine blades with dovetail roots are installed in the wheel through a hole or a gap in the circular root form of the disk. For the internal dovetail configuration of Fig.
The filling slot must then be filled either with a special blade, usually referred to as a locking bucket, or with a blank spacer piece, which is retained by one or more pins through the wheel rim and the locking bucket root blank. A similar installation for straddle roots is shown in Fig. This gap is then closed using a locking bucket that, like that for an internal root form, is retained by one or more pins through the root of the blade and the wheel rim.
Internal dovetail root; blades Straddle root; blades are are installed through filling slot installed via gap in wheel see inset , locking blade fixed rim teeth, fixed in position by one or more locking pins by one or more locking through the wheel rim. Even though the wheel has been designed for a specified integer number of blades, the tolerance stack up for a large number of blades, perhaps or more in many wheels, rarely results in a bladed disk that closes exactly as designed.
Most assembly specifications for wheel assembly call for the locking bucket to exhibit a slight circum- ferential interference with the adjacent blades when it is inserted in the filling slot. This interference tends to force the blades radially out- ward against the root locks.
This effect reduces uncertainty and potential errors in the rotor balancing process due to loose blades. It also tends to create a stiffer blade support during the tenon peening process, where riveted shroud bands are used. Turbine design engineers recog- nize the design advantage of the friction damping that results when adjacent blade roots move relative to one another.