EP4662387A1

EP4662387A1 - Method for improving a low-pressure section rotor blade for a steam turbine, low-pressure section rotor blade for a steam turbine, rotor and steam turbine power plant

Info

Publication number: EP4662387A1
Application number: EP23716897.6A
Authority: EP
Inventors: Sergey Kostyuchenko; Luca Maria Ripamonti; Christoph Pfoster
Original assignee: Ge Vernova Technology GmbH
Current assignee: Ge Vernova Technology GmbH
Priority date: 2023-04-06
Filing date: 2023-04-06
Publication date: 2025-12-17
Also published as: WO2024208427A1; TW202441062A

Abstract

The present disclosure relates to low-pressure section rotor blades (1) for steam turbines as well as to steam turbine rows, steam turbine rotors and to steam turbines. A method for improving one or more low-pressure section rotor blades (1) for a steam turbine comprises determining at least one of vibrations and aeromechanical instabilities of said one or more blades (1) and providing one or more holes (11) in said one or more blades (1) while the blades (1) are connected to a rotor. The one or more holes (11) extend from the tip end (4) towards the root end (2). The aerodynamic contour (3) of the one or more blades (1) remains unchanged.

Description

METHOD FOR IMPROVING A LOW-PRESSURE SECTION ROTOR BLADE FOR A STEAM TURBINE, LOW-PRESSURE

SECTION ROTOR BLADE FOR A STEAM TURBINE, ROTOR AND STEAM TURBINE POWER PLANT

FIELD

[0001] The present disclosure relates to low-pressure section (cylinder) rotor (moving) blades for steam turbines. The present disclosure further relates to rows of low-pressure section rotor blades for steam turbines, to rotors for steam turbines and to steam turbines as well as to power plants. The present disclosure further particularly relates to methods for improving one or more low-pressure section rotor blades for steam turbines.

BACKGROUND

[0002] Steam turbines are rotary mechanical devices that generate mechanical energy from a steam that flows through the turbine. For example, electrical power may be generated when the turbine is connected to a generator. A steam turbine comprises a rotor which includes a shaft and a plurality of rows of blades. A row of blades may also be known as wheel. The shaft is arranged along an axial direction of the turbine, and the plurality of rows of blades is connected to the shaft. The rotor is supported by bearings and is housed in a casing, e.g. a cylinder-shaped casing.

[0003] A plurality of “nozzles” may be mounted to the casing and form part of a stator. The nozzles may also be called “vanes” or "diaphragms". The nozzles are arranged between the rows. The casing and the nozzles form the stationary parts of the steam turbine.

[0004] A row generally comprises a platform mounted on the shaft and a plurality of blades on the platform. These blades may be referred to as moving blades. Stator nozzles on the other hand comprise a plurality of blades attached to the casing and spaced along a circumferential direction of the casing. A pair of a row of nozzles and a subsequent row of moving blades is usually referred to as a stage of a steam turbine. A steam turbine typically comprises a plurality of stages.

[0005] During operation of the steam turbine, the nozzles have a fixed position. They direct the flow of steam between a preceding moving blade row and the next moving blade row. Rows are designed to turn as the steam passes through their moving blades. The rotation of the rows turns the shaft to which they are attached. And the shaft may for example turn a generator for producing electric power.

[0006] The dimensions and shape of the moving blades are relevant for their efficiency, reliability and lifespan. Both the moving blades and the nozzles may generally have an airfoilshaped cross-section and may comprise a leading edge, a trailing edge, a pressure side and a suction side. A cross-section usually changes along a length (i.e. the radial height) of a blade for optimizing an interaction of the blade with the steam and for increasing efficiency. Blades are therefore usually twisted. Blade shapes may also change from one stage to the next, since the thermodynamic conditions (temperature, speed, pressure etc.) will generally be different from one stage to the next.

[0007] A moving blade, i.e. a rotor blade, generally comprises two portions: a root and an airfoil portion. A moving blade may sometimes further include a shroud at its tip end for reducing the vibration of the moving blade. The shroud may extend along a circumferential direction of the moving blade row. The shrouds of neighboring moving blades may be loosely connected for stabilizing the moving blades. A moving blade which does not comprise a shroud at its tip may be referred to as non-shrouded moving blade or a free tip moving blade. Rotor blades having a shroud substantially mid-span (i.e. substantially half way along a length of the blade) are also known.

[0008] Many different factors are to be taken into account in the design of the rotor blade. A rotor blade is also called a moving blade. One important aspect is the tuning of the natural frequencies (also referred to as resonance frequencies or eigenfrequencies) of the moving blades for avoiding frequencies which coincide with certain specific driving or excitation frequencies of the steam flow. If an excitation frequency coincides with one of the natural frequencies of a moving blade, resonance will occur. The moving blade will therefore start to vibrate with a particularly high amplitude, which could damage and even destroy the moving blade and other parts of the turbomachine. Tuning the moving blades allows minimizing or at least reducing the probability of resonance vibrations.

[0009] Another important aspect are the aeromechanical instabilities such as fluttering, rotating stall and buffeting. The moving blades may be tuned for avoiding or at least reducing the presence of aeromechanical instabilities during turbine operation.

[0010] The severity of the vibratory response of the moving blades can be difficult to predict using theoretical tools as it depends on the strength of the excitation. Therefore, frequency tests are usually performed to determine the actual vibratory response of the moving blades. In some examples, the turbine is operated to see how the moving blades of the turbine rotor behave under different conditions, e.g. under different speeds of the steam flow. Blade vibrations may for example be measured by attaching strain gauges to the moving blades. In other examples, a test rig may be used or a rotor may be used. For instance, blades may be attached to the test rig or the rotor for performing blade frequency testing. After performing blade frequency testing, the moving blades, e.g. an outer shape of the airfoil portion of the moving blades, may be modified.

[0011] Blade tuning can be achieved in several ways. Blade tuning may for example include varying the profile of a tip end of the moving blade, or in general varying the outer shape of the moving blade. In order to tune a moving blade, a tip region of a moving blade may be cut, machined and I or grinded. However, such blade tuning may have a detrimental effect on other important parameters such as stress distribution throughout the blade. Cut, machining and grinding of blades can lead to irreversible damages of the blades. The aerodynamic performance of the rotor blades can also be significantly affected, reducing the overall performance of the turbine. Also, thermal efficiency may be reduced, and the moving blades may be made more susceptible to erosion. In addition, if the moving blades are tested when attached to the turbine rotor, the blades which require tuning need to be removed from the rotor to modify them, which adds complexity and makes the process of tuning laborious.

[0012] Another way of limiting the negative effects of resonance may be the use of mixtuning. Mix-tuning includes providing two groups of different moving blades for the same stage or row of blades. The blades of a first group have a different outer shape, e.g. a different airfoil profile of an airfoil portion, than the blades of the other group. Therefore, each group of moving blades has its own natural frequencies, which are different from the natural frequencies of the other group. The moving blades of both groups are arranged alternately around the shaft along a circumferential direction. In this manner, two moving blades which are adjacent in a circumferential direction have different natural frequencies. It is known in the art that aeromechanical instabilities such as flutter, rotating stall or buffeting may be reduced by using a mix-tuned group of moving blades. However, as the outer shape of the blade airfoils is different in the two groups, this may also negatively affect the aerodynamic performance of the blades, and therefore the efficiency of the turbine. Additionally, as two separate manufacturing process are required, e.g. two different molds for each type of blade, this mix-tuning may be time consuming and expensive.

[0013] Pitch-to-chord ratio is another relevant parameter for steam turbines, as it may greatly affect efficiency and losses, in particular of the moving blades of the last stage. A non- optimal pitch-to-chord ratio of the tip portions of the moving blades of the last stage of the turbine may reduce the aerodynamic efficiency. Increasing the chord of the moving blades may help to increase the efficiency, but mechanical integrity may be compromised. Dynamic and static criteria for mechanical integrity may for example include a resonance free blade, allowable bending stress and sufficient mechanical damping; and allowable tensile stress and local (notch) stress in the airfoil as well as a suitable root attachment, respectively. It should be noted that portions of blades are also called sections of blades.

[0014] Pitch-to-chord ratio may have an optimum value between 0.55 and 0.85 for different types of turbine profiles, e.g. depending on whether impulse and/or reaction is going to be used to move the moving blades of a row. Due to mechanical integrity constraints, the pitch- to-chord ratio may be close to 1.0, or may even exceed this value, at a tip region of the last stage blades. If the pitch-to-chord ratio is too high, there is no blade-to-blade channel, and effectively controlling the steam flow may become very difficult, which may lead to a drop in efficiency.

[0015] A steam turbine may comprise sections which operate at different pressures. Turbine sections are also known as cylinders. A steam turbine may for example comprise at least one of a high-pressure (HP) section, an intermediate pressure (IP) section and a low- pressure (LP) section. The size of the blades increases towards the low-pressure section. Throughout this disclosure, the moving blades for a low-pressure section of a steam turbine are generally referred to as low-pressure section rotor blades. It is also possible that a steam turbine comprises only low-pressure section rotor blades, and therefore the steam turbine may only have a single section which is a low-pressure section in these examples.

[0016] The previously discussed disadvantages may be particularly detrimental for low- pressure section rotor blades.

[0017] The present disclosure therefore provides low-pressure section rotor blades, rotors and steam turbines which can mitigate or eliminate one or more of the above disadvantages.

SUMMARY

[0018] In an aspect of the present disclosure, a method for improving of one or more low- pressure section rotor blades for a steam turbine is provided. The method comprises determining at least one of vibrations and aeromechanical instabilities of said one or more low- pressure section rotor blades. Said low-pressure section rotor blades have a root end for attaching the low-pressure section rotor blades to a rotor, a tip end and an airfoil portion between the root end and the tip end. The method further comprises providing one or more holes in the tip end of said one or more low-pressure section rotor blades while said one or more low-pressure section rotor blades are connected to the rotor thereby reducing at least one of the determined vibrations and the determined aeromechanical instabilities. Said one or more holes extend from the tip end towards the root end. An aerodynamic contour of said one or more low-pressure section rotor blades remains unchanged.

[0019] In embodiments, the aerodynamic contour can be understood as including or consisting of pressure and suction sides of a low-pressure section rotor blade. In this or in other embodiments, the aerodynamic contour includes or consists of the outer shape of the airfoil portion. In further embodiments, the aerodynamic contour includes or consists of the outer shape of the airfoil portion and of the outer shape of the root end.

[0020] Accordingly, at least one of vibrations and aeromechanical instabilities of one or more low-pressure section rotor blades are determined. Then, one or more holes are provided in one or more low-pressure section rotor blades when the blades are connected to the rotor. The aerodynamic contour of the blades is not modified, e.g. the blades are not grinded, machined or cut. The one or more low-pressure section rotor blades are therefore improved as at least one of the previously determined vibrations and aeromechanical instabilities are reduced or prevented due to the provision of the one or more holes. Additionally, the possibility of providing said holes while the blades are attached to the rotor avoids problematic and timeconsuming detaching said blades from the rotor and subsequently reattaching said blades to the rotor. These problems are due to how strongly rotor blades are connected to the rotor, i.e. , blades may be damaged even irreversibly during detaching from the rotor. Time is a very relevant factor for any servicing and upgrade as decreasing of time of servicing or upgrades allows to shorten any outages.

[0021] In a further aspect of the disclosure, a low-pressure section rotor blade for a steam turbine is provided. The blade includes a root end for attaching the low-pressure section rotor blade to a rotor, a tip end, an airfoil portion between the root end and the tip end. The blade also includes one or more holes for reducing at least one of vibrations and aeromechanical instabilities, said one or more holes are provided only in the tip end and extend from the tip end towards the root end. An aerodynamic contour of said low-pressure section rotor blade remains unchanged. In embodiments, the most convenient way to maintain intact the aerodynamic contour is by placing of said one or more holes on a camber line of the tip end. This is especially relevant for blades with complex shape, e.g., with higher degree of twist. This can be made, for example, by defining the camber line at the tip end and subsequently providing said one or more holes on the camber line. This allows for placing said one or more holes in an equal distance from both pressure and suction sides.

[0022] In all aspects of the disclosure, as the aerodynamic contour of the one or more low- pressure section rotor blades remains unchanged, the blades are improved without losing aerodynamic efficiency and resistance to erosion, e.g. in comparison to a situation in which the blades are cut, machined or grinded. Aerodynamically efficient low-pressure section rotor blades and an aerodynamically efficient rotor for a steam turbine may accordingly be obtained. As providing one or more holes may tune the rotor blades, a single mold, instead of two different molds, may be used for manufacturing the rotor blades.

[0023] Also, as the one or more low-pressure section rotor blades, and in particular a last stage blade, may be longer than the rotor blades for other sections of the steam turbine and may therefore be more prone to suffer damage from aeromechanical instabilities and vibrations, this method may be particularly suitable for improving said one or more low- pressure section rotor blades.

[0024] At least one of aeromechanical instabilities and vibrations can occur in a process aimed at reducing weight of rotor blades. In other words, a rotor blade that is undergoing weight reduction will require limiting or preventing at least one of aeromechanical instabilities and vibrations in order to be used in a steam turbine. The invention allows obtaining and be applied to weight reduction of rotor blades. This allows to obtain previously impossible to manufacture rotor blade designs.

[0025] The aeromechanical instabilities may include at least one of fluttering, rotating stall and buffeting. The vibrations may include at least one of a first, a second and a third mode of blade resonance vibrations.

[0026] The one or more low-pressure section rotor blades may be non-shrouded low- pressure section rotor blades. The method may be particularly suitable for this type of blades, as they may be more affected by aeromechanical instabilities and vibrations than shrouded low-pressure section rotor blades.

[0027] In a further aspect of the disclosure, a low-pressure section rotor blade for a steam turbine improved by the method of the previous aspect is provided. As previously explained, when the above method is performed in one or more low-pressure section rotor blades for a steam turbine, the one or more low-pressure section rotor blades are improved as at least vibrations and/or aeromechanical instabilities are reduced.

[0028] In a further aspect of the disclosure, a rotor including at least one low-pressure section rotor blade according to the previous aspects is provided.

[0029] In some examples, all blades in a row have been improved according to the methods described herein. In some examples, at least some low-pressure section rotor blades do not comprise any holes. In some examples, a distribution or dimensions of holes vary between low-pressure section rotor blades. [0030] In some examples, each row of the rotor including the low-pressure section rotor blade of the previous aspects also includes one or more further low-pressure section rotor blades. Each said further low-pressure section rotor blade includes a narrower tip end when compared to the tip end of said low-pressure section rotor blade of the previous aspects. In each row, every two low-pressure section rotor blades are separated with one further low- pressure section rotor blade.

[0031] In a further aspect of the disclosure, a steam turbine comprising a rotor according to the previous aspect is provided.

[0032] In a further aspect of the disclosure, a fossil, renewable-energy, waste-to-energy, combined-cycle or nuclear powerplant comprising the steam turbine of the previous aspect or the rotor of the precedent aspect is provided.

[0033] Elements of the aspects and examples can be combined together to from new aspects and examples which are within the scope of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1A schematically illustrates a schematic example of a low-pressure section rotor blade for a steam turbine.

[0035] Figure 1 B schematically illustrates a cross-section along the line A-B of the airfoil portion of the low-pressure section rotor blade of figure 1 A.

[0036] Figure 2A schematically illustrates a perspective top view of a low-pressure section rotor blade comprising holes.

[0037] Figure 2B schematically shows a perspective side view of a tip region of the low- pressure section rotor blade of figure 2A.

[0038] Figure 3 shows a flow chart of a method for improving one or more low-pressure section rotor blades for a steam turbine.

DETAILED DESCRIPTION OF EXAMPLES

[0039] Reference now will be made in detail to embodiments of the present disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation only, not as a limitation. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents.

[0040] A schematic example of a low-pressure section rotor blade, in particular of a nonshrouded low-pressure section rotor blade, is shown in figure 1A. A low-pressure section rotor blade 1 generally comprises two portions: a root or root end 2 and an airfoil portion 3. The root 2 serves for connecting the low-pressure section rotor blade to the shaft, e.g. by form fitting or welding the root to the hub. The airfoil portion 3 is the aerodynamic portion that is configured to interact with the steam that passes by. The interaction may cause a force that contributes to turning the row in which the low-pressure section rotor blade 1 is located. The tip end or tip 4 of the low-pressure section rotor blade 1 rotates close to the stator casing when the row of the steam turbine rotates.

[0041] The low-pressure section rotor blade 1 of the example of figure 1A may have an increased length as compared to more upstream pressure sections. A height or length of a low-pressure section rotor blade, in particular of a last stage blade of a steam turbine, may exceed one meter. A blade length may be measured between the root end and the tip end of the blade along a radial direction of the row, the rotor or the steam turbine. An increased length of the blades may make the blades more prone or vulnerable to vibrations and aeromechanical instabilities.

[0042] Figure 1 B schematically illustrates a cross-section along the line A-B of the airfoil portion 3 of figure 1A. When the blade 1 is mounted to the rotor, the line A-B would be perpendicular to the radial and axial directions of the steam turbine and thus corresponds to a circumferential direction.

[0043] The cross-section of figure 1 B shows the leading edge 5 and the trailing edge 6 of the airfoil portion 3. The pressure side 7 and the suction side 8 of the airfoil portion 3 are also shown. An imaginary line 9 connecting the leading edge 5 with the trailing edge 6 is referred to as the chord of the airfoil portion 3. A camber line 10 is also illustrated.

[0044] A low-pressure section rotor blade may be manufactured using metal alloys such as steel or titanium alloys. A low-pressure section rotor blade, as well as a nozzle, may for example be manufactured by investment casting. A negative die of the blade shape may be provided, and the internal walls of the die may then be covered with a material such as wax to form a blade pattern. The blade pattern may be coated with a heat-resistant material to make a shell, and then that shell may be filled with the metal alloy to form the blade. [0045] According to an aspect of the disclosure, a method 30 is provided. The flow chart of figure 3 illustrates method 30, which is a method for improving one or more low-pressure section rotor blades for a steam turbine.

[0046] The method comprises, at block 31 , determining at least one of vibrations and aeromechanical instabilities of said one or more low-pressure section rotor blades. As previously explained, the low-pressure section rotor blades 1 have a root end 2 for attaching the low-pressure section rotor blades to a rotor, a tip end 4 and an airfoil portion 3 between the root end 2 and the tip end 4. The method comprises, at block 32, providing one or more holes 11 in said one or more low-pressure section rotor blades 1 while said one or more low-pressure section rotor blades are connected to the rotor thereby preventing at least one of the determined vibrations and the determined aeromechanical instabilities. Said one or more holes extend from the tip end 4 towards the root end 2.

[0047] In this manner, one or more low-pressure section rotor blades are provided with one or more holes such that at least one of vibrations and aeromechanical instabilities are limited or prevented.

[0048] Also, instead of having to remove a plurality of low-pressure section rotor blades from a rotor to improve them, (for example to frequency tune them after determining at least one of vibrations and aeromechanical instabilities) e.g. by decommissioning the turbine, transporting the blades to a warehouse, changing an outer shape of the airfoil portions, transporting back the blades and mounting them again, the improving may be performed on site without removing the low-pressure section rotor blades. This may allow for a faster assembly of the rotor and a faster commissioning. Aeromechanical instabilities are tested using steam and so in a steam turbine, e.g., at a power plant. Vibrations, in particular resonance, can be tested outside of a steam turbine, e.g., a rotor is tested at a factory. The invention is to be applied both to tests done in a steam turbine (which includes tests at a power plant) and to tests done at a factory.

[0049] In addition, an overall centrifugal force of the low-pressure section rotor blades comprising holes may be reduced. Relief of the root attachment may also be obtained.

[0050] The aeromechanical instabilities may include at least one of fluttering, rotating stall and buffeting. The vibrations may include at least one of a first, a second and a third mode of blade resonance vibrations.

[0051] By providing the one or more holes, an eigenfrequency of the one or more low- pressure section rotor blades can for example be tuned, i.e. set to a desired value, without the need to modify an aerodynamic contour. As an aerodynamic contour of the one or more low- pressure section rotor blades remains unchanged, aerodynamic efficiency may be maintained, and the low-pressure section rotor blades may not be made prone to erosion.

[0052] Tuning or adjusting an eigenfrequency may refer to modifying one or more low- pressure section rotor blades and achieving that the blades have a desired eigenfrequency. The aerodynamic contour of the one or more blades is however not modified. I.e., tuning or adjusting an eigenfrequency goes beyond merely altering an eigenfrequency of the one or more blades and rather relates to achieving a desired eigenfrequency.

[0053] Determining at least one of vibrations and aeromechanical instabilities may comprise in some examples obtaining an indication of how the one or more low-pressure section rotor blades vibrate under loads. The loads may be representative for operational loads. In some examples, the determination may allow measuring or calculating one or more eigenfrequencies, i.e. one or more frequencies at which the one or more low-pressure section rotor blade resonate. The determination may also help to estimate in which conditions, e.g. under which loads, the one or more blades may get damaged or may break.

[0054] An example of a low-pressure section rotor blade 1 in which one or more holes 11 , e.g. a first plurality of holes 11 , has been provided is shown in figures 2A and 2B. Figure 2A schematically shows a perspective top view of a steam turbine blade 1 and figure 2B schematically shows a perspective side view of a tip region of the steam turbine blade 1 of figure 2A. A tip region of a turbine blade as described herein may refer to a blade region closest to the tip 4 of the blade which has a length of, or less than, a third of a length of the blade.

[0055] The one or more low-pressure section rotor blades may be non-shrouded low- pressure section rotor blades.

[0056] In some examples, the one or more low-pressure section rotor blades may have a same outer shape. I.e., within normal manufacturing tolerances, all the low-pressure section rotor blades, e.g. of a rotor row, may have a same outer shape. For example, all the low- pressure section rotor blades may have been manufactured by using a same mold.

[0057] The one or more holes 11 may longitudinally extend from the tip end 4 to a longitudinal position of the airfoil portion 12 in some examples. Providing longitudinally extending holes may be easier than providing holes in other directions. The holes may be provided by drilling or other machining. Throughout the present disclosure, “longitudinally extending” may refer to an extension along a radial direction of a row and therefore of a steam turbine. A longitudinal position may herein be understood as a position along the airfoil portion which is separated from a tip end of the low-pressure section rotor blade. In some examples, at least one of the one or more holes may at least partially extend to the root end. In some examples, the step of determining at least one of vibrations and aeromechanical instabilities of said one or more low-pressure section rotor blades may be conducted while said one or more low-pressure section rotor blades are connected to the rotor. For example, the method may further comprise connecting the one or more low-pressure section rotor blades to the rotor, e.g. to a steam turbine shaft, before the step of determining at least one of vibrations and aeromechanical instabilities.

[0058] The step of determining at least one of vibrations and aeromechanical instabilities of said one or more low-pressure section rotor blades may include rotating the rotor at a speed within a range between a design speed of the steam turbine and a speed that is equal to 150% of the design speed of the steam turbine. The method may further comprise tuning a torsional mode of the one or more low-pressure section rotor blades, specifically a third natural mode. Providing the one or more holes may tune a third torsional mode of the corresponding low- pressure section rotor blade. A third mode may therefore be tuned and positioned in the right frequency range without having to cut or grind the low-pressure section rotor blade. The low- pressure section rotor blade is therefore not made susceptible to erosion. And as the aerodynamic contour of the blade is linked to aerodynamic efficiency, if the aerodynamic contour of the blade remains unchanged, aerodynamic efficiency may be maintained while avoiding resonance vibrations, in particular of the third mode.

[0059] The one or more low-pressure section rotor blades may comprise a pitch-to-chord ratio below 0.9, optionally below 0.87, and optionally equal to 0.86. The efficiency of the low- pressure section rotor blades may be increased as the chord of the low-pressure section rotor blades also increases and no grinding/machining of the outer surface may be necessary. Providing the one or more holes also facilitates meeting mechanical integrity constraints: desired pitch-to-chord ratios may be adopted, and the efficiency may be increased without causing problems of mechanical integrity. This may be particularly applicable to a tip region of a low-pressure section rotor blade, and more in particular to a tip region of a last stage blade.

[0060] In some examples, the one or more holes may be provided on alternating blades of a row of low-pressure section rotor blades. For example, the one or more holes may be provided on each second low-pressure section rotor blade of the row of blades. A row of low- pressure section rotor blades may in general comprise a plurality of low-pressure section rotor blades connected to a shaft for a steam turbine.

[0061] By interleaving the provision of holes, mix-tuning may be performed. But instead of having two groups of low-pressure section rotor blades with different outer shapes, e.g. a different profile of an airfoil portion, it is the presence of the plurality of holes in alternating low- pressure section rotor blades which provides two groups of blades, each group having its own eigen frequencies different from the eigenfrequencies of the other group. A first group of blades may be formed by the low-pressure section rotor blades comprising the holes whereas a second group of blades may be formed by the low-pressure section rotor blades which do not include such holes. The low-pressure section rotor blades of both groups may be arranged alternately around the shaft along a circumferential direction of the shaft.

[0062] In some examples, the method may further comprise providing one or more additional holes 13 through the airfoil portion 3 of the one or more of the low-pressure section rotor blades, e.g. on each second (non-shrouded) low-pressure section rotor blade, such that the additional holes 13 extend from the longitudinal position 12 of the airfoil portion 3 to an additional longitudinal position 14 of the airfoil portion 3. The vibrational frequencies may be controlled more easily in this manner. The additional longitudinal position 14 may be closer to the blade root 2 than the (first) longitudinal position 12.

[0063] The one or more holes may be empty. E.g. the one or more first holes 11 and the one or more additional holes 13 may be empty.

[0064] At least one of the distribution, depth and size of the holes may be determined on the basis at least of one of excitation measurements and calculations. For example, the size, e.g. diameter, the depth, the separation between holes, e.g. along a direction of a camber line 10 of the blade 1 , and the number of holes may be adapted to the low-pressure section rotor blade which is to be improved, e.g. tuned. A shape and size of a low-pressure section rotor blade, e.g. a length of the blade, how twisted it is and/or an outer shape of the blade may influence one or more of the previously mentioned features of the holes. Some aspects which the first 11 and/or additional 13 of holes may incorporate will be indicated further below.

[0065] Besides being suitable for manufacturing and commissioning a rotor for a steam turbine or the steam turbine from scratch, method 30 may also be used to retrofit at least one row of a steam turbine. In some examples, all the old low-pressure section rotor blades of the row may be removed. The new low-pressure section rotor blades may be connected to the rotor shaft and tested, i.e. at least one of vibrations and aeromechanical instabilities may be determined. One or more holes may then be provided in one or more low-pressure section rotor blades, e.g. to each second low-pressure section rotor blade, to improve the low-pressure section rotor blades.

[0066] In other examples, half of the old low-pressure section rotor blades of the row may be removed. For example, the low-pressure section rotor blades which had previously been cut or grinded to e.g. be frequency tuned may be removed. Unmodified low-pressure section rotor blades, e.g. uncut and/or non-grinded blades, may be left attached to the rotor. New low- pressure section rotor blades having a same outer shape than the low-pressure section rotor blades that remain attached to the rotor may replace the old removed blades. The new blades may interleave with the old blades. After determining at least one of vibrations and aeromechanical instabilities, one or more of holes may be provided to the new or the old blades.

[0067] Still in other examples, no old blade may be removed, and one or more holes may be provided in one or more of the old blades of the row.

[0068] A more efficient row, and therefore a more efficient steam turbine, may be obtained in of the above examples of a method for retrofitting a steam turbine row. Further examples of retrofitting methods may be possible.

[0069] The row may be a row of a last stage of the steam turbine. The low-pressure section rotor blades of the row of the last stage of the steam turbine may be longer than the low- pressure section rotor blades of the other stages of the steam turbine and may therefore be more affected by aeromechanical instabilities, vibrations and resonance. The last stage low- pressure section rotor blades may particularly benefit from having one or more first, and optionally one or more additional holes, provided on them.

[0070] According to a further aspect of the disclosure, a low-pressure section rotor blade for a steam turbine improved by the method described herein is provided.

[0071] The blade may for example comprise a root end for attaching the low-pressure section rotor blade to a rotor, a tip end and an airfoil portion between the root end and the tip end. The blade may comprise one or more holes which are provided only in the tip end and which extend from the tip end towards the root end such that at least one of vibrations and aeromechanical instabilities are prevented or at least reduced when the blade is attached to a rotor for, or of, a steam turbine, and the rotor is rotated. The aerodynamic contour of said low- pressure section rotor blade is not affected by said one or more holes.

[0072] According to a further aspect of the disclosure, a rotor for a steam turbine which includes at least one low-pressure section rotor blade as described herein is provided.

[0073] The rotor may comprise a shaft and at least one row of low-pressure section rotor blades connected to the shaft. In some examples, each blade in at least one (low-pressure) row of the rotor is a low-pressure section rotor blade as described herein. I.e., in these examples each blade in at least one row comprises one or more holes and is therefore improved as explained herein.

[0074] In other examples, each (low-pressure) row of the rotor including said low-pressure section rotor blades may also include one or more further low-pressure section rotor blades. Each said further low-pressure section rotor blade includes a narrower tip end when compared to the tip end of said low-pressure section rotor blade. In each row, every two low-pressure section rotor blades are separated with one further low-pressure section rotor blade.

[0075] That is to say, one further low-pressure section rotor blade is arranged between two low-pressure section rotor blades improved as explained herein. The further blade has a tip end narrower than a tip end of the improved blades.

[0076] In some examples, the low-pressure section rotor blades of a row may have a same outer shape (within tolerances). In other examples, an outer shape may vary among the row blades.

[0077] In some examples, at least one row may comprise a first group of improved blades and a second group of non-improved blades. The first group of blades of the row of blades may comprise one or more first holes extending from the tip end towards the root end, optionally to a first longitudinal position of the airfoil portion. The second group of blades of the row of blades may not comprise the one or more first holes extending from the tip end.

[0078] The low-pressure section rotor blade(s) of the previous aspect may belong to the first group and the further low-pressure section rotor blade(s) may belong to the second group in some examples.

[0079] As previously explained in this disclosure, efficiency may be maximized while resonance risk may be minimized.

[0080] In some of these examples, both groups of low-pressure section rotor blades may only differ, within tolerances, in that first group of blades comprises the one or more first holes and the second group of blades does not include them. I.e., the blades of both groups may have a same aerodynamic contour. In other examples, the two groups of blades may have different aerodynamic contours.

[0081] In an improved blade, the one or more first holes may longitudinally extend from the tip end 4 to the first longitudinal position 12 of the airfoil portion 3.

[0082] In some examples, a blade of the second group of blades, optionally a further low- pressure blade, may comprise one or more second holes extending from the tip end towards a root end, e.g. to a second longitudinal position of the airfoil portion, the second holes being different from the first holes. The one or more second holes may longitudinally extend from the tip end to a different longitudinal position of the airfoil portion. Alternatively, or additionally, the size and/or shape and/or position along a chord of the holes in the second group of blades may be different from the holes in the first group of blades.

[0083] The blades of the first group of blades may be interleaved with the blades of the second group of blades.

[0084] The one or more first holes 11 may extend to a same first longitudinal position 12 of the airfoil portion 3, as illustrated in the example of figure 2B. In other examples, the one or more first holes may extend up to different first longitudinal positions. For example, some holes may be longer and some holes may be shorter. Or in other words, the one or more first holes 11 may have different depths in some examples.

[0085] A length of the one or more first holes 11 may be less than one half, in particular less than one third, and more in particular less than a 10% of a length of the blade 1. As a region close to a tip end 4 may be particularly susceptible to vibrations, providing holes in such a region may be beneficial and sufficient for avoiding or at least mitigating the risk of resonances.

[0086] The one or more first holes may be empty. I.e., additional material is not added to the holes. In this manner, for example the tuned natural frequencies of the low-pressure section rotor blade 1 may be kept in place and the vibrational behavior of the blade may remain predictable.

[0087] In some examples, the first group of blades may comprise one or more additional holes 13 extending, e.g. longitudinally extending, from the first longitudinal position 12 to an additional longitudinal position 14, e.g. of the airfoil portion 3. The additional holes 13 may help to adjust a blade behavior, for example to fine tune the eigenfrequencies of a corresponding low-pressure section rotor blade 1 , and in particular the third torsional mode, in some examples.

[0088] Like the one or more first holes 11 , the one or more additional holes 13 may extend to a same additional longitudinal position, e.g. of the airfoil portion. I.e., these holes may have different lengths or depths. The one or more additional holes 13 may also be empty. In some examples, one or more of the additional holes may reach the blade root and extend along at least a portion of the root.

[0089] A sum of a length of the longest of the one or more first holes 11 and of a length of the longest of the one or more additional holes 13 may be less than one half, in particular less than one third of a length of the low-pressure section rotor blades 1. If the one or more first holes and/or the one or the additional holes have a same length, it may be understood that all the first holes (or additional holes) are a longest hole, and the length of any of the holes may be taken for performing the sum. [0090] The number of first holes 11 may be lower than the number of additional holes 13. This may allow to properly tune the third mode of the blade more easily at least in some examples.

[0091] A dimension of the one or more first holes 11 along a direction of a camber line 10 may be larger than a dimension of the one or more additional holes 12 along a direction of a camber line 10. I.e., in some examples the one or more first holes 11 may be thicker or wider than the one or more additional holes 12. This may be beneficial for suitably adjusting a blade behavior, e.g. for suitably tuning high eigenfrequencies of the low-pressure section rotor blade.

[0092] The row of blades may be a last stage row. I.e., the one or more low-pressure section rotor blades may be last stage blades. As last stage blades may be longer and aeromechanical instabilities and vibrations may negatively affect them, improving the one or more low-pressure section rotor blades as described herein may decrease the risk of damage due to aeromechanical instabilities, vibrations and resonance.

[0093] In some examples, all the blades of a rotor row may be made of a same material. For example, the low-pressure section rotor blades may be metallic. Having a blade made of a single type of material, instead of e.g. including a metallic portion and a composite portion, may facilitate the manufacturing of the blade. Manufacturing the blade may also be easier and cheaper.

[0094] The one or more low-pressure section rotor blades may be non-shrouded.

[0095] In some examples, the low-pressure section rotor blades may have a pitch-to-chord ratio below 0.9, in particular below 0.87, for example equal to 0.86.

[0096] The explanations provided with respect to one or more low-pressure section rotor blades 1 and the holes may also be applicable to the method 30 described above.

[0097] According to a further aspect of the disclosure, a steam turbine comprising a rotor as described herein, e.g. with respect to the previous aspect, is provided.

[0098] A fossil, renewable-energy, waste-to-energy, combined-cycle or nuclear powerplant comprising a steam turbine as described herein or a rotor as described herein is also provided.

[0099] According to a further aspect, a low-pressure section rotor blade, optionally a nonshrouded low-pressure section rotor blade, for a steam turbine is provided. The low-pressure section rotor blade comprises a root 2, an airfoil portion 3 and one or more first holes 11 extending, e.g. longitudinally extending, from a tip end 4 of the airfoil portion 3 to a first longitudinal position 12 of the airfoil portion 3. The airfoil portion 3 further comprises one or more additional holes 13 extending, e.g. longitudinally extending, from the first longitudinal position 12 to an additional longitudinal position 14 of the airfoil portion 3. The features and explanations described with respect to the low-pressure section rotor blades comprising the first 11 and the additional holes 13 with respect to the previous aspects are also applicable to this aspect.

[00100] According to a further aspect of the disclosure, a non-shrouded low-pressure section rotor blade 1 for a steam turbine is provided. The low-pressure section rotor blade comprises a root and an airfoil portion. The airfoil comprises a plurality of holes which tune a third mode of a torsional frequency of the low-pressure section rotor blade. At least a portion of the holes longitudinally extend from a tip end of the low-pressure section rotor blade.

[00101] This written description uses examples to disclose a teaching, including the preferred embodiments, and also to enable any person skilled in the art to put the teaching into practice, including making and using any devices or systems and performing any incorporated methods. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. Elements from the various embodiments, aspects and examples described, as well as other known equivalents for each such aspects, can be mixed and matched by one of ordinary skill in the art to construct additional embodiments, aspects and examples in accordance with principles of this application which are part of the disclosure. If reference signs related to drawings are placed in parentheses in a claim, they are solely for attempting to increase the intelligibility of the claim, and shall not be construed as limiting the scope of the claim.

Claims

1. A method for improving one or more low-pressure section rotor blades for a steam turbine, the method comprising: determining at least one of vibrations and aeromechanical instabilities of said one or more low-pressure section rotor blades, wherein said low-pressure section rotor blades have a root end for attaching the low-pressure section rotor blades to a rotor for the steam turbine, a tip end and an airfoil portion between the root end and the tip end; and providing one or more holes in the tip end of said one or more low-pressure section rotor blades while said one or more low-pressure section rotor blades are connected to the rotor thereby reducing at least one of the determined vibrations and the determined aeromechanical instabilities; wherein said one or more holes extend from the tip end towards the root end and an aerodynamic contour of said one or more low-pressure section rotor blades remains unchanged.

2. A low-pressure section rotor blade for a steam turbine, wherein the low-pressure section rotor blade includes a root end for attaching the low-pressure section rotor blade to a rotor, a tip end, an airfoil portion between the root end and the tip end, and one or more holes for reducing at least one of vibrations and aeromechanical instabilities, said one or more holes are provided only in the tip end and extend from the tip end towards the root end and an aerodynamic contour of the low-pressure section rotor blade is not affected by said one or more holes.

3. The method of claim 1 or the low-pressure section rotor blade of claim 2, wherein the aeromechanical instabilities include at least one of fluttering, rotating stall, and buffeting.

4. The method of claims 1 or 3 or the low-pressure section rotor blade of claims 2-3, wherein the vibrations include at least one of a first, a second, and a third mode of blade resonance vibrations.

5. The method of claims 1 or 3-4 or the low-pressure section rotor blade of claim 2-4, wherein said one or more low-pressure section rotor blades are non-shrouded low- pressure section rotor blades.

6. The method of claims 1 or 3-5, wherein the step of determining at least one of vibrations and aeromechanical instabilities of said one or more low-pressure section rotor blades is conducted while said one or more low-pressure section rotor blades are connected to the rotor.

7. The method of claim 1 or 3-6, wherein the step of determining at least one of vibrations and aeromechanical instabilities of said one or more low-pressure section rotor blades includes rotating the rotor at a speed within a range between a design speed of the steam turbine and a speed that is equal to 150% of the design speed of the steam turbine.

8. The method of any of claims 1 or 3-7 or the low-pressure section rotor blade of claim 2- 5, wherein said one or more low-pressure section rotor blades comprise a pitch-to-chord ratio below 0.9 or 0.87 or a pitch-to-chord ratio equal to 0.86.

9. The method of any of claims 1 or 3-8 or the low-pressure section rotor blade of claim 2- 5 or 8, wherein at least one of said one or more holes at least partially extends through the airfoil portion or to the root end.

10. The method of any of claims 1 or 3-9 or the low-pressure section rotor blade of claim 2- 5 or 8-9, wherein said one or more holes are empty.

11 . The method of any of claims 1-10 or the low-pressure section rotor blade of claim 2-5 or 8-10, wherein at least one of the distribution, depth, and size of the holes is determined on the basis of at least one of excitation measurements, and calculations.

12. The method of any of claims 1-11 or the low-pressure section rotor blade of claim 2-5 or 8-11 , wherein the aerodynamic contour includes or consists of pressure and suction sides of the low-pressure section rotor blade, or wherein the aerodynamic contour includes or consists of the outer shape of the airfoil portion, or wherein the aerodynamic contour includes or consists of the outer shape of the airfoil portion and of the outer shape of the root end.

13. A rotor including at least one low-pressure section rotor blade of any of claims 2-5 or 8-

14. The rotor of claim 13, wherein each blade in at least one row of the rotor is a low-pressure section rotor blade of any claims 2-5 or 8-12.

15. The rotor of claim 13, wherein each row of the rotor including said low-pressure section rotor blade includes also further low-pressure section rotor blades, wherein each said further low-pressure section rotor blade includes a narrower tip end compared to the tip end of said low-pressure section rotor blade and wherein in each row every two low- pressure section rotor blades are separated with one further low-pressure section rotor blade.

16. A steam turbine comprising the rotor according to any of claims 13 - 15.

17. A fossil, renewable-energy, waste-to-energy, combined-cycle or nuclear powerplant comprising the steam turbine as claimed in claim 16 or the rotor as claimed in any of claims 13 - 15.