Claims:Claims
We claim
1. A novel method for testing natural frequency of rotor blades using special tools, wherein the method comprises steps of:
cleaning, rotor blades and grooves;
mounting, the said rotor blades by using specially designed technological pieces, wherein the mounting steps, includes:
fixing, plurality of specially designed technological pieces in rotor grooves;
tightening, the rotor blades by providing a plurality of specially made adopters, taper plate wedges with shorter handles to suit to the pneumatic hammering device;
fixing, accelerometers on the blades and the blade is excited with special hammer, which does not give any bouncing effects;
recording, the response signal in Fast Fourier Transform (FFT) analyzer; and
comparing, the recorded natural frequencies with given natural frequency.

2. According to the novel method as discussed in claim1, manual hammering is eliminated and hence the slippage is avoided and there is no chance of damaging the blades or rotor labyrinth rings while tightening the technological pieces and while removing the technological pieces after NFT test.

3. According to the novel method as discussed in claim1, man power requirement is reduced.

4. According to the novel method as discussed in claim1, safety to the operating persons is increased by avoiding manual hammering.

5. According to the novel method as discussed in claim1, more number of blades can be tested in one position resulting in reduction of number of rotor positions during the test.
6. According to the novel method as discussed in claim1, cycle time is saved at an average of 30 to 40 percent when compared with the conventional test method.

7. According to the novel method as discussed in claim1, no damage to the technological pieces while removing the same after test.

8. A novel natural frequency testing system and method, wherein the system comprises of:
a plurality of specially designed technological pieces that are used to mount on a plurality of rotor grooves used for tightening rotor blades;
a plurality of proprietary made special adopters for tightening & removing the blades;
a plurality of specially made taper plate wedges with shorter handles to fit with a pneumatic device used for tightening & removing the blades; and
the said method includes steps of tightening the blades using the specially designed technological pieces, specially made taper plate wedges with shorter handles and adopters to fit with help of a pneumatic hammering device.
, Description:Technical Field of the Invention

[0001] The invention relates to a novel method of using specially designed tools for testing natural frequency of steam turbine rotor blades. More particularly, relates to special tools for tightening and loosening of rotor blades for carrying out natural frequency test without causing collateral damage.

Background of the Invention

[0002] Any rotating component exhibits a natural frequency of motion which offers minimum resistance to the cyclic excitation of forces. Natural frequency is dependent on the mass of the component and the stiffness, it offers to external forces. The geometry and the material properties and fixity conditions dictate the natural frequency of any component. Important components like steam turbine blades particularly blades of large length are so designed that the natural frequencies are away from the prevailing excitation frequencies during the normal operating ranges of the machine. This is done for increasing the fatigue life of the component to avoid resonance conditions. Statics reveal that the LP turbine free-standing blades are generally more susceptible to failure. There are many mechanisms by which LP turbine blades can fail. The failures are related to fatigue, stress corrosion cracking and corrosion fatigue. Commonly these failures initiate at different locations like blade roots and rotor grooves where blades are fixed.

[0003] The two primary forces acting on the blades are the steady centrifugal forces due to rotation and the fluctuating steam bending forces. Though the natural frequencies can be theoretically established, it is necessary to check the same, so that the conditions of the natural frequencies are away from the exciting frequencies. The natural frequency, which is dependent on root contact, increases with the fixity of the blade and attain a constant value. For the purpose of natural frequency testing, the fixity condition is simulated in the stationary root by fixing specially made technological pieces in the slot at the root of the blade on the rotor hub. The natural frequency is measured in this condition to ensure that it lies within the acceptable limit by proper contact in the blade root. The natural frequency testing is usually done on free standing blades every time when the blades are removed after long operation before refitting. A typical LP turbine rotor with different stages of free standing blades where NFT is carried out is shownin Fig 1.

[0004] After cleaning, the LP rotor blades are tested for natural frequencies. During the natural frequency test, every blade is fixed on the rotor hub with specially made technological pieces to suit the slot in the rotor groove where the locking pins are mounted. In this natural frequency test, technological tightening pieces, copper rods, suitable taper plate tools, etc.; are used for tightening the blades.

[0005] Prior to the natural frequency test, all the free standing blade roots and the corresponding rotor grooves are to be cleaned thoroughly. After cleaning, the LP rotor blades are tested for natural frequencies. Every blade is tightly fixed on the rotor hub with specially made technological pieces to suit the slot in the rotor groove where the locking pins are mounted. With the technological pieces, suitable taper plates with long handles of length approximately 600 to 750 mm length are used. To tighten the technological pieces, force is applied by manual hammering on the taper plates with long handle.

[0006] An accelerometer is mounted on the blade and the blade is excited with special hammer which does not give bouncing effects. The response signal is recorded in FFT analyzer. Whenever the natural frequencies of the blade is not matching with the given natural frequency data range, the test is repeated 3 to 4 times with the improved cleaning and fixing the blade. If the natural frequencies of the tested blades are not matching with the given reference values even after repeated tests, the same are replaced with suitable new blades. Technological pieces are removed after the test with manual hammering.

[0007] Manual hammering in conducting this test, being a manual method leaves vacuum in terms of safety of the equipment and people working. One disadvantage with the discussed traditional method is, usage of heavy hammers, manual hitting on taper wedge plates with long handles to exert a force while tightening technological pieces for fixing the blades in the rotor groove and a slippage may cause damage to the blades. While tightening the technological pieces with copper rod, due to accidental slippage of hammer, labyrinth rings can be damaged as shown in Fig. 2. Technological pieces may bend and damage due to uncontrolled force as shown in Fig 7b.

[0008] Due to accidental slippage of hammer, there is a chance of injuries to the persons involved in the operation. In addition to the instrument operator, two to three persons are required to tighten and dismantling the blades for Natural frequency test.
.
[0009] Moreover, natural frequency test is carried out in a particular position of the rotor for the convenience of tightening technological pieces which requires number of rotations of rotor and consumes more time. There is a chance of bending the technological pieces while removing them after test with manual hammering.

[0010] In order to overcome the above mentioned disadvantages, there is a need to develop tools to use and a method of tightening and removing the technological pieces for natural frequency test on rotor blades.

Brief Summary of the Invention

[0011] The present invention recognizes the limitations of the prior art and the need for systems and methods that are able to provide assistance to users in a manner that overcomes these limitations.

[0012] According to a first aspect of present invention, a method of testing natural frequency of rotor blades comprising a plurality of rotor blades on a rotor is disclosed. A plurality of specially designed technological pieces are inserted and tightened in grooves on the rotor. A proprietary made adopters with suitable extension matching to pneumatic device is used for tightening the blades. A plurality of tapper plate wedges with shorter handles to fit with a pneumatic device used for tightening the blades.

[0013] According to the first aspect of the present invention, the testing comprising steps of mounting the rotor blades on the rotor hub with specially made technological pieces to match with the slots in the rotor grooves is disclosed. The rotor blades are provided with tapper plate wedges with shorter handles to fit with the help of pneumatic hammering device. Thereafter, accelerometer is mounted on the plurality of rotor blades.

Brief Description of the Drawings

[0014] Various objects and advantages of the present invention will become apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments, in conjunction with the accompanying drawings, wherein like reference numerals have been used to designate like elements, and wherein:

FIG. 1 illustrates a typical LP turbine rotor showing different stages of blades where natural frequency test (NFT) is carried out according to the present invention.

FIG. 2 illustrates a perspective view, depicting a damaged labyrinth rings in existing method according to the present invention.

FIG .3 illustrates a perspective view depicting a wedge handle to remove the technological piece on opposite labyrinth ring side in improved method according to the present invention.

FIGS. 4a-4b illustrates a perspective views, depicting an existing technological piece and an improved technological piece for tightening the freestanding blades of turbine according to the present invention.

FIGS. 5a-5b illustrates a perspective views, depicting an existing technological piece and an improved technological piece for tightening the turbine blades on rotor grooves labyrinth ring side according to the present invention.

FIGS. 6a-6b illustrates a perspective views, depicting an existing tapper plate tool and an improved tapper plate tool according to the present invention.

FIGS. 7a-7b illustrates a perspective view depicting an original technological piece and a damaged technological piece with the existing method.

FIGS. 8a-8b illustrates perspective view, depicting wedge handles for removing the technological pieces according to the present invention.

FIGS. 9a-9b illustrates a side views depicting a rotor with blades in existing and invented methods according to the present invention.

FIGS. 10a-10b illustrates a perspective views, depicting an improved (invented) special adopters according to the present invention.

FIGS. 11a-11b illustrates a perspectives view, depicting an existing natural frequency method and a specially designed natural frequency test method according to the present invention.

FIGS. 12 illustrate a perspective view, depicting rotor damage due to hammer slippage in existing method according to the present invention.

FIG. 13 illustrates a perspective view, depicting a natural frequency test operating with one person handling according to the present invention.

Detailed Description of the Invention

[0015] `The present invention is directed towards a method for testing natural frequency of rotor blades. Referring to the drawings, wherein like reference numerals designate identical or corresponding systems, preferred embodiments of the present invention are described.

[0016] Referring to the drawings, FIG. 1 illustrates a typical LP turbine rotor 100 showing different stages of blades where natural frequency test (NFT) is carried out according to the present invention. The natural frequency testing is usually done on free standing blades 102 every time when the blades 102 are removed after long operation before refitting.

[0017] After cleaning, the LP rotor blades 102 are tested for natural frequencies. During the natural frequency test, every blade 102 is fixed on the rotor hub 128 with specially made technological pieces to suit the slot in the rotor groove where the locking pins are mounted. In this natural frequency test, technological tightening pieces with special adopters to suit pneumatic tool, suitable taper plate tools, etc. are used for tightening & removal of the blades.

[0018] FIG. 2 illustrates a perspective view 200 depicting a damaged labyrinth rings in existing method according to the present invention. The existing natural frequency test in manual hammering method uses heavy hammers 206 to exert a force while tightening the technological pieces 210 on labyrinth ring side 208a for fixing the blades 202 in the rotor groove of blade root 202 in rotor hub 228 and, due to accidental slippage of hammer 206, may cause damage to labyrinth rings 208b.

[0019] FIG. 3 illustrates a perspective view 300 depicting a wedge handle to remove the technological piece on opposite labyrinth ring side in improved method according to the present invention. To remove the technological pieces 310b on the opposite side of labyrinth rings, a wedge handle 312 suiting to the pneumatic tool 322b is used. To remove the technological pieces 310a on the side of labyrinth ring, wedge handles 316a and 316b suiting to the pneumatic tools 322b are used.
[0020] FIGS. 4a-4b illustrates a perspective views 400a-400bdepicting an existing technological piece and an improved technological piece for tightening the freestanding blades of turbine according to the present invention. All the free standing blade roots and the corresponding rotor grooves are to be cleaned thoroughly. After cleaning, the LP rotor blades are tested for natural frequencies. Every blade is tightly fixed on the rotor hub with specially made technological pieces410b to suit the slot in the rotor groove where the locking pins are mounted during operation. In the existing method, tightening of the technological pieces410a may cause damage to the rotor.

[0021] FIGS.5a-5b illustrates a perspective views 500a-500b depicting an existing technological piece and an improved technological piece on labyrinth ring side for tightening the turbine blades on rotor groove according to the present invention. Every blade is tightly fixed on the rotor hub with specially made technological pieces510b to suit the slot in the rotor groove where the locking pins are mounted. In the existing method, tightening of technological pieces 510a may cause the damage to labyrinth rings and rotor.

[0022] FIGS. 6a-6b illustrates a perspective views 600a-600b depicting an existing tapper plate tool and an improved tapper plate tool according to the present invention. A tapper plate tool 614b with wedge plate and shorter handle is used to match the pneumatic tool for tightening the blades opposite to labyrinth ring side. In existing method, a tapper plate 614a with longer handle for tightening the technological pieces to suit the manual hammering. The tapper plate tool 614a with wedge plate with longer handle is used for tightening the blade may cause of hitting the rotor blade resulting to damage.

[0023] FIGS. 7a-7b illustrates a perspective views 700a-700b depicting an existing technological piece and a damaged technological piece according to the present invention. Technological piece 710a is tightened by applying force by manual hammering on the taper plates with long handle. The tapper plates with long handle for tightening the technological piece 710a itself may be damaged as shown in fig710b.

[0024] FIGS. 8a-8b illustrates perspective views 800a-800b depicting a wedge handle for removing the technological pieces according to the present invention. The technological pieces on labyrinth ring side after natural frequency test are removed by wedge handles 816a and 816b to suit the pneumatic tool are used.

[0025] FIGS. 9a-9b illustrates a side views 900a-900b depicting an existing a rotor and rotor with blades on two sides according to the present invention. In existing method, the position of the rotor with blades 902a on one side. In improved method, the position of the rotor with blades 902b on both sides for finding the natural frequency (NFT).

[0026] FIGS. 10a-10b illustrates a perspective views depicting special adopters in improved method according to the present invention. Specially designed technological tightening pieces are mounted on the locking grooves tightly in the slots provided for locking pins mounted between the rotor hub and blade root. Special adopter’s 1024a and1024b are mounted on both sides of the blades for tightening. In existing method, tightening of technological pieces on labyrinth ring side may cause labyrinth ring damage while hitting with hammer.

[0027] FIGS. 11a-11b illustrates a perspective view depicting 1100a-1100b depicting the existing natural frequency test method and a specially designed natural frequency test method according to the present invention. In existing method, technological pieces 1110a, opposite side of labyrinth side are tightened by applying force through manual hammering 1106a on the tapper plate with long handle. The manual hammering 1106a with uncontrolled force on tapper plate with long handle and any slippage of heavy hammer, there is a chance of hitting the rotor 1118a, blades 1102a causing damage and also injury to the operator. To avoid these disadvantages, the novel method as shown in FIG.11b introduces a tapper plate tool with wedge tool and shorter handle is used to match the pneumatic tool 1122b for tightening the blades 1102 opposite side of labyrinth ring.

[0028] In existing method, technological pieces 1110b on labyrinth ring side are tightened by applying force through manual hammering 1106b on the copper rod 1120a which drives technological piece. Manual hammering 1106b with uncontrolled force on copper rod and any slippage of heavy hammer, there is a chance of hitting labyrinth ring resulting to damage, FIG. 2 to avoid this damage, a novel method as shown in FIG. 11b, a special adopter 1120b to suit to pneumatic tool 1122a is used to tighten the blades on labyrinth ringside.

[0029] This novel method comprising a tapper plate with shorter handle for easy operation of removing the technological pieces 1110a and 1110b to suit the pneumatic tools 1122a and 1122b. For removing the technological pieces 1110b and 1110a on labyrinth ring side and opposite side after NFT, wedge handles to suit the pneumatic tool 1122a and 1122b are used.

[0030] FIG. 12 illustrates perspective view 1200 depicting rotor damage due to hammer slippage in existing method according to the present invention. A tapper plate with longer handle for tightening the technological pieces 1210 to suit the manual hammering1206 is used which may has chances of hitting the rotor blade causing damage and hammer mark on slippage 1204 is shown.

[0031] FIG. 13 illustrates a perspective view, depicting a novel natural frequency test operating with one person handling according to the present invention. In this novel method, operators can handle the test without any injuries with less man hours. Hammering, damage to the rotor blades, rotor labyrinth rings due to accidental slippage is avoided while tightening the technological pieces. The pluralities of blades are tested in a single position and safety is improved for the operating person 1326.