Patent Document

FIELD OF THE INVENTION 
       [0001]    The present invention relates to a nondestructive testing apparatus for a blade root in a steam turbine, e.g., that converts steam energy, generated from combustion heat of nuclear fuel or fossil fuel in a nuclear plant or a thermal plant, to mechanical rotative energy, the testing apparatus capable of reducing time for an automatic testing of the blade root and improving reliability of the testing. 
       BACKGROUND INFORMATION 
       [0002]    Generally, a steam turbine installed at nuclear plants and thermal plants is used to convert heat energy of steam to mechanical rotative energy, by colliding high-temperature high-pressure steam with blades arranged in radial directions. When normally operating, the steam turbine performs at about 30 to 60 rotations per second. Also, the steam turbine is structured in such a manner that first to third stages are arranged sequentially from the center. 
         [0003]    The blades of the steam turbine are not damaged so frequently during power generation. However, once the blades are damaged, the whole facility may be seriously damaged along with stoppage of the power generation operation. Great expenses are incurred to restore the facility. Also, a long repair time is required. 
         [0004]    According to statistics regarding power generation, damage of the blades by crack, corrosion, and parts damage occupies about 30% of accidents of the steam turbine. A centrifugal tension generated by rotation of a rotor of the steam turbine and a bending stress generated by influent steam are among the main stresses causing such damages of the blades. Furthermore, vibration at a nozzle inlet caused by unevenness of the influent steam induces continuous fatigue. 
         [0005]    In relation to design of the steam turbine, differently from fatigue cracks generated by resonant vibration of the blades, the cracks occurring on the blades are mostly caused by corrosion. Furthermore, such cracks are generated most frequently at a root of the blade, where a body of the steam turbine and the blade are fixed, rather than at the blade. 
         [0006]    As illustrated in  FIG. 1 , a steam turbine may be structured as follows. Blades  110  each having a steam turbine tenon  140  and fixed on a disc  120  are radially arranged through 360 degrees. The discs  120  are arranged in plural stages symmetrically with respect to an axis of a turbine rotor  100 . The disc  120  and the blade  130  are interconnected through a blade root  130 . 
         [0007]    Conventional methods for performing a nondestructive testing of the blade root  130  of the steam turbine can be classified into a manual testing and an automatic testing both using ultrasonic waves. According to the ultrasonic automatic testing, as illustrated in  FIG. 2 , the turbine rotor  100  is arranged on a roller  141  and rotated by the roller  141 , thereby testing the blade root  130 . More specifically, a rail  160  is installed parallel with the axis of the turbine rotor  100 , and a testing apparatus  150  tests the respective blade roots  130  by moving along the rail  160  in an axial direction of the turbine rotor  100 . Referring to  FIG. 2 , an ultrasonic probe is mounted to an end of a testing arm  170  to obtain signals corresponding to tested parts. 
         [0008]    As illustrated in  FIG. 3 , the testing can be performed remotely by attaching to the turbine  100  a driver  180  which is magnetically driven. 
         [0009]    However, the above conventional testing methods are believed to bear various problems as follows and therefore are believed to require certain improvements. 
         [0010]    For example, the testing apparatus  150  including the rail  160  weighs much and occupies a large space due to a large volume. Therefore, in a restricted steam turbine room of the power plant, much manpower and equipment are required to operate the system. Especially, since a prop for preventing shaking of the testing apparatus  150  is a kind of heavy goods weighing about 300 kg, it is difficult to handle the testing apparatus  150 . 
         [0011]    Furthermore, the testing arm  170  for reaching the inside of the steam turbine from the outside is long. Therefore, the ultrasonic probe mounted at the restless end of the testing arm  170  can hardly contact with a tested object by a proper pressure. Furthermore, transmission of ultrasonic energy to the tested object cannot be achieved effectively, thereby deteriorating reliability of the testing. 
         [0012]    Moreover, the roller  141  is necessary to rotate the steam turbine at a predetermined speed. In order to test the steam turbine through 360 degrees, a testing apparatus having access to the steam turbine through 360 degrees is required. However, since such a testing apparatus is unavailable, the steam turbine itself needs to be rotated by 360 degrees. 
         [0013]    Still further, in order to disassemble, withdraw and transfer the steam turbine to a testing place, significant manpower and time are required. In addition, there exists an operator&#39;s risk in handling such a heavy system. 
         [0014]    When transferring to a next tested object after completing the test of one steam turbine, reinstallation of the testing apparatus is necessitated, which takes significant time. 
       SUMMARY 
       [0015]    Example embodiments of the present invention provide a nondestructive testing apparatus for a blade root of a steam turbine in a power plant, capable of rotating the steam turbine at a regular speed using a motor attached to the steam turbine to operate the steam turbine with only a steam turbine casing detached instead of with the whole steam turbine withdrawn from an operation place, performing a test from the outside with an ultrasonic probe fixed to a static wing, and performing the test while recording position information by an encoder attached to a predetermined position of the steam turbine and storing ultrasonic signals corresponding to each position, thereby providing carriage of a testing system by minimizing the size of the whole testing system and saving time for the testing of the steam turbine, as well as improving reliability of the test. 
         [0016]    A nondestructive testing apparatus for testing a blade root may be divided largely into two parts. 
         [0017]    One of the parts is an ultrasonic probe fixing unit for testing the blade of the steam turbine, and the other is an encoder fixing unit for recording a rotation distance of a rotating turbine rotor. 
         [0018]    The ultrasonic fixing unit includes a magnetic body having a magnetic body switchably fixed to a fixture supporter, a first steel shaft fixed vertically to its own magnetic body, a second steel shaft connected to the first steel shaft through a first joint and having a three-dimensional degree of freedom to be rotatable and vertically movable, a contacting pressure controller mounted to the second steel shaft to maintain a predetermined pressure between an ultrasonic probe and the blade which is a tested object, and the ultrasonic probe mounted to a leading end of the second steel shaft through the medium of the contacting pressure controller and a second joint to directly contact the turbine rotor which is the tested object. 
         [0019]    The encoder fixing unit includes a magnetic body switchably fixed to the fixture supporter, a first steel shaft fixed vertically to its own magnetic body, a second steel shaft connected to the first steel shaft through a first joint and having a three-dimensional degree of freedom to be rotatable and vertically movable, and an encoder mounted to the second steel shaft rotatably to measure a rotation distance of the turbine rotor. 
         [0020]    The contacting pressure controller for controlling a pressure between the ultrasonic probe and the blade may include a load cell detecting the pressure transmitted from the ultrasonic probe through the second joint, a driving motor generating a rotative motion using values detected by the load cell, and a motional direction converter converting the rotative motion of the driving motor to a linear motion so as to detect and control the pressure applied to the ultrasonic probe. 
         [0021]    Further features and aspects of example embodiments of the present invention are described in more detail below with reference to the appended Figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a perspective view showing the structure of a steam turbine in a general power plant. 
           [0023]      FIG. 2  is a perspective view schematically showing a conventional nondestructive testing apparatus for a blade root of the steam turbine. 
           [0024]      FIG. 3  is a perspective view schematically showing a conventional nondestructive testing apparatus, different from that illustrated in  FIG. 2 , for a blade root of a steam turbine. 
           [0025]      FIG. 4  is a cross-sectional view showing an installation state of a nondestructive testing apparatus for the blade root of the steam turbine, according to an example embodiment of the present invention. 
           [0026]      FIG. 5  is a plan view showing an ultrasonic probe fixing unit of  FIG. 4  mounted to the steam turbine. 
           [0027]      FIG. 6  is a perspective view of the ultrasonic probe fixing unit of the testing apparatus. 
           [0028]      FIG. 7  is a perspective view of an encoder fixing unit of the testing apparatus. 
           [0029]      FIG. 8  is a plan view showing the testing apparatus being rotated by 180 degrees to test the blade root nearby. 
           [0030]      FIG. 9  is a structural view of the whole system for testing the steam turbine using the testing apparatus. 
           [0031]      FIG. 10  shows a screen of signal evaluation showing the result of the testing of the steam turbine using the ultrasonic probe and the encoder. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Exemplary embodiments of the present invention are described in more detail with reference to the accompanying drawings. 
         [0033]    In the following description, the same or corresponding components as in conventional systems will be provided with the same reference numerals. 
         [0034]    As illustrated in  FIGS. 4 and 5 , a nondestructive testing apparatus for a blade root of a steam turbine installed in nuclear plants and thermal plants, performing the testing using a probe, includes an ultrasonic probe fixing unit  200  for testing a blade  110  of the steam turbine and an encoder fixing unit  300  for recording a rotation distance of a rotating turbine rotor  100 . 
         [0035]    Referring to  FIG. 4  and  FIG. 6 , the ultrasonic probe fixing unit  200  includes a magnetic body  201  switchably fixed to a fixture supporter  151 , a first steel shaft  202  fixed vertically to its own magnetic body  201 , a second steel shaft  204  connected to the first steel shaft  202  through a first joint  203  in a horizontal posture with respect to the figure and having a three-dimensional degree of freedom, that is, rotatable and vertically movable, and a contacting pressure controller  205  including a driving motor  206 , a motional direction converter  207  and a load cell  208  that are mounted to the second steel shaft  204  to control a contacting force applied to an ultrasonic probe  210 , and the ultrasonic probe  210  connected to a leading end of the second steel shaft  204  through a second joint  209 . 
         [0036]    The encoder fixing unit  300 , as illustrated in  FIG. 5  and  FIG. 7 , includes a magnetic body  301  switchably fixed to the fixture supporter  151 , a first steel shaft  302  vertically fixed to the magnetic body  301 , a second steel shaft  304  connected to the first steel shaft  302  through a first joint  303  in a horizontal posture with respect to the figure and having a three-dimensional degree of freedom, that is, rotatable and vertically movable, and an encoder  305  mounted to the second steel shaft  304  to be rotatable by 360 degrees so as to measure the rotation distance of the turbine rotor  100 . 
         [0037]    In the above-structured ultrasonic probe fixing unit  200 , the magnetic body  201  is switchably attached to the fixture supporter  151  and the first steel shaft  202  is fixed to the magnetic body  201  in a vertical posture. Since the second steel shaft  204  is connected to the first steel shaft  202  in a horizontal posture through the first joint  203 , the second steel shaft  204  accordingly has a three-dimensional degree of freedom, that is, being movable vertically along the first steel shaft  202  through the first joint  203  and rotatable by 360 degrees. The second steel shaft  204  is equipped with the contacting pressure controller  205  for maintaining a predetermined pressure between the ultrasonic probe  210  and the blade  110  of the steam turbine, which is a tested object. The contacting pressure controller  205  reads a force value of the load cell  208  equal to the pressure transmitted from the ultrasonic probe  210  through the second joint  209 , and converts the rotational motion of the driving motor  206  to the linear motion at the motional direction converter  207  by comparing the read value with a preset value of the load cell  208 . As the second joint  209  is pushed and pulled by the linear motion, the contacting pressure of the ultrasonic probe  210  is controlled so that the ultrasonic probe  210  can contact to a desired position of the blade  110  of the steam turbine, which is the tested object. 
         [0038]    The ultrasonic probe fixing unit  200  is capable of bringing the ultrasonic probe  210  into contact with the blade  110  of the steam turbine without an additional operation in a state where a casing of the steam turbine is opened. When the ultrasonic probe  210  is in contact with the blade  110 , the steam turbine is rotated at a regular speed by an operation motor mounted to the steam turbine, thereby providing the testing to be performed omnidirectionally through 360 degrees. 
         [0039]    In order to perform the automatic testing, it is necessary to record the rotation distance of the turbine rotor  100 . For this purpose, the encoder  305  needs to be attached to a proper position on a body of the steam turbine, to be accessible to the rotating steam turbine at any position. As a device for attaching the encoder  305 , the encoder fixing unit  300  illustrated in  FIG. 7  may be appropriately applied. The encoder fixing unit  300 , while supporting the encoder  300 , brings the encoder  305  into contact with the rotating steam turbine by exerting a predetermined force in a desired direction. 
         [0040]    In the same manner as the ultrasonic probe fixing unit  200 , the encoder fixing unit  300  is also rotatable by 360 degrees by the first steel shaft  302 , the second steel shaft  304 , and the first joint  303 , and therefore is contactable to a desired position as illustrated in  FIG. 5 . 
         [0041]    After the fixture supporter  151  for the ultrasonic probe fixing unit  200  and the encoder fixing unit  300  is mounted, when the testing of one stage is completed, the second joint  204  is rotated by 180 degrees as indicated by an arrow in  FIG. 8  to perform testing with an adjoining stage. Therefore, the root  130  of the blade  110  disposed at the opposite side can be tested. 
         [0042]    As explained above, the testing apparatus hereof is able to perform testing with the roots  130  of the blades  110  of the discs  120  on both sides at one position. Consequently, the testing time and the manpower can be considerably saved. 
         [0043]    Meanwhile, as illustrated in  FIG. 8 , an ultrasonic signal from the ultrasonic probe  210  and the rotation distance according to rotation of the turbine rotor  100  are detected by the encoder  305 , and received by a general ultrasonic transceiver  530 . The received signal is stored in a general personal computer (PC)  510  through a local area network (LAN) to be analyzed and utilized as the result of the testing.  FIG. 9  is a graph displaying a state of the ultrasonic nondestructive testing performed on the PC  510  using the ultrasonic probe  210 . 
         [0044]    In order to perform the ultrasonic nondestructive testing, the ultrasonic probe  210  needs to contact with the tested object through a contacting medium so as to transmit ultrasonic waves to a surface of the tested object. Therefore, a general contacting medium is supplied from an outlet of a contacting medium supplier  550  through a hose  560 . 
         [0045]    Generally, in order to obtain signals with excellent sensitivity during the ultrasonic nondestructive testing, the ultrasonic probe  210  is required to contact with the surface of the tested object by a predetermined pressure. Accordingly, the load cell  208  and the driving motor  206  are provided at a rear end of the ultrasonic probe  210  such that the ultrasonic probe  210  operates the regular pressure against the disc  120 . 
         [0046]    When a conventional testing apparatus is used, it takes about 5 to 12 hours for only installing the testing apparatus. However, the testing apparatus hereof demands just 20 to 30 minutes, thus highly saving the installing time. In addition, it takes less than even 10 hours for opening the steam turbine casing, withdrawing the steam turbine and transferring the steam turbine to the testing place as preparation for the testing. Therefore, the time for preparing the testing can be remarkably reduced. 
         [0047]    Moreover, when using the conventional testing apparatus, it can take more than an hour to move to the next stage after the testing with one stage. On the contrary, the testing apparatus hereof may require only about 5 minutes to do the same. 
         [0048]    To summarize, the testing apparatus according to example embodiments of the present invention is capable of performing a nondestructive testing stably in a simple manner. Furthermore, the time for testing the steam turbine can be greatly reduced. 
         [0049]    As should be appreciated from the above description, example embodiments of the present invention may solve various problems occurring when the steam turbine is tested by the conventional testing apparatus. For example, since considerable time is required from withdrawing the steam turbine for the testing to completing the nondestructive testing, a testing term should sometimes be extended during the operation of the power plant. However, the nondestructive testing apparatus according to example embodiments of the present invention effectively saves time for being installed and removed. Reliability of the testing result can be highly enhanced. In addition, it becomes easier to find and evaluate a defect of the testing apparatus. 
         [0050]    Although example embodiments of the present invention have been described for illustrative purposes, those skilled in the art should appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the present invention.

Technology Category: g