Abstract:
An apparatus and a method are provided for the inspection of ferromagnetic components using magnetic particles, particularly for in-situ inspection of parts of power plants, with the apparatus including a movable chassis defining a magnetic yoke with a central opening over a surface of the component to be inspected and a discharge nozzle to inject the magnetic particles onto the surface, a magnetic field source for generating a magnetic field on at least part of the surface below the opening and a probe to measure a representation of the spatial distribution of the magnetic particles on the surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to PCT/EP2013/050113 filed Jan. 04, 2013, which claims priority to European application 12150392.4 filed Jan. 6, 2012, both of which are hereby incorporated in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to devices and methods for inspecting metal components and installation such as parts of large power generating plants, such as turbine blades, turbine rotors, electro-magnetic generators, boilers and pipes. 
       BACKGROUND 
       [0003]    The in-situ or site inspection of turbine components by means of non-destructive methods (NDT) is a service which is typically carried out at intervals, in accordance to scheduled outages. In order to get access to the components of interest, many inspections require the dismantling of components resulting in lengthy and costly operations. 
         [0004]    For NDT applications, the selection of the testing method and technique to use depends on the type of defects to be detected, the state of the part and its geometry. There are cases where the metallurgical characteristics of the components, their surface conditions, or the access to the area of inspection, limit the use of the available inspection technologies, requiring further disassembly or even the removal of the components from site. 
         [0005]    For ferromagnetic components there are only a few NDT methods commonly used for the detection of defects exposed at the surface, such as cracks, holes, fissures and the like. They include the testing with eddy currents, ultrasonic probes or by Magnetic Particle Inspection (MPI). 
         [0006]    Typically, the methods based on eddy currents use a small coil for scanning the inspection area within a band of three to five millimeter width. This relatively small area of sensitivity of a single sensor device results in time consuming routines when the inspection is performed manually. Though larger probes with multiple coils are available, they tend to be heavy and expensive and hence not easy to move and to use for site inspections. Further, the eddy currents are changed by the roughness and the geometry of the surface effectively reducing the sensitivity of the method for complex shaped components or for surfaces exposed to corrosion or erosion as many parts of a turbine. 
         [0007]    Another well-known method is the inspection with ultrasonic waves. The waves are generated using transducers and are projected into the component to be inspected. Surface and subsurface discontinuities in the material scatter or reflect the ultrasonic wave. The transducers can detect the scattered or reflected signal and determine the location and shape of the discontinuity. 
         [0008]    Metal Particle Inspection (MPI) on the other hand is used to detect surface and subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The process injects a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source. Small magnetic particles are spread in a dry or wet environment across the surface of the component to be inspected and attracted to areas around cracks and other surface defects as the the magnetic field becomes inhomogeneous at these defects. After a part has been magnetized, it is often required to be demagnetized again before use. This and other requirements for MPI methods and equipment are subject of several national and international standards, including for example the ASTM E 1444-05 standard. 
         [0009]    In some variants of MPI, a special illumination of the surface is used to cause fluorescence of the particles to increase visibility and hence the sensitivity of the method. 
         [0010]    Conventional MPI tools are commonly wet horizontal machines with large coils. Handheld devices using a simple yoke are also known. Both types of known instruments are either impossible or difficult to use for power plant inspection, if, for example, the turbine blades are kept in the rotor or if the stator of the turbine is closed. 
         [0011]    General inspection tools and methods for various purposes of relevance to the present invention can be found for example in the U.S. Pat. Nos. 6,316,845, 7,624,827 and 7,958,955 and the published U.S. applications 20080289421, 20090314089 and 20110174565. 
         [0012]    Specific MPI inspection tools are described in the U.S. Pat. Nos. 4,950,989 and 6,316,845 providing further background information relevant to the implementation of magnets, electromagnetic coils, yokes and magnetic particles. 
         [0013]    Magnetic switchable devices are described in several publications including F. Rochat, P. Schoeneich, M. Bonani, S. Magnenat, F. Mondada, H. Bleuler, and H. Christoph “Design of Magnetic Switchable Device (MSD) and applications in climbing robot” in: H. Fujimoto, M. O. Tokhi, H. Mochiyama, and G. S. Virk, editors,  Emerging Trends in Mobile Robotics , pages 375-382, Nagoya, 2010. World Scientific. 
         [0014]    In light of the known prior art, the present invention can be regarded as addressing problems relating to mobility, size and general improvement of the applicability of MPI tools and methods, particularly in the field of on-site and in-situ inspections of parts of power generation plants, turbines, boilers, pipes and similar devices. 
       SUMMARY 
       [0015]    According to an aspect of the present invention, there is provided an apparatus for the inspection of ferromagnetic components using magnetic particles, particularly for in-situ inspection of parts of power plants, with the apparatus including a movable chassis defining a magnetic yoke with a central opening over a surface of the component to be inspected and a discharge nozzle to inject the magnetic particles onto the surface, a magnetic field source for generating a magnetic field on at least part of the surface below the opening and a probe to measure a representation of the spatial distribution of the magnetic particles on the surface. 
         [0016]    At least part of the chassis is advantageously made of ferromagnetic material in a ring-like shape with a central opening. This design can be seen as forming a yoke in a plane essentially parallel to the surface. The ring includes extensions of ferromagnetic material extending out of the plane of the ring in direction of the surface to be inspected. The extension are preferably designed such that the gap between the ring and the surface is reduced locally to less than 5 mm, more preferable less than 2 mm and even more preferably less than 1 mm. In a variant the gap width can be controlled by lowering or raising the chassis using for example screws or other adjustment devices. 
         [0017]    For mobility the chassis can be mounted for example on three or more rolling elements such as rolls, wheels, spheres or on gliding elements such as skates. 
         [0018]    The overall dimensions of the device are suitable for use as a mobile device and preferably as a handheld device. To improve the mobility, the longest horizontal dimension is preferably less than 200 mm. Its height is preferably less than 100 mm. Its weight is preferably less than 5 kg. 
         [0019]    The discharge opening to inject the magnetic particles includes for example a nozzle attached to a reservoir of magnetic particles. This reservoir can be mounted on the chassis or, alternatively, it can be separated from the chassis. In the latter case, a tube connects the discharge opening with the remote reservoir. 
         [0020]    A preferred embodiment of the magnetic field source for generating a magnetic field includes at least two rotatably mounted magnets mounted onto or within the ring-shaped frame, preferably at juxtaposed positions across the opening of the frame. With the magnets in place, the ferromagnetic frame forms a continuous yoke without a gap and with a central opening. A preferred variant includes two pairs magnets mounted such that one magnet of each pair can be actively rotated with the frame either manually or be motor means while the other magnet of each pair can follow the rotation. The magnets can be permanent magnets or include electromagnetic coils. 
         [0021]    In a preferred embodiment of the invention, magnetization and demagnetization is achieved through rotation of one or more magnets, preferably in a sequence of rotations with decreasing amplitude of the rotating angles. 
         [0022]    A preferred embodiment of the probe to measure a representation of spatial distribution includes an optical detector such as a camera for recording images of the surface under inspection. 
         [0023]    In a variant of this preferred embodiment the probe to measure a representation of spatial distribution includes further a high intensity source of radiation, preferably electromagnetic radiation. This source can be for example a light emitting diode emitting ultraviolet (UV) light. 
         [0024]    The chassis can further include a sensor which is capable of measuring location information or derivatives of location information such as velocity and direction to determine an absolute or relative position of the apparatus on the surface. 
         [0025]    According to another aspect of the invention there is provided a method for inspecting of ferromagnetic components using magnetic particles, particularly for in-situ inspection of parts of power plants such as airfoils of a turbine being mounted onto a rotor, the method including the step of placing onto the airfoil an apparatus for the inspection of ferromagnetic components using magnetic particles with the apparatus comprising a movable chassis defining a magnetic yoke with a central opening over a surface of the component to be inspected and a discharge nozzle to inject the magnetic particles onto the surface, a magnetic field source for generating a magnetic field on at least part of the surface below the opening and a probe to measure a representation of the spatial distribution of the magnetic particles on the surface and measuring a representation of the spatial distribution of the magnetic particles on the surface. 
         [0026]    In a preferred variant of this aspect of the invention, the magnetic field is switched between a state where a significant part of the field is directed into the surface through out-of-plane extensions of a ring-shaped frame of ferromagnetic material and a state where at least most of the field is confined within the ring-shaped frame without entering the surface. 
         [0027]    These and further aspects of the invention will be apparent from the following detailed description and drawings as listed below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Exemplary embodiments of the invention will now be described, with reference to the accompanying drawings, in which: 
           [0029]      FIGS. 1A and 1B  show a magnetic switchable device for use in an inspection tool in accordance with an example of the invention; 
           [0030]      FIG. 2  shows another magnetic switchable device for use in an inspection tool in accordance with an example of the invention; 
           [0031]      FIGS. 3A-3C  illustrate a demagnetization process in accordance with an example of the invention; and 
           [0032]      FIG. 4  illustrates the arrangement of elements in an inspection tool in accordance with an example of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    Aspects and details of examples of the present invention are described in further details in the following description. The operating principles on which the following examples of the invention are based are first illustrated making reference to  FIG. 1A-1B . The figures show views of the top and the side of a frame structure and magnets as can applied in the examples. 
         [0034]    In  FIG. 1A  there is shown a ring-shaped frame  10  of ferromagnetic material. The thickness of the frame is 6 mm and its width is 45 mm and length 40 mm. The frame  10  has an approximately square opening  101  of 20 mm width and length. The frame has extensions  102  of ferromagnetic material in direction of a surface  11  to be inspected. The height of the extension is about 10 mm giving the top of the frame a 16.5 mm clearance to the surface  11 . The gap between the bottom face of the extensions  102  and the surface  11  can be adjusted within the range of 0 or 0.1 mm to 4 mm. 
         [0035]    Also embedded into the frame are two disk-shaped permanent magnets  12 ,  13  with a diameter of 10 mm diameter and a height of 6 mm. One magnet  12  carries a fin (not shown) which couples the magnet to a micromotor (not shown). The micromotor can rotate the magnet  12  within the ferromagnetic frame  10 . The other magnet  13  is similarly mounted within the frame. 
         [0036]    The magnets  12 ,  13  are arranged to form a magnetic switchable device which can be switched between a first state and a second state as is illustrated in the  FIGS. 1A and 1B  respectively. In the first state the poles of the magnets are rotated relatively to each other such that the magnetic field as indicated by arrows remains within the frame  10  which acts a yoke. In this state none or minimal parts of the magnetic field are directed into the ferromagnetic surface  11 . In the second state the orientation of the poles is changed such that the magnetic field is at least partly directed into the extensions  102  and through the extensions into the surface  11  providing a magnetization of the surface within the area between the extensions, i.e. the area beneath the opening. 
         [0037]    By rotating both magnets  12 ,  13 , the orientation of the north and south pole of the magnet can be interchanged, resulting in a magnetic field having a direction in the surface  11  opposite to the direction shown in  FIG. 1B . This switch of direction can be used for demagnetizing the surface in a manner described in more detail below. 
         [0038]    The operating principles illustrated in  FIGS. 1A and 1B  can be extended to more than two magnets. An example with four magnets is shown in  FIG. 2  adding strength and more control options to the switching process. In the example of  FIG. 2 , two magnets  21 ,  22  are designed to be rotated actively using motors, while two magnets  23 ,  24  are mounted such that they are capable of rotating within the frame  20 . The rotation of one magnet of a pair  21 ,  23 ,  22 ,  24  is sufficient to cause a rotation of the paired magnet, hence it is not always necessary to provide all magnets of a pair with a rotating motor or handle. As shown in  FIG. 2  the magnets  21 , 22 , 23 , 24  are rotated around a point outside of and also eccentric from the central opening. 
         [0039]    In the example, the magnets are oriented at an angle (for the first pair  21 ,  23  of magnets) or (for the second pair  22 ,  24  of magnets) with respect to the frame. As in the example above the poles of the magnets can be rotated to switch between the first and second state and also reverse the direction of the magnetic field to provide a demagnetization of the surface. 
         [0040]    As shown in  FIGS. 3 , the angles and hence the relative orientation of the magnets determines the strength of the magnetic field and its direction in the inspected surface. The two branches of  FIG. 3A  show how the angles α and β can be varied for a demagnetization process. The resulting magnetic field strength in the sample is shown in the curve of  FIG. 3B . A significant demagnetization of the sample can be already achieved within 6 cycles. The reduction in the remanent field strength after a given number of cycles is illustrated in  FIG. 3C . 
         [0041]    An inspection tool exemplifying the above principles is shown in  FIG. 4 . The frame structure described above is used as a chassis  40  to provide a small, light-weight and readily movable tool. The chassis and the elements mounted onto it are protected by a dome-shaped outer shell  41 . The outer shell  41  can be made of any nonmagnetic material such as plastics or non-magnetic metal. 
         [0042]    The chassis  40  itself includes the iron frame with the magnets  42 , a central opening  401  and extensions  402  as described above. Further mounted onto the chassis  40  is a nozzle  43  through which magnetic particles can be sprayed. The nozzle is charged through a tube  431 , which in turn is connected to a remote reservoir of dry magnetic particles or a (wet) suspension of magnetic particles  432 . The particles  432  are preferably fluorescent. Such particles are known and commercially available for example under the tradename Magnaglo® 14 A. 
         [0043]    In addition, the chassis carries the optics  44  of a camera system including a pair of UVA LEDs  441  which emit UV radiation causing fluorescence of the magnetic particles. The camera system includes further elements such as images sensor and image display units, which depending on size or energy consumption may or may not be installed as part of the apparatus. In the case of a remote image detection and display system, the link  442  between the optical components and the remote parts can be made using an optical fiber cable  442  as shown or wireless signal transmission. 
         [0044]    The inspection tool is further shown to include three spherical wheels  45 , mounted such that the apparatus can be moved into arbitrary directions on the surface  46  to be inspected. A position encoder  47  detects the motion of the apparatus on the surface  46  and its output signal can be used to record the position of the apparatus and hence the location of any defects  461  of the surface detected by it. The manner in which the encoder is mounted and operates allows for the recording of a map of defect location in two or three dimensions. 
         [0045]    In operation the inspection tool is positioned onto the surface  46  to be examined. This surface can be for example the blade of a steam turbine fixed to a turbine rotor within a power plant. As the apparatus is moved on the surface by hand or with a small motor, the magnetic particles  432  are sprayed onto the surface through the nozzle  43 . The cavity formed by the chassis around the nozzle  43  limits the spread of the magnetic particles beyond the part of the surface  46  immediately below the tool. 
         [0046]    The magnets  42  are switched to provide a magnetic field H within the surface and are switched to either demagnetize the surface again or to move the tool along the surface. 
         [0047]    The particles are illuminated using the pair of UVA LEDs  441  and when illuminated start to emit fluorescent light. The camera  44  provides a continuous monitoring of the surface  46  which is traversed by the apparatus and registers the spatial distribution of light emitted or reflected from the magnetic particles  432  thus representing their own spatial distribution. Through visual inspection or by using computer-based image processing the sequence of images can be evaluated for the presence of surface or near surface defects  461  in the material. 
         [0048]    The position related information as measured by the position encoder  47  can be utilized to determine and display of the location of any defects detected. The present invention has been described above purely by way of example, and modifications can be made within the scope of the invention, particularly as relating to the shape and design of the chassis and the arrangement of elements it carries. The invention also consists in any individual features described or implicit herein or shown or implicit in the drawings or any combination of any such features or any generalization of any such features or combination, which extends to equivalents thereof. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments. 
         [0049]    Each feature disclosed in the specification, including the drawings, may be replaced by alternative features serving the same, equivalent or similar purposes, unless expressly stated otherwise. 
         [0050]    Unless explicitly stated herein, any discussion of the prior art throughout the specification is not an admission that such prior art is widely known or forms part of the common general knowledge in the field.