Abstract:
A method &amp; apparatus for conveying an ultrasonic sensor about an outer peripheral surface of a tube to perform a non-destructive evaluation thereof is disclosed. The ultrasonic sensor is mounted on a flexible tension link and the method involves extending the flexible tension link while urging an extended portion of the flexible tension link towards the outer peripheral surface of the tube at a plurality of locations along it&#39;s length, thereby causing the ultrasonic sensor to move through an arc in a direction circumferentially around the tube while maintaining ultrasonic coupling between the ultrasonic sensor and the tube.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    This invention relates generally to non-destructive evaluation of tubes and more particularly to non-destructive evaluation of a tube using an ultrasonic sensor. 
         [0003]    2. Description of Related Art 
         [0004]    Processing plants, such as power generation petrochemical and plants for example, make extensive use of metal tubing for delivering steam and other fluids to various process steps. Commonly, such tubes are constructed by welding together sections of tubing. In most applications, and specifically in safety critical applications, each and every weld must be inspected to confirm the integrity of the weld. Such inspections may be performed during the construction or commissioning of a plant, but there is also a need for ongoing periodic inspection of welds to look for signs of corrosion and/or cracking, for example. 
         [0005]    In many plants pluralities of tubes are run in close proximity to each other, often so closely spaced apart that access to the tube for performing an inspection of the weld may be difficult. While it is convenient to use ultrasonic non-destructive evaluation techniques for examining defects in many types of metal structures, the accessibility has limited the use of ultrasonic examination for inspecting tubes. 
         [0006]    Accordingly, other inspection techniques such as gamma radiography have typically been used to perform such inspections. Gamma radiography involves the use of radioactive isotopes, and during radiographic inspections, radiographic shielding barriers must generally be erected to protect personnel from harmful radiation. Accordingly, the work of welders and other construction workers may be interrupted when it becomes necessary to clear an area in proximity to a tube being inspected. 
         [0007]    In general the use of radioactive inspection techniques may cause unacceptable health and safety risks, and regardless of storage, handling, and transportation practices that may be implemented, human error may still occur resulting in accidents. 
         [0008]    Accordingly there is a need for improved methods of inspection of tubes. 
       SUMMARY OF THE INVENTION 
       [0009]    In accordance with one aspect of the invention there is provided a method for conveying an ultrasonic sensor about an outer peripheral surface of a tube to perform a non-destructive evaluation thereof, the ultrasonic sensor being mounted on an extendible portion of a flexible tension link. The method involves extending the flexible tension link while urging the extended portion of the flexible tension link towards the outer peripheral surface of the tube at a plurality of locations along the extended portion to cause the ultrasonic sensor to move through an arc circumferentially around the tube while maintaining ultrasonic coupling between the ultrasonic sensor and the tube. 
         [0010]    Extending may involve extending the flexible tension link to cause the ultrasonic sensor to be conveyed from a first location on the outer peripheral surface of the tube to a second location the outer peripheral surface of the tube, the second location being spaced apart from the first location by substantially one complete revolution around the circumference of the tube, and the method may further involve performing the non-destructive evaluation while retracting the flexible tension link to cause the ultrasonic sensor to move from the second location to the first location. 
         [0011]    The method may involve causing a reference mark to be made on the tube at at least one of the first location and the second location. 
         [0012]    The method may involve introducing an ultrasonic coupling fluid between the sensor and the outer peripheral surface of the tube to further facilitate ultrasonic coupling between the ultrasonic sensor and the tube. 
         [0013]    Introducing the ultrasonic coupling fluid may involve delivering a flow of ultrasonic coupling fluid between the ultrasonic sensor and the outer peripheral surface of the tube. 
         [0014]    The flexible tension link may include a plurality of linked elements and the ultrasonic sensor may be mounted on at least one of the linked elements, and maintaining ultrasonic coupling between the ultrasonic sensor and the tube may involve urging the at least one of the linked elements towards the outer peripheral surface of the tube such that the ultrasonic sensor is urged into contact with the outer peripheral surface of the tube. 
         [0015]    The flexible tension link may include a plurality of linked elements and urging the extended portion of the flexible tension link may involve urging each of the plurality of linked elements toward the outer peripheral surface of the tube thereby causing the extended portion of the flexible tension link to wrap around the tube. 
         [0016]    The flexible tension link may be housed in a frame and the ultrasonic sensor may be mounted on a distal end of the flexible tension link, and extending may involve causing the distal end to be extended from the frame. 
         [0017]    A proximate end of the flexible tension link may be coupled to an actuator on the frame, and causing the distal end of the flexible tension link to be extended may involve actuating the actuator to cause the distal end of the flexible tension link to be extended from the frame. 
         [0018]    The method may involve clamping the frame to the outer peripheral surface of the tube before extending the flexible tension link. 
         [0019]    The outer periphery of the tube may include an accessible portion and an inaccessible portion, the inaccessible portion including a portion of the outer periphery of the tube having limited access due to a surrounding environment, and the clamping may involve clamping the frame to the accessible portion of the outer periphery of the tube. 
         [0020]    The ultrasonic sensor may include a first ultrasonic sensor and the flexible tension link may include a first flexible tension link and the method may further include a second ultrasonic sensor mounted on a second flexible tension link, the first and second ultrasonic sensors being spaced apart, the method further involving extending the second flexible tension link while urging a second extended portion of the second flexible tension link towards the outer peripheral surface of the tube at a plurality of locations along the second extended portion to cause the second ultrasonic sensor to move through an arc circumferentially around the tube while maintaining ultrasonic coupling between the second ultrasonic sensor and the tube. 
         [0021]    The method may involve locating a circumferential position of the ultrasonic sensor on the peripheral surface of the tube. 
         [0022]    Locating the circumferential position may involve determining a displacement of the flexible tension link. 
         [0023]    Determining the displacement may involve coupling an encoder to the flexible tension link, the encoder being operable to produce a position signal representing the displacement. 
         [0024]    The ultrasonic sensor may be coupled to an analyzer, the analyzer being operable to transmit electrical stimuli to the ultrasonic sensor and to receive electrical signals from the ultrasonic sensor representing results of the non-destructive evaluation of the tube, and the method may further involve receiving the position signal at the analyzer and correlating the results with the circumferential position of the ultrasonic sensor. 
         [0025]    In accordance with another aspect of the invention there is provided an apparatus for conveying an ultrasonic sensor about an outer peripheral surface of a tube to perform a non-destructive evaluation thereof. The apparatus includes a frame operably configured to engage the tube and a flexible tension link housed in the frame, the flexible tension link being operably configured to permit the ultrasonic sensor to be mounted thereto. The apparatus also includes an actuator coupled to the flexible tension link and operable to cause the flexible tension link to extend from the frame. The flexible tension link is operably configured to urge an extended portion of the flexible tension link towards the outer peripheral surface of the tube at a plurality of locations along the extended portion to cause the ultrasonic sensor to move through an arc circumferentially around the tube while maintaining ultrasonic coupling between the ultrasonic sensor and the tube. 
         [0026]    The actuator is operably configured to extend the flexible tension link to cause the ultrasonic sensor to be conveyed from a first location on the outer peripheral surface of the tube to a second location the outer peripheral surface of the tube, the second location being spaced apart from the first location by substantially one complete revolution around the circumference of the tube, the actuator further being operably configured to retract the flexible tension link to cause the ultrasonic sensor to move from the second location to the first location while performing the non-destructive evaluation of the tube. 
         [0027]    The apparatus may include a coupling fluid supply line operably configured to introduce an ultrasonic coupling fluid between the ultrasonic sensor and the outer peripheral surface of the tube to further facilitate ultrasonic coupling between the ultrasonic sensor and the tube. 
         [0028]    The ultrasonic coupling fluid may include water. 
         [0029]    The flexible tension link may include a plurality of linked elements and the ultrasonic sensor may be mounted on at least one of the linked elements, the at least one linked element further including a spring operable to urge the at least one of the linked element towards the outer peripheral surface of the tube such that the ultrasonic sensor is urged into contact with the outer peripheral surface of the tube. 
         [0030]    The flexible tension link may include a plurality of linked elements and the apparatus may further include a plurality of springs operating between the linked elements to urge the extended portion of the flexible tension link toward the outer peripheral surface of the tube such that the extended portion wraps around the tube when the flexible tension link is extended. 
         [0031]    The ultrasonic sensor may be mounted on a distal end of the flexible tension link and the actuator may be coupled to a proximate end of the flexible tension link and actuating the actuator may cause the proximate end to be displaced thereby causing the distal end is extended from the frame. 
         [0032]    The actuator may include a rotateable drum located in the frame, the proximate end of the flexible tension link being coupled to the drum and an un-extended portion of the flexible tension link may wrap around the drum. 
         [0033]    The frame may further include a clamp operable to clamp the frame to the outer peripheral surface of the tube. 
         [0034]    The clamp may be operably configured to clamp the frame to an accessible portion of the outer periphery of the tube. 
         [0035]    The ultrasonic sensor may include a first ultrasonic sensor and the flexible tension link may include a first flexible tension link and the apparatus may further include a second ultrasonic sensor mounted on a second flexible tension link, the first and second ultrasonic sensors being spaced apart, the apparatus further including a second flexible tension link housed in the frame, the second flexible tension link being operably configured to permit the second ultrasonic sensor to be mounted thereto, the flexible tension link being coupled to the actuator, the actuator being operable to cause the second flexible tension link to be extended from the frame. The second flexible tension link may be operably configured to urge a second extended portion of the second flexible tension link towards the outer peripheral surface of the tube at a plurality of locations along the second extended portion to cause the second ultrasonic sensor to move through an arc circumferentially around the tube while maintaining ultrasonic coupling between the second ultrasonic sensor and the tube. 
         [0036]    The apparatus may include an encoder, coupled to one of the flexible tension link, the actuator, and the ultrasonic sensor, the encoder being operably configured to produce a position signal representing a circumferential position of the ultrasonic sensor on the peripheral surface of the tube. 
         [0037]    The apparatus may include an analyzer coupled to the ultrasonic sensor, the analyzer being operable to transmit electrical stimuli to the ultrasonic sensor and to receive electrical signals from the ultrasonic sensor representing results of the non-destructive evaluation of the tube, the analyzer further comprising an input for receiving the position signal from the encoder, the analyzer being operably configured to correlate the results with the circumferential position of the ultrasonic sensor. 
         [0038]    In accordance with another aspect of the invention there is provided an apparatus for conveying an ultrasonic sensor about an outer peripheral surface of a tube to perform a non-destructive evaluation thereof. The apparatus includes a frame operably configured to engage the tube and a flexible tension link housed in the frame, the flexible tension link being operably configured to permit the ultrasonic sensor to be mounted thereto. The apparatus also includes provisions for extending the flexible tension link from the frame and provisions for urging an extended portion of the flexible tension link towards the outer peripheral surface of the tube at a plurality of locations along the extended portion to cause the ultrasonic sensor to move through an arc circumferentially around the tube while maintaining ultrasonic coupling between the ultrasonic sensor and the tube. 
         [0039]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0040]    In drawings which illustrate embodiments of the invention, 
           [0041]      FIG. 1  is a perspective view of an apparatus in accordance with an embodiment of the invention for conveying an ultrasonic sensor about an outer peripheral surface of a tube; 
           [0042]      FIG. 2  is a top view of the apparatus shown in  FIG. 1 ; 
           [0043]      FIG. 3  is a side view of the apparatus shown in  FIG. 1  with a frame portion removed; 
           [0044]      FIG. 4  is a perspective view of a portion of flexible tension link used in the apparatus shown in  FIG. 1 ; 
           [0045]      FIG. 5  is a cross sectional view of a welded tube; and 
           [0046]      FIG. 6  is a process flowchart schematically depicting a process for performing a non-destructive evaluation of a tube, in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0047]    Referring to  FIG. 1 , an apparatus for conveying an ultrasonic sensor about an outer peripheral surface of a tube  24  according to a first embodiment of the invention is shown generally at  10 . In the embodiment shown, the tube  24  includes a first tube portion  26  and a second tube portion  28 , the first and second tube portions being joined by a weld  30 . The apparatus  10  is used to evaluate the quality of the weld  30 , using ultrasonic non-destructive evaluation techniques. 
         [0048]    The apparatus  10  includes a frame  12  and in this embodiment, the frame  12  includes a first frame portion  40  and a second frame portion  42 . The first and second frame portions  40  and  42  are spaced apart by a plurality of cross members (only two cross members  44  and  45  shown in  FIG. 1 ). 
         [0049]    The frame  12  includes a pair of fixed clamp jaws  14  and  16  and a pair of spaced apart moveable clamp jaws  18 , only one of which is shown in  FIG. 1 . Each of the moveable clamp jaws  18  has an associated clamp actuator  20  and  22  respectively. The clamp actuators  20  and  22  are operable to actuate the moveable jaws  18  to clamp the frame  12  to the tube  24  on which it is desired to perform a non destructive evaluation. 
         [0050]    The clamp actuator  20  is shown in greater detail in  FIG. 2 , and includes a lever  80  which is coupled to the moveable jaw  18  by a linkage  82  fastened between the lever and the moveable jaw at first and second pivot pins  84  and  86 . The clamp actuator also includes an adjustment screw  88  which is coupled to the lever  80  at a third pivot pin  90  to move the third pivot pin toward and away from the first pivot pin  84 . 
         [0051]    The clamp actuator  20  operates as an over-center clamp, and when the lever  80  is shifted toward the frame  12 , the second pivot pin  86  is moved away from the frame  12  to a position  92  (shown in broken outline) causing the moveable jaw  18  to be urged toward the fixed jaw  14 , thus clamping to the tube  24 . Since the second pivot pin  86  is shifted past a center line  94  (drawn between the first pivot pin  84  and the third pivot pin  90 ) the lever  80  becomes locked in the location shown in broken outline. The adjustment screw  88  is operable to shift the location of third pivot pin  90  in the direction indicated by the arrow  96 , facilitating clamping to tubes of varying diameter, and also facilitating adjustment of a clamping force for a particular tube diameter. Applying a force to the lever  80  in a direction away from the frame  12 , causes the second pivot pin  86  to move past the center line  94  toward the frame, thus unlocking the moveable clamp jaw  18 . 
         [0052]    The apparatus  10  further includes a first flexible tension link  46  and a first drum  50 . The first drum  50  is located in the first frame portion  40 . In the embodiment shown in  FIG. 1 , the flexible tension link  46  includes a first portion  48 , which is wrapped around and coupled to the first drum  50 . The flexible tension link  46  also includes a second portion  52 , which is spaced inwardly from the first frame portion  40 . 
         [0053]    The apparatus  10  is shown in  FIG. 3  with the frame  12  removed to more clearly show elements which are obscured by the second frame portion  42  in  FIG. 1 . Referring to  FIG. 3 , the apparatus  10  further includes a second flexible tension link  47  and a second drum  51 . The second drum  51  is located in the second frame portion  42  (shown in  FIG. 1 ). The second flexible tension link  47  includes a first portion  49 , which is wrapped around and coupled to the second drum  51 . 
         [0054]    Referring back to  FIG. 1 , the second tension link  47  also includes a second portion  54 , which is spaced inwardly from the second frame portion  42 . 
         [0055]    The apparatus  10  further includes a bar  56 , which is coupled to the first portions  48  and  49  of the first and second flexible tension links  46  and  47 . The second portions  52  and  54  of the first and second flexible tension links  46  and  47  are slideably mounted on the bar  56  and each include a pair of fasteners  53  and  55  for securing the second portions at desired positions on the bar  56 . Each of the second portions  52  and  54  may be positioned on the bar  56  by loosening the fasteners  53  and  55 , sliding the second portions to respective desired positions on the bar  56 , and then tightening the fasteners, thus securing the second portions to the bar in a desired spacing. 
         [0056]    The spaced apart second portions  52  and  54  of the first and second flexible tension links permit the second portions to be extended through substantially a complete revolution around the circumference of the tube  24 , without being impeded by the first portions  48  and  49  of the first and second flexible tension links  46  and  47 . 
         [0057]    As best shown in  FIG. 3 , the apparatus  10  also includes an actuator shaft  58  and an actuator knob  59 . The actuator knob  59  is coupled to the actuator shaft  58 . The actuator shaft  58  is located between, and coupled to, the first drum  50  and the second drum  51 . When the actuator knob  59  is rotated thus rotating the first and second drums  50  and  51 , the first and second portions  48  and  49  of the first and second flexible tension links  46  and  47  are extended (or retracted) thus extending (or retracting) the bar  56  and the second portions  52  and  54  of the first and second flexible tension links respectively. 
         [0058]    Referring back to  FIG. 1 , the first frame portion  40  further includes a first guide  61  and the second frame portion  42  includes a second guide  63 . Referring now to  FIG. 2 , the first guide  61  includes a channel  65  (shown in hidden detail in  FIG. 2 ), which guides the flexible tension link  46  between the drum  50  and the tube  24 . Similarly the second guide  63  includes a channel (not shown), which guides the flexible tension link  47  between the drum  51  and the tube  24 . 
         [0059]    Referring now to  FIG. 4 , the second portion  52  of the first flexible tension link  46  includes a distal linked element  60 . The distal linked element  60  includes a mounting face  69  for mounting an ultrasonic sensor  62 . The ultrasonic sensor  62  includes a threaded hole (not shown) and is secured to the mounting face  69  by a threaded fastener  67  through the distal linked element  60 . 
         [0060]    Referring back to  FIG. 1 , the second portion  54  of the second flexible tension link  47  also includes a distal linked element  64  (similar to the element  60  shown in  FIG. 4 ) for mounting an ultrasonic sensor  66 . 
         [0061]    In accordance with one aspect of the invention, the flexible tension links  46  and  47  are operably configured to urge an extended portion of each of the flexible tension links towards an outer peripheral surface  68  of the tube  24  at a plurality of locations along the extended portion to cause the ultrasonic sensor  62  and  66  to move through an arc circumferentially around the tube while maintaining ultrasonic coupling between the ultrasonic sensor and the tube. 
         [0062]    Referring to  FIG. 4 , the second portion  52  of the flexible tension link  46  is shown in greater detail. The second portion  52  includes the distal linked element  60 , and four linked elements  100 ,  102 ,  104 , and  105 . In general the flexible tension link  46  may include many linked elements similar to the linked elements  100 - 104 . The distal linked element  60  is pivotally connected to the linked element  100  using a pin  106 . Similarly the linked elements  100  and  102  are pivotally connected using a pin  108 , and the linked elements  102  and  104  are pivotally connected using a pin  110 . 
         [0063]    In the embodiment shown in  FIG. 4 , the linked element  104  is connected to a linked element  105  using a pin  109 . The linked element  105  includes a slot  107 , which is shaped to receive the bar  56  for facilitating sliding connection of the linked element  105  to the bar  56  (as shown in  FIG. 1  and as described above). 
         [0064]    The second portion  54  of the second flexible tension link  47  is identical to the second portion  52  of the flexible tension link  46 . 
         [0065]    The second portion  52  of the flexible tension link  46  also includes first, second third, and fourth helical spring pairs  112 ,  114 ,  116  and  117  at first, second, third and fourth locations  118 ,  120 ,  122 , and  123  respectively, along the length of the flexible tension link. In general, further spring pairs may be located between other linked elements along the flexible tension link  47 . The first spring pair  112  is operable to urge the distal linked element  60  and the linked element  100  apart, the second spring pair  114  is operable to urge the linked element  100  and the linked element  102  apart, the third spring pair  116  is operable to urge the linked element  102  and the linked element  104  apart, and the fourth spring pair  117  is operable to urge the linked element  104  and the linked element  105  apart, thus causing the flexible tension link  46  to take on an arc shape that generally wraps around and conforms to the peripheral surface  68  of the tube  24  being evaluated, thus causing the ultrasonic sensor  62  to move through an arc circumferentially around the tube. The first spring pair  112  is further operable to urge the ultrasonic sensor  62  into contact with the peripheral surface  68  of the tube  24 , thus facilitating ultrasonic coupling between the sensor and the tube. 
         [0066]    Still referring to  FIG. 4 , in one embodiment the ultrasonic sensor  62  includes first and second coupling fluid ports  124  and  126  for introducing an ultrasonic coupling fluid between the sensor and the outer peripheral surface  68  of the tube to further facilitate ultrasonic coupling between the ultrasonic sensor and the tube. In this embodiment the ultrasonic coupling fluid comprises water, and the water is allowed to flow between the sensor and the outer peripheral surface  68  while performing the non destructive evaluation of the tube  24 . 
         [0067]    The ultrasonic sensor  62  also includes an electrical signal connector  128  for connecting a signal cable (not shown) for transmitting electrical stimuli to the ultrasonic sensor for receiving electrical signals from the ultrasonic sensor representing results of the non-destructive evaluation of the tube. 
         [0068]    In this embodiment the linked elements  100  and  102  further include conduits  130  and  132  through which the coupling fluid supply line and the signal line may be threaded. Other linked elements may also include a conduit, thus allowing the coupling fluid supply line and the signal line to be run along the length of the flexible tension links  46  and  47  to couple to the sensors  62  and  66 . 
         [0069]    Referring back to  FIG. 1 , the apparatus  10  further includes a main coupling fluid supply line  70 . The main coupling fluid supply line  70  includes a “Y” or “T” coupling (not shown) which splits the main fluid supply line into two separate supply lines for supplying coupling fluid to each of the sensors  62  and  66  respectively through the first and second coupling fluid ports. 
         [0070]    The apparatus  10  also includes a main signal line  72 , which includes a plurality of signal conductors including first signal conductors for coupling to the sensor  62 , second signal conductors for coupling to the sensor  66 , and third signal conductors for coupling to a rotary encoder  76 , which is described later herein. 
         [0071]    The main signal line  72  is coupled to an analyzer  74 . The analyzer  74  includes circuitry which is operable to transmit electrical stimuli to the ultrasonic sensors  62  and  66  and circuitry which is operable to receive electrical signals from the ultrasonic sensor representing results of the non-destructive evaluation of the tube. The analyzer further includes circuitry and/or microprocessor circuits for analyzing the return signals and displaying an inspection result. Such analyzers for interfacing to one or more phased array ultrasonic sensors are commonly available, and in one embodiment the analyzer used may be a 32-channel ultrasound phased-array instrument (model number X-32) available from Harfang Microtechniques of Quebec Canada. 
         [0072]    Referring to  FIG. 3 , the apparatus  10  further includes a rotary encoder  76 , having a position signal output  77 . The rotary encoder is coupled to the actuator shaft  58  and produces a position signal at the output  77  representing a displacement of the first and second flexible tension links when the actuator knob  59  is rotated. In this embodiment the encoder used is model number S6S supplied by US Digital of Vancouver, Wash. The 6S6 encoder is a high resolution rotary to digital converter which produces TTL-compatible signal outputs representing an angular position and/or velocity of the actuator shaft  58 , and hence the position of first drum  50  and the second drum  51 , which are coupled to the actuator shaft. 
         [0073]    The position signal output  77  is coupled to the analyzer  74  via the third signal conductors in the main signal line  72  for transmitting the position signal to the analyzer. The analyzer  74  is operably configured to receive the position signal and to calculate the displacement of the first and second flexible tension links using the diameter of the first and second drums to convert angular position received from the encoder into a corresponding linear displacement of the flexible tension links. 
         [0074]    The sensors  62  and  66  are shown in greater detail in  FIG. 5 . Referring to  FIG. 5 , each of the sensors  62  and  66  is mounted on wedges  140  and  142  respectively. Each of the wedges  140  and  142  includes an angled face  144  and  146  respectively. In one embodiment sensors  62  and  66  include a plurality of sensor elements (not shown) arranged in an array proximate the angled faces  144  and  146 . Such multiple-element sensors are suitable for operation as a phased array, where the timing of excitation pulses sent to each of the sensor elements by the analyzer  74 , may be controlled to focus and direct resulting ultrasonic beams  148  and  150  within a wall  152  of the tube  24 , thus facilitating scanning of the sides of the weld  30  for defects. 
       Operation 
       [0075]    The operation of the apparatus  10  shown in  FIG. 1  is described with reference to  FIG. 6  and  FIGS. 1-5 . The operation of the apparatus  10  is described in connection with the inspection of a weld joining two tubular sections although it should be understood that the apparatus may be used to evaluate defects in tubes other than weld related defects. 
         [0076]    Referring to  FIG. 6 , a process for performing an ultrasonic non-destructive evaluation of a tube weld is shown generally at  160 . As shown at  162 , the process begins with preparing the surface of the tube to be inspected, which involves ensuring that the peripheral surface  68  over which the sensors  62  and  66  will be conveyed is free of weld spatter, dirt, grease, loose scale etc. 
         [0077]    As shown at  164 , where the weld  30  is only accessible from one side due to the surrounding environment, the apparatus  10  may be configured for single sided inspection at  166  by configuring the analyzer  74  to ignore the result from one of the sensors, and/or by removing the inactive sensor from the apparatus  10 . The remaining description of the operation assumes that the apparatus  10  is configured for double sided inspection as shown at  168 . 
         [0078]    As shown at  170 , the process continues with configuring the analyzer  74  and the apparatus  10  for correct sensor spacing, the type of sensor being used, etc. For example, referring to  FIG. 5 , the spacing “D 1 ” and “D 2 ” between the wedges  140  and  142  and a centerline  154  of the weld  30  should be within certain ranges, depending on a thickness “T” of the wall  152 . Accordingly, if the spacing of the wedges  140  and  142  is not within the acceptable range, then the spacing of the second portions  52  and  54  of the first and second flexible tension links  47  and  47  may require adjustment as described above. It may be necessary to enter other information regarding the sensor configuration, such as angle of the angled faces  144  and  146 , into the analyzer prior to performing the inspection. 
         [0079]    Returning to  FIG. 6  as shown at  172  the apparatus is then clamped to the tube  24  as described above, taking care to ensure that the spacing “D 1 ” and “D 2 ” (shown in  FIG. 5 ) remain in the appropriate ranges (i.e. the apparatus is clamped in alignment with the centerline  154  of the weld  30 ). Alternatively, the configuration shown at  170  may be performed after clamping the apparatus  10  to the tube  24 , should the access be sufficient to permit configuration of sensor spacing after clamping. 
         [0080]    As shown at  174 , the process continues by extending the flexible tension links  46  and  47  by rotating the actuator knob  59 . The flexible tension links  46  and  47  are extended by one full revolution around the circumference of the tube  24  to a starting position for the inspection of the tube  24 . 
         [0081]    Referring to  FIG. 4 , as the flexible tension links are extended, the extended portion (e.g. the second portion  52 ) is urged towards the outer peripheral surface  68  of the tube  24  at a plurality of locations (e.g. the locations  118 ,  120  and  122 ) along the extended portion. This causes the sensor  62  to be urged into contact with the tube  24  while the second portion  52  wraps around the tube thus moving through an arc circumferentially around the tube. Advantageously, this allows the apparatus  10  to be used to perform inspections on a range of different tube diameters. 
         [0082]    Referring back to  FIG. 6 , as shown at  176 , a reference mark is then made to indicate the position of the sensors  62  and  66  at the starting position for the inspection. The reference mark may be useful if a defect is found in the tube  24 , since the location of the defect indicated by the analyzer  74  is generally referenced to the start position. Accordingly should it be necessary to correct the defect or to cut the tube  24  for further inspection, the position of the defect, as indicated by the analyzer, may be located on the tube by measuring from the reference mark. 
         [0083]    As shown at  178 , at this stage in the process, the ultrasonic coupling fluid supply is activated to supply coupling fluid between the sensors  62  and  66  and the peripheral surface  68  of the tube  24 . 
         [0084]    As shown at  180 , the inspection then commences by activating the analyzer  74 . Generally activating the inspection involves pressing a “Start”, “Record” or other button on the analyzer  74  to cause the analyzer to generate excitation signals and receive return signals from the sensors  62  and  66  and the encoder  76 . 
         [0085]    As shown at  182 , the actuator knob  59  is then rotated to cause the flexible tension links  46  and  47  to be retracted into the frame  12 , thus causing the sensors  62  and  66  to move through an arc in a direction circumferentially around the tube while maintaining ultrasonic coupling between the ultrasonic sensors and the tube  24 . Once inspection has commenced the analyzer  74  monitors the position signal received from the encoder  76 , from which the analyzer is able to determine a real time velocity of the sensors along the peripheral surface  68 , and to provide feedback to the operator. The feedback may take the form of a plurality of indicator lights that show, for example, an indication such as “too fast”, “too slow”, or “correct” retraction speed. If the sensors  62  and  66  are conveyed too fast over the peripheral surface  68 , defects may be missed. 
         [0086]    As the flexible tension links  46  and  47  are retracted into the frame  12 , the analyzer provides excitation signals to the sensors  62  and  66 , which cause the ultrasonic beams  148  and  150  to be scanned over the weld  30  (as shown in  FIG. 5 ). The sensors  62  and  66  also generate return signals, which are received by the analyzer  74 , correlated with the position signal, and presented to the operator as data or a graphical interpretation thereof showing the result of the inspection. 
         [0087]    While retracting the flexible tension links  46  and  47 , the extended portions of the flexible tension links are urged towards the outer peripheral surface of the tube at a plurality of locations along the extended portion, as described above in connection with the extension, shown at  174 . 
         [0088]    Once the flexible tension links  46  and  47  have been fully retracted into the frame  12 , the analyzer  74  is de-activated to indicate the end of the inspection, as shown at  184 . The apparatus may then be unclamped and removed from the tube  24 . 
         [0089]    It has been discovered that it is advantageous to perform the inspection while retracting the flexible tension links  46  and  47 , since the retraction movement is smoother than the extension movement. While it is possible to perform the inspection when extending the flexible tension links  46  and  47 , it was found that the sensors  62  and  66  are more likely to stick while being extended, resulting in a less smooth movement and thus affecting the inspection result. 
         [0090]    Advantageously, the relatively low profile of the sensors  62  and  66 , shown in  FIG. 5 , and the mounting of the sensors on distal ends of the flexible tension links  46  and  47 , permits inspection of a tube  24 , even where the tube is surrounded by other tubes (such as tubes  98  and  99  shown in  FIG. 2 ). In one embodiment the sensors  62  and  66  and the flexible tension links  46  and  47  have a profile height of less than 0.5 inches, allowing inspection of tubes that are spaced apart from the tube  24  by as little as 0.5 inches. 
       Alternative Embodiments 
       [0091]    While the apparatus  10  has been described with reference to a hand operated actuator knob  59  for extending and retracting the flexible tension links  46  and  47 , the actuator shaft  58  may also be coupled to a motor or other mechanism for automatically extending and retracting the flexible tension links. 
         [0092]    While the apparatus  10  has been described as being configured using two phased array sensors  62  and  66 , the apparatus may also be configured with a single sensor (or more than two sensors), and the sensor need not be a phased array ultrasonic sensor. Furthermore while the apparatus  10  includes a flexible tension link  46  and  47  having first and second spaced apart portions the sensor may also be mounted on a flexible tension link having only a single, in-line portion. However, such a configuration may not permit a full scan of the circumference of the tube  24 , without unclamping, shifting circumferentially, and then re-clamping the apparatus. 
         [0093]    While specific embodiments of the invention have been described and illustrated, such embodiments should be considered illustrative of the invention only and not as limiting the invention as construed in accordance with the accompanying claims.