Abstract:
The invention provides an improved method and device for inspecting heat exchanger tubes from within the tube inside diameter that overcomes the shortcomings of the prior art. It adapts a guided-wave probe approach that makes use of a torsional wave mode instead of a longitudinal wave node disclosed in the prior art. The torsional wave mode has many advantages over the longitudinal wave mode for detecting defects. When energized by suitable instrumentation, the probe is caused to generate a torsional mode signal that is transmitted to the heat exchanger tube from the waveguide tube. When reflected signals from defects in the heat exchanger tube walls are returned to the inspection opening end of the heat exchanger tube, the reflected defect signals are transmitted to the probe waveguide tube for amplification, detection and characterization of the reflected signal.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/481,636, filed on Nov. 13, 2003, which is incorporated herein by reference. 

   BACKGROUND 
   The invention relates generally to a method and system for inspection of heat exchanger tubes using long-range guided-wave inspection techniques, and more particularly to a method using torsional guided waves for quickly inspecting heat exchanger tubes from the inside diameter of a tube. 
   Heat exchangers are used extensively in processing plants such as refineries, chemical plants and electric generation facilities, and typically comprise a multiplicity of heat exchanger tubes supported in a fixed position by a tube sheet. Each heat exchanger tube is accessible from one end for inspection and maintenance purposes. The heat exchanger tubes carry a first liquid or gas at a first temperature and are surrounded by a second liquid or gas at a second temperature, whereby heat is transferred from the first liquid or gas to the second liquid or gas, or visa-versa. In order to minimize forced shutdowns of equipment and processes due to heat exchanger failures and associated operating and maintenance costs, heat exchanger tubes are inspected on a regular basis. Since heat exchanger tubes are bundled in close proximity, making it difficult to inspect them from their outer diameter, inspections of heater exchanger tubes are conducted from the inside diameter of the tube. Because of the large number of heat exchanger tubes in a heat exchanger and the high cost of inspection, heat exchanger tubes are generally maintained based on sampled inspection data from a small number of tubes. This current maintenance practice does not result in high reliability of heat exchangers. 
   There have been a number of past efforts to find solutions for inspecting heat exchanger tubes. Many of these efforts have relied on inducing guided waves in the heat exchanger tube walls and detecting induced guided wave signals reflected from defects in the tube walls. Some of these solutions provided a means for inspecting a long length of tubing from a fixed probe location on the outside diameter of tubes for detection of reflected guided wave signals. The difficulty in accessing the outside diameter of heat exchanger tubes has rendered this approach impracticable. However, these methods have provided recognition that a potential solution for quickly surveying heat exchanger tube integrity, provided that the guided waves could be launched and detected from the inside diameter of the heat exchanger tubes. Because of various shortcomings of solutions disclosed in the prior art, including poor efficiency, poor defect detection capability and mode control difficulties, these solutions have not been widely accepted for practical heat exchanger tube inspections in the field. 
   There is a need for a nondestructive testing method in processing industries for enhancing reliability and reducing costs by quickly surveying all heat exchanger tubes in a heat exchanger and providing data suitable for determining appropriate subsequent inspection and maintenance actions. A desirable technique is one that can be applied from an inside diameter of a heat exchanger tube, can quickly inspect the entire length of a tube from one end, and requires cleaning only in an area of probe placement. 
   SUMMARY 
   The present invention provides a nondestructive testing method for enhancing reliability and reducing costs by quickly surveying all heat exchanger tubes in a heat exchanger and providing data suitable for determining appropriate subsequent inspection and maintenance actions. It provides for inspection of the entire length of all of the tubes from one tube end inside diameter, requiring cleaning of only a probe placement area on each tube. 
   The disclosed invention provides an improved method and system for inspecting heat exchanger tubes from within the tube inside diameter that overcomes the shortcomings of the prior art. The invention relies on a magnetostrictive transmitter and a tubular waveguide to generate torsional waves in the waveguide that are coupled to the inside of a heat exchanger tube for transmitting the torsional waves into the heat exchanger tube. The torsional waves reflect from defects and irregularities in the heat exchanger tube walls, return to the coupled waveguide and are sensed by the magnetostrictive sensor. It adapts a guided-wave probe approach that makes use of a torsional wave mode instead of a longitudinal wave node disclosed in the prior art. The torsional wave mode has many advantages over the longitudinal wave mode for detecting defects. Among the advantages of the use of torsional wave mode are greatly reduced wave dispersion, easier mode control, reduced interference from other wave modes, immunity to the presence of liquid product, and improved defect detectability. The invented probe generates and detects the torsional waves in a waveguide tube, which is in close contact with the inside diameter of a heat exchanger tube for coupling of torsional waves between the waveguide tube and the heat exchanger tube. The waveguide tube makes use of damping material to minimize reverberation of a guided-wave signal within the waveguide tube. Axial slits near the working waveguide tip end of the waveguide tube allow expansion for providing close contact with the inside wall of a heat exchanger tube for coupling the torsional waves between the heat exchanger tube and the waveguide tube. When energized by suitable instrumentation, the probe is caused to generate a torsional wave signal that is coupled to the heat exchanger tube from the waveguide tube. When reflected signals from defects in the heat exchanger tube walls are returned to the inspection opening end of the heat exchanger tube, the reflected defect signals are coupled to the probe waveguide tube for detection of the reflected signals. 
   It should be noted that within the context of this specification, the term “sensor” is used to describe a device attached to a waveguide that is capable of transmitting and receiving torsional waves in the waveguide. Furthermore, it is recognized that a transmitting sensor may also be a separate distinct device from a receiving sensor. 
   An embodiment of the present invention is a method for inspection of heat exchanger tubes using reflected torsional waves comprising the steps of inserting a cylindrical waveguide probe into an open end of a heat exchanger tube, a coupled end of the waveguide probe being located a distance from the open end by at least the distance from the open end to a heat exchanger tube sheet, applying an electronic transmit pulse to a magnetostrictive sensor mounted on the waveguide probe, generating and transmitting a torsional wave pulse in the waveguide probe by the magnetostrictive sensor, coupling the transmitted torsional wave from the waveguide probe to an inside wall of the heat exchanger tube for propagation along the length of the heat exchanger tube, coupling reflected torsional wave signals from defects and a far end of the heat exchanger tube to the waveguide probe, sensing the reflected torsional wave signals by a magnetostrictive sensor, and electronically processing the sensed signals for determining a location and characteristics of the defects in the heat exchanger tube walls. The step of generating a torsional wave pulse and the step of sensing the reflected torsional wave signals may be performed by the same magnetostrictive sensor including an integrated magnetostrictive transmitter and receiver. The step of generating a torsional wave pulse and the step of sensing the reflected torsional wave signals may be performed by separate magnetostrictive sensors including a magnetostrictive transmitter and separate magnetostrictive receiver. The step of generating torsional waves may comprise applying an electric current pulse of a fixed frequency to a coil wound over a ferromagnetic strip of the magnetostrictive sensor cylindrically affixed on the cylindrical waveguide probe. The ferromagnetic strip may be selected from the group consisting of a nickel strip and a strip of material having good magnetostrictive properties. The method may further comprise the step of magnetically polarizing the ferromagnetic strip in a circumferential direction. The step of coupling the torsional waves between the waveguide tube and the heat exchanger tube may comprise expanding the coupled end of the waveguide tube to make intimate contact between the coupled end and the inside diameter of the heat exchanger tube by applying a force from inside the waveguide tube using an expansible device. The method wherein the step of applying an electronic transmit pulse may comprise activating a function generator by an output of a control processor for generating a transmit pulse, connecting the transmit pulse at an output of the function generator to a power amplifier input for amplifying the transmit pulse, and applying the amplified output pulse from the output of the power amplifier to the magnetostrictive sensor, wherein the step of electronically processing the reflected torsional waves may comprise amplifying a signal from the magnetostrictive sensor in a preamplifier, connecting the amplified signal at an output of the preamplifier to an input of an analog-to-digital converter, and connecting an output of the analog-to-digital converter to an input of the control processor, and may further comprise the step of determining locations and characteristics of defects in the heat exchanger tube walls by the control processor using signal characteristics from the analog-to-digital converter output and the time differences between applying the electronic transmit pulse and sensing of the signal characteristics from the analog-to-digital converter output. 
   Another embodiment of the present invention is a system for inspection of heat exchanger tubes using reflected torsional waves, comprising a cylindrical waveguide probe inserted into an open end of a heat exchanger tube, a coupled end of the waveguide probe being located a distance from the open end by at least the distance from the open end to a heat exchanger tube sheet, means for applying an electronic transmit pulse to a magnetostrictive sensor mounted on the waveguide probe, means for generating and transmitting a torsional wave pulse in the waveguide probe by the magnetostrictive sensor, means for coupling the transmitted torsional waves from the waveguide probe to an inside wall of the heat exchanger tube for propagation along the length of the heat exchanger tube, means for coupling reflected torsional wave signals from defects and a far end of the heat exchanger tube to the waveguide probe, means for sensing the reflected torsional wave signals by a magnetostrictive sensor, and means for electronically processing the sensed signals for determining a location and characteristics of the defects in the heat exchanger tube walls. The means for applying an electronic transmit pulse may comprise a control processor for activating a function generator to produce an output pulse, a power amplifier for amplifying the output pulse to provide an electronic transmit pulse and the electronic transmit pulse being connected to the magnetostrictive sensor. The means for generating a torsional wave pulse and the means for sensing the reflected torsional wave signals may be performed by the same magnetostrictive sensor including an integrated magnetostrictive transmitter and receiver. The means for generating a torsional wave pulse and the means for sensing the reflected torsional wave signals may be performed by separate magnetostrictive sensors including a magnetostrictive transmitter and separate magnetostrictive receiver. The means for generating torsional waves may comprise means for applying an electric current pulse of a fixed frequency to a coil wound over a ferromagnetic strip of the magnetostrictive sensor cylindrically affixed on the cylindrical waveguide probe. The ferromagnetic strip may be selected from the group consisting of a nickel strip and a strip of material having good magnetostrictive properties. The system may further comprise means for magnetically polarizing the ferromagnetic strip in a circumferential direction. The means for coupling the torsional waves between the waveguide tube and the heat exchanger tube may comprise expanding the coupled end of the waveguide tube to make intimate contact between the coupled end and the inside diameter of the heat exchanger tube by applying a force from inside the waveguide tube using an expansible device. The means for coupling the torsional waves between the waveguide tube and the heat exchanger tube may comprise a drawbar mechanism being repositioned for actuating an expanding collet on the coupled end of the waveguide probe, the actuated expanding collet for expanding the coupled end of the waveguide probe to create a firm mechanical contact with the inside wall of the heat exchanger tube, the generated transmitted torsional wave being propagated from the magnetostrictive sensor to the coupled end of the waveguide probe, and the propagated torsional wave being coupled from the coupled end of the waveguide probe to the inside wall of the heat exchanger tube. The means for coupling reflected torsional wave signals may comprise the reflected torsional wave signals being coupled from the inside wall of the heat exchanger tube to the coupled end of the waveguide probe, and the reflected torsional wave signals being propagated from the coupled end of the waveguide probe to the magnetostrictive sensor. The system wherein the means for applying an electronic transmit pulse may comprise a function generator being activated by an output of a control processor for generating a transmit pulse, the transmit pulse at an output of the function generator being connected to a power amplifier input for amplifying the transmit pulse, and the amplified output pulse from an output of the power amplifier being applied to the magnetostrictive sensor, wherein the means for electronically processing the reflected torsional waves may comprise a signal from the magnetostrictive sensor being amplified in a preamplifier, the amplified signal at an output of the preamplifier being connected to an input of an analog-to-digital converter, and an output of the analog-to-digital converter being connected to an input of the control processor, and may further comprise locations and characteristics of defects in the heat exchanger tube walls being determined by the control processor using signal characteristics from the analog-to-digital converter output and the time differences between applying the electronic transmit pulse and sensing the signal characteristics from the analog-to-digital converter output. 
   Yet another embodiment of the present invention is a method for inspection of heat exchanger tubes using reflected torsional waves, comprising generating, transmitting and coupling a torsional wave pulse to an inside wall of a heat exchanger tube for propagation along the length of the heat exchanger tube, coupling and sensing reflected torsional wave signals from defects and a far end of the heat exchanger tube, and electronically processing the transmitted and sensed torsional waves for determining defect location and characteristics. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings wherein: 
       FIG. 1  shows a functional block diagram of a system for implementing the present invention; 
       FIG. 2  shows typical representations of a transmitted torsional mode guided-wave signal and a reflected torsional mode guided-wave signal from a defect in a wall of a heat exchanger tube; 
       FIG. 3  shows dispersion curves for longitudinal mode and torsional mode waves in different diameter tubes; 
       FIG. 4  shows embodiments of a torsional mode guided-wave probe; 
       FIG. 5  shows an enlarged sensor configuration of an embodiment of a torsional mode guided-wave probe; 
       FIG. 6  shows enlarged waveguide tip configurations as embodiments of a torsional mode guided-wave probe; and 
       FIG. 7  shows details of a magnetostrictive sensor for generating and detecting torsional mode guided wave signals. 
   

   DETAILED DESCRIPTION 
   Turning to  FIG. 1 ,  FIG. 1  shows a functional block diagram of a system  100  for implementing the present invention. A control processor  110  commands a function generator  130  to initiate an electric current pulse of a fixed frequency that is amplified by a power amplifier  150 . The amplified electric current pulse  155  is sent to a magnetostrictive sensor  165  positioned on a probe  160 . The amplified electric current pulse  155  causes the magnetostrictive sensor  165  to generate and transmit a torsional wave pulse into the probe waveguide tube that is coupled to a heat exchanger tube. When the torsional wave encounters a defect in the wall of the heat exchanger tube, a reflected torsional wave signal is returned to the waveguide tube coupled to the heat exchanger tube. The reflected signal produces an electrical voltage signal in the magnetostrictive sensor  165  that is amplified by a preamplifier  140 . The amplified signal is then converted to a digital signal using an analog-to-digital converter  120  and sent to the control processor  110  for analysis and interpretation. 
   Turning to  FIGS. 2A and 2B ,  FIG. 2A  shows a typical representation  200  of a transmitted torsional mode guided-wave signal  230  and a reflected torsional mode guided-wave signal  240  from an end of a heat exchanger tube. The signals  230 ,  240  are represented in amplitude  210  and distance  220 , as measured by a magnetostrictive sensor  165 . As shown in  FIG. 2 , the incident wave  230  is transmitted into a heat exchanger tube, and after a time determined by a round-trip excursion between transmission of the incident signal  230  and receipt of the reflected defect signal  240  by the magnetostrictive sensor  165 . The magnetostrictive sensor  165  is positioned on a suitable probe, as shown in  FIG. 1 .  FIG. 2B  also show the transmitted pulse  230  and returned pulse from an opposite end of a heat exchanger tube. In the top trace  270 , reflections from a notch  250  and corrosion  260  are indicated. In the bottom trace  280 , the heat exchanger tube is tested from the opposite end, showing corrosion  260  and a notch  250 . 
   Turning to  FIG. 3 ,  FIG. 3  shows dispersion curves  300  for longitudinal mode and torsional mode waves in different diameter tubes. This  FIG. 3  illustrates the advantage of the use of torsional waves over the use of longitudinal waves for defect detection.  FIG. 3  depicts how the group velocity  310  of various longitudinal waves and torsional waves changes with frequency  320  for different size tubes. The velocity of longitudinal wave mode L( 0 , 1 ) for tubes of ½ inch outside diameter  350 , ¾ inch outside diameter  340  and 1 inch outside diameter  330 , varies between approximately zero and more than five kilometers per second over the frequency range shown. The velocity of longitudinal wave mode L( 0 , 2 ) for tubes of ½ inch outside diameter  380 , ¾ inch outside diameter  370  and 1 inch outside diameter  360 , also varies between zero and more than five kilometers per second over the frequency range shown. In comparison to the wide variation in the longitudinal wave velocities, torsional wave mode T( 0 , 1 ) for the three tube sizes  390 , shows no variation in group velocity  310  independent of frequency  320 . 
   Turning to  FIGS. 4A and 4B ,  FIG. 4A  shows an embodiment of a torsional mode guided-wave probe  400  inserted into one of many heat exchanger tubes  410 , positioned by a tube sheet  420 . The probe  400  is similar to the probe disclosed in U.S. Pat. No. 5,892,162 having a patent date of Apr. 6, 1999, which is incorporated herein by reference. The patent discloses a guided-wave probe for inspection of tubes and pipes from the inside diameter, which relies on longitudinal guided waves that are generated in the probe&#39;s waveguide from an area outside of a heat exchanger tube. In contrast with the previously patented invention, the present invention relies on torsional wave signals for defect detection rather than longitudinal wave signals, because the torsional wave signals are much less dispersive than the longitudinal wave signals and have a velocity that is independent of tube size and wave frequency, as described above. The torsional mode signals are easier to control and have better defect detection capability than the longitudinal mode signals. The sensors used in the patented invention is different from the sensors used in the present invention. In the present invention, waves generated by a sensor  450  are propagated along the probe&#39;s waveguide  460 , and mechanically coupled to the inside wall of a tube  410  at the working tip area of the probes waveguide by an expanding collet  470 . The coupled area of the probe&#39;s waveguide  460  is split along its length near the collet  470 , so that when tightened, the collet  470  causes the split end to expand to be in close proximity with the tube inside wall for coupling signals between the waveguide  460  and the tube  410 . The present invention discloses the use of a collet  470  for expanding the working tip area of the probe  400  when a collet nut  430  is tightened down on a threaded end of a collet draw bar  435  that is rotationally connected to the expanding collet  470 . The probe  400  also includes a damping material  440  positioned on the waveguide tube  460  to minimize reverberations of the guided-wave signals in the waveguide tube  460 . 
   According to  FIG. 4A , the present invention uses a sensor  450  for transmitting and detecting torsional mode signals that relies on a magnetostrictive sensor approach similar to the magnetostrictive sensor disclosed in U.S. Pat. No. 5,581,037 having a patent date of Dec. 3, 1996, which is incorporated herein by reference. The patent discloses a magnetostrictive sensor for inspecting tubes from an inside diameter that is applicable to ferrous tubes, and uses longitudinal waves generated directly in the tube walls. The magnetostrictive sensor  450  of the present invention generates and detects torsional mode signals in the probe&#39;s waveguide  460  utilizing a thin ferromagnetic strip, similar to the magnetostrictive sensor disclosed in U.S. Pat. No. 6,396,262 having a patent date of May 28, 2002, which is incorporated herein by reference. 
   As shown in  FIG. 4A , to conduct an inspection of a heat exchanger tube  410 , the probe  400  is inserted into an open end of the heat exchanger tube  410  such that the coupled region  470  of the probe  400  is at or just beyond the tube sheet  420 . The probe  400  is then coupled to the heat exchanger tube  410  inside surface by tightening the collet nut  430 , repositioning the drawbar mechanism  432 . The magnetostrictive sensor  450  is then activated and torsional waves are coupled from the tip  470  of the probe  400  to the heat exchanger tube  410 . The reflected signals from tube defects are returned to the sensor  450  and acquired by the associated instrumentation shown in  FIG. 1 . An electrical connector  442  is shown on the barrel  442  of the probe  400  for connecting the sensor signals to associated instrumentation. 
     FIG. 4B  illustrates another embodiment  490  of the present invention. To conduct an inspection of a heat exchanger tube  410 , the probe  490  is inserted into an open end of the heat exchanger tube  410  such that the coupled region  474  of the probe  400  is at or just beyond the tube sheet  420 . The probe  400  is then coupled to the heat exchanger tube  410  inside surface by pulling the handle  434  such that the handle  434  pivots about a pivot point  435 , causing drawbar mechanism  438 , connected to a connecting rod  437  connected to the handle  434 , to reposition. The repositioning action of the drawbar mechanism  438  causes the coupled region of the probe  474  to be tightened against the inside wall of the heat exchanger tube  410 . The magnetostrictive sensor  450  is then activated and torsional waves are coupled from the tip  474  of the probe  400  to the heat exchanger tube  410 . The reflected signals from tube defects are returned to the sensor  450  and acquired by the associated instrumentation shown in  FIG. 1 . An electrical connector  442  is shown on the barrel  440  of the probe  400  for connecting the sensor signals to associated instrumentation. 
   Turning to  FIG. 5 ,  FIG. 5  shows an enlarged sensor configuration  500  of an embodiment of a torsional mode guided-wave probe  400 .  FIG. 5  depicts a portion of the probe  400  shown in  FIG. 4 . The probe  400  is positioned in one of many heat exchanger tubes  410  fixed in position by a tube sheet  420 . A sensor  450  comprises a sensor coil  452  and a cylindrical ferromagnetic strip  454 , such as nickel or other material with suitable magnetostrictive properties, that is securely affixed to an outside wall of the waveguide tube  460  for providing magnetostrictive properties necessary for the generation and detection of torsional wave signals according to the present invention. A time-varying magnetic field generated by the sensor coil  452  generates torsional waves in the cylindrical ferromagnetic strip  454  via the magnetostrictive effect. The cylindrical ferromagnetic strip  454  and the sensor coil  452  are also used to detect reflected waves from a tube defect via the inverse magnetostrictive effect. The damping material  440  is also shown positioned on the waveguide tube  460  to minimize reverberations of the guided-wave signals in the waveguide tube  460 . 
   For generation and detection of torsional mode signals, a DC bias magnetic field is required in the circumferential direction. Bias magnetization in a circumferential direction may be accomplished by inducing residual magnetization in the ferromagnetic strip  454  along its length prior to circumferentially attaching or wrapping the strip  454  to the waveguide tube  460 . Another method for creating a circumferential bias magnetic field in the ferromagnetic strip  454  is, after attaching the strip to the waveguide tube, to pass a DC electric along the longitudinal axis of the guided wave probe tube  460 . The electric current flow is in a direction of a longitudinal axis of the waveguide tube  460 . 
   Turning to  FIGS. 6A and 6B ,  FIG. 6A  shows an enlarged waveguide working tip configuration  600  of an embodiment of a torsional mode guided-wave probe  400 . The tip of the waveguide tube  460  contains axial slits positioned around the tube circumference in the region near the working tip of the waveguide tube  460 . Torsional waves generated in the waveguide tube  460  are mechanically coupled to a heat exchanger tube  410  using a collet drawbar mechanism  435 .  FIG. 6A  shows how the collet  470  is expanded when the nose piece  475  is forced into the collet  470  by the drawbar  435  when a user tightens the drawbar nut  430  to reposition the drawbar  435 , mechanically coupling the waveguide tube tip  460  to the inside of the heat exchanger tube  410 . 
   Similarly,  FIG. 6B  shows another embodiment of an enlarged waveguide working tip configuration  650  of a torsional guided wave probe. The tip of the waveguide tube  460  contains axial slits positioned around the tube circumference in the region near the working tip of the waveguide tube  460 . Torsional waves generated in the waveguide tube  460  are mechanically coupled to a heat exchanger tube  410  using a drawbar mechanism  474 .  FIG. 6B  shows how the drawbar nose piece  476  causes the tip of the waveguide tube  460  to be expanded when the nose piece  476  is forced into the tip of the waveguide tube  460  when a user repositions the drawbar  474 , mechanically coupling the waveguide tube tip  460  to the inside of the heat exchanger tube  410 . 
   Turning to  FIG. 7 ,  FIG. 7  shows details of a magnetostrictive sensor for generating and detecting torsional mode guided wave signals.  FIGS. 7A and 7B  illustrate a more detailed view of the magnetostrictive sensor ( 450  shown in  FIGS. 4 and 5 ).  FIGS. 7A and 7B  show a cylindrical ferromagnetic strip  754  affixed to the waveguide tube  760 . The sensor coil  752  is wound over the cylindrical ferromagnetic strip  754  that contains bias residual magnetization. The ferromagnetic strip  754  is typically an inch wide and wrapped around the waveguide tube  760 . The width of the ferromagnetic strip  754  may be adjusted according to the frequency of the guided waves, being narrower for high frequencies and wider for lower frequencies. The ferromagnetic strip  754  may be made from any material that has good magnetostrictive characteristics, such as nickel, grain-oriented silicon steel, or other magnetostrictive material that has the ability to retain residual magnetism and a high magnetostrictive coefficient. The residual magnetic field is induced in the ferromagnetic strip  754  by applying a suitable energizing magnetic field to the ferromagnetic strip  754  and removing the energizing magnetic field such that the direction of the induced magnetic field is circumferential when cylindrical ferromagnetic strip  754  is wrapped around the waveguide tube. After affixing the cylindrical ferromagnetic strip  754  to the waveguide tube  760 , the magnetostrictive sensor coil  752  is placed around the cylindrical ferromagnetic strip  754 . For generation of torsional mode waves, the DC bias magnetic field must be in a circumferential direction around the waveguide tube  760  as described. In contrast, for generation of longitudinal mode waves, the DC bias magnetic field must be in a longitudinal or lengthwise direction of the waveguide tube  760 . 
   Another method of creating a circumferential bias magnetic field in the cylindrical ferromagnetic strip  754  is illustrated in  FIG. 7C . As shown in  FIG. 7C , a DC current is caused to be passed between conductive rings  782 ,  784  around an electrically conductive waveguide tube  760 . This current causes a circumferential magnetic field to be induced in the cylindrical ferromagnetic strip  754 . If the waveguide tube  760  is not electrically conductive, a heavy gauge copper wire may be inserted through the waveguide tube and a DC current applied to the copper wire to achieve the same result. 
   Although the present invention has been described in detail with reference to certain preferred embodiments, it should be apparent that modifications and adaptations to those embodiments might occur to persons skilled in the art without departing from the spirit and scope of the present invention.