Abstract:
Apparatus and method to identify chafing of a conduit, thereby reducing the failure of any system which would be damaged or whose function would be impaired by abrasion of the conduit. Such a system may carry electrical power, fuel, other fluid, hydraulics, pneumatics, optical, or electromagnetic signals. Wear caused by rubbing against external structures is detected by wrapping the conduit with a sensing element, which may be a conductive wire, waveguide, fiber optic cable, or a tube (wound around the conduit or enclosing it) that holds a fluid under pressure. The sensing element is positioned so that chafing on the conduit electrically contacts, breaks, or punctures the sensing element well before the conduit fails. Measuring the end-to-end integrity of the sensing element or performing other tests on it determines whether a chafing object is present, the nature of the chafing object, and where on the conduit the chafing has occurred, thereby indicating that the conduit&#39;s integrity will be compromised unless remedial measures are taken.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to reducing the failure of a system that carries electrical, optical, or electromagnetic (as in waveguide) signals or that conveys fluids. In particular, it relates to failures of such a system that result from chafing of the conduits within it. Such a system may carry electrical power; fuel, refrigerant, other fluid; or optical and/or electromagnetic signals. Or the system may be hydraulic or pneumatic. Degradation or failure of such a system could also expose the systems around the conduit to adverse effects, as when a spark ignites the contents of a fuel container. 
     The outer surfaces of electrical, optical, and electromagnetic cables and cable bundles are frequently subject to wear caused by rubbing against external structures. If allowed to proceed, this wear can cause grounding, shorting, or breaking of the cable&#39;s internal structures (conductors or optical fibers). Hydraulic, pneumatic, and fuel or other fluid lines, pipes, and hoses are also susceptible to failure from chafing. 
     Each of these structures susceptible to chafing can be called a “conduit”. This application uses the term conduit for any structure that can fail from chafing, such as a cable, cable bundle, hydraulic hose or pipe, pneumatic hose or pipe, or fuel/fluid hose or pipe. 
     A conduit often fails before a system&#39;s operator knows that the conduit has a chafing problem. Currently nothing is in wide use to detect when a conduit experiences chafing and thus becomes subject to failure. The disclosure of U. S. Pat. No. 4,988,949 is limited to detecting chafing on electrical cables against an electrically grounded structure under constant monitoring. It teaches nothing about either periodic testing or detecting chafing on any conduits other than electrical cables, nor does it detect chafing against a non-electrically grounded structure. 
     Thus there exists a need for a simpler apparatus and method of detecting chafing on any conduit that is likely to chafe against a non-electrically grounded structure or that requires only periodic monitoring. 
     SUMMARY OF THE INVENTION 
     Therefore, one object of the present invention is to provide a mechanism to detect chafing. 
     Another object of the present invention is to provide a mechanism to detect chafing prior to any damage to the internal structure of a conduit or to systems in the vicinity of the conduit. 
     Briefly stated, the present invention provides apparatus and method to detect chafing in a conduit, thereby reducing the failure of any system which would be damaged or whose function would be impaired or degraded by abrasion of the conduit. Such a system may carry electrical power, fuel, other fluid, optical or electromechanical signals, or it may be hydraulic or pneumatic. Wear caused by rubbing against external structures is detected by wrapping the conduit with a sensing element (conductive wire or fiber optic cable, or a tube to hold fluid under pressure; such a tube may be wound around the conduit, or it may enclose it.) The sensing element is positioned so that chafing on the conduit breaks or punctures the sensing element well before the conduit fails. Measuring the end to end integrity of the sensing element or performing other tests on it determines whether it has failed, thereby indicating that the conduit&#39;s integrity will be compromised unless remedial measures are taken. 
     According to an embodiment of the invention, a method for detecting chafing of a conduit comprises the steps of: placing adjacent the conduit an effective length of a medium, the medium being capable of conducting a signal, the medium being located so that a chafing object cannot substantially abrade the conduit without causing substantial damage to the medium; determining if there is end-to-end integrity of the medium, whereby lack of the integrity implies chafing; and if at least one of the conduit and the chafing object has an outer surface comprising electrically conductive material and the medium is electrically conductive, a further step, prior to the step of determining, of electrically isolating the medium from any other conductor. 
     According to a feature of the invention, a method for detecting chafing of a conduit comprises the steps of: placing adjacent to the conduit an effective length of electrically conductive wire, the wire being held substantially against the conduit so that a chafing object cannot substantially abrade the conduit without causing substantial damage to the wire; determining if there is end-to-end integrity of the wire, whereby lack of the integrity implies chafing; and if at least one of the conduit and the chafing object has an outer surface comprising electrically conductive material, a further step, prior to the step of determining, of electrically isolating the wire from any other conductor. 
     According to another feature of the invention, apparatus to detect chafing in a conduit, comprises: a sensing element placed adjacent to the conduit so that a chafing object cannot substantially abrade the conduit without causing substantial damage to the sensing element; and means for determining end-to-end integrity of the sensing element. 
     These and many other objects and advantages of the present invention will be readily apparent to one skilled in the pertinent art from the following detailed description of a preferred embodiment of the invention and the related drawings, in which like reference numerals designate the same elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a preferred embodiment of the invention whose sensing elements can be either conductive or optical. 
     FIG. 2 shows how to detect chafing by a manual read-out that uses a conductive sensing element. 
     FIG. 3 shows a conductive sensing circuit of the present invention. 
     FIG. 4 shows a cable bundle configured with a sensing element that has extensions. 
     FIG. 5 shows a circuit of the present invention for automatic read-out. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, in a first embodiment of the present invention, chafing of a conduit  120  is detected by means of a manual readout. Conduit  120  can be any item that is susceptible to chafing, e.g., conductive or optical cable, cable bundle, fluid line (pipe or hose), hydraulic line or hose, pneumatic line or hose. 
     The manual read-out embodiment of the present invention uses an optical or conductive sensing element configured as follows. Conduit  120  is spirally wrapped with a sensing element  100 . If sensing element  100  is electrically conducting, and conduit  120  has an outer layer that is electrically conducting, conduit  120  must be electrically insulated from sensing element  100 . This can be done either by wrapping conduit  120  with an electrically insulating layer or by insulating sensing element  100  before wrapping it around conduit  120 . Sensing element  100  extends from a first end  200  and is wrapped around conduit  120  so that it doubles back on itself at a point  220 . Sensing element  100  has a second end  210  in close proximity to first end  200 . 
     If sensing element  100  is conductive, it should not touch itself at any spot along the way. To do so would make it impossible to determine continuity beyond that point. 
     After conduit  120  is wrapped with sensing element  100 , the whole assembly is covered with a layer of tape or other wrap to hold it in place. If sensing element  100  is conductive, this layer must be electrically insulating unless sensing element  100  is separately insulated. 
     Sensing element  100  is wrapped around conduit  120  so that any external structure that causes chafing contacts sensing element  100  and causes it to break. The fragility of sensing element  100  is chosen so that it breaks only after significant chafing has occurred but well before conduit  120 &#39;s functional failure. 
     Sensing element  100  is configured relative to conduit  120  so that external structures in proximity to conduit  120 , if they can cause chafing, impinge upon sensing element  100  before they significantly damage conduit  120  itself. Also, if sensing element  100  comprises an optical fiber, care must be taken to assure that the losses due to bending are not great enough to make it appear as if the fiber is broken. Thus optical fiber may not be usable on a small-diameter conduit. 
     In this embodiment, testing for end-to-end continuity of sensing element  100  detects chafing. This test is performed with simple optical or electrical test instruments, depending on the composition of sensing element  100 . A simple optical test comprises injecting light into one end of the fiber and determining whether the light reaches the other end. A broken sensing clement  100  indicates that chafing has occurred on conduit  120 . Additional tests can be performed if sensing element  100  is conductive. Testing for voltage on conductive sensing element  100  or electrical grounding of conductive sensing element  100  can also indicate chafing. 
     Alternate Embodiment 1. It is not necessary to spirally wrap sensing element  100  around conduit  120 . Sensing element  100  can extend in the axial direction of conduit  120  as long as sensing element  100  is in position to be chafed by any external structures that can cause chafing on conduit  120 . By turning back on itself, sensing element  120  can make multiple passes axially along the conduit. Both first end  200  and second end  210  can be at the same end of conduit  120 . 
     Alternate Embodiment 2. It is not necessary to have sensing element  100  double back on itself. Having first and second ends  200 ,  210  of sensing element  100  in close proximity to each other increases the ease of testing continuity of sensing element  100 . Complex testing techniques, such as time-domain reflectometry, allow the verification of the end-to-end integrity of sensing element  100  without accessing both ends of sensing element  100 . Doubling back of sensing element  100  also increases the likelihood of a conductive sensing element  100  touching itself, thereby disabling chafing detection beyond the point where sensing element  100  touches itself. 
     Alternate Embodiment 3. A plurality of sensing elements  100  can be especially helpful if sensing element  100  is not wrapped spirally around conduit  120 . A plurality could sense chafing at different locations along conduit  120 . 
     Alternate Embodiment 4. Sensing element  100  comprises a tube that, if punctured, indicates chafing of conduit  120 . The test for chafing for this embodiment is whether the tube holds fluid pressure without significant leaks. Access anywhere along the tube suffices, provided a test point is available. Thus there is no advantage to having the tube double back on itself to end up in close proximity to where it started. 
     Alternate Embodiment 5. A tube concentric to conduit  120  could surround it and act as sensing element  100 . Failure of the tube to hold fluid pressure without significant leaking indicates potential chafing on conduit  120 . 
     Alternate Embodiment 6. Making sensing element  100  conductive allows several other implementations: Referring to FIG. 2, chafing of conduit  120  is detected by a manual read-out from conductive sensing element  100 . Conduit  120  can be any item susceptible to chafing, e.g., conductive or optical cable, cable bundle, fluid line (pipe or hose), hydraulic line or hose, pneumatic line or hose. 
     A manual read-out apparatus of the present invention whose sensing element  100  is conductive is configured as follows. Conduit  120  is spirally wrapped with sensing element  100  that is thin-gauge, electrically conducting, and uninsulated. If conduit  120  has an outer layer that is electrically conducting, conduit  120  must be insulated from conductive sensing element  100 , either by wrapping conduit  120  with an electrically insulating layer or by insulating sensing element  100  before wrapping it around conduit  120 . Conductive sensing element  100  extends from a first electrical ground  130 , connected or in proximity to conduit  120 , to a second electrical ground  130 , connected or in proximity to conduit  120 . A bridge  110  of known resistance makes each of conductive sensing element  100 &#39;s connections to electrical grounds  130 . Each end of conductive sensing element  100  typically terminates with the same resistance between it and electrical ground  130 . After conduit  120  is wrapped with conductive sensing element  100 , the whole assembly is covered with an electrically insulating layer. If conductive sensing element  100  is separately insulated, this layer is unnecessary if sensing element  100  is held rigidly in place. 
     Conductive sensing element  100  is wrapped around conduit  120  so that any external structure that causes chafing comes into contact with conductive sensing element  100  and causes it to break. The gauge of conductive sensing element  100  is chosen so that it breaks only after significant chafing has occurred but well before conduit  120 &#39;s functional failure. 
     Conductive sensing element  100  must be configured relative to conduit  120  so that external structures in proximity to conduit  120 , if they cause chafing, impinge upon conductive sensing element  100  before they damage conduit  120  itself. 
     Referring to FIG. 3, the resistance of bridge  110  is selected so that one-half its value is much larger than the resistance of the entire length of conductive sensing element  100 . (This value minimizes the degree of accuracy required of the test equipment.) The resistance of bridge  110  should also be large enough that shorting conductive sensing element  100  to a power line does not damage any of the systems involved. That is, where R B =R 1 =R 2 =resistance of each bridge, and R S =total resistance of conductive sensing element  100 , R S &lt;&lt;R B /2. 
     Two general tests are applied to the configuration: 
     Test 1. Measure electrical resistance from any point on conductive sensing element 100 to electrical ground. The result should be approximately equal to, or slightly greater than R B /2. Any other value indicates failure (allowing for normal drift and measurement errors). 
     Test 2. Measure the resistance from one end of conductive sensing element  100  to the other. The result should be small (but not zero) if we have chosen conductive sensing element  100  correctly; i.e., 0&lt;R S &lt;&lt;R/2, where R S  is the resistance of conductive sensing element  100 . It is desirable (but not necessary) that conductive sensing element  100  have a resistance from about 20 ohms to 100 ohms. This value allows one to determine the approximate location of the chafing if one of the broken ends of conductive sensing element  100  is grounded. Unfortunately, in many cases this value requires that conductive sensing element  100  be excessively thin. Any measured value that differs from R S  indicates failure. 
     To determine whether these tests apply, one must consider (1) the location of the rupture point of conductive sensing element  100  when it breaks from chafing and (2) the conductive properties of both conduit  120  and the chafing member. In a first possible case, the chafing member wears through the insulation on conductive sensing element  100  but has not yet broken it. In a second case, a line that carries electric current chafes conductive sensing element  100 . In this case, the above tests must be preceded by a voltage test from conductive sensing element  100  to electrical ground. Any voltage found on conductive sensing element  100  shows a current is present. Since conductive sensing element  100  is probably insulated, presence of a current indicates chafing, possibly in its early stages. Both tests might detect chafing and grounding to either the chafing element or to conduit  120 . 
     If chafing breaks conductive sensing element  100  into two portions, there are several possible cases: 
     Case 1: Both broken ends of conductive sensing element  100  are electrically floating. This means that they are not conductively contacting either conduit  120  or the chafing member. This is the most likely case. Both Test 1 and Test 2 (see above) work. 
     Case 2: One of the broken ends of conductive sensing element  100  is electrically floating (not connected to an electrical ground). The other is touching either an electrically conductive spot on conduit  120  or the chafing member. This is the second most likely case. Test 1 (see above) always works from one of the two portions of broken conductive sensing element  100 . Test 1 also works from the other portion of conductive sensing element  100  unless the circumstances are exceptional. If the broken end of conductive sensing element  100  touches the conducting surface of either conduit  120  or the chafing member, and the broken end has a resistance through that surface to ground that almost exactly equals R B , Test 1 will fail. But the probability is very small that conductive sensing element  100  is grounded through the exact resistance R B . Test 2 (see above) always works. 
     Case 3. Both broken ends of conductive sensing element  100  touch either conduit  120  or the chafing member. This case breaks down into several variations. 
     Case 3, variation 1: Both broken ends contact an electrically insulating surface. Both Test 1 and Test 2 (see above) work. 
     Case 3, variation 2: One broken end of conductive sensing element  100  contacts an electrically insulating surface. The other broken end contacts an electrically conducting surface. Test 1 always works from one of the two portions of broken conductive sensing element  100 . Test 1 also works from the other portion of conductive sensing element  100  unless the broken end (1) touches the conducting surface of either conduit  120  or the chafing member AND (2) has a resistance through that surface to ground almost exactly equal to R B . Test 2 always works. 
     Case 3, variation 3: Both broken ends of conductive sensing element  100  are touching an electrically conducting surface. This variation breaks down into several versions. 
     Case 3, variation 3, version A: Both broken ends of conductive sensing element  100  are touching an electrically conducting surface that is electrically grounded. This is the most likely example of Case 3. Test 1 works. Test 2 will fail. 
     Case 3, variation 3, version B: Both broken ends of conductive sensing element  100  are touching a conducting surface. One broken end is floating (not connected to an electrical ground), and the other is grounded. Test 1 always works from one of the two portions of broken conductive sensing element  100 . Test 1 also works from the other portion of conductive sensing element  100 , unless the broken end of conductive sensing element  100  (1) is touching the conducting surface of either conduit  120  or the chafing member, AND (2) has a resistance through that surface to ground almost exactly equal to R B . Test 2 always works. 
     Case 3, variation 3, version C: Both broken ends of conductive sensing element  100  contact an electrically conducting surface that is electrically floating (not connected to an electrical ground). Because electrically conducting elements are usually grounded, this version has a low probability. Since one does not expect either broken end to be in good contact with either conduit  120  or the chafing member, the probability that both are so connected is also low. This version breaks down into several examples. 
     Case 3, variation 3, version C, example 1: Both broken ends of conductive sensing element  100  contact different electrically conducting surfaces that are electrically floating. Both Test 1 and Test 2 work. 
     Case 3, variation 3, version C, example 2: Both ends of conductive sensing element  100  contact the same electrically floating conducting member. Or the broken ends contact different electrically floating conductive members that are connected non-resistively. Both Test 1 and Test 2 fail to detect the chafing that caused this condition. This example has a very low probability of occurring. If the two members are resistively coupled, both tests work if the resistance is high enough. If we ground the floating electrically-conductive member, Test 1 works; Test 2 does not. 
     Case 4: Conductive sensing element  100  is not broken, but chafing has worn through the insulation that covers it. 
     Case 4, variation 1: If conductive sensing element  100  is not in electrical contact with an electrically conducting chafing member, or the chafing member conducts but is electrically floating (not in contact with an electrical ground), Tests 1 and 2 will be unable to detect that chafing has occurred. Detection of chafing in this situation must wait until (1) conductive sensing element  100  comes into electrical contact with an electrically grounded member, (2) the test operator grounds the chafing member, or (3) conductive sensing element  100  breaks from the chafing. 
     Case 4, variation 2: If conductive sensing element  100  is in electrical contact with a grounded chafing member, Test 1 will indicate chafing, and Test 2 will indicate no chafing. 
     Case 4, variation 3: If conductive sensing element  100  is in contact with a conductive chafing member that is grounded through a resistance, Test 1 will indicate chafing; test 2 will not. 
     An example of this alternate embodiment used #24 AWG copper wire for sensing element  100  and 1000-ohm resistor bridges  110 . Conductive sensing element  100  was wrapped spirally around conduit  120  with a 0.25 inch pitch, defined as the distance between consecutive windings of conductive sensing element  100  when measured parallel to the axis of conduit  120 . The total length of conductive sensing element  100  was determined from the following formula: 
     
       
           S= ( L/P )(Square Root of ( p   2 +(7 r*D ) 2 )) 
       
     
     where L=the length of conduit  120  to be wrapped with conductive sensing element  100 , D=the diameter of conduit  120 , and S=the total length of the conductive sensing element  100 . For conduit  120  of length 10 feet and diameter ½, the length of conductive sensing element  100  would be approximately 64 feet. For #24 AWG copper wire, the resistance of conductive sensing element  100  is thus 1.65 ohms. This amount of resistance easily satisfies the criterion that the total resistance of conductive sensing element  100  be much less than one-half the resistance of resistive bridges  110 . 
     The weight of sensing element  100  is also critical in some applications. For the above example, the weight of conductive sensing element  100  needed to wrap the ten-foot long, ½-inch diameter conduit  120  with a 0.25 inch pitch is approximately 1.24 ounces. Additional weight is added to the system by resistive bridges  110  and by the insulating wrap applied over the entire conduit  120 /conductive sensing element  100  system. Teflon tape is a good choice for the insulating wrap. It is extremely light in weight, strong, and common in aircraft applications. 
     Alternate Embodiment 7. Referring to FIG. 4, to detect chafing by automatic readout we configure a system as follows. Conduit  120  is spirally wrapped with conductive sensing element  100  as in the first embodiment. Alternatively, sensing element  100  can be configured in other than a spiral wrap. All that is required is that a chafing member impinges on sensing element  100  prior to significantly damaging conduit  120 . Conductive sensing element  100  is again a thin gauge, electrically conducting wire. If the outer layers of conduit  120  are electrically conducting, it must first be wrapped with an electrically insulating layer. Both ends  140  of conductive sensing element  100  are brought out to junction box  150 , wherein circuitry automatically detects chafing. After conduit  120  is wrapped with conductive sensing element  100 , the whole assembly is covered with an electrically insulating layer as in the previous embodiment. 
     In this embodiment, conductive sensing element  100  is wrapped around conduit  120  in the same manner as in the manual read-out embodiment. The difference between the two embodiments is that, for automatic readout, both ends  140  of conductive sensing element  100  are brought out to junction box  150 , wherein automatic testing determines periodically whether chafing has occurred on conduit  120 . Referring to FIG. 5, ends  140  of conductive sensing element  100  enter junction box  150 , wherein circuits  160 ,  170 , and  180  test for conditions that might be caused by chafing against conduit  120 . Any or all of circuits  160 ,  170 , and  180  can be used, depending on the types of chafing that are potential threats to conduit  120 . 
     Circuit  160  tests for floating voltage caused by chafing, through the outer insulating layer surrounding conductive sensing element  100 , that short-circuits it to a voltage source. Any voltage measured by a voltmeter in circuit  160  indicates chafing on conduit  120 . 
     Circuit  170  tests for chafing on conduit  120  that has caused a break in conductive sensing element  100 . A small voltage is applied to circuit  170 , and the current is measured. No current means there is a break in conductive sensing element  100 , probably from chafing. 
     Connecting one end  140  of conductive sensing element  100  to circuit  180  allows testing for chafing that results from electrical grounding. Any current in circuit  180  detected when a small voltage is applied indicates chafing has grounded out conductive sensing element  100 . 
     If circuit  170  determines that there is a break in conductive sensing element  100 , the distance of the break from the location of the read-out circuitry can be found with a Time Domain Reflectometer (“TDR”). If circuit  180  determines that conductive sensing element  100  is shorted to ground, a simple test of resistance determines the distance to the ground point, if the resistance per unit length of conductive sensing element  100  is known. 
     Alternate Embodiment 8. The automatic mode is reconfigured for manual readout. Referring again to FIG. 4, ends  140  of conductive sensing element  100  are fed to junction box  150 . There the tests for chafing described above for Embodiment 2 are performed with a hand-held device. 
     Alternate Embodiment 9. To detect chafing by automatic readout we configure a system as follows. Sensing element  100  is an optical fiber placed adjacent to conduit  120  so that a chafing member impinges on sensing element  100  prior to significantly damaging conduit  120 . We shine a continuous optical signal into one end of sensing element  100 . An optical detector at the other end issues an alarm if the signal is interrupted. 
     Which embodiment to employ depends on the application. The choice should be made by system engineers familiar with the system to be protected. 
     All of the embodiments described above offer the following advantages over present techniques. The present invention detects chafing against virtually any structure external to conduit  120 , whether or not conducting, whether or not grounded. The sensitivity of the present invention can be adjusted so that the detection of chafing allows a lead-time adequate to correct chafing prior to conduit  120 &#39;s failure. The present invention can be implemented with varying instrumentation from fully automatic to fully manual. The manual embodiment of the present invention does not require a power source. The present invention detects chafing on a wide variety of conduits: electric or optical cables, fluid lines (pipes and hoses), hydraulic lines and hoses, and pneumatic lines and hoses. 
     The embodiments of the present invention that employ manual readout require only a commercially available handheld tester. Testing by automatic readout can be instrumented in one or more of several implementations, depending on which failure mechanisms are likely to be present in the particular application. In many cases, the present invention allows rapid location of the chafing site with a simple electrical or optical TDR or resistance measurer. 
     Clearly many modifications and variations of the present invention are possible in light of the above teachings. It should therefore be understood that, within the scope of the inventive concept, the invention may be practiced otherwise than as specifically claimed.