Abstract:
A system and a method are provided for monitoring the condition of an interface. Although not limited to this particular application, the system and method are particularly suited for monitoring the interface of two liquid lubricated mechanical seal faces. The system monitors an interface by using a wave source to produce an ultrasonic shear wave, directing the wave at the interface, detecting the wave after it interacts with the interface, and comparing the detected wave to predetermined wave characteristics. Based on the comparison, an alarm may be triggered. The alarm may indicate that the mechanical seal is failing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to copending U. S. provisional application entitled, “CONDITION MONITOR FOR A LIQUID MECHANICAL SEAL, ” having Ser. No. 60/162, 940, filed Nov. 2, 1999, Georgia Tech Docket No. 2137PR, which is entirely incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The U.S. government may have a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of GIT Project No. E-25-T-46 awarded by the U.S. Office of Naval Research under research grant N00014-95-1-0539, entitled Integrated Diagnostics. 
    
    
     TECHNICAL FIELD 
     The present invention is generally related to methods and equipment for monitoring the interface of two surfaces and, more particularly, is related to a system and method for monitoring the interface of two liquid lubricated mechanical seal faces. 
     BACKGROUND OF THE INVENTION 
     Although not limited to any particular interface, the background of the condition monitor is provided with reference to the interface between the two faces of liquid lubricated mechanical seals. 
     Condition monitoring of liquid lubricated mechanical seals is based on the observation that the proximate cause of most seal failures is excessive contact between the faces. Such contact leads to mechanical and thermal damage to the faces, and ultimately, to seal failure. Thus, excessive face contact is a precursor to seal failure. Therefore, the detection of contact and measurement of the severity of contact would allow preventive action to be taken to avoid seal failure. 
     The most extensively investigated approach to condition monitoring of liquid lubricated mechanical seals is the acoustic emission (AE) method. AE relies on the emission of ultrasonic acoustic waves by a pair of surfaces when they are in sliding contact. This approach has not proven to be commercially feasible for a variety of reasons. First, it is very difficult to distinguish seal emissions from the emissions generated by other sources (noise). Second, the seal emissions characteristics are not known a priori, so that one does not know what frequencies to listen for without first testing a particular seal. Third, seal emissions characteristics can differ from seal to seal. Fourth, even with the same seal, the emissions characteristics can change with a change in operating conditions. And fifth, with the same seal the emission characteristics can change with time as the face surfaces change (e.g. , due to wear). Thus, even if the seal emissions can be isolated and identified, their interpretation is extremely difficult. 
     Attempts have also been made to use alternate techniques. In one such technique, emitted audible acoustic waves are monitored instead of the ultrasonic waves, described above. Four microphones (placed outside the subject machine) are used in conjunction with a sophisticated signal-processing scheme to isolate the seal emissions from noise. The placement requirements of multiple microphones may make this approach impractical in an industrial environment. In addition, this method still suffers from the other drawbacks of the classic AE method. 
     Another approach involves the use of multiple conventional eddy current proximity probes to monitor the shape and power spectrum of the orbit plot of the rotating face angular misalignment. This technique can indicate whether or not contact occurs, but it has not been shown to indicate the severity of contact. 
     Finally, attempts have been made to use multiple conventional sensors to monitor such operating characteristics as sealed pressure, sealed temperature, housing vibration and motor current. The data from these sensors are fed into an elaborate data processing system (e.g., containing a neural network) to determine if a seal is in danger of failing. This complexity of this technique limits its application. 
     The benefits of condition monitoring include the reduction in the probability of catastrophic failure, the reduction or elimination of scheduled maintenance, and an increased machine or component life. The application of such condition monitoring to liquid lubricated mechanical seals has been prevented by the lack of a proven commercially available seal monitor. Although attempts have been made to develop such a monitor, none have proven successful. 
     Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system and a method for monitoring the condition of an interface. Although not limited to this particular application, the invention provides a system and method for monitoring the interface of two liquid lubricated mechanical seal faces. 
     Briefly described, in architecture, the system for monitoring an interface can be implemented as follows. A wave source produces a shear wave (transverse wave). The shear wave is directed at the interface. A wave sensor detects the wave after it interacts with the interface. A wave analyzer compares the detected shear wave to predetermined wave characteristics. The system produces an output containing information regarding the comparison. 
     The present invention can also be viewed as providing a method for monitoring an interface. In this regard, the method can be broadly summarized by the following steps: producing a shear wave; directing the wave at the interface; detecting the shear wave after the shear wave interacts with the interface; analyzing the detected wave in comparison to predetermined wave characteristics; and producing an output containing information regarding the comparison. 
     In the preferred embodiment, which is meant as a nonlimiting example, the approach involves detecting the collapse of the lubricating film between the seal faces and detecting excessive asperity contact. The collapse of the lubricating film and excessive asperity contact are precursors to seal failure. An ultrasonic transducer is placed behind one of the seal faces and used to produce ultrasonic shear waves (at a known frequency and amplitude) which propagate toward the interface between the two seal faces. By monitoring the amplitudes of the waves transmitted through or reflected by the interface, one can detect film collapse and the degree of contact between the faces. 
     This approach avoids the difficulties of the acoustic emissions method and the other methods described above. Actively generated ultrasonic shear waves are used to diagnose the condition of the sealing interface, indicating the occurrence and severity of contact. This method uses very simple hardware and signal processing software, making it especially suitable for commercial use. 
     Some systems, methods, features, and advantages of the present invention have been described in the following publications which are entirely incorporated herein by reference: Anderson, W. B. , Salant, R. F. , and Jarzynski, J. , “Ultrasonic Detection of Lubricating Film Collapse in Mechanical Seals, ” STLE Tribology Trans, Vol. 42, pp. 801-806, (1999); and Salant, R. F. , Anderson, W. , and Jarzynski, J. “Condition Monitoring of Mechanical Seals Using Actively Generated Ultrasonic Waves, ” BHR Group Fluid Sealing, pp. 271-289 (2000). 
     Other systems, methods, features, and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. In accordance with the invention, the following figures are provided. 
     FIG. 1 is a cut-away view of a machine housing and a single transducer for monitoring the interface of two seal faces in a first type of mechanical seal and a block diagram showing an analyzer. 
     FIG. 2 is a cut-away view of machine housing, a first transducer for producing a shear wave, and a second transducer for detecting the shear wave after the shear wave interacts with the interface of the mechanical seal of FIG. 1 and a block diagram showing an analyzer. 
     FIG. 3 is a cut-away view of a machine housing and a single transducer for monitoring the interface of two seal faces in a second type of mechanical seal and a block diagram showing an analyzer. 
     FIG. 4 is a cut-away view of a machine housing, a first transducer for producing a shear wave, and a second transducer for detecting the shear wave after the shear wave interacts with the interface of the mechanical seal of FIG. 3 and a block diagram showing an analyzer. 
     FIG. 5 is a cut-away view of a machine housing and a single transducer for monitoring the interface of two seal faces in a third type of mechanical seal and a block diagram showing an analyzer. 
     FIG. 6 is a cut-away view of an alternative arrangement for the first and second transducers in which the first and second transducers are on the same side of the interface of a mechanical seal. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The condition monitor for an interface, and associated method(s), will be specifically described in the context of preferred embodiments for liquid lubricated mechanical seals. The embodiments are nonlimiting examples of implementations for the condition monitor for an interface. Numerous other embodiments are envisioned and are possible such as journal bearings and static joints. Other embodiments will be apparent to those with skill in the art. In general, the particular embodiment will be determined by the interface to be monitored. 
     A piezoelectric transducer is placed behind a non-rotating seal face and used as a source to produce ultrasonic shear waves at a known frequency and amplitude. Those waves propagate toward an interface between the non-rotating seal face and a rotating seal face. If there is no contact between the faces, most of the ultrasonic energy is reflected at the interface; very little is transmitted across the interface. Conversely, if there is mechanical contact between the faces, less ultrasonic energy is reflected at the interface and more is transmitted. As the contact becomes more severe, asperities are increasingly compressed and deformed, and the real area of contact increases. Consequently, the amplitudes of the reflected waves are decreased, and those of the transmitted waves, increased. Therefore, either the transmitted or the reflected wave amplitudes can be measured to determine if contact occurs and the severity of contact. 
     For dual seals with a single rotating face between two non-rotating faces, it is convenient to measure the transmitted waves. However, for most applications involving single, double or tandem seals, the transmitted wave technique would be difficult to implement because two transducers are required for each seal, one source and one receiver. One of those transducers would have to be mounted behind the rotating seal face and some method must be used to transmit a signal between that transducer and ground (e.g. slip rings or telemetry). Therefore, for most applications the reflected wave technique is more practical, since both the source and the receiver can be mounted behind the non-rotating face. 
     The condition monitor for an interface offers important advantages over the previous methods described above. Since the ultrasonic shear waves are actively generated, their frequency and amplitude can be chosen such that they can be easily distinguished from emissions generated by other sources (noise). Further, since the frequency is chosen, one knows what to listen for, and therefore an optimum transducer for the receiver can be selected and a very simple signal-processing scheme can be used. One form of a signal processing scheme is to store the reflected wave characteristics of the mechanical seal during normal operation and compare the stored wave characteristics to operating wave characteristics. The output would then be the triggering of an alarm when significant differences are noted between the stored wave characteristic and the operating wave characteristics. 
     A second form of a signal processing scheme is to store several reflected wave characteristics of the mechanical seal during normal operations over a period of time and compare the stored wave characteristics to operating wave characteristics and produce an output indicating the wear on the seal. The output could then be used to more efficiently maintain the seal. 
     Ultrasonic shear waves (transverse waves) are particularly well suited for monitoring liquid lubricated mechanical seals because the mismatch between acoustic impedances of the sealed liquid and the seal face materials is low. In shear waves, either particles of the medium in which the waves travel vibrate at right angles to the direction of the wave propagation (e.g. sonic transverse waves) or energy fields oscillate at right angles to the direction of propagation (e.g. electromagnetic waves). 
     Although transducers producing shear waves are particularly well suited for liquid seals, gas seals (“dry gas seals”) have become increasingly popular over the last few years, especially for sealing compressors. Gas seals are particularly vulnerable to face damage caused by excessive contact and therefore would also benefit from a condition monitoring system that may use transducers producing shear waves. 
     FIG. 1 shows a cut-away view of a machine housing and a single transducer for monitoring the interface of two seals in a first type of mechanical seal. The machine housing and condition monitoring system as a whole is shown by reference numeral  10 . In detail, FIG. 1 shows a machine housing  12 . A seal chamber  14  is located in the machine housing  12 . The seal chamber  14  contains a fluid. A shaft  16  is also shown. 
     Around the shaft  16 , and within the machine housing  12 , is an annular fixed portion  18  of the mechanical seal. Also around the shaft  16 , and within the machine housing  12 , is an annular rotating portion  20  of the mechanical seal. The annular fixed portion  18  includes a front with a fixed face  22 . The annular rotating portion  20  includes a front with a rotating face  24 . The fixed face  22  and the rotating face  24  form the two parts of the mechanical seal that slide near each other during normal operation. During normal operation, a thin liquid film forms at the interface of the fixed face  22  and the rotating face  24 . The mechanical seal also includes a spring  26 , a collar  28 , a first O-ring  30 , and a second O-ring  32 . The components and operation of a mechanical seal are well known to those with skill in the art. 
     FIG. 1 also shows a transducer  34 , for example a PZT-5A poled to operate in the transverse mode, mounted on the back of the fixed portion  18  of the mechanical seal. The transducer  34  may be driven at 4 MHz by a function generator, to produce an ultrasonic shear wave  36  directed at the interface of the fixed face  22  and the rotating face  24 . During normal operation, the ultrasonic shear wave  36  would primarily be reflected from the interface. The double arrowhead line representing the ultrasonic shear wave  36  indicates the dual operation of the transducer  34  as both a wave source and a wave sensor for the reflected ultrasonic shear wave  36 . 
     In a non-contacting seal, when the thin liquid film breaks down the asperities in the fixed face  22  and the rotating face  24  make contact. Under these circumstances a greater portion of the ultrasonic shear wave  36  passes across the interface. A measurably lesser portion of the ultrasonic shear wave  36  is reflected back to the transducer  34 . The measurably lesser portion may be shown by a change in amplitude of the reflected wave. In a partially contacting seal, there is contact during normal operation that becomes more severe when the film breaks down. In a partially contacting seal, as in a non-contacting seal, when the thin film breaks down a greater portion of the ultrasonic shear wave  36  passes across the interface and a measurably lesser portion of the ultrasonic shear wave  36  is reflected back to the transducer  34 . 
     Also shown in FIG. 1 are an analyzer  38   a  and an analyzer lead  40   a  running between the transducer  34  and the analyzer  38   a . The analyzer  38   a  may be any device capable of controlling the transducer  34  and analyzing the signal received from the transducer  34 . The analyzer  38   a  may include an oscilloscope, a memory element, a personal computer, and digital signal processor. If a computer is used as an analyzer, the signal can be filtered and transformed to the frequency domain with a Fast Fourier Transform (FFT) and the peak amplitude recorded, tracked and temperature compensated. The operation of the analyzer  38   a  in combination with a transducer is well known in the art. Among others devices, the analyzer lead  40   a  may be copper wire or the analyzer lead  40   a  may be wireless. 
     The analyzer  38   a  of the condition monitor for an interface can be implemented in hardware, software, firmware, or a combination thereof. In the preferred embodiment(s), the analyzer is implemented in software or firmware that is stored in a memory and that is executed by a suitable instruction execution system. If implemented in hardware, as in an alternative embodiment, the analyzer can be implemented with any or a combination of the following technologies, which are all well known in the art: a discrete logic circuit(s) having logic gates for implementing logic functions upon data signals, an application specific integrated circuit (ASIC) having appropriate combinational logic gates, a programmable gate array(s) (PGA), a field programmable gate array (FPGA), etc. 
     Mounted to the rear of the transducer  34  is a thermocouple  42   a  for temperature correction of the transducer  34 . The thermocouple  42   a  may be mounted anywhere as long as it is subject to the same temperature variations as the transducer  34 , i.e. in thermatic connection. The thermocouple  42   a  is connected to the analyzer  38   a  by thermocouple lead  44   a . The instantaneous temperature and calibration curves are used to provide the temperature correction. The calibration curves are obtained from measurements inside a temperature-controlled oven. 
     FIG. 2 shows a cut-away view of the machine housing of FIG. 1, a first transducer for producing a shear wave, and a second transducer for detecting the shear wave after the shear wave interacts with the interface of the mechanical seal. The machine housing and condition monitoring system as a whole is shown by reference numeral  50 . Other than changes noted immediately below, FIG. 2 is identical to FIG.  1 . FIG. 2 shows a first transducer  52  mounted on the back of the annular fixed portion  18  of the mechanical seal and a second transducer  54  mounted on the back of the rotating portion  20  of the mechanical seal. The first transducer  52  produces an ultrasonic shear wave  56  and the second transducer  54  detects the ultrasonic shear wave  56  after it passes through the interface of the fixed face  22  and the rotating face  24 . The second transducer  54  of the condition monitor of FIG. 2 detects a measurably greater ultrasonic shear wave  56  when the liquid film in the interface breaks down and the asperities in the fixed face  22  and the rotating face  24  make contact. 
     Also in FIG. 2, an analyzer  38   b  controls first transducer  52  through one of two analyzer leads  40   b  and receives a signal from second transducer  54  through the second of two analyzer leads  40   b . Although FIG. 2 shows the wave producing first transducer  52  mounted on the back of the annular fixed portion  18 , the operation of the transducers could be reversed in which case the transducer mounted on the back of the annular fixed portion  18  would detect a wave produced by the transducer mounted on the back of the rotating portion  20 . 
     Mounted to the rear of the first transducer  54  is a first thermocouple  42   b  for temperature correction of the first transducer  54 . Mounted to the rear of the second transducer  54  is second thermocouple  58  for temperature correction of second transducer  54 . The first thermocouple  42   b  is connected to the analyzer  38   b  by thermocouple lead  44   b  and the second thermocouple  58  is connected to the analyzer  38   b  by thermocouple lead  59 . 
     FIG. 3 shows a cut-away view of a machine housing and a single transducer for monitoring the interface of two seals in a second type of mechanical seal. The machine housing and condition monitoring system as a whole is shown by reference numeral  50 . In detail, FIG. 3 shows a machine housing  52 . A seal chamber  54  is located in the machine housing  52 . The seal chamber contains a fluid. A shaft  56  is also shown. 
     Around the shaft  56 , and within the machine housing  52 , is an annular fixed portion  58  of a mechanical seal. Also around the shaft  56 , and within the machine housing  52 , is an annular rotating portion  60  of the mechanical seal. The annular fixed portion  58  includes a front with a fixed face  62 . And the annular rotating portion  60  includes a front with a rotating face  64 . The fixed face  62  and the rotating face  64  form the two parts of the mechanical seal that slide near each other normal during operation. During normal operation, a thin liquid film forms at the interface of the fixed face  62  and the rotating face  64 . The mechanical seal also includes a spring  66 , a collar  68 , a first O-ring  70 , and a second O-ring  72 . The components and operation of a mechanical seal are well known to those with skill in the field. 
     FIG. 3 also shows a transducer  74  mounted on the back of the annular fixed portion  58  of the mechanical seal. The transducer  74  produces an ultrasonic shear wave  76  directed at the interface of the fixed face  62  and the rotating face  64 . During normal operation, the ultrasonic shear wave  76  is primarily reflected from the interface. The double arrowhead line representing the ultrasonic shear wave  76  indicates the dual operation of the transducer  74  as both a wave source and a wave sensor for the reflected ultrasonic shear wave  76 . Also shown in FIG. 3 are an analyzer  78   a  and an analyzer lead  80   a  running between the transducer  74  and the analyzer  78   a.    
     Mounted to the rear of the transducer  74  is a thermocouple  82   a  for temperature correction of the transducer  74 . The thermocouple  82   a  is connected to the analyzer  78   a  by thermocouple lead  84   a.    
     FIG. 4 shows a cut-away view of the machine housing of FIG. 3, a first transducer for producing a shear wave, and a second transducer for detecting the shear wave after the shear wave interacts with the interface of the second mechanical seal. The machine housing and condition monitoring system as a whole is shown by reference numeral  90 . Other than changes noted immediately below, FIG. 4 is identical to FIG.  3 . FIG. 4 shows a first transducer  92  mounted on the back of the annular fixed portion  58  of the mechanical seal and a second transducer  94  mounted on the back of the rotating portion  60  of the mechanical seal. The first transducer  92  produces an ultrasonic shear wave  96  and the second transducer  94  detects the ultrasonic shear wave  96  after it passes through the interface of the fixed face  62  and the rotating face  64 . The second transducer  94  of the condition monitor of FIG. 4 detects a measurably greater ultrasonic shear wave  96  when the liquid film breaks down and the asperities in the fixed face  62  and the rotating face  64  make contact. 
     In FIG. 4, an analyzer  78   b  controls first transducer  92  through one of two analyzer leads  80   b  and receives a signal from second transducer  94  through the second of two analyzer leads  80   b . Although FIG. 4 shows the wave producing first transducer  92  mounted on the back of the annular fixed portion  58 , the operation of the transducers could be reversed in which case the transducer mounted on the back of the annular fixed portion  58  would detect a wave produced by the transducer mounted on the back of the rotating portion  60 . 
     Mounted to the rear of the first transducer  92  is a first thermocouple  82   b  for temperature correction of the first transducer  92 . Mounted to the rear of the second transducer  94  is a second thermocouple  98 . The first thermocouple  82   b  is connected to the analyzer  78   b  by thermocouple lead  84   b  and the second thermocouple  98  is connected to the analyzer by thermocouple lead  99 . 
     FIG. 5 shows a cut-away view of a machine housing and a single transducer for monitoring the interface of two seals in a third type of mechanical seal. The machine housing and condition monitoring system as a whole is shown by reference numeral  100 . In detail, FIG. 5 shows a first portion of a machine housing  102 . A seal chamber  104  is located in the machine housing  102 . The seal chamber contains a fluid. A shaft  106  is also shown. 
     Around the shaft  106 , and within the machine housing  102 , is an annular fixed portion  108  of a mechanical seal. Also around the shaft  106 , and within the machine housing  102 , is an annular rotating portion  110  of the mechanical seal. The annular fixed portion  108  of the mechanical seal includes a front with a fixed face  112 . And the annular rotating portion  110  of the mechanical seal includes a front with a rotating face  114 . The fixed face  112  and the rotating face  114  form the two parts of the mechanical seal that slide near each other during normal operation. During normal operation, a thin liquid film forms at the interface of the fixed face  112  and the rotating face  114 . The mechanical seal also includes a spring  116 , a collar  118 , a rubber bellows  120 , and an O-ring  122 . 
     FIG. 5 also shows a transducer  124  mounted on the back of the annular fixed portion  108  of the mechanical seal. The transducer  124  produces an ultrasonic shear wave  126  directed at the interface of the fixed face  112  and the rotating face  114 . During normal operation, the ultrasonic shear wave is primarily reflected from the interface. The double arrowhead line representing the ultrasonic shear wave  126  indicates the dual operation of the transducer  124  as both a wave source and a wave sensor for the reflected ultrasonic shear wave  124 . 
     Also shown in FIG. 5 are an analyzer  128  and an analyzer lead  130  running between the transducer  94  and the analyzer  128 . Mounted to the rear of the transducer  124  is a thermocouple  132  for temperature correction of the transducer  124 . The thermocouple  132  is connected to the analyzer  128  by thermocouple lead  134 . 
     FIG. 6 is a cut-away view of an alternative arrangement for a first and a second transducer in which the first and second transducers are on the same side of the interface of a mechanical seal. The alternative arrangement as a whole is shown by reference numeral  140 . A first metal mount  142  and a second metal mount  144  are shown. Although metal mounts are shown in FIG. 6, the mounts may also be constructed from ceramic or other materials. A first transducer  146  and a second transducer  148  are mounted to a first surface of the first metal mount  142  and the second metal mount  144  respectively. An opposing second surface of the first metal mount  142  and an opposing surface of the second metal mount  144  are at an angle to the first surface to allow for the positioning of the first transducer  146  and the second transducer  148  at angles to the back of the stationary portion  150  of a mechanical seal. Positioning the first transducer  146  and the second transducer  148  at an angle allows the first transducer  146  to direct a shear wave  152  at an angle to the fixed face  154  of the stationary portion  150  of the mechanical seal and allows the second transducer  148  to detect the reflected shear wave  156  from the interface  158  of the fixed face  154  and the rotating face  160  of the rotating portion  162  of the mechanical seal 
     It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.