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.
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.
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. , xe2x80x9cUltrasonic Detection of Lubricating Film Collapse in Mechanical Seals, xe2x80x9d STLE Tribology Trans, Vol. 42, pp. 801-806, (1999); and Salant, R. F. , Anderson, W. , and Jarzynski, J. xe2x80x9cCondition Monitoring of Mechanical Seals Using Actively Generated Ultrasonic Waves, xe2x80x9d 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.