1 10th International Conference on Wear of Materials ACOUSTIC EMISSION IN MONITORING SLIDING CONTACT BEHAVIOUR Lic.Tech. Juha Miettinen Professor Veli Siekkinen Tampere University of Technology, Machine Design, P.O. Box 589, SF-33101 Tampere, Finland telefax: +358-31-3162307, email: miettinen@ruuvi.me.tut.fi Abstract This paper describes the usage of acoustic emission measurement in monitoring sliding contact behaviour in mechanical face seal. The mechanical face seal works normally in boundary lubrication region. If the leakage rate of the seal rises hydrodynamic lubrication takes part in the sliding contact. Correspondingly if the cooling water disappears it can lead in to dry contact situation. These kind of situations can appear in failures of seals. Usually the cooling liquid in mechanical face seal is water. In high temperatures the sliding contact becomes unstable because of cavitation in the lubrication region. The cavitation is result of the boiling of the lubricant in the contact. This cavitation can affect severe wear in the sliding materials. The measuring gauge was situated quite near the sliding contact because of the high frequency acoustic emission signal attenuates very quickly in boundary surfaces. The seal has been tested when the sliding contact was normal and dry and also when the leakage was high. Also tests in high temperatures has been made and the temperature region where sliding contact becomes unstable has been determined using temperature and acoustic emission measurement.
2 1. Introduction When mechanical face seal works correctly the leakage of the seal can not be observed visually. For example in seal which diameter is 50 the magnitude of leakage is mm between 0.1-10 ml/hour when the rotation speed is 3000 1/min /1/. When the seal is damaged leakage occurs trough the gap of sliding faces or trough the rubber secondary seals. One important application of mechanical face seal is the axle seal of the centrifugal pump. In favourable circumstances mechanical face seal can run harmless 10-20 years in the pump. It does not need any maintenance as e.g. stuffing box which we must adjust regularly. The problem in the use of mechanical face seal is that if the failure happens it is usually total. We must stop and open the pump. One estimate of the useful life of mechanical face seals is that only 5% reaches the designed useful time 15% are damaged because of wrong seal type 20 % are damaged because of wrong assembly and 60% are damaged because of the mistakes during running time /2/. The aim of this investigation was to study the use of acoustic emission in condition monitoring of the sliding contact of mechanical face seal in centrifugal pump. 2. Sliding contact in mechanical face seal In mechanical face seal the sliding contact can run in boundary lubrication in mixed lubrication or in fluid film lubrication region (Fig 1.). In full fluid film regime runs e.g. hydrostatic face seal. There we have a cap between the faces and the leakage is high. If we use hydrodynamic grooves in one of the faces (Fig 2.) then hydrodynamic lubrication takes part in the sliding contact. Usually mechanical face seal runs in mixed lubrication regime. There we have minimum wear of the faces with acceptable leakage. η v b G = F G is duty parameter, η is kinematic viscosity, v is average sliding speed of the face, F is load of the face, b is seal face width. Figure 1. Mechanical face seal running regimes. Usually seal runs in mixed lubrication region /3/.
3 Figure 2. Hydrodynamic groove in sliding face /4/. In boundary lubrication and mixed lubrication region the most influential factors are materials of the faces, coatings and geometric properties like surface roughness and surface waviness. If the flatness of the face is not good enough hydrodynamic lubrication takes part in the sliding contact /5/. If the temperature of the sliding faces rises too high the lubrication film starts to cavitate and it becomes unstable. That will cause starvation in the film. It leads in to high wear of the faces. If the temperature becomes furthermore higher the film starts to vaporise /6, 7, 8/. 3. Acoustic emission in sliding contact Acoustic emission (AE) means a short time stress wave which is induced by rapid relaxation of strain energy /9, 10/. These stress waves can be measured with vibration transducer from the surface of the object. The frequency of the vibration is from 50 khz to 1 Ghz. The measurement principles are usually to count the number of pulses which are over some threshold level to measure the peak amplitude to measure the signal hold on time or to measure the energy if the vibration (Fig. 3 ). Frequency analysis is used very seldom because of the nonlinearity of the frequency response. Figure 3. Some measuring principles of acoustic emission /9/. In this study it has been measured AE signal RMS. value.
There are in the sliding contact many wear mechanisms going on at the same time. That makes difficulties to use e.g. AE pulse count method in measurements. In monitoring sliding contact where many wear mechanisms are going on at the same time we can use AE signal RMS (V RMS ) measurement principle /9, 11/. The RMS value represents the energy of the AE activity. 4 4. Methods The tested seal was situated in 15 kw centrifugal pump. Pumping situations were like in normal pumping use. The AE measurement method is in principle not sensitive to the vibrations exited by pumping process or to other vibrations which comes from environment. To eliminate the mistake in measurements caused by other vibrations a reference vibration transducer were situated on the spiral and on the frame of the pump. 4.1 Test seal In the measurements we used double acting mechanical face seal (Fig 4). Buffer fluid was water. Here is the place for figure 4 Figure 4. Seal used in the measurements. Double acting mechanical face seal. Sliding face materials: rotating face Cr steel, fixed face carbon graphite. Seal manufacturer Tiivistetekniikka Oy, Finland.
The high frequency vibration attenuates very strongly in the boundaries. The AE transducer was situated on the carbon graphite fastening ring (Fig. 5 and Fig. 4). The place is quite near the sliding contact. 5 Figure 5. The fixing of AE transducer on the seal. 4.2 Measuring instruments AE transducer in the seal was resonance type accelerometer. The resonance peak was 220 khz. Amplifier gain was 46 db and signal was filtered with a 200 khz octave bandwidth filter. The maximum output voltage was ±6V at 200 khz frequency. From the gained AE voltage signal it was measured RMS value. Reference AE transducers were the same type but their amplifier output was DC voltage which was proportional to the AE activity. 4.3 Measurements The measuring situations were: Test 1: Test 2: Test 3: Test 4: Normal running. New faces and low face temperature. Leaking seal. One small radial channel in one of the faces. Low face temperature That describes abnormal leakage of the seal. Vaporisation and dry running. The seal was drained slowly from buffer fluid. That describes high vaporisation of fluid film and dry running. Long time tests in different sliding face temperatures.
6 5. Results In figure 6 are results from test 1 (normal running, AE1) and test 2 (leaking seal, AE 2). The values of the leaking seal are on an average 25% lover than values in normal running. The radial channel in the face works like hydrodynamic groove. Hydrodynamic lubrication takes part in the sliding contact. 0.6 AE [V] 0.5 0.4 0.3 0.2 0.1 AE1 Seal, Test 1 AE1 Ref 1 AE1 Ref 2 AE2 Seal, Test 2 AE2 Ref 1 AE2 Ref 2 0 0 10 20 30 40 50 60 70 Running time [ min ] Figure 6. Measuring situations: Test 1 (normal running, AE1) and test 2 (leaking seal, AE 2). The temperature of seal faces were between 44 C and 79 C. Same pumping situation in all measurements, rotation speed 1400 1/min and puffer fluid pressure 0,8 MPa. In figure 7 are results from measuring situation test 3 (vaporisation and dry running). The seal s drained slowly from buffer fluid. when the temperature rises the vaporisation rises in the fluid film. In temperature over 200 C the contact runs dry. AE [V] 3.5 3 2.5 2 1.5 1 0.5 0 AE Seal AE Ref 1 AE Ref 2 50 100 150 200 250 Seal face temperature [ C ] Figure 7. Measuring situation: Test 3 (vaporisation and dry running). Rotation speed 1400 1/min and buffer fluid pressure 0 MPa.
In figure 8 are results of long time tests (test 4) in temperatures 43 C, 54 C 67 C, 113 C and 151 C. The t st running time in each temperature was three hours. 7 AE [V] 1.2 1 0.8 0.6 0.4 0.2 0 1 2 3 4 5 6 7 8 Measurement n = 1400 1/min 43 C 54 C 67 C AE [V] 1.2 1 0.8 0.6 0.4 0.2 0 1 3 5 7 9 11 13 15 Measurement n = 1600 1/min 113 C 151 C Figure 8. Results of long time tests (Test 4) in temperatures 43 C, 54 C, 67 C 113 C and 151 C. The test running time in each temperature was three hours. The maximum AE value is near 1.2 V RMS. We can approximate that the maximum allowable running temperature of the seal is abouth 150 C. 7. Conclusion The use of acoustic emission measurement in monitoring sliding contact behaviour in mechanical face seal in centrifugal pump has been studied. With acoustic emission measurement it is possible to discover the leakage and dry running of the seal and also cavitation in the face sliding contact. The vibration transducer must be correctly mounted and it must be located as near as possible the sliding contact. If the pumping situation varies a lot the use of reference AE transducer increases the reliability of the measurement.
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