Feb 27, 2006 - duration of contact in liquid lubricated mechanical seals. D. Mba. 1 ... [6] applied AE measurements for detecting deterioration of seals. A pilot ...
Application of Acoustic Emission technology for detecting the onset and duration of contact in liquid lubricated mechanical seals D. Mba1 , Tom Roberts 2 , Easa Taheri2 , and Alan Roddis2 1
School of Engineering, Cranfield University, Cranfield, Bedfordshire, United Kingdom. 2 AESSEAL, United Kingdom.
Abstract The continuous quest for higher efficiency has lead machinery to be designed closer to their limits and with minimum clearances in order to reduce leakage and consequently increase efficiency. This is particularly the case in the seal industry where the inappropriate selection or deterioration of seals can greatly influence the life and efficiency of machinery. Typically a mechanical seal unit consists of rotating and stationary parts separated by a fluid film. The aim of this investigation was to assess the applicability of Acoustic Emission (AE) technology for detecting the onset and duration of seal-to-seal contact; a possible result of breakdown of the separating film. It was concluded that the AE technology successfully identified the onset and duration of seal-to-seal contact. Key words- Acoustic emission, condition monitoring, mechanical seals.
Introduction A recent review of sealing technology [1] reiterated the important role of mechanical seals in rotating machinery. In order to maintain the integrity of sealing surfaces it is desirable to maintain separation of the faces with a lubricating film. At the same time the separation of these faces must be minimized to reduce leakage [2]. The operation of these seals is influenced by friction, thermal conductivity and abrasion resistance, etc. The former has been proved to have direct correlations with AE [3, 4]. Acoustic Emission is defined as transient elastic waves generated due to a rapid release of strain energy caused by structural alteration in or on a solid material under mechanical or thermal stress. Primary sources of AE are crack initiation, crack propagation, impacting and friction. As AE only detects high frequency elastic waves (50 KHz to 1MHz), it is insensitive to structural resonances and typical mechanical background noise (less than 20 kHz). Attempts to apply this technique to monitoring the performance of rotating machinery started in the late 1960’s as noted in a recent review article [5]. The main concern with application of the AE technology is the attenuation of the signal during propagation, as such the AE sensor has to be as close to its source as possible. This limitation may pose a practical constraint when applied to certain rotating machinery. The application of the acoustic emission technology can provide powerful diagnostic capability, which is safe and cost-effective. Traditionally the most commonly measured AE parameters for monitoring are amplitude, r.m.s and energy.
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Despite the early published work, the application of AE technology to monitoring mechanical seals has been slow to develop and remains in its infancy. Holenstein et al [6] applied AE measurements for detecting deterioration of seals. A pilot system was developed and installed in a thermal power plant. Holenstein applied different data analysis techniques in an attempt to correlate operating condition and absolute wear. It was shown that a rise in AE levels was due to deformation of the seal surfaces and periodic contact. Kataoka et al [7] stated that AE technology could be applied to monitoring mechanical seals without interfering with its operation. Furthermore, it was stated that AE could monitor seals in pumps. A correlation between AE levels and the mechanical integrity of the seals was determined based on laboratory and field tests by taking measurements on the actual seals and on their casings. Kataoka went on to apply the technology for assessing seal leakage. Anderson et al [8] noted the development of the AE technology for monitoring the integrity of seals. The main drawback with the technology, Anderson stated, was the difficulty in distinguishing between AE’s from mechanical seals to those from other sources (bearings, motor, etc). Yet another observation by Andesron was that a universal based AE system that could be applied to the wide range of seal types and materials was non-existent. However, following some experimental investigations Anderson did state that the simplicity of an AE based system suggested its applicability to field operation. In summary it has been shown that when seal faces make contact a rise in temperature of the seals has been noted. In addition, measurements of AE have also been shown to rise due to seal contact. The aim of this investigation was to re-affirm the applicability of Acoustic Emission (AE) technology for detecting the onset and duration of seal-toseal contact, and to assess the ability of the technology to infer the type and nature of seal contact. Test rig and data acquisition system The seal test unit used for this assessment employed Carbon Carbide and Silicon Carbide (Car-Sic) seal materials. The sealing arrangement meant one seal remained stationary whilst the other rotated with the main shaft. A variable speed motor was employed. A defined test sequence was undertaken to simulate intermittent and continuous contact of the different combination seal faces. Two commercially available piezoelectric type AE sensors were employed for this investigation. The first was a ‘Physical Acoustics’ miniature type sensor (PICO) with an operating frequency range of 200 KHz – 1000 KHz. The second sensor (Type WD), had an operating frequency range of 100 KHz – 1000 KHz. The ‘PICO’ type sensor was placed on the stationary seal whilst the ‘WD’ sensor was placed on the seal casing. Pre-amplification was set at 20dB and the signal output from the preamplifiers was connected directly to a commercial data acquisition card. In addition, anti-aliasing filters were built into the data acquisition card. During the tests continuous AE ASL (Average Signal Level) values were calculated in real time by the analogue-to-digital converter (ADC) controlling software at a sampling interval of 10ms. The ASL is calculated from the r.m.s measurement and is given as:ASL in dB= 20 x Log (1.4 x r.m.s in V / 100)
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(1)
Furthermore, a thermocouple was fixed on the stationary seal and a torque measuring system, placed on the main shaft, allowed for instantenous changes in torque to be measured. As seal contact will result in increased frictional resistance, this in turn will cause a change in torque transmitted along the drive shaft.
Experimental procedure The purpose of this test was to simulate intermittent and constant contact of the seal faces. All tests were undertaken under atmospheric conditions with water as the lubricating film. The rest-rig was operated at 3600rpm with a fixed volume of water held within the test-rig. To simulate seal contact the lubricant was slowly drained from the test-rig. This ensured that the lubricant temperature increased steadily between the contact faces eventually causing contact of the seal faces. Immediately contact was established, based on observing trends in temperature and torque, the testrig was completely re-filled with the lubricant. The addition of the lubricant served to develop a lubricating film between the faces; ensuring no contact. This procedure was repeated three times for each speed condition. Results, observations and discussions Observations of temperature and torque changes are detailed in figure 1. For the tests undertaken at 3600rpm three distinct contact periods were observed on both temperature and torque readings. After approximately 1750 seconds of operation the operating speed was reduced to 500rpm, see figure 1. At the reduced rotational speed the process of ensuring contact was repeated three times as described earlier. However, only changes in AE levels and torque values highlighted the contact as instantaneous increases in temperature as a direct result of contact were indiscernible from the overall increase in seal temperature over the operating time; figure 1 also shows the instantaneous changes in torque as a direct consequence of seal contact at the lower operating speed. At the instances of increased seal temperature and torque there was a corresponding increase in AE, from the seal and seal casing, of up to 60% in some instances. Interestingly, after the lubricant allowed back into the rig to establish a lubricating film between the seals, AE values reduced to the original levels, see figure 1. Based on the observations of temperature and torque measurements noted during this test it was postulated that the steep increases in AE levels were attributed to seal-to-seal contact. The duration of relatively high AE levels during intermittent contact demonstrated that the technology offered the potential to measure contact duration. Furthermore, observations of the AE waveform (time series) highlighted that the nature of the contact may be ascertained in real-time. For instance figure 2 shows a continuous type AE wave, with transient AE bursts superimposed, gradually reducing in amplitude as the lubricant film between the seals redeveloped.
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110
25
315
137
87
21
280
124
64
17
245
111
41
13
210
98
18
9
175
140
85
-5
Torque (Nm)
150
Temperature (C)
350
ASL seal face (dB)
ASL seal casing (dB)
Increases in seal temperature due to contact
Speed 500rpm
Temperature
Torque (Nm)
5
72
-28
1
105
59
-51
-3
70
46
-74
-7
35
33
-97
-11
AE on seal face
AE on sealing casing 0
20
-120
-15 0
310
620
930
2006-Feb-27 13:35 H:\AESSEAL\Car Sic.dat H:\AESSEAL\Plot Car Sic.dat
Figure 1
1240
1550
1860
2170
2480
2790
Time (seconds)
Observations of temperature, instantaneous changes in torque and AE recordings from the seal and seal casing
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3100
0.06
Amplitude (volts)
0.04 0.02 0 -0.02 -0.04 -0.06 -0.08
Figure 2
0
0.5
1
1.5 2 Time (µ seconds)
2.5
3
3.5 x 10
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Observations of AE waveform during re-development of the lubricating film
Conclusions The initial assessment of the applicability of the AE technology to detecting the onset and duration of seal-to-seal contact has been a success. Interestingly, the opportunity to monitor the duration, and nature, of contact is very encouraging. AE activity associated with contact was readily observed on the seal casing implying that direct contact with the seal is not a pre-requisite in employing this technology for monitoring seal contact in operational environments. References 1. Bob Flitney, Review of features in Sealing Technology during the last year , Sealing Technology, Volume 2005, Issue 5, May 2005, Pages 6-11 . 2. Orcutt, F. K., An investigation of the operation and failure of mechanical face seals, 4th International conference on fluid sealing, ASLE Annual meeting, Philadelphia, sponsored by ASME, USA, 1969, session 5, 205-217. 3. Bones, R. J., McBride S.L., and Sobczyk, M., 1990 Wear studies using Acoustic Emission techniques. Tribology International, 23(5), 291-295. 4. Bones, R.J. and McBride, S.L., 1991 Adhesive and abrasive wear studies using acoustic emission techniques. Wear. Vol. 149, 41-53. 5. D. Mba & Raj B.K.N. Rao, Development of Acoustic Emission Technology for Condition Monitoring and Diagnosis of Rotating Machines; Bearings,
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Pumps, Gearboxes, Engines and Rotating Structures. The Shock and Vibration digest, 38/1, Jan 2006, 3-16. 6. Holenstein, A.P. and Flepp, B., Diagnosis of Mechanical Seals in Large Pumps. Proceedings of COMADEM 1996 (International congress on Condition monitoring and Diagnostic engineering management), Sheffield Academic Press, Sheffield. 73-84. 1 – 85075 – 635 – X, 441-460. 7. Kataoka, T; Yamashina, C., Komatsu, M., Development of an incipient failure detection technique for mechanical seals, Proc. 4th International pump sysmposium, Texas A & M University, 1987, 121-129. 8. Anderson, W., Jarzynski, J., Salant, R. Monitoring the condition of liquidlubricated mechanical seals. Monitoring the condition of liquid-lubricated mechanical seals. Sealing Technology, Volume 2002, Issue 2, February 2002, Pages 6-11
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