1、PDF外文： http:/ 45 页 共 57 页 中文 2940 字 出处： NDT & E International, 2005, 38(5): 354-358 The application of Acoustic Emission for detecting incipient cavitation and the best efficiency point of a 60KW centrifugal pump； case study
2、; L. Alfayez, D. Mba, G. Dyson July 2005AbstractPumps play a significant role in Industrial plants and need continuous monitoring to minimize loss of production. To date, there is limited published information on the application of Acoustic Emission (AE)to incipient pump cavitat
3、ion. This paper presents a case study where AE has been applied for detecting incipient cavitation and determining the best efficiency point (BEP)of a 60KW centrifugal pump. Results presented are based on NPSH (Net Positive Suction Head)and performance tests. In conclusion the AE technique was shown
4、 to offer early detection of incipient cavitation, furthermore, the technique has demonstrated the ability to determine the BEP of a pump Keywords: Acoustic Emission； best efficiency point； cavitation； condition monitoring； Pump performance 1. Introduction Typically the pump manufacturer will
5、undertake performance and NPSH(Net Positive Suction Head)tests on supplied pumps, the significance of the latter is to determine the 3%drop in head at which serious cavitations will occur. The NPSH can be expressed as the difference between the suction head and the liquids vapour head. The concept o
6、f NPSH was developed for the purpose of comparing inlet condition of the system with the inlet requirement of the pump. Cavitation causes a loss of pump efficiency and degradation of the mechanical integrity of the pump. It must be noted that cavitation starts to develop before the 3%drop in head. I
7、t is generally accepted that the critical pressure for inception of cavitation is not constant and varies with operation fluid physical properties and the surface roughness xx 大学机械工程学院毕业设计（论文） 第 46 页 共 57 页 of the hydraulic equipment. Application of the high freq
8、uency Acoustic Emission (AE)technique in condition monitoring of rotating machinery has been growing over recent years1-9.Typical frequencies associated with AE activity range from 20 KHz to 1MHz.The most commonly used method for identifying the presence of cavitation is based on observations of the
9、 drop in head. Whilst other techniques such as vibration analysis and hydrophone observations for pump fault diagnosis are well established, the application of AE to this field is still in its infancy. In addition, there are a limited number of publications on the application of AE to pump health an
10、d cavitation monitoring. Derakhshan et al 10investigated the cavitation bubble collapse as a source of acoustic emission and commented that the high amplitude pressure pulse associated with bubble collapse generated AE. With the AE sensor was placed on the actual specimen experiencing cavitation &nb
11、sp;Derkhshan observed increasing AE r.m.s levels with increased pressure of flow and cavitation. However, with the AE sensor mounted on the tank wall the reverse was observed, decreasing AE r.m.s levels with increasing pressure and cavitation. This was attributed to a visible bubble cloud that incre
12、ased with pressure. It was commented that this cloud attenuated the AE signature prior to reaching the transducer on the wall casing. Neill et al11,12assessed the possibility of early cavitation detection with AE and also noted that the collapse of cavitation bubbles was an impulsive event of the ty
13、pe that could generate AE. It was observed that when the pump was under cavitation the AE operational background levels dropped in comparison to non-cavitating conditions. In conclusion Neill stated that loss in NPSH before the 3%drop-off criterion was detectable with AE and evidence of incipient ca
14、vitation was detectable in the higher frequency band(0.5 to 1MHz). The papers reviewed above have clearly associated AE with the collapse of cavitation bubbles. The presence of cavitation has been shown to increase or decrease operational AE noise levels10,11,12.This paper presents a case study to a
15、scertain the applicability of the AE technique for detecting incipient cavitation, and, to access the opportunities offered by the AE technique for determining the best efficiency point(BEP)of a pump. 2. Experimental setup xx 大学机械工程学院毕业设计（论文） 第 47 页 共 57 页 A seri
16、es of performance and NPSH tests were undertaken on a two stage DavidBrown60KW centrifugal pump(Model DB22)with a maximum capacity of 204m/h at an efficiency of 70.6%.These tests were undertaken using a closed loop arrangement with a vacuum facility in accordance with BS 9906.It must be noted that b
17、est endeavours were undertaken to reduce the time taken to reach the required flow rate during performance and NPSH tests. Acoustic Emission sensors were located at a distance of 0.5 m from suction flange； at the suction flange； on the pump casing in the vicinity of impeller suction eye； on the casi
18、ng in the vicinity of the impeller discharge tip；0.5m from discharge flange, see figure 1. 3. Data acquisition systems The AE sensors used for all of the experiments were broadband type sensors with a relative flat response in the region between 100 KHz to 1MHz (Model:WD,Physical Acoustics Cor
19、poration).The output signal from the AE sensors was pre-amplified at 40dB.Continuous AE r.m.s values were calculated in real time by the Analogue to Digital Converter(ADC).The sampling rate for acquisition was set at 100ms for all tests and the time constant for calculating the AE r.m.s was also set
20、 at 100 ms. 4. Experimental results and observation 4.1 Performance test Figure 2 details the performance characteristics of the pump, highlighting the BEP at 94.5m/hr. The performance test were undertaken twice to ensure repeatability. Observations of AE r.m.s activity during the performance test a
21、re displayed in figure 3.During the performance test, AE activity from the sensor located in the vicinity of impeller on pump casing was found to have the largest magnitude, providing the best position for correlating AE activity to pump performance. It was observed that the minimum AE r.m.s value w
22、as obtained for a flow rate of 94.5m/hr. At this flow rate the AE activity generated from the fluid flow within the pump and pipes was lowest in comparison to other flow rates. Either side of this flow rate resulted in increasing AE r.m.s activity with increased flow rates. Based on these observations it was concluded that the BEP must occur where there was minimal 3 3
