## 4 Conclusions  
1. Based on our improved AE system, structural AE monitoring scheme is developed as SHM-system. Examples of its successful application are given. 2. Oil refinery: Overheating induced the formation of defects at the hydrotreater at the oil refinery plant. The use of SHM-system allowed to extend the lifetime of the hydrotreater by 6 months and to prevent the destruction of the hydrotreater during operation. Cluster analysis method of the AE data was developed and carried out. 3. Bridges: As a result of a series of experimental studies, the basic principles have been formulated, which make it possible to improve the efficiency of AE method as applied to the SHM of bridge structures. The solutions for their implementation have been suggested. The acoustic noises arising in monitoring of bridge structures have been analyzed and the causes of noise such as vibrations and friction due to the passing vehicles and wind gusts have been revealed. The multi-stage algorithm of noise filtering has been offered that includes selection by AE parameters. 4. Pipelines: Experimental work was carried out on the AE monitoring of the high-temperature process pipeline. The most suitable was the use of the frequency range 150-500 kHz and the threshold in the region of 40-55 dB\({}_{\text{AE}}\). It was found that the activity and amplitudes of noise increase significantly when the rate of pipeline temperature change exceeds 1\({}^{\circ}\)C/min. It is revealed that the attenuation coefficient essentially increases with the accumulation of sediments in the internal volume of the pipeline. The possibility of detecting melt leakage through flange joints is shown, even if it is invisible under a layer of thermal insulation and is not noticeable. 5. Gas adsorbers: In the upper part of the gas adsorber at the refinery, non-permissible defects were found. A decision was made to install a system for SHM for the upper part of the gas adsorber. The main task of the installed monitoring system was to ensure safe operation of the gas adsorber until it can be replaced in a year. 6. Support rollers of rotary kiln: The possibility to conduct AE testing of support rollers of rotary kilns in operating mode has been investigated. It was found that in the range above 30 kHz the acoustic noises generally are no more than 32 dB\({}_{\text{AE}}\). For full testing of all 3 bearings it is necessary to mount 3 AE sensors on each of 2 bearing enclosures. To perform full AE testing of shaft it is sufficient to provide access to the shaft by removing protective caps and to mount 2 AE sensors: one on the side of thrust bearing and one on the opposite side. The signals with amplitude of about 100 dB\({}_{\text{AE}}\), duration of more than 100,000 \(\upmu\)s allows us to estimate the needed lubrication intervals. The higher values of AE signal RMS are observed on the shafts of support rollers with  
Figure 28: Dispersion curves for channel 29 (**a**); and for channel 30 (**b**). long lifetime. While testing the support rollers, the AE sources activity was registered on 2 shafts in the area of fillet. They presumably correspond to active fatigue cracks. * Dragline excavator: Work was carried out on AE monitoring of boom, column and struts of a walking dragline excavator. To remove noise, the use of a frequency range of 150-500 kHz and a threshold of 50 dB\({}_{\text{AE}}\) was most appropriate. With these settings, the AE activity acquires a cyclic character synchronized with the excavator operation cycle. By the results of attenuation measuring the recommended distance between the AE sensors is from 4.5 to 9 m. The AE data was collected for 2 days. Six sources of AE have been detected on the left and right struts. When performing ultrasound testing, 2 of them were classified as 20 and 30 mm cracks. S.A.E. managed the project and developed the software for the SHM-system. V.A.B. carried out the data acquisition and provided data filtering and clusterization.