## 3 Results and Discussion
### High-Temperature Process Pipeline  
On the active high-temperature pipeline without decommissioning, studies were produced to assess the feasibility of conducting AE monitoring. Application of AE monitoring of high-temperature or cyclic thermal loaded structures is an effective method for defects detection. Several interesting applications are described in the papers [15; 16; 17; 18]. The tested object consisted of pipe sections with a diameter of 50 to 80 mm with a total length of 8 m. Individual pipe sections were connected with both flanged and welded joints. The working medium of the pipeline was molten salt, the temperature of which could reach 600 \({}^{\circ}\)C. To synchronize the AE data with the operating mode of the pipeline, one thermocouple was installed. 8 AE sensors were installed through waveguides. Simulations of AE signals were carried out once per day by emitting from a sensor to monitor the attenuation factor.  
When the pipeline was cold, the propagation velocity and attenuation coefficient were measured, which were 4900 m/s and 1.7-2.7 dB/m, respectively. Then the pipeline was put into operation. AE data were taken in various operating modes of the pipeline for 63 days, including in the total amount for 15.5 days in a stationary high-temperature regime. The process of heating the pipeline was long and took up to several days. During the heating process, the melt began to flow into the pipeline. The most suitable was the use of the frequency range 100-500 kHz and the threshold in the region of 40-55 dB\({}_{\text{AE}}\). It was found that the activity and amplitudes of noise increased significantly with the pipeline temperature rising at a rate exceeding 1 \({}^{\circ}\)C/min, especially when clamping fixtures for waveguides were used (Figure 11a). Amplitudes of noise reached values of up to 100 dB\({}_{\text{AE}}\). It is assumed that the occurrence of noise is associated with the thermal expansion of the pipeline, which induced friction on the thermocouple and other items on the surface of the pipe. During the work in the stationary mode, the noise level was reduced to an acceptable one and amounted to about 50 dB\({}_{\text{AE}}\).  
It is revealed that the attenuation coefficient increases with the accumulation of sediments on the inner surface of the pipeline and can reach 58 dB/m. With this level of attenuation, the maximum distance between the AE sensors could not be more than 0.7 m. The value of the propagation velocity for acoustic waves was unchanged.  
Figure 10: (a) Scatter plot diagram of AE signal energy and duration; (b) scatter plot diagram of amplitude and parameter “counts/duration.” Impulses corresponding to noise are marked black, while impulses from the Hsu-Nielsen source emitted at distances of 0–2 m, 2–7 m and 7–15 m from AE sensors are marked green, yellow and red, respectively.  
In the course of the work, the melt leakage was observed as a result of the destruction of the flange joint gasket. In this case, the AE system recorded an increase in the duration of the AE signals a few hours before the leak invisible under the layer of thermal insulation began to be observed visually (Figure 11b).  
As a result of the conducted studies, the possibility of AE monitoring of the most critical sections of the pipeline was confirmed: the detection and location of crack-like defects and detection of leaks on this pipeline by the AE method can be accomplished.  
Figure 11: Amplitudes of noise depend on the pipeline temperature (**a**); the melt leakage (**b**). Colors of points indicates different channels.