3.1.2. Cluster Analysis of AE Data

After preliminary filtering, a cluster analysis of the AE data was carried out. AE signals were clustered for each measuring channel separately. As a measure of the similarity for each pair of signals, the correlation coefficient of their waveforms was used (threshold value was set to 0.7), or a measure based on the similarity between their AE parameters. As a result, "clusters of signals" were formed [10].

In total, about 500,000 AE signals were analyzed, including 65,000 waveforms. For AE signals registered by the channels testing zone #5, 23 clusters were formed. While conducting cluster analysis, 14 clusters was attributed to process-induced AE events, 4 clusters corresponded to the correlated noise and 5 clusters were characterized as the sources of AE with high probability.

The most representative clusters were formed for signals registered by channelś 8 and́ 17 (Figure 3, Table 1). These signals had an amplitude of more than 70 dBAE with a high probability they were generated by AE source located in zone #5.

**Figure 3.** Waveforms corresponding to clusters centers of mass, for acoustic emission (AE) signals from channels: (**a**) channel #8; (**b**) channel #17.


**Table 1.** Parameters of clusters for channels 8 and 17.

#### 3.1.3. Testing of the Dismantled Object

Thus, the results of the cluster analysis confirmed a previously determined degree of danger of AE source located in zone #5. A final decision to stop the hydrotreater was made. The dismantled hydrotreater was investigated by NDT in order to establish the reasons of its transition to the critical state. Internal examination revealed numerous disintegrations of the cladding layer (up to its complete absence in some areas) (Figure 4). Dimensions of damages were up to 5 mm. Ultrasonic testing revealed numerous discontinuities in welds and the base metal shell. The lengths of discontinuities were 100 mm along and 15 mm in depth. In one of the most active zones, discontinuities were detected both in welds and in the heat-affected zone. Certain discontinuities occurred at different depths, from 26 to 80 mm. These damages demonstrated significant degradation with loss of mechanical properties of the metal, which leads to brittle fracture of the structure [11].

Thus, the use of SHM-system allowed, on the one hand, to extend the lifetime of the hydrotreater by 6 months and on the other hand, to prevent the destruction of the hydrotreater during operation with all the ensuing consequences.

**Figure 4.** Breach of the cladding layer.
