3.1.1. SHM-System Location

Overheating caused by a run-away of the process induced the formation of defects at the hydrotreater at the oil refinery plant. During periodic AE testing, significant AE sources were detected and it was assumed that it is the destruction of the cladding layer. In December 2006, SHM-system was put into operation, the task of which was to ensure the safe operation of the hydrotreater with an extended lifespan until it is possible to replace it.

Structurally, the hydrotreater is a thick-walled vertical steel vessel. The inner surface of the hydrotreater is protected from the influence of an aggressive medium by a cladding layer. The high operating temperatures of the hydrotreater (from 320 ◦C to 425 ◦C), the aggressive environment and high operating pressure are factors that can lead to the generation and catastrophically rapid flaw growth, especially when the cladding layer is destroyed, which can induce hydrogen embrittlement and a decrease in the strength of the material.

The main diagnostic method of SHM-system was the method of AE testing. The AE handling part was represented by 18 AE sensors GT200UB (130–200 kHz), grouped into 6 belts, 4 on the hydrotreater shell and 1 belt on each lid. Sensors were mounted on waveguides to ensure the temperature allowed for the sensor is not exceeded. Temperature, vibration and pressure sensors were also installed. A threshold data acquisition algorithm was used. The threshold value was about 40 dB. The analog-to-digital converter (ADC) sampling rate was equal to 2 MHz.

By May 2007, the state of the hydrotreater had deteriorated. It was found to have 9 active zones, corresponding to AE sources with varying degrees of danger (Figure 2). The activity of some zones increased significantly, which indicated a progressive flaw growth and the possibility of the hydrotreater destruction. The source found in zone #5 and characterized by 370 events, was considered to be critically active.

**Figure 2.** Location map. Color of points mean quantity of location events. Blue points: 1–2 events, yellow: 3–10 events, orange points: 11–25 events, red points: 26–100 events, brown points: more than 100 events.

In order to resolve the issue of the time of decommissioning, it was decided to conduct additional analysis of monitored AE data.
