## 2 The Process Plant Inspection and Maintenance System
### Potential Non-Destructive Testing Techniques for On-Stream Inspection  
The Ultrasonic Thickness Gauging (UTG) technique serves as a primary method for on-stream inspections in plants and effectively detects corrosion [66]. Its notable advantage lies in its ability to operate at elevated temperatures, with some equipment functioning even at 500 \({}^{\circ}\)C [12]. However, UTG is not without limitations, such as reduced productivity in spot thickness measurements and limited sensitivity to localized corrosion. Addressing these limitations is crucial for further enhancing UTG's effectiveness in corrosion detection during plant inspections [67]. Phased Array Ultrasonic Testing (PAUT) has emerged as a compelling alternative, notably surpassing conventional Ultrasonic Testing (UT) and Radiography Testing (RT) techniques, both of which fall under volumetric inspection methods [68; 69]. Figure 2 illustrates the common process of the PACM. The first step in the process is to calibrate the PACM equipment based on the selected material and thickness. The variation in ultrasonic velocity in different materials is a key factor in obtaining accurate results, so it is important to calibrate the equipment accordingly. Figure 2: The common process flow of PACM. During on-site data collection, it is crucial to ensure that the test surface is smooth and free of obstructions to improve the accuracy of the results. Sufficient couplant should also be applied to the surface to improve the acoustic contact between the probe and the surface. This can help to enhance the signal-to-noise ratio and improve the overall quality of the data collected [70]. Once the data have been successfully collected, they can be transferred to a computer for further analysis. This may involve the use of specialized software to identify defect dimensions, depth, and location. The results can then be used to assess the severity of corrosion and determine appropriate maintenance or repair actions. The UT is an NDT technique that uses ultrasonic waves to measure the thickness of a material. They are widely used for inspecting welds and detecting corrosion in thin-walled structures. However, the UT has limitations in detecting localized corrosion and is less suitable for high-temperature environments. RT is a NDT technique that uses radiation to inspect materials for defects. They are widely used for inspecting welds and detecting corrosion in thick-walled structures [71]. However, RT has some limitations. Radiation exposure can pose a risk to human health, which may require other trade workers in the area to stop working, thereby causing inconvenience. Additionally, RT requires that both surfaces be accessible for inspection, which may not always be the case in industrial settings. The PAUT circumvents radiation hazards, making it safe for simultaneous on-site use with other trades. This technique offers rapid result processing, three-dimensional defect information, and permanent records, similar to RT [72]. Its superiority over UT is evident in its multiple-element probe, which permits flexible beam steering, rendering superior scan coverage and speed [69; 73; 74]. The proficiency of PAUT is reflected in its ability to provide three-dimensional defect information through S-scan, C-scan, D-scan, and A-scan data, as shown in Figure 3[75]. However, it is important to compare PACM with other NDT techniques in order to understand its strengths and limitations in a broader context. One such technique is eddy current testing (ET), which is commonly used to detect surface and subsurface defects in conductive materials [76; 77]. While ET can be sensitive to small defects, it is not as effective at detecting localized corrosion as other techniques such as PAUT or acoustic emission testing (AET). ET is less suitable for high-temperature environments and requires the material to be electrically conductive, which may not be the case for all the materials used in processing plants. Figure 3: Phased array ultrasonic testing data presentation (adapted from [75]).