## 3 Results and Discussion
### Practical Guidelines for HDPE AST Integrity Evaluation  
The performance of the HDPE tank depends on many parameters, such as the location (e.g., weather conditions and the number of freezing cycles experienced during winters), the liquid (e.g., the concentration of the liquid and type of the stored liquid), the frequency of the filling and emptying, the maintenance program, etc. Therefore, the frequency and interval between the inspections of an HDPE AST tank should be determined by its service history unless outstanding circumstances call for an earlier inspection. A tank's service history can be obtained by reviewing the performance of a tank in a similar service, ideally at the same location and with the same material and capacity. The inspector, inspection company, or owner should develop detailed checklists that identify, record, and document all aspects of each inspection. To evaluate an HDPE AST, the authors propose the following inspection guideline:  
1. Review prior formal and periodic inspections, and identify the locations that need closer inspection. Figure 16: An example of a less confident signal return detected using PAUT. Figure 17: Return signal indistinguishable from background noise detected using PAUT. 2. Visually inspect the AST and look for any abnormality or unusual signs, such as cracking, crazing, brittle appearance, holes, dents, and abrasions. 3. Visually check around the fittings, particularly focusing on the areas around the steel bolts. Check fittings, hoses, gaskets, and all connections for any signs of general corrosion (in steel bolts and gaskets), deterioration, or leaks. Note changes from the original design and installation information, if available. 4. Walk around the AST and look for any deformation, buckling, or distortion. Pay attention to where the upper section of the wall meets the lower section of the wall, which is usually waist height. This connection separates the thicker wall in the lower section from the thinner wall in the upper section. Compare changes from the current inspection with the last inspection report. 5. Look closely for signs of brittleness in the tank's dome. Chemicals that produce fumes can cause the dome to oxidize and embrittle without being in direct contact with the brine solution. To avoid walking or standing on the dome surface, the evaluation of the dome should be completed by safety-certified personnel using lift equipment. 6. Investigate the base of the AST to make sure it rests on a firm and even base. Animals can burrow underneath the AST, causing the base to settle unevenly. Note changes from the original design and installation information, if available. 7. Use the IR camera without using the external heat source and walk around the AST to check for any anomalies that may interfere with the defect detection. If there is something on the external wall, wipe off the AST wall to remove any moisture and wait for at least 10 min until a uniform temperature forms on that area before starting the evaluation. In addition, pay attention to see if there is any leakage from the AST, especially close to fittings. 8. Power on the external heat source, and allow it to reach maximum power. Record the ambient temperature, relative humidity, and wind speed. Measure the temperature of the liquid inside the AST, and measure the AST wall temperature at different spots to calculate a representative \(\Delta\)T (\(\Delta\)T = Twall_surface \(-\) Tliquid). 1. If \(\Delta\)T > 0, then IRT can be conducted. If the AST is located outdoors, then the best time frame for conducting the IRT evaluation is between 9:00 a.m. and 6:00 p.m., preferably between 2:00 p.m. and 6:00 p.m. ASTs located indoors can be inspected using IRT at any time of day. 2. If \(\Delta\)T < 0, then do not proceed with IRT investigations. 9. Start the IRT experiment by selecting the heating-cooling combination according to the recommendations provided in Table 8.
10. Heat the first area, and move to heat the next area. Heating successive areas limit downtime.