Ultrasonic gauging is the most common method of measuring the thickness of tank shells. It generates the data on which to base the calculations showing that shells are fit for purpose.
It must be appreciated, however, that external ultrasonic thickness gauging is historically performed manually with a grid technique, which yields a very low probability of detection (PoD) and may therefore not detect isolated areas of thinning. Ergo, it is not a substitute for the occasional internal inspection and gauging of the tank.
Instruments used for ultrasonic testing (UT) should be of an acceptable standard to accomplish this task. Instrument operators should be certified on an approved standard and it is important for procedures to be in place describing how to perform inspections, and what instruments, instrument settings, probes, couplant, access, etc. that must be used.
External gauging used to monitor the extent of general metal loss in various areas of the tank provides useful information about the condition of the tank, but the extent of necessary gauging depends on the product stored in the tank, the tank’s age, and historical inspection results.
Methods of ultrasonic thickness gauging include remote-access crawlers such as Scorpion2 or manual thickness gauges. Advantages of using Scorpion2 include:
- Removes the need for costly scaffolding and rope access
- Improves the ability to detect corrosion by performing continuous scans from the bottom to top, which can be viewed as A-scans and B-scans UT reports
- Produces permanent hard copies and software records of results that can be retrieved and containing extremely accurate X and Y coordinate locations of any corrosion for future risk-based inspection (RBI)
- Eliminates the need for gridding (required in manual thickness gauging)
- Assists with future risk-level assessment and RBI forecasting by permanently storing data and eliminates the risk of not being able to duplicate inspected areas in the future
- Detect delamination and corrosion
- Collects data more quickly and enables the real-time analysis of inspections
- Generates digital reports instead of manual reports
- Reduces the risk of safety incidents related to corrosion issues by minimizing the use of scaffolding and tank roof walk boards
Tank Shell Inspection Recommendations
For a better PoD, coverage, and UT corrosion mapping of tank shells, we recommend RMS2. When customers request a first-course shell inspection, it is possible to screen plates with the versatility of Handscan MFL system to further improve the PoD of corrosion over conventional UT thickness gauging.