Ultrasonic thickness gauging is the most common method of measuring the thickness of the shell of a tank. It providing the data on which to base the calculations that show that the shell is fit for purpose. It must be appreciated however that external ultrasonic thickness gauging, is historically carried out manually using a grid gauging technique which has a very low probability of detection (POD) and may not detect areas of isolated thinning and is therefore not a substitute for the occasional internal inspection and gauging of the tank.
Instruments used for ultrasonic inspection should be of an acceptable standard. The operators of the instruments should be certified as being of an approved standard. It is important that there is a procedure in place that describes how the inspection will be carried out and specifies the instruments, instrument settings, probes, couplant, access etc. that will be used.
External gauging can be used to monitor the extent of general metal loss in various areas of the tank and provide useful information on the condition of the tank.
The extent of gauging required is dependent on the product stored in the tank, the tank age and previous inspection results.
Method of ultrasonic thickness gauging include remote access crawlers such as the range of
Scorpion crawlers or manual thickness gauge’s such as the
T-Scan300 DL. The Advantages of the Scorpion range include:
- Removes the requirement for costly scaffolding or rope access.
- Will improve the ability to detect corrosion by taking a continuous scan from the bottom to top which can be viewed as an A and B scan UT report.
- Permanent hard and soft copy record of the results that can be retrieved at any future date with extremely accurate X and Y coordinate location of any corrosion for future RBI comparison.
- Eliminate the need to apply any grid lines matrix as per the requirement of manual grid gauging.
- UT data is permanently stored to assist with future RLA and RBI forecasting and eliminates the risk of not being able to duplicate the inspected area at a future date.
- Will detect de-laminations as well as corrosion
- Quicker data collection and real time analysis of the inspection.
- Computer generated report in lieu of totally manual reports.
- Reduced safety incident risk with corrosion issues by minimising the use of scaffolding and tank roof walk boards on suspect areas.
Tank Shell Inspection Recommendations
For an improved POD, increased coverage and UT corrosion mapping of the tank shell then a
RMS2 system would be recommended.
When client request the 1st course shell inspection it is possible to screen the plate using the versatility of our MFL HandScan system to improve the POD of corrosion detection over conventional thickness gauging.