Quality Aboveground Storage Tank inspection
What is ‘quality’ Aboveground Storage Tank (AST) inspection and how do you achieve it? In industry today, you will get as many answers as questions asked. This presentation provides one view of current industry practice and guidance regarding AST inspection from the view of an experienced inspection contractor.
AST inspection is normally based on API 653 or other international guidelines for inspection intervals. AST inspection requirements are based on known AST inspection history or on what inspection interval needs to be met. Methods and approaches to such inspection vary greatly. External (in-service) inspection can consist of a few shell and roof Ultrasonic (UT) thickness readings and Visual (VT) inspection to more extensive data collection used to establish corrosion rates and maximum safe fill heights. Internal (either out-of-service or robotic in-service) inspection also varies from random bottom and shell UT readings to more extensive data collection using bottom scanning equipment with UT prove-up and more complete evaluation of shell corrosion.
While some owner/operators are satisfied establishing minimum condition only, inspection equipment and techniques are available today that allow for more detailed data collection at reasonable and cost-effective rates. Bottom scanning equipment differs from scanners used in the early 1990’s but there is still much industry concern over the quality of bottom inspection. There are an abundance of stories regarding past tank inspections that resulted in the need to take an AST back out of service after inspection for immediate additional inspection and repair. This issue can almost always be related to the quality of the AST inspection and the qualifications and experience of the AST inspection contractor. Typical bottom scanning equipment in use today is depicted in Figures 1 and 2.
Before any inspection work takes place, the issue of bottom cleanliness/preparation must be addressed as it has a tremendous bearing on the ability of any inspector/inspection to achieve the desired ‘quality’ result. A tank that is out of service and made suitably clean and gas-free for personnel entry is not necessarily ready for inspection. Tank cleaning for entry purposes is different than tank cleaning for inspection purposes. The tank cleaner needs to be advised that cleaning is for the purpose of internal inspection or they will not do an adequate job. This means that all sludge, residual product build-up, loose scale and other deposits on the bottom plates and along weld seams must be properly removed to allow for suitable inspection. The preparation process may be made easier (or more complicated) by the presence of coating. An intact coating may make bottom cleaning easier. A failed coating may need to be partially or completely removed prior to inspection. The degree of cleanliness achievable may not be known until the tank is entered and cleaning proceeds. The owner/operator has to be prepared to deal with the unexpected.
How does an owner/user establish that he is getting ‘quality’ inspection? Inspection must be performed by experienced AST inspection personnel who have appropriate credentials and knowledge of AST inspection issues. Since the issuance of API 653 in 1991 and the establishment of the API 653 Inspector Certification Programme in the US (and now including such certification for international users of this inspection guideline), the owner/operator should understand that not all AST inspectors are equal. Those with API 653 certification have demonstrated a degree of knowledge of the guideline and of AST inspection issues that sets them apart from AST inspectors not so certified. Does this mean that all API 653 AST inspectors are equal? The answer to that question is no. There can be significant differences in the knowledge and level of experience that an individual API 653 inspector/inspection company brings to the AST inspection project. How long has the inspector been certified? Has he/she been involved continuously in AST inspection since certification? Have they been re-certified over this time period?
Re-certification is intended to strengthen the API 653 programme and process to help provide better assurance to the owner/operator that the API 653 certified inspector has adequate knowledge to get the job done in an effective manner, but there can still be some disparity between individual inspectors. A certified inspector with 30 years continuous experience in AST inspection issues and someone with only 1 year of experience after certification are not necessarily equal just based on the fact that both are certified API 653 inspectors. Certification alone is not a guarantee of ‘quality’ AST inspection. The owner/operator must make some choices regarding who is doing the inspection and ensuring that there is adequate depth of knowledge and experience inserted into the process so that the desired goal of ‘quality’ inspection is achieved.
Larger owner/user organisations often have certified API 653 inspectors on staff. This helps reduce their out-of-pocket inspection costs and provides some flexibility in AST inspection scheduling/ execution but does it insure ‘quality’ inspection? Again, the answer is no. Even certified, knowledgeable, experienced staff inspectors may not actually collect the data, leaving it to contractors hired to provide NDE services. (This may not be the best approach to AST inspection depending on the job requirements.) Unless the staff Authorised Inspector (AI) carefully monitors what is done in the field, the data that is evaluated may be not be as accurate or as complete as necessary to properly evaluate the AST.
If internal inspection is performed, how is the scanning and proveup of the bottom indications validated? Bottom scanning equipment is expensive to own and maintain. Rental equipment is available but variable as to quality and not normally rentable with a qualified operator. If owned, how often is the equipment used to do AST bottom scanning and is the operator qualified to use the equipment and correctly interpret the scan results? Can the operator adequately prove-up the indications using UT or is another NDE technician doing that portion of the process? Is he certified to acceptable criteria (ASNT or similar) to adequately establish the depth and extent of bottom corrosion (especially soil side)? What type of UT is being used (A-scan vs. B-scan vs. hand-held D-meter)? Is the NDE technician familiar with how the scanner operator marked the bottom indications so that he knows what to look for?
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