By Vince Carucci
API 653, “Tank Inspection, Repair, Alteration and Reconstruction,” was first published in January 1991, and a short supplement was provided in January 1992. The Second Edition of API 653 was published in December 1995, and an errata was issued in May 1996. This article summarizes several of what this writer feels to be the more significant changes that were made to this widely-used standard. Please note that this is not an all inclusive summary of the changes made.
The original weld joint efficiency (E) must be used when determining the minimum required shell plate thickness (Ref. Para. 188.8.131.52 of API 653 and the March 1996 issue of the Carmagen Report). API 653 previously required that an “E” of 0.7 be used if the original “E” was unknown. This assumption could result in a severe fill height restriction even with a minimal amount of corrosion in situations where the original “E” was actually much higher but could not currently be documented due to the loss of design and fabrication records. The revised API 653 has relaxed this blanket requirement a great deal for most applications by looking at how storage tanks were actually designed. Weld joint efficiency requirements are now contained in a new Table 2-1.
The key to this new, and realistic, approach is knowing the year of tank construction and being able to confirm that the tank was originally designed and constructed to API 650 or API 12C. The tank nameplate (assuming it is still there) provides this information. Once this information is in hand, Table 2-1 specifies the required “E” considering how the tank would have been designed at that time to meet the design standard that was used. There will still be some tanks where an E = 0.7 (or even lower) must be used. However, an E = 0.85 or even 1.0 can now be used for many cases where design documentation no longer exists.
In assessing the suitability of a tank bottom for continued service, the bottom plate and butt-welded annular plate thicknesses must be compared to acceptable values. In all cases, the acceptable annular plate thickness is larger than the acceptable bottom plate thickness (Ref. Tables 2-3 and 4-1 of API 653). Thickened, butt-welded, annular plates are not used in every tank. For these cases, API 653 previously used the same minimum required thickness for the portion of a bottom that was near the shell (i.e., the sketch plate) as it did for the rest of the bottom. This was an unconservative approach.
Para. 184.108.40.206 of the revised API 653 specifies a minimum required thickness in the “critical zone” of the tank bottom (i.e., within 12 in. of the shell in this case) as:
This requirement forces all tank bottoms to be divided into at least two zones for the purpose of thickness evaluation (i.e., near the shell and away from the shell) since different acceptance criteria may be required for each. As in other thickness evaluations, API 653 permits performing a detailed stress analysis instead of following these thickness limits.
The following highlights several other changes that were made: