API RP 652-2020 pdf free download

API RP 652-2020 pdf free download.Linings of Aboveground Petroleum Storage Tank Bottoms.
4.8? Fretting-related? Corrosion Fretting-related corrosion may occur in hydrocarbon service on the bottoms of external foating roof tanks. When the tank is emptied, the foating roof is typically supported on roof-support legs constructed of open-ended pipe. Most bottom designs require “striker plates” under each roof support leg. When the foating roof is landed, the pipe legs rest on the striker plates supporting the weight of the roof. Repeated, frequent contact between the striker plate and the open end of the pipe leg removes any protective layer of rust scale that may have formed on the striker plate surface. When the roof is foated again, any water on the tank bottom causes corrosion at the location on the striker plate where the coating and/or any protective rust scale has been damaged. Experience has shown that frequent roof landings over a long period of time causes corrosion severe and localized enough to corrode a hole through the striker plate and the bottom plate like a cookie cutter cuts dough. 4.9? Generalized? vs? Localized? (Pitting)? Corrosion Two common types of corrosion found on tank bottoms are general and localized (pitting) corrosion. In general corrosion, thousands of microscopic corrosion cells occur on an area of the metal surface resulting in relatively uniform metal loss. In localized (pitting) corrosion, the individual corrosion cells are larger and distinct anodic and cathodic areas can be identifed. Metal loss in this case may be concentrated within relatively small areas with substantial areas of the surface unafected by corrosion. Corrosion of this type can often be found when MIC is present. 4.10 Stress Corrosion Cracking Stress corrosion cracking (SCC) has been identifed in tanks storing fuel grade ethanol. Owner/operators should take measures to protect welds from this phenomenon. One method identifed to prevent SCC is to coat welds with internal lining materials.
5 Determination of the Need for Tank Bottom Lining 5.1 General The bottom plates of aboveground storage tanks are susceptible to internal and external corrosion. Storage tank bottoms are generally fabricated from carbon steel plate sections that are typically 0.25 in. (6 mm) thick. Annular bottom plates of storage tanks frequently have thicker plate sections ranging from 0.25 in. to 1.0 in. (6 mm to 25 mm). The bottom plate sections and the attachment fllet lap welds are intended to function as a membrane and prevent leaks. Uniform soil support beneath the bottom plate minimizes stress in the bottom plate. The need for an internal tank bottom lining in an aboveground storage tank is generally based on several considerations: a) corrosion prevention; b) tank design; c) tank history; d) environmental considerations; e) reduction of time and efort for future tank cleaning; f) federal, state, and local regulations; g) product quality; and h) considerations under API Standard 653 with respect to next inspection interval. 5.2 Linings for Corrosion Protection The proper selection, application, and maintenance of tank bottom linings can prevent internal corrosion of the steel tank bottom. However, unless means of corrosion prevention are used on the soil side, perforation of the tank bottom may still occur. The minimum thickness of the steel tank bottom should be determined according to API 653. An internal tank bottom lining may be deemed necessary if corrosion is expected to proceed such that the steel thickness may reach this minimum thickness, generally 0.100 in. (2.5 mm), prior to the next scheduled inspection.
If the minimum bottom thicknesses at the end of the in-service period of operation are calculated to be less than the minimum bottom renewal thicknesses given in API 653, or less than the minimum bottom renewal thicknesses providing acceptable risk as determined by an RBI assessment per API 653, the tank bottom shall be lined, repaired, replaced, or the interval to the next internal inspection shortened. When using API 653 to determine appropriate internal inspection intervals for aboveground storage tanks, the anticipated life of the lining as well as the corrosion rate anticipated in the event of premature lining failure should be considered.