One of the most important steps involved with regard to tank inspection, using API Standard 653, is establishing the internal inspection interval. API 653 says,
“Section 188.8.131.52 Internal inspection is primarily required to:
Ensure that the bottom is not severely corroded and leaking.
Gather the data necessary for the minimum bottom and shell thickness1 assessments detailed in Section 6. As applicable, these data shall also take into account external ultrasonic measurements made during in-service inspections (see 6.3.3)..........”
The discussion of similar service in this article is limited to consideration of the tank bottom thickness.
API 653 allows a tank to operate up to 10 years when there is no knowledge of the tank bottom internal (topside) or external (underside) corrosion rate between internal inspections or when a tank is new. The implied basis for this is that the combined, i.e. floor topside and underside, corrosion rates for perforation through the tank bottom is less than 25 mils per year. For the most part, data and experience bear this out2.
The fundamental challenge in determining the internal inspection interval for tanks is based upon establishing the corrosion rate. This subject deserves a lot of thought, especially when the operator wants to justify tank internal inspection intervals greater than 10 years.
Until a tank has been internally inspected for the first time it can be argued that there is no data to establish specific corrosion rates for that tank. However, corrosion rates can be established based on the accumulated knowledge and inspection performed on tanks in the past. This practice is called establishing the inspection interval based on “similar service experience3” in API 653.