Calculating intervals for tank inspection

By Carlos Molina

One of the things about API 653 certification is that doesn´t ensure that you are competent in tank inspection. You can pass the exam, and still you don´t always know or remember all you studied in the course, especially if not practiced on a daily basis. One of those things to remember is tank internal inspection interval calculation .

I was wandering the eng-tips forum the other day and I found a question about calculating minimum remaining thickness for a tank bottom.  I saw that forum and decided to write this post in order to clarify the subject more. Besides, this is part of the Body Of Knowledge of the examination, and you know you have to get it right…

Internal inspection is neccesary usually when you don´t have access to the underside of the bottom of a tank, so it is very important that you can know in advance the next due inspection date.

To know how many years you will be operational until next inspection, you need to go to of the API 653 standard. This numeral is used to calculate next interval inspection interval after bottom repairs, although calculations can be done at any time during tank operation.

The formula in goes this way:

Calculate minimum remaining thickness

Calculate minimum remaining thickness

This is the explanation for all of those variables

MRT calculation variables

MRT calculation variables

But you are looking for an answer in years, so you will have to reorder the equation the following way

Calculate time until next internal inspection

Calculate time until next internal inspection

Resolving for this last equation, you can obtain when is next internal inspection due.

When the formula puts a value on any of these variables, is not a media between the measurements taken in a tank bottom, but the most critical spot value measured.

We have a tank that contains water; its original bottom thickness is 1/4″. There is one point in the bottom that is 5mm thick. Corrosion rate for the top (product) side of the bottom is 0,5mm per year, and the corrosion rate for the underside (soil side) of the bottom is 0. The owner/operator ask if the tank can resist 10 more years in service without lining or maintenance. If the tank is new, when should be next inspection done?

To know the answer, you can open this educational animation (Forgive my poor animation skills.)


If you can´t see the video above, you can go to the “How to know next internal inspection interval to API 653” video I uploaded to Youtube.

Remaining life calculations show that the tank in its current state will not reach the estimated remaining life time of ten years. It is recommended to repair all corrosion. The repair work shall be carried out in accordance with API 653, 9.10.

Well, that was the little contribution to this subject. I would love if you have something to say about how helpful this was.


BONUS FOR API EXAM TAKERS. API 652 6.2.2 says required remaining remaining thickness is 0.05in for tank bottoms. This is plain wrong. API 653 remains true.



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{ 6 comments… add one }
  • majid August 9, 2015, 5:59 pm

    Thanks it is very helpful. I want to know that MRT will always 2.54 in every case?

    • August 10, 2015, 1:02 pm

      Not always. The different values can be found on table 4.4 and numeral It depends on the design of the bottom/foundation. Also, it is different from minimum required thickness of the annular plate

  • melvin fernandes October 23, 2015, 7:19 pm

    Dear Sir
    This was very helpful to understand specially the animation…thank you and think there is a typographical error in the above formula as mentioned on api 653 its (Stpr + Upr).

  • Joseph Muniz July 28, 2016, 8:09 pm

    For an above ground storage tank fabricated in 1966 with no history documented and then having the entire bottom replaced. Does the API code mandate a 5 year due date the same as the Pressure Vessel Code requires for the next inspection after major repairs ?

    • Carlos F. Molina July 28, 2016, 8:49 pm

      For a new tank bottom, inspection frequencies are given by “The initial internal inspection intervals for newly constructed tanks and/or refurbished tanks shall be established either per or”

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