Question: To make the evaluation of the weak roof of a tank, what are the equations to use or is it based on the inspection? The API 653 standard only asks that there is no thickness less than 0.09 in an area of ​​100 in²?

Answer: For the evaluation of the roof the only criteria available is the one you mention …. “there is no thickness less than 0.09 in. In an area of ​​100 in²”

I will illustrate it with a real case. We had severe atmospheric corrosion in the roof of a tank, due to the proximity of the tank to the sea.

Of all the corroded spots, which were in the hundreds, 70% were a little over 0.09in ‘thick. But the size of the corroded area at neither point was 100in2. The roof was in enough shape to allow the operators to step on it.

Yet, it was recommended to patch or change the sheet in 8 sections. The area at these points was at least 80in2, close to 100in2. Some of these corroded areas were the sum of 2 or 3 smaller areas.

In some other areas, especially near the top angle, it was requested to fill the area with welding. In general, the roof, including corroded areas which weren´t replaced or fill welded, had to be coated more generouslyy.

A human foot has an area close to 100in2. This is an arbitrary size but serves as a guide.

What we put in the report was..

<<< Roof corrosion was compared against API 653 4.1.4.2 requirement (replace any area less than 0.09in thick in any 100in2 area), thus replacing 8 roof sheet sections and fill-weld the rest of the areas marked during the inspection. >>>

The roof had 2 perforations below the railing, and it continued to function that way until inspection without problems. Of course that had to be sealed.

The entire roof was recoated using the strip-coating technique heavily.

The maintenance contractor wanted me to recommend the replacement of the entire roof. The most he got was replacement of some areas. That was better for him than just patching the same areas, because he had more work.

You can also patch instead of replace.

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