API 653 Tank bottom corrosion rates/inspection interval

[Declan86]

Good day,

I am just looking for a little direction of which corrosion rates to use when calculating the remaining life of an API 653 tank bottom.

An example would be if you have a tank that has 2 soil side pits. The deepest pit of 0.150in after 10 years on a 0.250in nominal floor and a shallower pit of 0.050in. The maximum corrosion rate for this tank bottom would be 0.015in. If the client wants a 10 year interval and you use that corrosion rate you you would have to repair both pits to meet the MRT of 0.100in. Or at least that's the way I was thought it and most people I ask agree with that assessment.

I have a co-worker who disagrees and says that once the 0.150in pit is repaired your corrosion rate for the interval calculation drops and therefore you don't have to repair the second pit.

I was just looking for some insight on who is right, or are we both wrong?

 

[Dhurjati Sen]

Your co-worker is WRONG.

He has used the general corrosion rate in evaluating the shallower pit.

He needs to use pitting corrosion rate for evaluation which could greatly differ from the general corrosion rate.

DHURJATI SEN
Kolkata, India