Fixed wall thickness inspection refers to the time interval between two thickness measurements; how long is this specific interval?
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I calculate the corrosion rate by subtracting the currently measured minimum wall thickness from the original wall thickness, and then dividing that result by the service life. But one of the reviewers said that it is necessary to use the difference in wall thickness measured over the last two times, divided by the interval between detections. The corrosion rate is determined. Is that the case?
The thickness measurement method directly measures the thickness reduction of the material due to corrosion. The formula is: Corrosion rate (mm/a) = Δd / t, where Δd is the difference in thickness before and after corrosion ； t is the corrosion time. That is, the time interval between the two measurements.
The formula is the same; there’s no difference, but is it necessary to use the values from the two most recent measurements?
Yes, such a calculation method is more accurate. By dividing the difference in wall thickness measured during the last two tests by the time interval between those two tests, the actual corrosion rate over that period can be determined, which helps to more accurately assess the rate of wear on the equipment and its remaining service life. Using the entire service life to calculate the average corrosion rate may mask instances of accelerated or slowed corrosion during certain periods. .
It doesn’t have to be nearby; only the time interval between the two times matters. And the wall thickness difference between the two times.
Mainly, where did you get the data on this original wall thickness? If it were based on the drawings, it would definitely not be allowed, because the wall thickness specified in the drawings is only a guide for production; the actual wall thickness can vary.
Also, the service life you mentioned is definitely incorrect; the designed service life differs from the actual one, and factors such as when material flow starts and how long there is a pause in use are not taken into account.