Averaging thickness readings in a pitted exchanger shell

[mechengr12]

I have a coded hair pin exchanger with general pitting. If certain pits are below tmin, what guidance should I follow for taking an average of the area? API 510 7.4.3? Can someone give me a quick understanding of general corrosion and pitting guidance that is a good practice to follow for understanding at what point further measures must be taken?

Also, if the hairpin exchanger is a 5" diameter and 30' long is this required to be a coded vessel (plant is in a non-coded state). All I have been able to locate is ASME Sect VIII regarding equipment greater than 6" must be coded and stamped.

Thank you

 

[racookpe1978]

I've faced that problem with pitting before but NEVER tried to find a way to justify thickness average rather then thickness minimum (thickness at the bottom of the "average" pit.)

Your first failure point will be across the line of least thickness ... the sum of the biggest pit in a line across a point... near where the maximum stress in the whole surface is. On that line, the thinnest material will fail first. Then the rest very quickly will follow.

 

[metengr]

Look at API 579.

 

[EdStainless]

If all that you are worried about is strength then you need to look at pits in groups, either lines or clusters.
If leaking is your concern then the very deepest ones are all that matter.
An average is meaningless.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

Here is another standard
National Board Inspection Code, NBIC Part 2, 4.4.8.7. All of the information is there for evaluating corrosion pits.