Pressure vessel internal and external corrosion - effective as one side corroded vessel ? 1

[mariolucas75]

Dear Forum,

If there is a pressure vessel with some internal and some external corrosion and individually these internal / external corrosions are less than corrosion allowance, but together are more... can I consider this vessel one side corroded with net corrosion depth as a sum of internal + external, thus going deeper than corr allowance ?
And if so ... where is that written ?

Thks

 

[dig1]

Hello Mariolucas,

I think the best way to look at this is to determine if the remaining thickness is less than the required thickness (by calculation). Depending on the Post Construction Code/Jurisdiction/etc. there may be additional requirements which will be written in those documents. This is a starting point for the evaluation.

 

[SnTMan]

mariolucas75 "And if so ... where is that written ?"

I doubt it is written anywhere, but the relevant equations tell the story.

What you propose, treating both allowances as internal allowance is approximately correct, but only approximately, due to the dimensionial differences.

One way to do this is to adjust the external dimensions by the external corrosion allowance, i.e. OD - 2 X CA (external). For all features of interest. Then perform the calculations with the given CA (internal). See if it then meets design pressure. This should be fairly easy to do in most software. If the software calculates MAWP (New and Cold) this will be incorrect.

Alternately, you can adjust dimensions per both CA's and perform the calculations with zero CA. Again, MAWP (N&C) will not be correct.

Regards,

Mike





The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[GD2]

OP,
There are two options to go ahead with it:
1. If opposing corrosion depth from opposing surfaces does not overlap - a layer of uncorroded material will exist. Take this thickness and evaluate for available thickness Vs required thickness.
2. If opposing corrosion depth from opposing surfaces overlap on the section profile, do a FEA.

GDD
Canada

 

[r6155]

It is clear that the corrosion is throughout the thickness: the remaining strength.

Regards

 

[GD2]

OP,
The attached pic will clarify what I meant. It is depicted for pitting corrosion but you can also use for LTA. [URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1725551725/tips/Corosion_on_both_sides_o3b71f.pdf[/url]

GDD
Canada

 

[EdStainless]

The problem with two-sided corrosion is that you can end up with bending stresses on the shell.
If you have external corrosion adjacent to an area of internal corrosion then simple wall thickness may not tell you enough.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Cobra17]

If you use UG-27 (inside radius based calculation), 'R' should be based on the inside corroded thickness, then add the outside corrosion allowance to your calculated 't'

 

[Dhurjati Sen]

Is the pressure vessel insulated?

Is it externally painted?

You can always eliminate the external corrosion and do your calculations based ONLY on internal corrosion.
As a matter of fact, the causes for external and internal corrosion are entirely different.

DHURJATI SEN
Kolkata, India