Hi,

Consider a pipe system that was stressed checked & have been built already. Now, the iso's have to be updated (as-built status) - and while going through the CII calc., it is discovered that the incorrect ca (corrosion allowance) was used previously. If the correct ca is now used, the system fails.
What recourse is there?

Waleed

Is it failing in sus or thermal?

Sus stress % Exp stress %
Design conditions: 100.2 243.8
Operating cond.: 93.1 132.1

Dear Waleed,

Beside fail in EXP load case, In which SUS it fail? cold SUS or hot SUS?
and what value of WT did you use? WT in construction stage or WT measured by today?

If you are designing to B31.3, corrosion allowance normally has no effect on expansion stress, which is based on nominal wall. Although Caesar can be set up to use the corroded wall in an expansion case, the code does not require this. Consequently it looks like your 243.8% expansion stress was there to begin with.

waleed,

Samsul P.M. has a good point about the actual wall thickness of the installed piping. The mill tolerance might be backed out of the calculations to offset the increased corrosion allowance factor. You state, "If the correct ca is now used" - what was the earlier ca , and what is the corrected ca? What is pipe diameter, material, ppressure and temperature conditions? Did a warning message advise of a required wall thickness exceed wall thickness available for pressure?

The corrosion allowance could be considered a somewhat arbitrary value that is intended to provide a number of years of service life based on an estimated annual loss of wall thickness due to corrosion. Many times the corrosion is reduced after a stable passivated film is established inside the piping. Maybe the reduced ca piping might be accepted with an extended warranty to be provided by the designer / builder.