Calculation of retirement thickness for a furnace tube

[APItips]

Hi Experts
I have been evaluating a UT thickness survey report of furnace tubes which were out of service for inspection, I noticed a metal loss due to internal corrosion, the corrosion allowance is not mentioned on the data sheet.
the tube OD size is 8" , the original thickness is 12.7mm.
the minimum actual thickness found is 6.6

My questions is what is the applicable code or paragraph for retirement thickness calculation for a furnace tube?
Should I use the Barlow formula in API 574? but this is for piping I believe!

your replies are highly appreciated.

 

[metengr]

look at API 579 fitness for service for guidance. There is no specific retirement thickness because this involves significant risk for codes and standards. You need to use various references and determine the risk from an owner's perspective.

 

[chicopee]

If it is a NB/ASME stamped boiler, check out the power or heating boiler code to which it was constructed. In these two codes, formulae abound about tube thickness vs. MAWP. I believe that there is a reduction percentage that's allowed, however, that value eludes me without further research.

 

[APItips]

Thanks for your advice metengr and chicopee

 

[Duwe6]

Barlow works fine for internal pressure. Now you need to factor in the structural load [can the tube still hold itself up?] anf any point loads from supports and restraints. Even for tubes as large as 8-inches, the sum of the structural loads usually overshadow the internal pressure load.

ps. VERY unusual for petrochem furnaces to be Code-stamped.

 

[chicopee]

The formulae shown in the NB/ASME power code I and heating code IV will have tube thicknesses based on MAWP which in all likely hood will not be commercially available; it will be necessary to select a commercially available tubes with slightly thicker walls, normally based on the Birmingham Wire Gage. Obviously that selection will be a slight overkill, therefore, metal loss as stated in your OP should not be less than the calculated tube thickness. There are reduction percentages but none that I read dealt with tube thicknesses.