API 653 allowed thickness different between courses 2

[rab1944]

I am being asked to replace a 1/2" thick carbon steel course with a 5/16" thick stainless course. The course above the replacement is 1/2" carbon steel. I know you would not do this with a new tank but would this be allowed in API 653 or would section 9.3.3.2 disallow this by the "no more than 1/8" statement (referring to the max. perimeter difference).
Thanks in advance.

 

[JStephen]

I don't think the standards cover composite carbon/stainless construction for starters. Normally, this would not be allowed on new tanks, and I assume would not then be allowed in 653 either, as it intends to maintain equivalence with new construction.

 

[IFRs]

Two comments

API 650 states:
"The calculated stress for each shell course shall not be greater than the stress permitted for the particular material used for the course. When the allowable stress for an upper shell course is lower than the allowable stress of the next lower shell course, then either a or b shall be satisfied.
a. The lower shell course thickness shall be no less than the thickness required of the upper shell course for product and hydro-static test loads"
b. An elastic analysis per 5.6.5 using final plate thicknesses shall be performed.

The inside of an upper shell course shall not project beyond the inside surface of the shell course below"

Appendix SC of API 650 covers mixed material tanks. This will be part of the next release of API 650.

 

[JStephen]

In API-653, Add. 3, Section 9.2.1, "The minimum thickness of the replacement shell plate material shall be calculated in accordance with the as-built standard."

In 9.1.1, "The basis for repairs and alternations shall be an API Std 650 equivalence."

Then in API-650, 11th Ed., 5.6.1.3, "No shell course shall be thinner than the course above it."

I assume the original tank had a large corrosion allowance and the intent is to replace with the stainless plate without a corrosion allowance. I suppose that could be done, but the standards just aren't set up for that situation. The corrosion allowance is generally assumed to be uniform over the whole shell, and the whole tank is assumed to either be carbon steel or stainless, per the current standard.