There is an option in Caesar for including Corrosion allowance for expansion stress. The default in Caesar is without corrosion allowance
for expansion stresses. I would like to know if there is any code case or code inerpretation available for excluding corrsosion allowance for expansion stress calculation in ASME B31.3

Wall thickness has very little impact on displacement stresses, so having a corrosion allowance applied or not applied is not going to make very much difference.

I could not agree to Mr. Klein's view on wall thickness in displacement stress range.

In expansion case we are actually looking into a case of thermal fatigue and corrosion has got a serious role to play in that.

I am personally not aware of any 31.3 interpretation where it is suggested not to use corrosion allowance in expansion case.

A.Bhattacharya

Stress Analyst

Bechtel Corporation

In describing the stress calculations for sustained and occasional stresses in paragraph 302.3.5(c), the code clearly states that the thickness used in these calculations shall include corrosion. But with expansion stresses there is no clear indication. Paragraph 319.4.4(a) defines the equation (17) for expansion stress but defines Z simply as the section modulus of pipe. It makes no statement on the use of corrosion. In fact, the definitions for Effective Section Modulus used for branch comnections in Equation (20) clearly include the use of nominal wall thicknesses.

It appears that the process piping code found reason to include corrosion when checking burst and collapse but not fatigue. That is not to say corrosion should not be included and, as stated previously, CAESAR II configuration allows the user to corrode all stress cases. The irony is that corrosion can directly affect fatigue strength.

Here's a little tidbit...
Interpretation 4-10 of May 1985:
"Question (3): For thermal flexibility analysis in accordance with 319, is it required to determine stresses by applying the forces and moments produced by pipe of nominal wall thickness to the pipe section in the corroded condition?

Reply (3): No. Nominal wall thicknesses shall be used in flexibility calculations. Refer to 319.3.5, 300(c)(3), and 300(c)(5)."

These interpretations can be superceded but I see no other reference to corrosion and expansion stress.

The calculated value of expansion stress depends upon 1- Nominal value of moment 2- SIF 3- Section Modulus. While it is clear that in Caesar section modulus and SIF are corrected to include effects of corrosion, what is about the effects of corrosion on the calculated nominal value of the moment? In other words is the stiffness matrix adjusted to cater for corrosion? As I observe it is not, since same deflections appear whether corrosion is applied or not.

Paragraph 319.3.5 states that nominal dimensions (OD & thickness) be used in flexibility calculations.
So all expansion loads in CAESAR II are set without corrosion but, as a user option, the stress can include corrosion in the section modulus.

And, if you need more convincing, you can do, as I did, this little test in Caesar:

Create a one element model, say 10 feet long, of 4" diameter pipe. Anchor one end and apply a 1" displacement at the free end.

Forget about corrosion allowance. Put in some wall thickness for the segment and run a thermal case with D1. Do this for a couple of different wall thicknesses and you'll find that the stress does not vary all that much.

Yes, your section modulus is somewhat smaller as the wall thickness decreases. But, the stiffness is also lower, so the moment imposed is lower as well.

The expansion stress range is independent of the pipe's wall thickness. This is why if you are overstressed in the Expansion case, changing (increasing) the wall thickness won't solve the problem.

Hi Richard,

I think allowable expansion stress range i.e 1.25Sc+0.25Sh is independent of thickness(hence corrosion). Once you include libral stress in the stress range it becomes thickness(Corrosion) dependent, hence expansion stress range should change with or without corrosion.

Computed stress range((Sb**2+4St**2)^0.5] is also dependent on thickness(hence corrosion) because of section modulus of pipe

Jaswinder S Bansal
Stress Analyst
Bechtel Corporation

We can conclude that the calculated expansion stress will increase with corrosion allowance due to the reduced section modulus of the pipe.

The allowable expansion stress range without liberal stress option will not change as it is independent of thickness of pipe.

Similarly allowable expansion stress with liberal stress option will reduce with increased corrosion allowance due to lesser margin between code allowable and sustained stresses which will be added to allowable expansion stress range.
Also it is to be noted that sustained stresses shall be calculated only with corroded thickness and it is not permitted by code to exclude corrosion allowance .

ASME B31.3 code expects to use nominal pipe without corrsion allowance for calculating expansion stresses and Caesar allows to include corrosion allowance in the expansion stress calculation, by choosing the option in the config set up file.

For conservatism, use uncorroded section modulus for moment loading and corroded section modulus for resulting stresses!

The old guided cantilever formulas do not include wall thickness for expansion stress calculations, just diameter. As previously stated the effect of wall thickness on moments cancels out the effect on stress. However that assumes uniform corrosion. Localized wall thinning can cause an "SIF" effect. This may be the result of erosion, pitting, non-uniform chemical concentrations, etc. You won't find an appendix D formula for these and they might not even be reflected in your corrosion allowance.