Greatings,

I have not thought about it before, but one of my fellow engineers asked me after I handed in my spreadsheet for SIF calculation, and my calculation spreadsheet did not include the corrosion allowance either.

I have checked ASME B31.3 Table D300 for Flexibility factor and SIF, and could not find the answer I was looking for. The table Notes (4) is defines:

Tbar = for elbows and miter bends, the nominal wall thickness of the fitting,
Tbar = for Tees, the nominal wall thickness of the matching pipe.

I have thought that the code calculations are based on the corroded wall thicknesses, and the code does not support it in any way as far as I checked.

In case there is a small corrosion allowance available the value does not change much. But if the corrosion allowance is 3 mm and greater the changes become very large with small diameter pipe sizes.

At the moment I do not have Caesar II software to check to see what Caesar II considers for the SIF calculation.

Therefore, I would like to get your valuable opinion on the issue.


Thanks in advance and kind regards,

Ibrahim Demir

Greetings again,

It is interesting that the only code CODETI C3.2.6.3 Notation for "ef" gives the following explanation for the thickness in SIF applications:

"ef = Thickness of reference used to calculate the constraints: the half-thickness a range of tolerances and c1- c1+ less corrosion allowance or possible erosion (see Note 1 and 2).

Note1. The determination of the stress from a well defined thickness can be very conservative in so far as the corrosion or erosion leading to sub-thickness are in the vast majority of localized cases.
In agreement with the Principal validation behavior for the network can be based on a verification of the constraints taken into account these phenomena only for areas potentially affected."

The translation into English may not be 100% correct but the concept is more or less defined.

I guess this is valid for all the standards; however I still can not get this validated by ASME B31.3.

Regards,

Ibrahim Demir

Unless a Code specifically says to deduct the corrosion allowance from the SIF calc, the corrosion allowance is not considered. Most Codes consider corrosion only in the Section Modulus when making the stress calc.

Dear Ibrahim.

IMO, what Codeti says is that the rule is to consider a reference thickness, defined as a median value in tolerance range (that means considering both "plus" and "minus" tolerances) minus the corrosion allowance.

This Codeti reference thickness is used in calculation for both Flexibility Factor, k, and Stress Intensification Factor, i. So you have to count also the flexibility consequences not only stress intensification.

However, by Note 1 of C3.2.6.3, Codeti makes a step back recognizing this approach (i.e. "ef") can be very conservative since, for the majority of cases, the corrosion/erosion can be a quite local phenomenon.
The rest of Note 1 is not so clear and seems to be written in French diplomatic language; I would interpret it as I can obtain the Project Authority agreement to base my stress evaluation applying "ef" only in zones I'm concerned about the uniform corrosion/ erosion phenomena. In practice, this approach would be quite difficult because I have to predict where it is the case and where is not. Finally, I would say important is I can obtain an agreement to infringe the basic rule.

You said "I guess this is valid for all the standards".
I think it is not the case to be so worried; EN 13480 also requires the nominal thickness for k and i calculation...

Regards.

Thank you for your answers. So we agree that:

1. ASME B31.3, ASME B31.1 and EN 13480 (although is mostly copied from Codeti) do not consider the corrosion allowance.
2. Codeti is a bit diplomatic about it, however recognises the corrosion allowance reduction for flexibility and SIF calculation and calculation of stresses.

Our basic problem is the large uniform corrosion allowances that is taking almost more than half the thicknesses, the pressure is very low, but temperature is about 200 deg C. The thicknesses were selected for the pressure including corrosion allowances only. There are extreme cases like this with carbon steel application. The pipe specifications do not consider the stress analysis requirements for this issue unfortunately.

We know that there are significant changes in flexibilities and SIFs in case we consider this uniform corrosion allowance such as the case above.

I think this issue should be recognised by the codes, and the stress engineer should be the decision maker for the application of that in line with process requirements or engineer.

We use FEA softwares to prove the piping behaviour under the code recognised cases, and make pipe take all the loads, but ignore the largest part of the disappearing thickness under uniform corrosion, unbelievable.

I still think using the corrosion allowance in flexibilities and SIFs should be optional in the analysis softwares against the extereme condition. The user should probably run both case scenario to see the piping passes under both conditions, whether the code recognises or not.

Kind regards,

Ibrahim Demir

Ibrahim,

You said "The user should probably run both cases scenario to see the piping passes under both conditions, whether the code recognises or not."

I understand you would like to run a separate non-Code case, based on "large uniform corrosion" prediction.
I think you would perform a separate Caesar run imposing the corroded thickness as "nominal" thickness . Maybe this approach would be theoretically questionable because we mix on event belong to "end-life" of pipe with the "fatigue life"- expansion, however any other method has theoretical weaknesses...

Do you think this "procedure" would be not satisfactory for your case/ your Client?

best regards

Mariog,

I do not want to speculate the condition of the code that far. But, the stress enginer as a designer is responsible to make sure the system is safe under the considered conditions, the codes are minimum requirements, and my idea is not against the code either if we think a bit further.

Note 1 above in Codeti already explains the idea behind. So it leaves the application to the designer.

Flexibility and SIF calculations under code formulas with and without corrosion allowance makes substantial change in the values with large corrosion (uniform ) allowance. When the pipe corroded the fittings become more flexible with large stress intensification factors. This means when the systems starts corroding the fatigue life is going to be reduced faster. In case the client asks certain life expectation you need to be more careful with the corrosion.

You may say why the piping procedure does not consider different material with less corrosion allowance or no corrosion. This is another economical issue that the client/user of piping takes care.

Kind regards,

Ibrahim Demir