Hello.

The formula for thickness calcualtion for internal pressure is P X R / (SE - 0.6 P). If R = 30" and corrosion allowance is 0.625", in some cases I see people take R = 30.625 ( i .e. they add the CA to the radius) while in other cases they do not (i.e they take R = 30 only).

What is the rationale behind this?

Thanks a lot. Appreciate it.

Sharjeel

Those who add ca make, in my opinion a strictly theoretical correct assuption. But it is not a Code requirement and on pratical point of view the difference is not relevant. This behaviour comes from the easy calculation made by computer program. If they had to work like we were used to work in the old times of logarithmic rule or tables they would not make these useless picknicky calculations.
Mauro

You'll find the answer in UG-16(e) which deals with the corrosion allowance in design formulas. The dimensional symbols used in all design formulas throughout Section VIII, Div. 1 represent dimensions in the corroded
condition. This means, that in case of the calculation of a cylindrical shell, as in your example, the CA must be added to the internal radius in order to be correct.

Thanks a lot for your responses.

It is very well accepted, that the Code formulas represent dimensions in the corroded condition.

What still baffles me (and what I meant by my earlier post) was that in the corroded condition the radius would be 30" and not 30.625". How does 30.625" represent the corroded condition.

Once again, thanks a lot.

Have a nice day.

Sharjeel

30' is the internal radius when it is new, and when there is no corrosion yet. After (internal) corrosion has taken place, the material gets thinner from the inside out, and the radius gets bigger.

Thanks a lot. So if we have lets say, an internal corrosion allowance of CA1, external corrosion allowance of CA2, and radius "R",

then the radius term in the circumferential seam/longitudinal joint stress calculation would be (R + CA1 + CA2), even though the corrosion from outside is making the radius smaller, while the corrosion from inside is making the radius bigger.

Appreciate it.

Sharjeel

Thanks a lot. So if we have lets say, an internal corrosion allowance of CA1, external corrosion allowance of CA2, and radius "R",

then the radius term in the circumferential seam/longitudinal joint stress calculation would be (R + CA1 + CA2), even though the corrosion from outside is making the radius smaller, while the corrosion from inside is making the radius bigger.

Appreciate it.

Sharjeel,
No, only the internal corrosion is affecting the internal diameter, so the term would be R + CA1. The external corrosion would reduce the thickness in addition, and would have to be taken into account as well but is not involved in the calculation for the required thickness based on the internal diameter or internal radius.