## Why Don't The Required Thickness Don't Match?

## Why Don't The Required Thickness Don't Match?

(OP)

For vessels with a specified corrosion allowance, when I run a calculation for the minimum required thickness of a cylinder based on the I.D. formula per UG-27(c)(1), I get one result.

However, when I run a calculation for the minimum required thickness of a cylinder based on the O.D. formula per Appendix 1-1(a)(1), I get a slightly different result, even though all of the geometry in both cases is exactly the same.

Which result is correct?

What am I doing wrong?

Thanks.

However, when I run a calculation for the minimum required thickness of a cylinder based on the O.D. formula per Appendix 1-1(a)(1), I get a slightly different result, even though all of the geometry in both cases is exactly the same.

Which result is correct?

What am I doing wrong?

Thanks.

## RE: Why Don't The Required Thickness Don't Match?

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## RE: Why Don't The Required Thickness Don't Match?

## RE: Why Don't The Required Thickness Don't Match?

You may not be doing anything wrong. Both equations give essentiall same answer, but they are not identical. Remember also that ASME VIII requires that all calcs be made in the fully corroded condition. Therefore, you must deduct a full corrosion allowance when performing the Code calcs. In your case the inside radius INCREASES by the amount of the corrosion allowance. The outside radius remains unchanged. I am assuming that the corrosion allowance is for internal corrosion purposes.

Steve Braune

Tank Industry Consultants

www.tankindustry.com

## RE: Why Don't The Required Thickness Don't Match?

## RE: Why Don't The Required Thickness Don't Match?

## RE: Why Don't The Required Thickness Don't Match?

The formula for the required thickness in terms of the ID is

T = PRi/(SE-0.6P)

The formula for the required thickness in terms of the OD is

T = PRo/(SE+0.4P)

These two formulas give different results unless T = Treq (or P = MAWP). This difference is NOT due to rounding errors.

## RE: Why Don't The Required Thickness Don't Match?

_{o}=R_{i}+Tprex

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## RE: Why Don't The Required Thickness Don't Match?

If Ro does not equal Ri+t, then the two equations HAVE to give DIFFERENT results.

## RE: Why Don't The Required Thickness Don't Match?

The result is that you end up with inconsistent results from the two formulas.

## RE: Why Don't The Required Thickness Don't Match?

I get the first equation back, so the equations appear to be very consistent.

TTFN

## RE: Why Don't The Required Thickness Don't Match?

## RE: Why Don't The Required Thickness Don't Match?

Just for the record, what are the thickness requirements when calculated both ways. I suspect that both answers are very close.

Steve Braune

Tank Industry Consultants

www.tankindustry.com

## RE: Why Don't The Required Thickness Don't Match?

Yes, for most of the cylinders I've calculated, the required thicknesses are VERY close, within a few thousandths of an inch.

However, in running nozzle reinforcement calculations, it is not uncommon to use a nozzle that is significantly thicker than the required thickness for internal pressure (as per UG-45). With a large difference in the required vs. actual thickness, the results of these two formulas diverge enough to affect the MAWP for the reinforcement calculations by a few psi.

I plan to stick with the more accurate I.D. formula from now on.

-Christine

## RE: Why Don't The Required Thickness Don't Match?

_{i}based formula.However it is not a matter of accuracy. The point is that pressure is of course assumed to act at R

_{i}in both, but in one R_{i}is given, in the other one must be inferred in a safe way. And as the formula doesn't know the actual thickness, it is inevitable to assume pressure is acting at R_{o}-T (if you inspect closely the formula, this would be in fact R_{o}-0.4T, but that's another story).Anyway I agree with you: the R

_{o}based formula is useless.prex

http://www.xcalcs.com

Online tools for structural design