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ASME VIII Div 1, UG-27 required thickness formula validity

ASME VIII Div 1, UG-27 required thickness formula validity

ASME VIII Div 1, UG-27 required thickness formula validity

(OP)
Formulas are given in UG-27 (c) for calculating the required thickness (t) for cylindrical shells.  The paragraph states the formulas are valid "When the thickness does not exceed one-half of the inside radius, or P does not exceed 0.385SE"

I am reviewing calculations done by a consultant and thick wall formulas were applied.  The part under consideration has overly designed very thick walls; however, the required thickness (t) as calculated with the thin wall formulas does not exceed one-half the radius.

What drives the selection of thick walled formulas - the calculated required thickness (t) using standard thin wall formulas, or the part design thickness?

Thanks.

RE: ASME VIII Div 1, UG-27 required thickness formula validity

Hi beh188

Any vessel can be calculated using the thick walled cylinder theory and that is, I understand the most accurate method.
The thin walled theory was developed assuming that the hoop stress throughout the cylinder was constant and because it varied very little with vessels of certain proportions also the thin walled vessel theory is easier to apply.

desertfox

RE: ASME VIII Div 1, UG-27 required thickness formula validity

As the sentence you quoted says '...or...', the thin shell formulae are mandatory when one of the two conditions (or both)  is satisfied.
So, if the thickness does not exceed one-half of the inside radius, UG-27(c) must be satisfied.
Which thickness (req'd or design)? This comes from the formulae:
- if you are calculating the required thickness from the design pressure, then simply must be
P/(SE-0.6P)<=0.5
otherwise you revert to thick wall
- if you are calculating the MAWP, t is the design thickness and
t/(R+0.6t)<=0.385
An interesting consequence of all this is that, in your situation, the thickness must be calculated with the thin wall formula, but the MAWP is to be calculated with the thick wall one.
 

prex
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RE: ASME VIII Div 1, UG-27 required thickness formula validity

I think the real plate thickness shall be used to decide whether thick wall fomula or thin wall formula is to be used for calculation. The stresses in the shell depend on the real thickness, not the theoritical required thickness.

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