Here is an old chestnut that has been asked about again and may be of general interest. Which condition is more severe for a horizontal vessel on saddles, the new or the corroded condition?
 
Certainly, in the "new" (uncorroded) condition there is more mass due to the vessel material than is present in the corroded condition. On the other hand, for a "full" vessel there will be slightly more liquid volume (and mass) in the corroded condition than in the new condition. So on the one hand there is more mass of metal in the "new" condition, but on the other hand there may be more mass of liquid in the "corroded" condition, these might counteract each other.
 
But for the sake of discussion let us assume that the total weight of the vessel and contents is greater for the new condition than for the corroded condition. What effect does this have? Certainly the weight and seismic loads acting on the vessel will be greater in the new condition; wind loads will not change (discounting any external corrosion on the shell).
 
However, consider that stress in the shell is a function of the shell thickness. The available shell thickness is less in the corroded condition than in the new condition. The result is that the slightly higher loads in the new condition applied to the corresponding greater thickness result in lower stresses than do the slightly lower loads and lesser thickness in the corroded condition.
 
It would be difficult to derive a mathematical proof that the corroded condition always results with greatest stress. But the condition is simple to illustrate in general, and to verify explicitly for any particular case:   Create the vessel model including loads and corrosion allowance. Perform calculations for the Zick analysis. Repeat the procedure with corrosion allowance set to "0". Compare the calculated Zick stresses for the corroded and uncorroded cases; invariably the corroded condition will result with higher stresses in the shell.
 
In some situations, depending upon the liquid level and density, the saddle splitting stress may be higher for the new condition than the corroded condition. However, COMPRESS checks the splitting stress for the test condition as well, which will govern for all cases unless the vessel has a very high operating liquid level. In any case, the calculated splitting stresses are general very small in proportion to the allowable stress.
 
 

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