The paradox of pressure vessel wall thickness calculation

Abstract

The calculation of pressure vessel wall thickness is made with formulae usually imposed by national codes. Although the Lamés formulae give actual stresses and leave the decision on the safety factor to the designer, these codes impose certain safety factors in the calculation of allowable stresses, or even include the safety factor in the wall thickness calculation formula. If we take as an example a pressure vessel made of 316 SS with 125 mm ID, and designed for 400 bar, the methods of calculation which are most commonly used are: • -Lamé • -Stoomwezen (Dutch Code), • -AD Merkblatt (German Code), • -CODAP (French Code), • -ASME Code Section VIII, Division 1 or 2 (USA) . Obviously, the thicker the wall, the lower the stresses due to the pressure. However, some other factors need to be considered, the most important of them probably being thermal stresses. The thicker the wall, the higher the thermal stresses, all other parameters being equal. When such a vessel is heated from the outside, a temperature gradient is created across the wall and the temperature is higher on the outside than the inside. Consequently, the thermal stresses are in compression on the outside, and in tension on the inside, and come in addition to the pressure stresses. The designer must choose the wall thickness so as to minimize the total of pressure and thermal stresses.