The maximum allowable stress values to be used in the calculation of the vessel’s wall thickness are given in the ASME Code for many different materials. These stress values are a function of temperature. Section VIII of the ASME Code, which governs the design and construction of all pressure vessels with operating pressures greater than 15 psig, is published in two divisions. Each sets its own maximum allowable stress values. Division 1, governing the design by Rules, is less stringent from the standpoint of certain design details and inspection procedures, and thus incorporates a higher safety factor of 4. For example, if a 60,000 psi tensile strength material is used, the maximum allowable stress value is 15,000 psi. On the other hand, Division 2 governs the design by Analysis and incorporates a lower safety factor of 3. Thus, the maximum allowable stress value for a 60,000 psi tensile strength material will become 20,000 psi.
Many companies require that all their pressure vessels be constructed In accordance with Division 2 because of the more exacting standards. Others find that they can purchase less expensive vessels by allowing manufacturers the choice of either Division 1 or Division 2. Normally, manufacturers will choose Division I for low-pressure vessels and Division 2 for high-pressure vessels.
The maximum allowable stress values at normal temperature range for the steel plates most commonly used in the fabrication of pressure vessels are given in Table 12-3. For stress values at higher temperatures and for other materials, the latest edition of the ASME Code should be referenced.