This is the second in a series of articles on pressure vessels that we will publish on our website.
Our first article introduced the ASME code and Section VIII, Rules for Construction of Pressure Vessels, which is divided into three divisions. This article covers the main characteristics of Divisions 1 and 2.
Division 1 is the simplest. It focuses mainly on primary membrane and bending stresses, with limited exceptions for localized stresses in some equipment components.
Under paragraph U-1(d), Division 1 rules apply without changes to equipment with pressures up to 20 MPa (3,000 psi). For higher pressures, Division 2 is generally more suitable, while Division 3 is recommended above 68.95 MPa (10,000 psi).
Because of this simplified approach, Division 1 uses the highest safety factor when establishing allowable stresses for materials and welds. As a result, it generally leads to thicker equipment for the same design conditions.
Division 2 is more sophisticated and defines different allowables for general primary membrane stress (Pm), primary bending stress (Pb), localized primary membrane stress (PL), secondary stress (Q) and peak stress (F). Class 1 equipment is sized by formulas, while Class 2 equipment may also use stress analysis.
For Class 1 equipment, allowable stresses use a factor of 3.0 applied to material tensile strength at design temperature. Class 2 equipment uses a factor of 2.4. Welding allowables are generally higher than in Division 1, although execution and testing requirements are more rigorous.
In all cases, Division 2 allowable stresses are greater than or equal to those in Division 1. Equipment designed to Division 2 requirements is therefore generally lighter.