In our first article we present the ASME code and its section VIII called “Rules for Construction of Pressure Vessels”. We also comment that this section is composed of 3 divisions. In today's article we'll cover the main features of Divisions 1 and 2 of the code.
Division 1 is the simplest and is mainly concerned with the determination of primary membrane and bending stresses acting on equipment, with minor exceptions presented in paragraph UG-23(e) where local stresses (usually secondary or peak) acting on some parts of the equipment.
As per paragraph U-1(d), Division 1 rules apply without change for equipment with pressures up to 20 MPa (3,000 psi). For higher pressures, special care must be taken when using Division 1 and it is more appropriate to use Division 2, although there are no recommendations for this in the code. For pressures above 68.95 MPa (10,000 psi), as per paragraph 1.2.2.2 of Division 2, it is recommended to use Division 3.
Due to this approach, Division 1 is the one that uses the highest safety coefficient to obtain the allowable material and welds, that is, it is the one that presents the lowest allowable values for the materials used in the manufacture and the lowest values for the allowable welding (according to the UW-12 table of the code, this value can be up to 0.45) and, consequently, that which implies thicker equipment for the same design condition.
For this division, tables 1A and 1B of Section II, Part D of the code apply. Basically, up to a certain specific temperature for each material, the admissible adopted is the lowest value between 2/3 of the yield strength and 1/3.5 of the rupture strength of this material, at the equipment design temperature.
Division 2 is more sophisticated in engineering terms and determines different allowables for different types of stresses, classified as: general primary membrane stresses (Pm), primary bending stresses (PB), localized membrane primary voltages (PL), secondary voltages (Q) and peak voltages (F). In this division, there are two ways to perform the sizing of equipment: one through formulas, when the equipment is classified as Class 1, and the other, through formulas or through stress analysis, when the equipment is classified as Class 2.
For Class 1 equipment, tables 2A and 2B of Section II, Part D are used to obtain the allowable stresses, which use a factor of 3.0 dividing the material's rupture stress at the design temperature. For this Class it is not possible to use the materials in the creep region. As for Class 2 equipment, tables 5A and 5B of Section II, Part D are used, which use a factor of 2.4 dividing the material's tensile strength.
Furthermore, the allowables for welding are lower than in the case of Division 1; they generally range between 0.85 and 1.0. However, the execution level and the tests to be applied for the welds are more rigorous.
In any case, the allowable stresses for Division 2 are greater than or equal (if the allowable is a function of the material yield stress) to those defined for Division 1, that is, in general, equipment built in accordance with the requirements of Division 2 are generally lighter than those built to Division 1 requirements.
