Maximum allowable stress

Division 2. With the advent of higher design pressures the ASME recognized the need for alternative rules permitting thinner walls with adequate safety factors. Division 2 provides for these alternative rules it is more restrictive in both materials and methods of analysis, but it makes use of higher allowable stresses than does Division 1. The maximum allowable stresses were increased from one-fourth to one-third of the ultimate tensile stress or two-thkds of the yield stress, whichever is least for materials at any temperature. Division 2 requkes an analysis of combined stress, stress concentration factors, fatigue stresses, and thermal stress. The same type of materials are covered as in Division 1. 

Subsection C This subsection contains requirements pertaining to classes of materials. Carbon and low-alloy steels are governed by Part UCS, nonferrous materials by Part UNF, high-alloy steels by Part UHA, and steels with tensile properties enhanced by heat treatment by Part UHT. Each of these parts includes tables of maximum allowable stress values for all code materials for a range of metal temperatures. These stress values include appropriate safety fac tors. Rules governing the apphcation, fabrication, and heat treatment of the vessels are included in each part. 

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. 

Maximum Allowable Stress Value for Common Steels... 

R = temperature, absolute, degrees Rankin r = rc = ratio of back pressure to upstream pressure, P2/Pi, or critical pressure ratio, Pc/Pi rj = relative humidity, percent S = maximum allowable stress in vessel wall, from ASME Code, psi., UCS-23.1-23.5 UHA-23, UHT-23... 

For design purposes it is necessary to decide a value for the maximum allowable stress (nominal design strength) that can be accepted in the material of construction. 

Pressure vessels are subjected to other loads in addition to pressure (see Section 13.4.7) and must be designed to withstand the worst combination of loading without failure It is not practical to give an explicit relationship for the vessel thickness to resist combined loads. A trial thickness must be assumed (based on that calculated for pressure alone) and the resultant stress from all loads determined to ensure that the maximum allowable stress intensity is not exceeded at any point. 

Lifting lugs attached to the equipment shall be designed using a maximum allowable stress of one-third of the specified minimum yield strength of the material. 

Maximum allowable stress depends sharply on temperature. 

Maximum Allowable Stresses per ASME Code Section VIII, Division 1, (1980) Para UCS-23, for -20 to +650 F except SA-537 A-36 Modified is made to fine grain practice with manganese in range 0.80 to 1.20 percent by ladle analysis. 

Estimate the safe working pressure for a 4 in. (100 mm) dia., schedule 40 pipe, SA53 carbon steel, butt-welded, working temperature 100°C. The maximum allowable stress for butt-welded steel pipe up to 120 °C is 11,700lb/in (79.6N/mm ). 

Note Tensile stress and proof stress are not the same as the maximum allowable stress permitted by design code. See Tables 7.5 and 7.7 for maximum allowable stress values. 

Proof stress is the stress to cause a specified permanent extension, usually 0.1%. The maximum allowable stress specified by the ASME Boiler and Pressure Vessel (BPV) Code is calculated from these and other material properties at the design temperature, and allows for suitable safety factors. The basis for establishing maximum allowable stress values is discussed in Chapter 13 and is described in detail in the ASME BPV Code Section 11 Part D, Mandatory Appendix 1. 

The quantity of a material used will depend on the material density and strength (maximum allowable stress) and these must be taken into account when comparing... 

Note The maximum allowable stress values are at 40°C (100°F) and are taken from ASME BPV Code Sec. II Part D. The code should be consulted for values at other temperatures. Several other grades exist for most of the materials listed. 

Cost ratings, relative to the rating for mild steel (low carbon), are shown in Table 7.5 for mid-2006 prices. Materials with a relatively high maximum allowable stress, such as stainless and low alloy steels, can be used more efficiently than carbon steel. Note that the simplified formula given in equation 7.2 does not take into account different corrosion allowances for the different materials. 

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