Allowable Stress Values

Many companies require that all their pressure essels lie 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 1 for low -prcssure 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. 

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In shaded areas, allowable-stress values wliich are printed in italics exceed two- tliirds of the expected yield strength at temperature. All odier allowable-stress values in shaded areas are equal to 90 percent of expected yield strength at temperature. See ANSI B31.3. 

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. 

Maximum Allowable Stress Value for Common Steels... 

The bases for establishing design stress values for bolting materials and allowable stress values for other metallic materials in this Code are as follows ... 

IP-2.2.7(d). The strength reduction factor represents the reduction in yield strength with long-term exposure of the material to elevated temperatures and, in the absence of more applicable data, shall be taken as 1.0 for austenitic stainless steel and 0.8 for other materials. For castings, the basic allowable stress shall be multiplied by the casting quality factor, Ec. Where the allowable stress value exceeds two-thirds of yield strength at temperature, the allowable stress value must be reduced as specified in para. IP-2.2.7(c). Wind and earthquake forces need not be considered as acting concurrently. At temperatures warmer than 427°C (800°F), use 1.33... 

If the allowable stress for the branch pipe wall is less than that for the run pipe, its calculated area must be reduced in the ratio of allowable stress values of the branch to the run in determining its contributions to area A3. 

No additional credit may be taken for a material having higher allowable stress value than the rim pipe. 

If material is welded, brazed, or soldered, the allowable stress values for the annealed condition shall be used. 

If no welding is employed in the fabrication of piping from these materials, the allowable stress values may be increased to 230 MPa (33,3 Idp/in ),... 

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. 

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. 

Maximum allowable stress values from ASME BPV Code Sec. 11 Part D. 

In addition to the BPV Code Sec. VIII, the process design engineer will frequently need to consult Section II Part D, which lists maximum allowable stress values under Sec. VIII D.l and D.2, as well as other materials properties. A comprehensive review of the ASME code is given by Chuse and Carson (1992) and Yokell (1986) see also Perry and Green (1997). 

The basis for establishing the maximum allowable stress values in the ASME BPV Code is given in ASME BPV Code Sec. II Part D, Mandatory Appendix 1. At temperatures where creep and stress rupture strength do not govern the selection of stresses, the maximum allowable stress is the lowest of... 

The maximum allowable stress values for ASME BPV Code Sec. VIII D. 1 are given in ASME BPV Code Sec. II Part D Table lA for ferrous metals and Table IB for nonferrous metals. Maximum allowable stress values for Sec. VIII 0.2 are given in Sec. II Part D Table 2A for ferrous metals and Table 2B for nonferrous metals. Different values are given for plates, tubes, castings, forgings, bars, pipes, and small sections, as well as for different grades of each metal. 

Typical maximum allowable stress values for some common materials are shown in Table 13.2. These values may be used for preliminary designs. The ASME BPV Code should be consulted for the values to be used for detailed vessel design. 

The vessel wall thickness must be sufficient to ensure the maximum stress intensity does not exceed the maximum allowable stress (nominal design strength) for the material of construction, at any point. The AS ME BPV Code Sec. II Part D should be consulted for the maximum allowable stress values in tension or in compression. 

UG-23 Maximum Allowable Stress Values UG-27 Thickness of Shells Under Internal Pressure UG-32 Formed Heads, Pressure on Concave Side UG-34 Unstayed Flat Heads and Covers... 

C = allowance for threading and grooving D = outside diameter of pipe P = internal pressure in psi S = maximum allowable stress value in psi T = wall thickness... 

There are other designations that should be considered for other types of service such as high pressure/high heat etc. These designations are encountered in tables used for calculating pressure allowable based on maximum allowable stress values (S-values). 

Table 1 Allowable stress values for selected materials...

The maximum allowed stress value in the tubes is strongly influenced by the maximum tube wall temperature and by the maximum heat flux. Even a slight increase in the maximum tube wall temperature may result in a serious decline of the expected tube life. 

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