Design strength , pressure stress

Internal or External Fluid-pressure Stresses. A safe design can be made by use of ASA B 31.1 Code as explained in the section on Pipe Strength and Wall Thickness [see Eqs. (9-1) and (9-2), p. 344]. 

Step 5 Since the seismic design for pressure vessels is based on allowable stress rather than ultimate strength, the base shear may be reduced by a factor of 1.4. 

Safety Factor The ratio of design burst pressure over the maximum allowable working pressure (MAWP) or design pressure it can also be expressed as the ratio of tensile or yield strength over the maximum allowable stress of the material. 

Two basic modes of failure are assumed for the design of pressure vessels. These are (a) elastic failure, governed by the theory of elasticity and (b) plastic failure, governed by the theory of plasticity. Except for thick-walled pressure vessels, elastic failure is assumed. When the material is stretched beyond the elastic limit, excessive plastic deformation or rupture is expected. The relevant material properties are the yield strength and ultimate strength. In real vessels we have a multiaxial stress situation, where the failure is not governed by the individual components of stress but by some combination of all stress components. 

Division 1. Below the creep range, design stresses are based on one-fourth of the tensile strength or two-thkds of the yield, or 0.2% proof stress. Design procedures are given for typical vessel components under both internal pressure and external pressure. No specific requkements are given for the assessment of fatigue and thermal stresses. 

For EPSR design, the stress level to contain an explosion is set at the yield strength, a design factor of 1. Thus, for an alloy, the design stress level would be about 1.5 times the ASME code design stress. So a pressure vessel rated at 6 bar for the ASME code (EPR) would have an EPSR rating of 9 bar. 

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