Design stress

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. 

At high temperature, the behavior is different. A stmcture designed according to the principles employed for room temperature service continues to deform with time after load apphcation, even though the design data may have been based on tension tests at the temperature of interest. This deformation with time is called creep because the design stresses at which it was first recognized occurred at a relatively low rate. 

Such programs generally concentrate on the technical parts of designing an experiment, and provide limited guidance on the important, softer aspects of experimental design stressed in this article. Also, most computer routines do not allow one to handle various advanced concepts that arise frequently in practice, eg, spHt plot and nested situations, discussed in the books in the bibhography. In fact, some of the most successful experiments do not involve standard canned plans, but are tailored to fit the problem at hand. 

Sa metals, excluding factor E, or bolt design stress Allowable stress range for MPa Idp/im (ksi)... 

ASTM and the Plastics Pipe Institute, a division of the Society of the Plastics Industry, have established identifications for plastic pipe in which the first group of letters identifies the plastic, the two following numbers identify the grade of that plastic, and the last two numbers represent the design stress in the nearest lower (0.7-MPa (lOO-lbfiin ) unit at 23°C (73.4°F). 

This is a product specification. No design stresses are necessary. Limitations on metal temperature for materials covered by tliis specification are ... 

The SE values in Table 10-49 are equal to the basic allowable stresses in tension S multiplied by a quality factor E (see subsection Pressure Design of Metallic Components Wall Tliick-ness"). The design stress values for bolting materials are equal to die basic allowable stresses S. The stress values in shear shall be 0.80 times the allowable stresses in tension derived from tabulated values in Table 10-49 adjusted when applicable in accordance widi Note 13. 8tress values in bearing shall be twice those in shear. 

The maximum operating temperature is arbitrarily set at 260 C (500 F) because harder temper adversely affects design stress in the creep-rnptnre-temperatnre ranges. 

Pressure Vessel Code as it existed in the past (and will continue). Division 2 was brought out as a means of permitting higher design stresses while ensuring at least as great a degree of safety as in Division I. These two divisions plus Secs. HI and X will be discussed briefly here. They refer to Secs. II and IX. 

Part AM This part lists permitted individual constnic tion materials, apphcable specifications, special requirements, design stress-intensity vafues, and other property information. Of particular importance are the ultrasonic-test and tou ness requirements. Among the properties for which data are included are thermal conduc tivity and diffusivity, coefficient of theiTnal expansion, modulus of elasticity, and yield strength. The design stress-intensity values include a safety factor of 3 on ultimate strength at temperature or 1.5 on yield strength at temperature. 

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. 

Times-to-failure are normally presented as creep-rupture diagrams (Fig. 17.9). Their application is obvious if you know the stress and temperature you can read off the life if you wish to design for a certain life at a certain temperature, you can read off the design stress. 

Pressure Vessels. Refineries have many pressure vessels, e.g., hydrocracker reactors, cokers, and catalytic cracking regenerators, that operate within the creep range, i.e., above 650°F. However, the phenomenon of creep does not become an important factor until temperatures are over 800°F. Below this temperature, the design stresses are usually based on the short-time, elevated temperature, tensile test. 

The recommended maximum design stress in tension is one-quarter the ultimate tensile strength (for cast irons a value up to 185 N/mm (12 tonf/ in. )). The fatigue strength is one-half the tensile strength. Notched... 

The reeommended maximum design stresses for a life of 5 to 10 years based on long-time ereep tests are given in Table 3.24. 

Because borosilicate is a brittle material, its design stress is restricted to less than 7 N/mm. Borosilicate glass is attacked by hydrofluoric acid even when a solution contains only a few parts per million of fluoride ions, and at... 

The physical and mechanical properties of the principal thermoplastics of interest for process plant applications are listed in Table 3.42. Table 3.43 gives typical hydrostatic design stresses for different types of thermoplastic pipe. Plastics widely employed in piping systems are described briefly below. 

Example 3.17 Short carbon fibres with a diameter of 10 fim are to be used to reinforce nylon 66. If the design stress for the composite is 300 MN/m and the following data is available on the fibres and nylon, calculate the load transfer length for the fibres and also the critical fibre length. The volume fraction of the fibres is to be 0.3. 

The design stress is limited in praetiee by the material of eonstruetion. For a given material of eonstruetion, the separating effeet at the maximum stress ean be derived... 

The factors Uiat enter into tlie design of vessels include type of material, configuration, inetliod of construction, design stresses, and tliickness of Uie metal. As with any equipment, design pressures and temperatures should take into consideration Uie most severe combination of conditions anticipated. Vessels must be completely drainable. Liners and wear plates may be required to prevent corrosion. 

Ref. [27] presents a thorough discussion of limits to structural components strengths, and these should be observed. Ductile design practices should be used. The maximum allowable design stress should not exceed 25% of the ultimate strength. The strength of the enclosure should exceed the vent relief pressure by at least 0.35 psi. 

Curing studies-pot life Engineering data for design Stress relaxation at low loads under different exposures... 

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