The Code provides allowable stresses for metallic piping covered by the base Code and Chapter VHI (Category M) based on criteria listed in para. 302.3. These are, with certain exceptions, the lowest of the following:
One-third of the specified minimum tensile strength (which is at room temperature)
One-third of the tensile strength at temperature (times 1.1)
Two-thirds of the specified minimum yield strength (which is at room temperature)
Two-thirds of the "minimum" yield strength at temperature
Average stress for a minimum creep rate of 0.01%/1000 hours
Two-thirds of the average stress for creep rupture in 100,000 hours
80% of the minimum stress for a creep rupture in 100,000 hours
Specified values are the minimum required in the material specifications. For unlisted material, the "minimum" at temperature is required in para. 302.3.2(f) to be determined by multiplying the specified (room temperature) values by the ratio of the average strength at temperature to that at room temperature. The allowable stresses listed in the Code are determined by the ASME Boiler and Pressure Vessel Code Sub-committee II, and are based on trend curves that show the effect of temperature on yield and tensile strengths. (The trend curve provides the aforementioned ratio.) An additional factor of 1.1 is used with the tensile strength at temperature.
These paragraphs were updated in the 2000 Addendum to provide wording that is more consistent with Section I of the BPVC. However, there was no change in the allowable stress basis, even though the factor of 1.1 for tensile strength at temperature was added to the wording [in para. 302.3.2(d)(8)], whereas it had not previously been mentioned.
An exception to the above criteria is made for austenitic stainless steel and nickel alloys with similar stress-strain behavior, for which one can use as high as 90% of the yield strength at temperature. This is due not to a desire to be less conservative, but to a recognition of the differences in behavior of these alloys. The quoted yield strength is determined by drawing a line parallel to the elastic loading curve, but with a 0.2% offset in strain. The yield strength is the intercept of this line with the stress-strain curve. Such an evaluation provides a good yield strength value of carbon steel and alloys with similar behavior, but it does not represent the strength of austenitic stainless steel, which has considerable hardening and additional strength beyond this strain value. However, the additional strength is achieved at the cost of additional deformation. Thus, the higher allowable stresses relative to yield are only applicable to components that are not deformation-sensitive. Thus, although one might use the higher allowable stress for pipe, it should not be used for flange design. The allowable stress tables use a style that lets the Code user know when the allowable stress exceeds two-thirds of the yield strength at temperature, as will be described in Section 11.8.
Use of the higher allowable stress can create some complications, because the Code has limitations in the form of stress in the piping relative to yield strength. For example, variations above what would otherwise be considered to be the maximum allowable working pressure of the piping are permitted, to as high as 33% above the allowable stress. However, the Code requires that the stress not exceed yield stress. Thus, if the pipe was designed using 90% of yield stress, the allowable variation would be much less than 33%. Similar circumstances occur for occasional loads (see para. 302.3.6; stress is limited to yield).
The allowable stress for bolting is similar, except that it is based on one-fourth of the tensile strength rather than one-third, and special consideration is given to bolting materials for which the strength has been enhanced by heat treatment or strain hardening. They are limited to one-fifth of the specified minimum tensile strength and one-fourth of the specified minimum yield strength, unless these are lower than the values of allowable stress for the annealed material, in which case the annealed material values of allowable stress are used. Note that "these" in the preceeding sentence, and Code wording, refers to the value of allowable stress calculated using the relevant factors.
For cast and ductile iron materials, the behavior is brittle and the allowable stress differs accordingly. For cast iron, the basic allowable stress is the lower of one-tenth of the specified minimum tensile strength (at room temperature) and one-tenth of the "minimum" strength at temperature, also based on the trend of average material strength with temperature. For ductile iron, a factor of one-fifth is used rather than a factor of one-tenth. For malleable iron the same criteria are used as for other metallic materials (e.g., carbon steel). #Little_PEng
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