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The basic allowable stresses are also called code table stresses because they are tabulated in the code book. They are used directly in the calculation of pipe wall thickness subject to design pressure.

We call them basic allowable stresses because they are the design stresses for the most important but basic sustained loads. For other loadings, the design stresses are modified from these basic allowable stresses by applying factors and/or combinations. These allowable stress tables are the backbones of the code. They are established based on the following criteria.

4.3.1  Bases for Establishing Allowable Stresses

Allowable stresses for piping materials are established differently for different groups of materials, which are generally categorized as: (1) bolting materials, (2) cast iron, (3) malleable iron, and (4) other materials. Group 4, which covers most of the common piping materials, will be discussed as a com-parison of different codes. The basic allowable stress values at a given temperature for materials other than bolting materials, cast iron, and malleable iron are taken as the lowest of the following:

(1) The lower of one-third of ultimate strength at room temperature and one-third of ultimate strength at temperature. B31.1 and Class 2 nuclear piping use a 1/3.5 factor instead of 1/3. EN-13480 uses a 1/2.4 factor instead of 1/3.

(2) Except as provided in item (3) below, the lower of 2/3 of yield strength at room temperature and 2/3 of yield strength at temperature.

(3) For austenitic stainless steels and nickel alloys having similar roundhouse stress-strain behavior, the lower of 2/3 of yield strength at room temperature and 90% of yield strength at temperature.

This 90% yield allowable is not recommended for flanged joints and other components in which slight deformation can cause leakage or malfunction.

(4) 100% of the average stress for a creep rate of 0.01% per 1000 hours.

(5) 67% (2/3) of the average stress for rupture at the end of 100,000 hours.

(6) 80% of the minimum stress for rupture at the end of 100,000 hours.

(7) For structural grade materials, the basic allowable stress shall be 0.92 times the lowest value determined from items (1) through (6).

The allowable stresses for bolting materials are generally smaller. This is partially because bolt stresses are somewhat more unpredictable. It is also a well-known fact that bolts are often stressed above the design value in the field to ensure a proper seating of the gasket.

The above bases for allowable stress provide safeguards against (1) gross deformation and exces-sive strain follow-up, (2) rupture, and (3) creep. To safeguard against gross deformation, the stress is limited to 2/3 of the yield strength, and to guard against rupture, the stress is limited to 1/3.5 to 1/3 of the ultimate strength. The margin against rupture is generally referred to as the rupture safety fac-tor. In other words, the allowable stress is set with a rupture safety factor of 3 or 3.5 depending on the code used.

The safety factor against creep for systems operating in the high temperature domain is not easy to put a number on. However, because a system normally experiences the full design temperature in only a small portion of its operating life, the 100,000-hour benchmark is generally considered conservative, although it is much lower than the expected plant life. The time to rupture at a given stress level is very sensitive to the actual operating temperature. A 20°F temperature decrease at a design temperature of 1000°F will more than double the 100,000 hours of time to rupture. Creep rupture is time-dependent. By combining the low creep allowable stress together with the expected corrosion allowance throughout the operating life, an unusually thick pipe wall may be initially re-quired if the pipe is designed for full operating life. Some high temperature systems, therefore, are purposely designed to have them replaced, according to schedule, once or twice throughout the life of the plant.

Allowable stresses for B31.3 are generally higher than those for other codes. This is due to the unique industries it serves as discussed in Chapter 1. However, in high temperature ranges where creep damage governs, allowable stress becomes the same for all codes. It should also be noted that the allowable stress for Class 1 nuclear piping is set higher than the allowable stress for B31.1 power piping and Class 2 and Class 3 nuclear piping. This is mainly due to the additional design calcula-tions, quality controls, and quality assurances required by Class 1 piping. The combined effort makes the real safety factor in Class 1 piping higher than that of Class 2 and Class 3 piping, even though the basic allowable stress value is higher in Class 1 piping. The extra calculations and quality assurances of Class 1 piping eliminate many of the uncertainties that might compromise the quality of the piping system.

4.3.2  Code Allowable Stress Tables

The basic allowable stresses for the approved material are tabulated in the code book as appendices.

These tables list the approved materials together with the established allowable stresses at selected temperature marks. The materials not listed are unapproved, and shall be used only through special qualifications. The materials are applicable only in the temperature ranges that have allowable stresses listed subject to limitations given by the accompanied notes. Besides the allowable stresses, the table also contains related information as follows:

(1) Spec. No.: These are the American Society for Testing and Materials (ASTM) [7] specification numbers. The ferrous materials are prefixed with the letter “A,” and the non-ferrous materials are prefixed with the letter “B.” The ASTM specification comprises a set of rules for manu-facturing and testing a group of pipe materials of similar characteristics. Each specification number covers numerous different materials. For instance, the most common specification, A-106, configured for “seamless carbon steel pipe for high-temperature service” has several grades. Therefore, in addition to Spec. No., we also need other classifications to identify a given material.

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(2) Grade: Most specification numbers have more than one grade. Generally, the grade is associ-ated with material compositions and strength. The plate material used in welded pipe may also be classified as grade in some codes.

(3) Type or Class: Types and classes are associated with different manufacturing methods and the scope of inspections required. For instance, Type S means seamless, Type E means electric resistance welded, and Type F means furnace butt-welded. In B31.3, the Type or Class is not listed in the main table. The type and class together with joint efficiency are listed in a separate table in B31.3.

(4) Material Composition: This shows the alloy type and compositions of relevant materials.

(5) P-Number: This the welding qualification number to which the material belongs. This number corresponds to the one given in ASME B&PV Code Section IX [8].

(6) Notes: The table contains many special notes. The applicable notes are listed in this column.

These notes generally are related to the limitations and extra attentions required for the given material.

(7) Specified Minimum Tensile Strength (8) Specified Minimum Yield Strength

(9) Joint Efficiency or Quality Factor: For welded pipe, this is the longitudinal or spiral weld joint efficiency. For castings, this is the casting quality factor. These factors are to be applied to the allowable stress for calculating the wall thickness. They are not used in the evaluation of the piping flexibility. In B31.3, this column, together with type and class, is listed in a separate table.

(10) Minimum Temperature: This the design minimum temperature for which the material is nor-mally suitable without impact testing beyond that required by the material specification. This column is not available in B31.1, which generally considers -20°F (-29°C) the minimum ap-plicable temperature. (See also 1.3.1 (b) Brittle Rupture.)

(11) Maximum Allowable Stress in Tension: The stress values are given for the benchmark temper-ature points generally spaced by every 100°F at low tempertemper-ature ranges and spaced by every 50°F at high temperature ranges. The stress at a temperature in between the benchmark points can be linearly interpolated. The material should not be used for temperatures outside the two extreme temperatures within which the allowable stresses are given.

In most code tables, the given allowable stresses include also the corresponding longitudinal joint efficiency, E, and casting quality factor, F. The values are generally referred to as SE values. However, the values given in B31.3 allowable stress tables do not include the joint efficiency and quality factor.

For the pressure design of B31.3 components, the allowable stress value given in the code table has to be multiplied with the applicable joint efficiency or quality factor. B31.3 gives the joint efficiencies and quality factors in a separate table.

To identify a material for design and analysis, we only need the specification number, grade, and type or class, if applicable.

4.3.3  Weld Strength Reduction Factor

The material at weld-affected zone is weaker than the unaffected zone. This weakness at weld is already taken care of by applying a joint efficient on the allowable stress, and also by applying a stress intensification factor or stress index on the calculated stress.

At high temperature ranges, the weld weakens further, thus accelerating creep failure. This deterio-ration against creep is attributed to weld residual stress, weld material discontinuity stress, and weld shape discontinuity stress. The effect of these minor stresses is not significant when the pipe fails at a stress higher than the yield strength. However, when the pipe fails at a stress much lower than the yield strength, these minor stresses have a very significant effect such as in the case of high cycle fatigue discussed in Section 1.3.2. At creep range, the pipe also fails at a stress much lower than the yield

strength. Therefore, the contribution of these minor weld stresses is expected to be significant too.

The fraction of the weld strength reduction, in comparison with base pipe material, at creep range is called weld strength reduction factor, or simply, W factor.

The weld strength reduction factor is a very complex function of temperature, pipe base material, weld material, welding process, smoothness of the weld, and heat treatment of the weld. The code provides certain W factors for some materials. The listed factors should be used if available. As a gen-eral idea, the W factor for austenitic stainless steels and creep strength enhanced ferritic (CSEF) steels with normalizing plus tempering (N + T ) post-weld heat treatment (PWHT) is 1.0 for 950°F (510°C) and below, and is 0.5 at 1500°F (815°C). The factor can be linearly interpolated for temperatures between 950°F and 1500°F.

The application of W factor depends on weld location and load category. The longitudinal weld and spiral weld, which are used in the production of the pipe, work only against the hoop stress in pressure design of the pipe and its components. Circumferential welds, which are used to assemble the piping, work only against longitudinal stress. Therefore, the W factor for circumferential weld is used in reducing the allowable stress for the sustained longitudinal stress. However, the W factor does not apply to occasional loads due to the load’s short duration, which has little effect on creep failure.

The W factor is also not applicable to expansion and displacement stresses due to their non-sustained natures, which have little effect on creep failure.

Application of the circumferential W factor would require the identification of all weld locations in the design analysis. This is simply impractical for non-nuclear industries with short project schedules.

Therefore, the same approach used in setting the evaluation procedure for thermal expansion stress might be used here. In the development of thermal expansion stress evaluation procedure [9], we have assumed that the piping has a circumferential weld everywhere. With this assumption, we simply ig-nored the existence of circumferential weld and adjust the stress intensification factor and allowable stress accordingly. For the W factor, we can also assume that the circumferential weld exists at every point. In the case of W factor, we do not even have to adjust anything. The factor is used “as is” for all points.