Allowable Stresses, ASME B31.1 vs B31.3

Type of Stresses in a Piping System

In the article Types of Loads and Stresses in a Piping System, the types of loads and the resulting individual and compound stresses was discussed. ASME codes classify the compound stresses as per the load types that generate them, as shown in Figure 1.

Types of Loads and compound Stresses in a Piping System
Figure 1, Types of Loads and compound Stresses in a Piping System

In this article we will have a closer look at ASME B31.1 and ASME B31.3, comparing how these two codes calculate these stresses and their allowable limits. We will have a look to some examples of these calculations by CAESAR II In another article.

Allowable Stresses Table

The values of metals allowable stresses at cold temperature, Sc, and hot temperature, Sh, have been listed in appendix A of both ASME B31.1 and B31.3. Below the definition of Sc, Sh and some other related stresses have been provided form the codes nomenclature. The definition use different wording but the concept is the same. However the values for Sc and Sh are not the same for both codes. Figures 2 and 3 show these values for A53 Grade B from both codes as an example


ASME B31.1-2018, Appendix G

S   Basic material allowable stress
Sc   Basic material allowable stress at minimum (cold) temperature
Sh   Basic material allowable stress at maximum (hot) temperature
SA   Allowable displacement stress range
SE   Computed displacement stress range
SL   Stress due to sustained loads


ASME B31.3-2020, Appendix J

S   Allowable stress for metals
Sc   Basic allowable stress at minimum metal temperature expected during the displacement cycle under analysis
Sh   Basic allowable stress at maximum metal temperature expected during the displacement cycle under analysis
SA   Allowable stress range for expansion stress
SE   Computed thermal expansion stress range
SL   Longitudinal stress due to pressure, weight, and other sustained loads

A53 Grade B Allowable Stresse, ASME B31.1-2018, Part-1
A53 Grade B Allowable Stresse, ASME B31.1-2018, Part-2
Figure 2, A53 Grade B Allowable Stresse, ASME B31.1-2018
A53 Grade B Allowable Stresse, ASME B31.3-2020, Part-1
A53 Grade B Allowable Stresse, ASME B31.3-2020, Part-2
Figure 3, A53 Grade B Allowable Stresse, ASME B31.3-2020

Code Stresses and the Limits

Both ASME B31.1 and B31.3 discuss three types of stresses and use the same designation for them. The sections that these stresses are discussed and their limits are as follows:

ASME B31.1

Discussed in sections 102.3 and 104.8 with the following limits:


ASME B31.3

Discussed in sections 302.3, 319.4.4 and 320.2 with the following limits:


Tables 1 to 3 compare related sections from both codes with the calculating equations and the limits. These tables present the main equations only. For more details see these sections in the code text.


Table 1 comparing Sustained Stresses-ASME B31.1 vs B31.3
Table 1, Sustained Stresses-ASME B31.1 vs B31.3
Table 2 comparing Displacement Stresses-ASME B31.1 vs B31.3
Table 2, Displacement Stresses-ASME B31.1 vs B31.3
Table 3 Comparing Occasional Stresses-ASME B31.1 vs B31.3
Table 3, Occasional Stresses-ASME B31.1 vs B31.3

Related Articles

Types of Loads and Stresses in a Piping System

References

[1] ASME B31.1-2018, Power Piping
[2] ASME B31.3-2020, Process Piping


Disclaimer: The contents of this page are for educational purposes only and are not intended to provide practical advice.