Tip of the Month September 2006

Most piping codes incorporated into piping stress analysis programs stipulate that piping systems (new or existing) meet certain minimum engineering requirements thought necessary for safe design. Users are well-advised to follow their own more rigorous analysis (in lieu of such code requirements) if the validity of such an approach can be shown. Either way, the code does not limit conservatism, nor on the other hand, does it do away with the need for competent engineering judgment.

With regard to stresses, there are three main types of stresses that a designer must check for:

Primary Stresses resulting from primary loads (Sustained)
1. from external loading
2. not self-limiting and cause distortion
3. limit these loads to prevent plastic deformation
Examples of loads: internal pressure, weight.
Sustained stresses must be less than sustained allowable stresses.

Secondary Stresses resulting from secondary loads (Expansion):
1. by displacement constraints
2. self-limiting and cause no distortion
3. limit these loads to prevent fatigue failure
Examples of loads: thermal load, transient, support movement, vibration.
Expansion stresses must be less than the thermal allowable stress range.

Peak Stresses resulting from occasional loads (Occasional):
1. cause the highest stresses but cause no distortion
2. limit these loads to prevent fatigue crack initiation
Examples of loads: wind, earthquake, impact, fluid hammers.
Occasional stresses must be less than the occasional allowable stresses.

These allowable stresses are all computed based on material allowable stresses that are input into your piping models either from a material library on hand-input by the user. Not all codes refer to these stresses by the names mentioned above. ANSI B31 terminology is used above. Please refer to either Appendix A of CAEPIPE User's Manual or a copy of the piping code for more details.