Hello Mucour

Q. My question is does CEASER-II uses calculated Principal stresses, based on the Tresca theory, in determining the B31.3 code limit for longitudinal stresses due to sustained load?

A. Caesar II follows the ASME B31 Codes for Pressure Piping and it includes the specific equations that these Codes provide for calculating stresses. That being the case, the better question is “Do the ASME B31 Codes use the Tresca failure criteria for combining calculated principal stresses?”. And the answer to that is yes.

Note that the circumferential stress (also known as “hoop stress”) due to internal (to the pipe) pressure, is addressed in accordance with the B31 Codes by calculating the required minimum allowable pipe wall thickness (e.g., B31.1, Paragraph 104). This circumferential stress is not required by the Codes to be calculated. The Codes state that the criterion for limits on internal pressure stress are satisfied (as far as circumferential stresses are concerned) when the wall thickness of the piping component, including any reinforcement, meets the minimum wall thickness requirements. Caesar II checks the user specified pipe wall thickness in accordance with the Code minimum wall thickness requirement and if the specified wall thickness is inadequate, a warning is printed.

Q. Since Principal stresses is a combination of the longitudinal stress, hoop stress and torsional shear stress and this (S1-S3) must not exceed the material yield based on the Tresca theory. Is it that B31.3 is not interested in the principal stress.

A. In May of 2005, a Code Case (B31 Case 178 – “Providing an Equation for Longitudinal Stress for Sustained Loads in ASME B31.3 Construction”) was issued that addresses the B31.3 evaluation of “sustained longitudinal stresses” (i.e., principal stresses). Obviously then, ASME B31.3 “is interested in” (i.e., adequately addresses) principal stresses.

The ASME B31 Codes do not use 100 percent of the material yield stress for a comparison (limit) for any calculated stress. The sustained (principal) stresses due to weight and internal pressure are limited to Sh at the material temperature (Sh at temperature is provided in Appendices “A” of the Codes). The values for Sh provided by the Code include the consideration of the fact that circumferential stresses are not included in the calculation of “sustained stresses”.

Q. Is it that the moment calculate the Longitudinal stresses due to sustained load and make sure it is less than the Basic allowable stress due to yield at the max metal temperature, then the piping will not fail for single point load. So what is the significance of principal stresses

A. Principal stresses are of primary importance in that they are “non-self limiting stresses”. These principal stresses will not diminish as the piping system deflects under the loadings of pressure and weight. In B31.1 the equation for the calculation of the (“additive”) sustained stresses includes two terms. One term addresses the longitudinal stresses due to internal pressure and the other term addresses the longitudinal stresses due to the resultant of the bending moments and the torsional moment. The absolute values of the stresses calculated by each term are added and the sum is the (unsigned) total sustained stress. The total sustained stress is compared to Sh – the maximum allowable stress at temperature. Again, this allowable stress (Sh) is a fraction of the material yield stress. The B31.3 equation for sustained stresses (from Code Case 178) is slightly different but it accomplishes the same thing.

Note that shear stresses are not often the primary concern in piping systems. However when shear stress IS the limiting factor the ASME B31 Codes provide adequate limits for its evaluation (e.g., B31.1 Paragraph 102.3.1 (B)).

All the above is my opinion and not that of ASME International or any ASME Code Committee.

Regards, John.