Evaluation of Limit Load Analysis Methods Applied to High Temperature Design: Part 2 — Analysis and Results

Abstract

Limit Load Analysis (LLA) is a powerful tool for design at temperatures below the creep range, and there is desire to extend the method to the elevated temperature (creep) regime. However, there is no direct relationship between LLA and elevated temperature allowable stresses and failure modes, such that the basic LLA methods or results must be manipulated in some way to be generally meaningful for elevated temperature design. The most direct way to judge simplified methods is against a rigorous solution; this requires an inelastic material model consistent with allowable stresses. Such a creep model was described in Part 1 of this work (the Omega model), and subsequently applied in a rational way to the problem of primary load carrying capacity and design using a simple sample problem. In this paper, extension is made to the case of a typical steam header, and existing simplified (LLA) methods are compared against further detailed transient inelastic finite element analysis. Recommendations are then made for the application of LLA to evolving ASME high temperature design rules. Paper published with permission.

Creep-Fatigue Design Applied to Molten Salt Solar Receivers

“…In fact, material supplier rupture data was used along with the literature data to construct such a parametric representation for use here. This process is described in detail in [10], but the basis LMP data (and corresponding curve fit) is shown in Figure 17, and examples of creep strain vs. time curves generated from the model for various conditions are shown in Figure 18. Note that primary strain was assumed to be negligible in the reviewed literature creep curves and resulting creep model.…”

“…This creep model was implemented into the FEA described above in the section, THERMAL AND STRESS ANALYSES, using an Abaqus user subroutine (see [10]) -all other inputs remained unchanged. The results are shown in Figure 19 in terms of stress relaxation and damage and creep strain accumulation at the crown of the tube.…”