By Nadia Strikovski
Staff Systems Architect, CAESAR II Development
A typical piping system is subject to various internal and external loading. For proper design of the piping system, two different distinct failure criteria need to be considered—one for primary loads where even a single application may lead to catastrophic failure, and one for secondary loads which lead to fatigue failure.
Primary loads are usually force-driven and include system weight, pressure, and spring forces. Secondary loads are usually displacement-driven, almost always self-limiting and cyclic in nature, and typically include thermal expansion, imposed anchor movements, and vibration. Most piping codes recognize this distinction and recommend evaluating the sustained (SUS) stresses (caused by primary loads) and expansion (EXP) stress ranges (caused by secondary loads), and provide guidance for calculating the stresses and the corresponding allowable stresses for each failure criteria.
Let’s concentrate on the expansion (EXP) stress range and how to set up the appropriate load cases in CAESAR II. Note that the expansion stress range considers the temperature change between two states of the system, where a state is the physical position of the system (with regards to nonlinear restraints and presence of other loads) under the temperature.
Let’s also review the Load Case Templates available with the CAESAR II installation. The Static Load Case Editor will recommend a set of load cases for a given job based on the Load Case Template selection in the configuration file. The CAESAR II Configuration Editor contains various settings to fine-tune the calculations, assist in initializing inputs, or enhance usability of the software. The Load Case Template setting in the Configuration Editor>Database Definitions section contains the following templates (among others):
- LOAD_BASIC.TPL - Recommends the set of basic EXP load cases between each of the operating (OPE) cases and the SUS case.
- LOAD_EXP.TPL - Includes additional EXP stress range load cases for better coverage of multiple operating conditions.
Other templates, such as LOAD_ALT.TPL and the default LOAD.TPL, will not be discussed in this article. (Refer to the CAESAR II User’s Guide for details.) It is worth mentioning, however, that all other Load Case Templates provided with the CAESAR II installation are equivalent to the LOAD_EXP.TPL with regards to the expanded coverage of multiple operating conditions.
Please join us for next month’s newsletter for the conclusion of this article, where we will discuss systems with multiple operating conditions.