By Nadia Strikovski
Staff Systems Architect, CAESAR II Development
In last month’s newsletter, we defined load case templates. Let’s now take a look at systems with multiple operating conditions.
Consider a system that has only one operating temperature (or operating temperature fluctuations are insignificant/uncommon). For such a system, we would only need to consider one expansion range condition for either the start-up or shutdown, where temperature cycles between the ambient/installed to the operating. The single expansion load case could be set up by subtracting the SUS state case from OPE state case. This may be easily set up manually, or recommended by the CAESAR II Static Load Case Editor (regardless of the particular Load Case Template selection).
Most piping systems, however, have more than a single operating condition. Some systems may experience temperature fluctuations in the operating cycle based on daily (day/night) or seasonal (summer/winter) external temperature changes. Other systems may include deviations in operating temperature due to varying the quality/consistency of the matter flowing inside the pipe. Even other systems may have more than one operating cycle (i.e., pump A on, pump B on, both pumps on, both pumps off; or systems with a bypass line), although the actual operating temperature magnitude is not changing. Each operating condition in these systems will produce its own unique state.
For systems with multiple operating conditions, we would need to first consider the set of basic EXP ranges between each of the operating temperatures and the ambient/SUS case. However, we can’t expect the system to completely cool down to ambient from the first temperature condition before heating up to the second temperature condition. In fact, the system would most likely cycle between two given operating conditions.
Another situation where the range between two operating conditions will be required, is when one operating temperature is above ambient and another operating temperature is below ambient, and the extreme range would produce higher EXP stresses than individual basic EXP ranges. This situation requires setting up additional expansion ranges between each and every one of those operating conditions.
If there are only two operating conditions, T1 and T2, then you can expect three EXP load cases: the two basic ranges, OPE1-SUS and OPE2-SUS, and the additional cycling between the two temperature cases OPE1-OPE2. For a system with six operating conditions, however, there would be 21 EXP load case permutations. And for a system with nine operating conditions, the number of EXP permutations will become 45.
In these cases, the manual setting up of the appropriate EXP load cases may become tedious. This is where the LOAD_EXP.TPL (or equivalent) Load Case Template comes in handy. This expanded Load Case Template has been the default choice in CAESAR II since the 2014 (v7.00) release in April 2014 (it comes under the default name LOAD.TPL).
Although the typical piping systems may experience multiple operating conditions, and it is clear that all permutations of ranges would be considered with the CAESAR II default Load Case Template, in reality, not every combination of every operating condition is likely to happen to the system. Additionally, some ranges may be indirectly covering/including other ranges.
Some companies or power users may prefer setting up the expansion stress ranges manually, since they know exactly what combinations of which operating conditions to consider based on the specific piping system and design requirements. They can still take advantage of the software recommending the load cases.
Also, while we are discussing EXP stress ranges, there can also be a number of OPE and SUS load cases to consider. In this situation, the users can select the LOAD_BASIC.TPL from the configuration file, open the Static Load Case Editor, and then select Recommend Load Cases to read and update or replace the load cases.
As a user, you have many options when considering expansion stress ranges in a wide variety of operating conditions. Using the built-in CAESAR II Load Case Templates, in conjunction with the CAESAR II Load Case Editor’s Recommend Load Cases option, the decision-making becomes easier when it comes to solving your analysis needs.