What is Hot Sustained Stress?
In Layman’s terms, Sustained means always present. So sustained stresses are the stresses which are present in the system throughout its operating cycle. The weight of the piping system and Pressure inside the pipe are examples of sustained loads that generate sustained stresses in the system. So what is a hot sustained case?
While analyzing a piping system, many times you will come across some supports which will take a load in sustained cases but are not taking a load in operating and design temperature cases (Refer to Attached Fig. 1 and Fig. 2 for one such typical example). The support is lifting at that point in temperature case i.e. supports are not contributing to load and stress distribution while in operating condition. Still in that situation, the weight of the pipe and pressure inside the system will induce sustained stresses. So in my opinion, hot sustained stress is the sustained stress in pipe operating situations. And we must ensure that the system stress will not fail because those supports not sharing any load. That is why many organizations make it mandatory to check sustained stresses.
Methods of Hot Sustained stress checking
I have come across two different methods of hot sustained stress checks in the various organization based on old philosophy:
1. Conventional Method of Hot-Sustained Check
- In the first method, the analyst has to run the static analysis as per the conventional method.
- Now go to the restraint summary and note down the support nodes which are lifting or not taking any vertical load (Sometimes small positive value may be there due to guide and line stop frictions, in that case, check the vertical displacement if it shows positive value consider the same as lifting).
- Make a separate Caesar file with the name FILE NAME_HOT SUSTAINED.
- Open the input screen and delete all lifting supports from the nodes you noted down. Delete only +Y support, Guide and line stops will be there.
- Run the analysis and check sustained stress.
- If sustained stress is within allowable limit accept the file as it is else change the support location or routing to make the system safe.
2. Second method of Hot Sustained Checking
- In this method, the analyst will check the hot sustained stress in the same main file (No need to create a separate file). Some additional load cases are required. Let’s assume we will check hot sustained stress in design temperature, T1 condition (means we will check which supports are lifting in design temp case). So the below-mentioned cases are required for hot sustained stress checking
L1: W+T1+P1 OPE
L2: T1 OPE/EXP
L3: L1-L2 SUS
- Check the stresses for load case L3, if the same is within the allowable limit then accept the file else make the system safe.
3. Hot Sustained Check Philosophy from Caesar II 2018 version onwards:
From Caesar II version 2016 onwards, based on code requirements they have created a load case type known as ALT-SUS. So, in the single file, by adding ALT-SUS load cases, Caesar II automatically calculates hot-sustained stresses for each operating condition. Refer to Fig. 3 below to understand the load cases which has been introduced from ASME B 31.3-2014 onwards as Alternate sustained stress checking.
So, the above stresses are sufficient for hot-sustained stress checking. Load case L3 is checking sustained stress for temperature T1; which means in load case L3 all supports which are lifting or not taking load will be removed by the software automatically and sustained stress will be calculated after that. A similar situation will arise for load cases L5 and L7 as shown above and they are the hot sustained cases with respect to temperature cases T2 and T3 respectively.
- Now you may be thinking about whether to mark deleted supports in isometric or not. You must mark those supports. As we have not deleted the supports in actual practice. Supports will be there at site, We simply ensured that without those supports also the system will be safe. However, if you want to delete those supports that can be done if all other stress criteria can be met.
- Whether we need to check expansion stresses in a hot sustained file too? In my opinion, if we are using liberal stress for expansion stress range checking then it is better to check expansion stress (along with sustained stress) in the hot sustained file. Otherwise, it is not required as the system won’t fail in an expansion cases even after removing those supports.