Temperature and Sustained Loading
Temperature and Sustained Loading
(OP)
Can anyone explain to me why temperature change is not considered a sustained load, when the system is operating for a long period of time without shutdown? Or is there an accurate way to consider the full operating case(T1,P1,G) and what method of calculating the allowable would be used?





RE: Temperature and Sustained Loading
Which code are you designing to (if any) ? B31.3 does not cover full operating case to calculate an allowable but there may be other codes that do.
RE: Temperature and Sustained Loading
I am designing to B31.3. It just seems odd to me that you account for gravity and pressure for sustained, and temperature change as expansion, but not all 3 together. Especially, since the system will be operating for long periods of time.
I am working on a very hot system ~1000-1200 deg F, and I am concerned with the full operating case since it will be running for a long duration.
I am working with AutoPIPE, and maybe you can help clear up an issue for me. For the sustained code case it uses the allowable stress as the materials allowable at Tmax. But for the expansion code case it uses something different, something much higher. My question is if the expansion happens in the hot case then why does the code not check the stresses to the allowable at Tmax?
Thank you for your help, I am trying to understand the code as much as possible and any help is appreciated.
RE: Temperature and Sustained Loading
The topic is covered adequately in various textbooks on piping, such as the one by Becht.
RE: Temperature and Sustained Loading
Primary failure = bang (like a gunshot). Secondary failure = psshhhhhhh (like a leak tire running flat). That's the reminder I use.
RE: Temperature and Sustained Loading
RE: Temperature and Sustained Loading
RE: Temperature and Sustained Loading
RE: Temperature and Sustained Loading
Because it doesnt require you to do so, they're reviewed different. Review the Code. Check B31.3 para 302.3.5 eq. 1a/1b.
Also, reviewing the help file of AutoPIPE with respect to how it calculates codes stresses is useful. Check Help -> Contents -> Reference Information -> Code Compliance Calculations -> ASME B31.3 20xx -> ...
RE: Temperature and Sustained Loading
Example: A very hot pipeline laid in a pipe rack will expand. As it is restrained in axial elongation by the friction of the pipe supports, the pipe tends to take on a snake shape in the rack due to the resulting eccentricities and bending stress. That's all that happens. A snake shape. If the same were to happen to a vertical structural column, bending would not necessarily be limited, eccentricities and bending stressses would increase and the structure could easily collapse to the ground. With a properly designed piping system, that would most likely never happen.
RE: Temperature and Sustained Loading
RE: Temperature and Sustained Loading
Do you understand the difference between a primary stress (Weight & Pressure) and a secondary stress(Thermal)?
RE: Temperature and Sustained Loading
Also Don't forget that theoretically a thermal load on an unrestrained pipe generates no stress, only expansion. That is why lines that exhibit enough flexibility in their design (pass the flexibility check) do not actually require any pipe stress analysis at all.
RE: Temperature and Sustained Loading