Temperature and Sustained Loading 3

[ctmecheng]

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?

 

[NovaStark]

The sustained load represents the piping system in its installed state without being up to temperature. Temperature change is considered in the expansion case.


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.

 

[ctmecheng]

NovaStark,

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.

 

[TGS4]

This has to do with the fact that different stresses from different sources relate to different failure modes. Unfortunately, in my opinion, the discussion of this topic is more involved than I am willing to do (for free).

The topic is covered adequately in various textbooks on piping, such as the one by Becht.

 

[XL83NL]

Sustained loads, or stresses if you like, are primary. Thermal displacements stresses are secondary.
Primary failure = bang (like a gunshot). Secondary failure = psshhhhhhh (like a leak tire running flat). That's the reminder I use.

 

[ctmecheng]

Ok, so if I compute my stress analysis and my system stresses are under code allowable for the sustained case (G+P) and the expansion case (amb to T1) independently, then my piping system is acceptable? Assuming my system is properly restrained.

 

[XL83NL]

Assuming you fullfill all Code requirements, yes.

 

[NovaStark]

Also at that temperature you may be encountering creep depending on the material which I believe there is a section in the code that elaborates on this.

 

[XL83NL]

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?

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 -> ...

 

[BigInch]

As XL83NL mentioned, the reason is because of the difference in primary and secondary stresses. For example, thermal deflections associated with axial stress can introduce bending stress. When that occurs the axial stresses are usually reduced. Additionally, after the piping system has gone through a few on and off cycles, the accompanying hysteresis effects produce a ratcheting which usually moves the neutral point of the overall displacement range that will happen in the next cycle, thereby reducing the maximum thermal stress that will be developed during next cycle. Secondary stresses in piping usually do not produce a failure. Most of the time they only result in a self-limiting deflection and any temporary eccentric stresses that cause bending usually result in a reduction of maximum primary stress.

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.

 

[XL83NL]

A good reference, besides what TGS4 already mentioned, is Peng & Peng's book on "Pipe Stress Engineering". For 111USD, it's a bargain. All knowledge in there is soo valuable, I would consider this to be the default work of reference for any pipe stress engineer to be at his/her disposal on the desktop. For this particular subject, see paragraph 3.3 and 4.5.

 

[DSB123]

ctmecheng,
Do you understand the difference between a primary stress (Weight & Pressure) and a secondary stress(Thermal)?

 

[BigInch]

At least he's trying to understand and ... asking good questions.

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.

 

[ctmecheng]

Thank y'all for the help and advice it is much appreciated.