Thermal Expansion Coefficient (e); B31.3 Appendix C

[HP.E]

Hi guys,

As we learned in academic textbooks, we can calculate linear thermal expansion of a metal part by the following formula:
ΔL = α.L. ΔT

where α is linear thermal expansion coefficient for a specific material.

When we go to the table C1 of Appendix C in B31.3, we have: linear thermal expansion coefficient e for base temperature 70 F.

It seems that e = (α. ΔT) and ΔL = e.L

For example, my design temperature is T2=600 F. I can find e=4.7 in/100ft linear thermal expansion for it in the mentioned table.

My question is why T1=70 F? and where can I find the real T1 when I calculating a line expansion on the pipe-way? Then where is correct coefficient factor for (T2-T1)?

 

[EdStainless]

T1 is 70F because when most equipment is fabricated or idle it is about 70F.
So you need the CTE for 70-whatever your temperature of interest is.

I have used these tables to fine the CTE between 300F and 800F when a process will be running in that range for most of its life.
Just subtract the RT-300 value from the RT-800 one.
These are set up this way because the CTE is non-linear with temperature, and different materials have different curves.
You can find some data for various materials that give fourth power curve fits for CTE.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Ehzin]

Another good resource would be ASME Section II, Part D, assuming you have it available to you. Instead of just mean or total expansion from 70°F to an indicated temperature it has instantaneous thermal expansion at a given temperature. If you wanted to find the growth from 300°F to 800°F you could find incremental expansion by finding the growth of a 300°F pipe to 350°F, then finding the growth from 350°F to 400°F using the instantaneous coefficients at each step. It could be built in Excel fairly easily or other similar spreadsheet program. A bit cumbersome but it's fairly accurate since you have no major shifts in the coefficient throughout the temperature increments in the standard. May not be the most convenient but it should be the most exact when trying to jump between non ambient temperatures.

Thanks,
Ehzin