Local overheating 1

[ANK72]

Hello all,
We are trying to assess fitness for service of a A106-B, 30 inch pipe that was originally designed to operate at 23barg MAWP and 150deg C temperature (to ASME B31.3). The piping is currently running at the max design pressure and temperature. Thermographic examination has shown several hot spots ranging from 200 to 270deg C. Given the higher temperature, the strength of material in the high temp localities will be lower than in the rest of the pipe.
Is there a manual stress analysis method that I can use to determine the structural stability of the hot spots. At this stage we are not interested in using FEA. I'm currently looking through Timoshenko's textbooks but haven't found anything applicable yet.
Can someone recommend an analytical method please?
Thanks

 

[codeeng]

270C does not appear to be very hot. You can check your wall thickness based on a reduced allowable stress (20ksi @ 150C and 18.6ksi @ 270C).

 

[jte]

ANK72-

I tend to agree with codeeng that the temperatures are probably not hot enough to be a substantial concern. However... the hot spots are in compression as the surrounding relatively cold steel doesn't want to expand. I guess I'd just run a quick FEA, but that's simply 'cus I have that tool readily available.

You might want to get in touch with the folks at

http://www.equityeng.com/fitness/api579.html

to discuss the issue with them. The next edition of API 579 is alleged to include a chapter on hot spots, but I'm not aware of when it is due to be published. If you're real nice to the folks developing it they might be able to get you a draft of that section which might include a hand calc approach.

jt

 

[ANK72]

codeeng, jt,
Thanks for your input. Jt, you are correct. The pipe is under longitudinal compression and that's what complicates the matters. Under the combined load (pressure, longitudinal force, gravity, occasional load, etc.) the combined stress is higher than the code allowable. However, the hot spots are not greater than 1/3 of the pipe diameter and the cooler material around the hot spots provides some inherent reinforcement. Now, what I'm trying to establish is whether this reinforcement is sufficient to maintain the overall structural stability.
Jt, I've written to "equity eng" and am praying for a prompt response.
Thanks again

 

[Xianglu]

These locallized stress is secondary stress, they are self limiting and the evaluation shall be according to the Allowable Displacement Stress Range - B31.3 or B31.1
eg. B31.3 (1a or 1b)
SA=f(1.25Sc+0.25Sh) or if Sh>Sl
SA=f(1.25(Sc+Sh)-Sl)
Where Sc is basic allowable stress at min temp
Sh is basic allowable stress at max temp
f is stress reduction factor
f=6.0(N)^-0.2
N is displacement cycle number
Sl is longitudinal stress

 

[jte]

These "localized" stresses cover nearly 1/3 of the perimeter of the pipe. The pipe is in longitudinal compression and the hot spot aggravates the situation for potential buckling. For a compressive loading, 1/3 of the perimeter may not be at all localized. This "secondary" stress may be not at all self limiting.

On the other hand, all buckling is (initially) localized, and all loadings are secondary: The load is self limiting when the pipe depressurizes due to the hole and/or hits the ground!

More seriously, a paper by George et. al.

http://store.asme.org/product.asp?c...Papers&category_name=&product_id=PVP2004-2613

presented at last year's ASME Pressure Vessels and Piping Conference discussed the realism of assuming that all thermally induced loads are secondary. Turned out that in at least one example, the thermally caused nozzle loads on a vessel would fail the vessel before enough deflection took place to relieve the piping system. Food for thought.

jt

 

[ANK72]

Thanks everyone. I guess I should summarise the basis for my investigation.
ASME B31.3 is concerned with stresses due to the following four load combinations:
1. Hoop stress (due to Max P)- this is coincident with the max temperature.
2. Sustained (due to Gravity + Max P)
3. Expansion (due to temperature change from Tmin to Tmax)
4. Occasional (Sustained + Earthquake)

Now, the hoop stress along at the hot spot temp is below the code allowable...Just!!! Under the sustained loads the hoop stress is combined with the stresses due to the longitudinal forces and moments induced by the self weight of pipe and content plus pressure. The combination of the stresses is what takes it over the limit. However, like I mentioned before, the hot spots are surrounded by cooler material with higher strength. Because the Young's modulus at the higher temperature in the hot spot is higher than that of the surrounding cooler material stress redistribution will occur. THIS is what I need to work out, the redistributed stress in both.
If anyone knows a calculation procedure, please help.
Cheers