Evaluation of stress in 'hot spots' 1

[ANK72]

Hello all,
We are trying to assess fitness for service of a A106-B, 30 inch refractory lined pipe which is opearting at its max design conditions - 23barg (MAWP) and 150deg C temperature (designed to ASME B31.3). Thermographic examination has shown several hot spots of 200 to 270deg C.
Is there a manual stress analysis method that I can use to determine the structural stability of the pipe in the hot spot localities.
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

 

[metengr]

For what its worth, the ASME B&PV code allowable stress for SA 106 Grade B pipe material does not change below 345 deg C (650 deg F). Typically, carbon steel above 650 deg F begins to experience time dependent deformation (creep), and a drop in mechanical properties. Therefore, from a mechanical properties viewpoint nothing changes at 270 deg C for carbon steel.

Is your question related to exposure to accelerated corrosion and possible internal wastage of the pipe wall effecting structural integrity because of the increase in metal temperature?

 

[ANK72]

metengr,
Thanks for your input. The pipe is designed to ASME B31.3 which has its own allowable stress tolerances different from ASME B&PV sect II. The allowable stress in B31.3 starts dropping from 400deg F (204deg C).

I'm not concerned about accelerated corrosion. You see, the pipe is under longitudinal compression and that's what complicates the matters. Under the combined load (pressure, longitudinal compressive force, gravity, occasional load, etc.) the combined stress is greater than the code allowable value (for the temperature of the hot spot). 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. Neither Roark, nor Timoshenko offer a suitable solution. I'm still searching for the mythical calculation......not surrendering to FEA just yet!
Cheers

 

[unclesyd]

My curiosity is up.

What are the process gas/gases in the line?
What is the process temperature?
What is the gas velocity?
Is it anything that might affect the piping material or cause damage to the surrounding structure if it ruptures?

I ask because I've seen 2 recent situations with H2 plant tail gas lines that had hot spots and there was an attempt to limp along until a scheduled outage neither made it.
The problem with some refractory lined pipes that there are two or sometimes 3 layers of refractory. a hot side refractory then an intermediate (bonding) layer and the insulating layer next to the pipe. The insulating layer is the weakest component for several reasons, the main one is that it isn't very erosion resistant.

 

[prex]

Not sure to understand what you are looking for.
If you want to determine the stress distribution due to a temperature hot spot, well something may be done by formula, but I have nothing ready at the moment (did that in a very far past, don't remember where that formula may be now).
However don't see how this could help you: those stresses would be in the peak category, so would not enter in the B31.3 stress evaluations.
On the other side, if the compressive load you mention is of mechanical origin (not an expansion stress) so that you are beyond the limit for sustained (mechanical) loads or for occasional loads, then I think your pipe is not complying with code, whatever you do with the analysis of the hot spot, as you need to use the highest local (average) wall temperature.
To solve this problem, besides repairing the insulation of course, you could make a finer analysis, carefully classifying the category of stresses (mechanical or expansion) and also determining the actual stress at the exact location where the hot spots are (provided you continue to survey them in time): you don't need to use the same temperature for the entire piping system.

prex

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[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

 

[codeeng]

I don't think you should be combining hoop pressure stress with longitudinal bending stress, but rather longitudinal pressure stress with longitudinal bending stess.

 

[ANK72]

Codeeng,
Thanks for your posting. As far as the code goes, I think you are correct. Would you suggest I consider the hoop stress on its own then, without combining it with any other stresses?

 

[prex]

Well, why not? This is exactly what is required by the code: only the longitudinal stress due to pressure is to be combined to sustained loads.

prex

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