Yield Strength, Elastic Limit, Creep Effect 3

[bulkhandling]

HI, How can I get the yield strength of A335 P22 at temperature of 955F? Is the following correct?
30000/20000x11000=16500
Where,
Yield strength at normal temp=30000psi
Allowable stress at normal temp by ASME B31.3=20000psi
Allowable stress at 950F by B31.3=11000psi

My problem is that the pipe expansion loop does not resume the original installation position when it cools down after
normal operation at 955F. Does the calculated stress has to be larger than the yield strength at the deformation points?
Many thanks
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I hope to find a source for an accurate yield strength because the calculated thermal stress is pretty close to the yield strength I calculated using that formula.
I agree on your understanding about ASME codes, And in my situation, I think the original designer missed one support at the loop so that each time the pipe to expand, it takes the deformed position as the cold position and when it cools down again the pipe yield further more.
After years of operation, the deformation became significant till we have to take care of.
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The value tabulated in ASME II D for SA-335 P22 yield strength at 955°F is 24.69 ksi: don't forget that this is a tabulated minimum, and that actual strength might be considerably higher.
You get a lower value from the allowable stress because at that temperature long term (creep) effects are accounted for in the allowable.
I don't understand the described behaviour: if your calculated stress is close to 16 ksi, then you shouldn't have any accumulation of plastic strain, that would start somewhere beyond the double of the yield strength. Of course creep also might come into effect, as you refer to years of operation.
Your explanation on the pipe not coming back could be a good one, however this still requires a somewhat higher stress and also that, on coming back, some permanent deformation would occur at a location outside your expansion loop.
prex

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Prex,
It seems that the piping stress is much smaller than the yield strength and the piping deformation was supposed not to happen. And I'm sure that no stress can be higher than 18ksi arround the loop.
I'm interested in your words "creep effect" that may be the only cause for the deformation but how can I theorically explain the behavior when the stress is lower than the yield strength?
The piping loop is kind of special and if you could send me your email address, I will be very happy to send you the graphic picture...
Bulk

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Creep is of course the accumulation of plastic deformation just because of sustained stress: only the time spent with high stress is of importance, not the cycling.
One of the criteria for determining the allowable stress in the creep range by ASME is that it may be equal to the stress that will cause 0.01% deformation in 1000 hr. If your pipe stays at high temperature for long periods, say 5000 hr per year, and the accumulated service is 10 year, this could (as a maximum) lead to an accumulated strain of 0.5%: don't expect this can fully explain your observations.
prex

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MJCronin:
I strongly suggest that you re-post this question in the "Piping & fluid mechanics engineering" forum........people like "John Breen" and "cb4" could shed a lot of light on this question....



Also see:

thread391-79795

 

[bvi]

For ASME B31.3 piping and similar piping codes, permanent deformation after the first cycle of operation is a definite possibility. The piping self springs into a new "cold" position. The self springing can be due to yielding (and the stresses in the elbows are actually two time what you are calculating with the code equations, this is well known, not an code mistake)and creep. That would explain some permanent movement during the first few cycles. If it is continuing to get worse, then you have something different, perhaps a ratchet mechanism, where there is an increment of deformation each start up and shut down cycle. If you have ratchet, you may not have properly calculated the stresses, or you may have an unusual condition, such as severe thermal transients with the process fluid.

 

[unclesyd]

Your problem is quite common in the installation of complex steam piping distribution headers. Our installation was quite fortunate in that for the majority of construction of the 24" high pressure (650 psi) steam header, the construction piping superintendent and the plant engineer both knew about "Cold Spring" in steam lines that alleviates a lot of problems similar to your problem. All these lines were installed with cold spring to satisfy movement at the expansion loops. Running hot the hangers are vertical and cold(for only about 3 days in 50 years) they are maybe 10° from vertical. The last steam header installed (1983) was installed without any "Cold Spring". Being about 4000' long not counting the loops when the line was commissioned the results were disastrous. It took down numerous hangers and supports and reinforced concrete block fire wall. All the unanticipated movement while targeting had been ignored.
We also have a large vaporized heating fluid distribution system where all the piping was installed with “Cold Spring”. It worked too as all hangers are essentially vertical.