Where to find metal yield strength at 955F 1

[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

 

[iainuts]

Yes, I believe your equation for yield strength is reasonably accurate. ASME codes are very conservative though, so I'd expect the yield strength to be at least as high as this equation predicts, if not significantly higher on average.

Note however, that ASME B31.3 and all piping codes that I'm aware of, allow yielding during thermal expansion/contraction. The codes allow this on a one time basis only. Note that if you've designed the pipe correctly (per the code), it should only yield once. After that, it goes through cyclic stresses due to thermal expansion/contraction but never should yield a second time.

I won't say I'm an expert at the piping codes, I suspect there are others here that know the codes inside out, backwards and upside down. But I believe this is a correct interpretation of how piping stresses are handled and calculated by the codes.

 

[bulkhandling]

Thanks, iainuts,
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.
Thanks for your comments.

 

[prex]

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

http://www.xcalcs.com

Online tools for structural design

 

[bulkhandling]

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...
mikewu108@yahoo.com
Your comment is appreciated!
Mike

 

[prex]

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

http://www.xcalcs.com

Online tools for structural design

 

[bulkhandling]

I think my question should be:

What is the Elastic Limit of A335 P22 at temp 955F?

Since at yield stress the material already experienced some deformation (0.2%?).

Or is there a relation between Elastic Limit and Yield Strength?

 

[prex]

The elastic limit and the yield strength are somewhat theoretical quantities, as stress-strain relationships for metals are always plus or minus different from a pure elastoplastic behaviour.
The two quantities are normally to be considered the same and the minimum tabulated yield strength (or elastic limit) is defined in ASME IID as a function of temperature.
If you want to know more about your specific piece of metal you should take a sample and conduct a tension test at temperature, but I can't figure out what you would do with the resulting stress-strain curve.
In my opinion the explanation of your observations is to be found from the analysis of pipe behavior, trying to detect and analyze any effect of pipe sticking into the expanded position and not coming back when cooling.

prex

http://www.xcalcs.com

Online tools for structural design

 

[MJCronin]

bulk,

I think that "prex" is on the righ track based on my experience with piping in this temperature range (over 950F). It is not unusual for selected power plant piping systems operating in this range ( ie. main steam and hot reheat) to move to a new position on every cooldown.

Some power plant owners have administrative systems in place to measure this displacement and I believe ASME B31.1 has a Non-mandatory appendix that discusses setting up such a program.

You should also be aware that calculation of piping system stress levels (through use of CEASAR-II and similar programs) do not produce exact results !!! The stresses in your expansion loop may be more that the analysis program esitmate. CEASAR-II only produces a calculated stress level to be compared with ASME B31.1 code limits.......This is not the same type of stress calculation that a finite-element program like ANSYS or COSMOS performs.

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....


MJC

"There comes a time in the affairs of man when he must take the bull by the tail and face the situation." W.C. Fields