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B31.3 Allowable stress for stress range.

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mw2005

mw2005

Mechanical
Joined
Dec 6, 2011
Messages
3
Location
GB
Hi all,
I have a case where one branch is at high temperature while adjacent pipework cycles from cold to hot.
When calculating stress range, the allowable stress range at branch is calculated using SA=f(1.25Sh+0.25Sh) rather than f(1.25Sc+0.25Sh). Is it due to the fact that branch is at high temperature in both cases?
The Sc is defined as "basic allowable stress at minimum temperature expected during the displacement under analysis"
There is difference in opinion at our place regarding this. In my opinion it is correct to use SA=f(1.25Sh+0.25Sh)but some say it should be f(1.25Sc+0.25Sh) Sc being at ambient temperature.
I would like to have your expert opinion on this.
I have attached the sample CAESAR model herewith.

Many thanks and regards

MW
 
 http://files.engineering.com/getfile.aspx?folder=6bc1e136-2d21-4a23-a393-fd09fc2b0b67&file=MOMENT_REVERSE.C2
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Seems like you need to run four analysis cases: And I'm not instinctively sure which of the four is going to show the point of highest stress - which "I think" is going to be at the hot-side pipe connection on the pipe between cold pipe and hot pipe.

1. Hot pipe cold and cold pipe cold position.
Fabrication case, should be near-zero stress, unless you need to put in pre-stress loads to solve one of the future cases.
2. Hot pipe hot and cold pipe cold position.
3. Hot pipe hot and cold pipe hot positions.
Might be maximum movement, but I don't know the relative movement between the two pipe as they move. But hot-hot is probably weakest strength condition.
4. Hot pipe cold and cold pipe hot.
Probably not your usual operations condition. But needs to be checked.
 
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You also need to check all the differences between the cases above. The maximum range maybe between cases 3 and 4.
 
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Mw2005,
ASME B31.3 uses the following two equations:
SA=f(1.25Sc + 0.25Sh) is used for the entire piping system of the same material and temperature whereas SA=f(1.25(Sc+Sh)-SL) is a component allowable stress at temperature where SL has been caculated for that component.
I am not sure about the equation with two Sh.Allowable stress equations always considers Sc and Sh.

Ganga D. Deka, P. Eng
Canada
 
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