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if i do not make any change and just accept the result that CAESAR give, in what range the error of the calculation?

I can't say. If the Code knew that, they would adjust the equations to consider your D/t ratio.

Quote:
Is right way to modelling the elbow with single or double flange? can be this minimize the error?

CAESAR II gives this warning because the Codes don't address this condition (D/t > 100). CAESAR II does what the Code does. If the Code doesn't know how to do something, then neither does CAESAR II. The presence of flanges won't alter the fact that the Code equations don't address D/t ratios in excess of 100.
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Regards,
Richard Ay - Consultant