Dear Muth,
You can take as reference "Introduction to Pipe Stress Analysis" by Sam Kannnapan in Chapter V - Expansion Loop & Expansion Joint. There is a simple nomograph to determine bend length, L2 of it. Beside that there is also formula to have L2 = (3*E*D*dL/144*Sa)^0.5. Or you can use design of loops using M.W Kellogg chart.
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Many thanks & regards,
Sam Manik