I know I'm really just a graduate, but I'm thinking that your question is really ambiguous. The force due to pipe thermal expansion will depend highly on the piping geometry and boundary conditions.
Do you mean for a straight run of pipe anchored at both ends? If so, then it's probably gonna be huge!
Here's my guess : I'd calculate the expansion length first, based on the material's thermal coefficient of expansion, and the piping temperature. From that you get the pipe strain (change in length/length). Using the pipe material's Young's Modulus you can get the axial stress. Multiply this by the pipe cross-sectional area and you get the force.
Not sure if this is 100% correct, as the pipe could have a tendency to bend. A good estimate perhaps? Can't think why'd you want this though.
-Pete
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