Long steam pipes that are fixed at the ends often have a section in the shape of a U why?

Answer 1

When the pipe metal heats up or cools down, it expands and contracts. If the ends were free, it would just get longer and shorter as it heated or cooled. However, pipes are usually connected to other pipes at their ends. Even a little bit of expansion or contraction would set up large forces on the end sections that might make them bend, and then leak. The u-shaped piece allows the pipe to expand and contract a little bit without placing enormous stress on its ends. -----|__|-------------------------------- ^^ The elbows at the top of the "u" get slightly closer together when the pipe expands, and slightly farther apart when it cools. A "U" may also serve to trap bits of particulate matter (if it's pointing downward) or gas bubbles (if it's pointing upward) but generally it's there to permit thermal expansion.