The heated pipe is longer than the cold pipe.
Temperature also changes the dimension of a pipe
A variation worth considering from the specification stage.
As temperature rises, material tends to expand. As it drops, it tends to contract. In a piping system this change in length can matter, especially in long runs and in lines that operate at temperatures far from the installation temperature.
What defines the change
The change in length depends on the original length, the temperature difference and the material's coefficient of thermal expansion. That coefficient varies with the alloy, so carbon steel, stainless steel and other alloys do not expand in the same way.
Why check it in the specification
Expansion can put loads on supports, equipment and fittings if the design does not provide for absorbing it. Choosing the alloy and stating the operating condition in the requisition helps avoid surprises in assembly and operation.
Schematic illustration