When the temperature of a body rises, the agitation of its molecules increases. As a general rule, this agitation leads to the body increasing in size, which is expansion.
We will be particularly concerned by the expansion of piping when heating circuits are being brought up to operating temperature. Nothing can stop this process; the pipes will increase in length, even if they have to deform to do so. When the expansion is hindered it provokes banging in the circuit caused by the sudden displacement of the piping.
23 [mm] (0.9 ins) Explanation: The coefficient of expansion for copper is 0.017 [mm/m.K] This means that the pipe lengthens by 0,017 [mm] per meter, and per Kelvin. 0,017 x 15 x 90 = 22.95 [mm] (0.9 ins)
0.1 x 15 x 50 = 75 [mm] (2.95 inches)