The calculation of the pressure drop along the individual pipes of a gas network requires use of the flow equations. Many gas flow equations have been developed and a number have been used by the gas industry. Most are based on
the result of gas flow experiments.  The result of the particular formula normally varies because these
experiments were conducted over different range of flow conditions, and on varying internal surface
roughness. Instead, each formula is applicable to a limited range of flow and pipe surface conditions.