I am trying to better understand conceptually flow through a long pipeline. in this case the pipeline is 60miles long with a diameter of 13feet. the outlet is located lower than the inlet by about 600 feet and the pipe has several elevation increases and decreases over the length of the run. The entire system is gravity fed. I have seen measured pressure data over the length of the pipeline and observed rises and falls in the pressure which appear dependent on elevation gain, or pressure drop due to friction. The pipe maintains a constant diameter over it's entire length. Does Bernoulli's equation hold true for this case? What I am really trying to understand is whether or not there are fluid velocity changes associated with the observed pressure changes.