What is phase equilibrium and why do materials seek equilibrium structures? how do equilibrium structures differ from metastable structures?

Water flows at the rate of 200 i/s upwards through a tapered vertical pipe. the diameter at marks(3) clo5) the bottom is 240 mm and at the top 200 mm and the length is 5m. the pressure at the bottom is 8 bar, and the pressure at the topside is 7.3 bar. determine the head loss through the pipe. express it as a function of exit velocity head.