Say there's a pipe that pumps a liquid that empties into an elevated tank The discharge end of the pipe is 5 m higher up than the suction but it goes over an intermediate obstacle, say, 20 m higher than suction level. Is the pump head requirement 5m or 20m? I'm confused. My gut feeling says 20m otherwise how will one get the flow started. Let's assume liquid, incompressable flow, both suction & discharge atmospheric & negligible frictional / velocity heads etc. Basically, does one calculate a head difference between initial & final points or initial & highest points?