by rollingstein
 P: 305 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?
P: 1,086
 Quote by rollingstein Say there's a pipe that pumps a liquid
A pump pumps a liquid, a pipe constrains the flow.

The 20m up and 15m down is a siphon but it is only able to provide one atmosphere of suction.
 Let's assume liquid, incompressable flow,
I think it would be far more important to specify the density of the liquid. That will determine the functionality of the siphon.

You are really confused here. Where in the system is the pump ?
Is it a fixed displacement pump or a centrifugal pump ? They behave quite differently in this sort of situation.
 P: 305 Centrifugal pump and the liquid is water. Pump is right after the suction tank so that it always has a flooded suction.
P: 1,086

Will the potential siphon pipe that rises 20m and falls 15m remain full of water when flowing normally, or can air flow back in because the outlet to the reservoir is open above the reservoir water level.
P: 305
 Quote by Baluncore Will the potential siphon pipe that rises 20m and falls 15m remain full of water when flowing normally, or can air flow back in because the outlet to the reservoir is open above the reservoir water level.
Air could flow back in. At startup, all pipe will be full with air.

I guess that means size for head=20 m?
P: 1,086
 Quote by rollingstein I guess that means size for head=20 m?
A centrifugal pump can prime a greater head slowly, then once water starts to flow through the siphon the flow increases as the head is less. That requires the end of the pipe to be under the reservoir water surface or that it have something to prevent air entering to break the siphon once it is running. That could be a bucket attached to the end of the pipe to retain water.

The difference in inlet and outlet height of the siphon is not relevant because a siphon can only pull about 10m of water. Once running the pump would only see a 10m head so flow would be greater.
 P: 305 Understood. Thanks!