Piping pressure head estimation

[rollingstein]

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?

 

[Baluncore]

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.

 

[rollingstein]

Centrifugal pump and the liquid is water.

Pump is right after the suction tank so that it always has a flooded suction.

 

[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.

 

[rollingstein]

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?

 

[Baluncore]

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.

 

[rollingstein]

Understood. Thanks!

 

[Travis_King]

Just to add:

For your design flowrate/head, use the 5m+Frictin losses as your head.
For starting head, just make sure the system is on the pump's curve at 20m. You want a pump that can develop some flow without dead-heading at 20m of head and then run out to your design flow rate after the siphon takes over and leaves you with your (5 m + Friction losses + Discharge pressure).

 

[rollingstein]

Just to add:

For your design flowrate/head, use the 5m+Frictin losses as your head.
For starting head, just make sure the system is on the pump's curve at 20m. You want a pump that can develop some flow without dead-heading at 20m of head and then run out to your design flow rate after the siphon takes over and leaves you with your (5 m + Friction losses + Discharge pressure).

Understood. Thanks. So I need a pump curve with a shut off head of at least 20 m.

I'm confused about your 5m operating point. Must be more right? Siphon can only provide at most 10 m.

 

[Travis_King]

Yea, sorry, after you've filled the pipe, you'll have to get the water to 10m, the siphon will take over from there.