Pumping downhill
Pumping downhill
(OP)
I've been tasked with assisting in the design and manufacture of a (hypothetical) pipeline for water transport. From a reservoir, carried over a distance of 2km with a height drop of 30m, delivering approx. 30L/s. For many types readily available pipe (PVC, steel epoxy etc) the head loss seems very small compared to the height drop, but intuitively it seems very unrealistic that a pump will not be required. Am I missing something very fundamental? Apologies if my question is completely asinine, I am not particularly well versed, nor well practised in the world of fluid mechanics





RE: Pumping downhill
30m is 3 bar, so 1.5 bar per km is a reasonable pressure drop.
It will probably work for an 8" or 10" pipe, but not a 6" or 4"
Try giving us some sizes and pressure drops you've been looking at.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Pumping downhill
RE: Pumping downhill
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Pumping downhill
As others pointed out, it's not as simple as siphoning gas form a neighbor's fuel tank. You have to worry about hydraulic grade line, surge effects, servicing the line, and cleaning/scouring. A lot of those get easier for fluids that don't have solids that can built up, and systems that start and stop flow very slowly.
The 1.5% grade is pretty good for water. If you can control elevation well for how the pipe is laid, pumps might not be needed. Gravity flow is a whole lot more reliable than pump systems IMHO.
RE: Pumping downhill
Cross country water pipelines are typically designed for 1 to 1.5 m/s velocity for economical pumping. 50 cm is way too large which may result in solids settling in the pipeline at the low velocity. The larger pipeline will also be more expensive.
It is typically more reliable to pump than to rely on gravity. If you are pumping downhill, it will be difficult to remove air bubbles as the air will be moving against the flow. You would need a velocity of around 1.5 m/sec to flush the air out of the line.
Pressure pipelines have a little more flexibility with capacity as you can install a larger pump to push more flow.
If the pipeline does not have a constant grade, gravity lines will have difficultly because air may be trapped in humps along the pipeline route. A gravity pipeline with limited slope does not have flexibility to push the air out with higher flow.
A reliable gravity flow pipeline would have a larger pipe so that the pipe was not flowing full. You would also install the pipeline with a designated slope. Both of these would add to the cost.
The gravity line may work as long as the water continues to flow. The problem is stopped and must be restarted. It is much easier to restart a pumped pipeline.
RE: Pumping downhill
Also the line exit at the end must be submerged.
Else you'll have air and water in this line, and not all water.
RE: Pumping downhill
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RE: Pumping downhill
As you can see from the responses above there is a bit more to it than it appears.
The key things you need to decide are what are your design drivers.
Cost?
No power or cost for a pump?
Profile of the pipe - Very important - if there is a high point greater than your start reservoir level it might not be possible to flow without a pump
does the pipe need to be full?
How are you controlling flow?
Where are you controlling flow?
Do you have restrictions on supply of pipe?
Only then can you do a proper concept assessment and work out if a smaller pipe (say 110mm PE) with a small pump costs less than a larger pipe without a pump.
This is why designers exist to deliver what the client wants and within their widely differing requirements.
on the face of it, your pipe looks very big for what is quite a small flow rate and without further details it is difficult to know what is best in your instance. There are many parameters that you might think are better for your pipeline than someone elses, but we don't know what that it, but always remember you need to get a vertical prfile before you do anything else....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Pumping downhill
RE: Pumping downhill