Pumping downhill

[galapagos]

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

 

[LittleInch]

Probably not. That's only 110 m3/hr so at that flow the size as you go from one pipe size to the next one it will have huge impact on the pressure drop. Pressure drop is proportional to (d1/d2)^5

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.

 

[galapagos]

Thanks for your response! The piping we were looking at (large diameter, approx. 50cm pvc) seemed to keep major losses pretty low, much much less than 1.5bar

 

[katmar]

It's a bit more complicated than just matching the overall elevation change with the required flow rate. Search on Eng-Tips and also with Google for the subjects "Hydraulic Gradient" and "Slack Flow". If your pipeline ever goes above the hydraulic gradient line then you can run into problems.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[someguy79]

It is of course possible and feasible to transfer water over that distance without use of pumps. I hear the Roman aqueducts were pretty effective.

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.

 

[bimr]

That looks to be a 150 mm pipe which has a headloss of 29 m for plastic pipe. That would leave zero pressure at the outlet. Probably not doable.

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.

 

[georgeverghese]

Be careful with the configuration of the inlet at the reservoir end - there should be adequate submergence, some kind of anti vortex device, and the entire line must be fully liquid primed - a valve at the lower end should be closed, and the line filled up from the high point at the reservoir end.

Also the line exit at the end must be submerged.

Else you'll have air and water in this line, and not all water.

 

[EdStainless]

Check vents at the humps, good strong lines, it works for LA, they use gravity and siphons to come over some fairly large 'hills'.

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P.E. Metallurgy, Plymouth Tube

 

[LittleInch]

Galapagos,

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.

 

[bimr]

One alternative that may be attractive is locating a pump station along the pipeline route instead of at the starting point.

 

[georgeverghese]

A manual throttle butterfly valve at the downstream end (near the exit block valve) will also be required to burn off any excess static head. Line velocity should be kept at less than say 10ft/sec. For total eqvt length of say 2500m, a 150mm or 200mm PVC line should do.