Flow rate in a gravity fed water pipeline 1

[gilesm89]

Hi everyone,

Was hoping for some help from a pipeline guru.

I am looking into the maximum flow for a pipeline from a reservoir to an outlet about 40m lower.

The way I have been looking at it is what is elevation head of the reservoir surface and that is effectively the driving head for the flow. I am then looking at a pipe friction handbook and taking note of the head loss m/100m and then matching the total headloss to the elevation difference from reservoir surface to outlet. The book then gives me the flow rate in L/s for that value.

Am I on the right track? I did note that the longitudinal profile is a bit up and down (but never higher than the reservoir. Will this affect the flow rate? Must the hydraulic grade line be above the pipe elevation at all times?

Also the PN rating of pipe. As far as I understand PN8 could accommodate 80m of pressure. Does this vary with temperature?

Hope this makes sense!

 

[cvg]

rather than significant trapped air problems or multiple lift stations with multiple gravity lines with deep cuts, fills and tunnels all of which is costly and very problematic; I would suggest 1 pump station and force main all the rest of the way, starting at the tee into the existing pipeline, with the profile following the ground slope, and with a pressure sustaining valve at the end to prevent draining of the pipe and introduction of air when the pump shuts off. design the force main for sufficient velocity and trapped air will be unlikely.

 

[BigInch]

Isn't a force main, by definition, NOT a gravity pipeline.

you must get smarter than the software you're using.

 

[cvg]

yes, a force main is not a gravity line and from what I can see the whole gravity line idea is fraught with problems

 

[bimr]

My understanding is that the OP is working on a water supply project, not a sewage force main.

Where I am located, the term "force main" is a term for a pipelines that convey wastewater under pressure from the discharge side of a pump or pneumatic ejector to a discharge point.

I agree that the a force main is more difficult project and would require a pump.

 

[BigInch]

Not a problem, for a proper application and if done correctly.
As I understood it, we were trying to determine if the application is a proper one for a gravity flow solution.

you must get smarter than the software you're using.

 

[cvg]

ok, I guess we have a lack of information about what type of water is flowing from this reservoir, so perhaps raw water pipeline might be a more appropriate term, I don't know. If we are parsing words, than the term "lift station" is also commonly used for wastewater.

 

[BigInch]

Yes quite true, but "A rose by any other name would smell as sweet."
If you run out of head, be it gravity or otherwise, you need a pump to "lift it".

you must get smarter than the software you're using.

 

[bimr]

One would expect that if you maintained a flood suction on the pipeline (there is a reservoir) and kept the pipeline from draining with a back pressure valve, then the pipeline should work as a pressure pipeline.

 

[cvg]

BIMR, quite true, however it does not appear it can handle the design flow nor will it flow with enough velocity to handle the air that will enter from the reservoir. Seems like two fatal flaws

 

[BigInch]

Any fixed working pipeline, exposed to an acceleration field (gravity might be one) will have pressure anywhere the fluid depth is not zero. Technically all flowing pipelines are pressure driven, even if not flowing full. Hydrostatic pressure on one cross section is greater than the pressure on another, presto, flow. The key question; is riving pressure caused by gravity, hydrostatic, or some other driver.

you must get smarter than the software you're using.

 

[bimr]

230 l/s over 7,200 meters in a 450 mm pipe gives a headloss of 38.4 meters with a velocity of 1.8 m/s. The OP is showing a drop of 45 meters.

Flooded suction means exactly that. It will also be just as difficult to keep air out of a pumped system.

Looks like it would work to me.

 

[gilesm89]

Thanks for all the responses. The fact that this has sparked quite a lot of debate proves to me that this is much more complicated than I originally thought! I'm going to take some time to look into the trapped air issue, as this is something that a lot of you have mentioned. Thanks again!