Gravity feed to bottom of a lake (very much a newbie)

Forum about flow calculators available on pipeflowcalculations.com and how to find solution for your fluid flow problem.
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admin
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Re: Gravity feed to bottom of a lake (very much a newbie)

Post by admin » Thu Nov 07, 2013 11:03 pm

Flow rate through pipe is established based on the energy (mainly pressure) difference from start to the end of pipe. If you have bigger pressure difference flow rate will be higher. If you have same pressure difference flow will be higher when resistances are lower.
When you have the end of pipe under the zero water level, it is the same effect as if you have counter pressure on pipe exit. So for p2 in calculator you should enter pressure due to water depth which is p2=rho * g * h. On the other hand, as you have start of pipe above the zero water level you should use that elevation as inlet pressure, which is p1 = rho * g * (H + h).
h - the depth of pipe exit in the water
H - height of pipe entrance above the water.
So in the end you will have pressure difference of p1 - p2 = rho * g * H - that is the energy that you have for water to flow. That energy will be used to create water flow in pipe. Losses due to friction are depending on flow velocity, pipe roughness, pipe length, flow regime.
Of course if you have trapped air in pipeline, you will have possible loss of flow, at the end.
Pipe flow calculations - since 2000

admin
Site Admin
Posts: 371
Joined: Mon Feb 08, 2010 7:47 pm

Re: Gravity feed to bottom of a lake (very much a newbie)

Post by admin » Tue Nov 12, 2013 10:39 pm

Power of pump can be calculated as P = Q * (p1 - p2). Pressure drop is due to friction, local resistances and change in elevation between start and end of pipe.
So, if you calculate pressure drop in pipe (p1 - p2) for some flow rate, you can also calculate power of pump for that flow rate.
And that is it. You also should add some 5 and more % for energy loss in pump itself and it depends on the pump.
So, if you have changes in height in the pipe line, it doesn't affect pump power. Start and end pipeline elevation are important. It is only important to keep pressure in water pipe line above the pressure where water evaporates for given temperature to avoid chocked flow (vapor pressure).
If you are pumping water through the pipe line with pump and pipe line goes up, you can have pressure in pipe line bellow vapor pressure and that you want to avoid. In that situation you can have open tank at some point and than pressure at that point will be atmospheric avoiding possible water evaporation. Of course pump will have to work with much higher pressure and power to overcome height difference as well as resistances.

But if you have pipe line that goes up and than down to the same elevation as start of pipe, than your pump will only have to supply enough pressure to compensate for friction and not elevation difference, meaning pump will work with much lower power.
Pipe flow calculations - since 2000

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