Effect of gravity on a loop
Effect of gravity on a loop
(OP)
Hi. I have a question which is probably easy, but I can't get my head around it.
There is a body of water in which a pipe has both ends submerged.
The pipe is a loop, and extends 20 meters high.
The pipe is primed with water, and I want to move water from 1 end of the pipe to the other.
The siphon effect does not count as both ends of the pipe are at the same height.
Do I still need a pump with sufficient head to move the water, or does gravity get cancelled in this arrangement as I am being led to believe?
Thanks.
There is a body of water in which a pipe has both ends submerged.
The pipe is a loop, and extends 20 meters high.
The pipe is primed with water, and I want to move water from 1 end of the pipe to the other.
The siphon effect does not count as both ends of the pipe are at the same height.
Do I still need a pump with sufficient head to move the water, or does gravity get cancelled in this arrangement as I am being led to believe?
Thanks.





RE: Effect of gravity on a loop
You should start your pressure analysis at the outlet - bottom of downleg - and working backwards. By accounting for the static head and the friction losses you can work back up the downleg to get the pressure at the top. With a 20 m high loop you will probably find it is negative at the top. This is clearly impossible and nature solves the problem by boiling the water to lower the static head.
Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Effect of gravity on a loop
If the loop height was smaller so that the pressure at the top was still positive, would there still be an increase in temperature?
Or does the temperature only rise as a result of the negative pressure?
RE: Effect of gravity on a loop
I see you are a computer engineer, so perhaps a little further explanation is warranted. At any given temperature a liquid exerts a fixed vapor pressure. At higher temperatures the vapor pressure is higher. When the vapor pressure equals the surrounding pressure the liquid boils. At high pressures a higher temperature is required to boil the liquid (as in the domestic pressure cooker) and at low enough pressures the liquid will boil at ambient temperatures.
Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Effect of gravity on a loop
Good luck,
Latexman
RE: Effect of gravity on a loop
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Effect of gravity on a loop
Now imagine that you gradually increase the flow rate. Eventually you will reach a flow rate where the downleg runs full. When this is achieved the static head recovery in the downleg will aid the pump and it will "see" less than 20m. Increasing the flow rate beyond this point is what BigInch has described. The friction losses in the downleg cut into the pressure recovery and eventually a flow rate will be reached where the net recovery is zero. In practice, this is a very high flow rate and it is rare (but not unknown) to employ such high flow rates in process plants. At such flow rates the friction in the upleg is also significant.
The nasty part is in the low flow zone where the downleg is not yet full. This is a frequent circumstance. It can cause severe vibration. The cure is leave the top open, or use a vacuum breaker/air vent.
Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Effect of gravity on a loop
RE: Effect of gravity on a loop
There may be no reason to keep the down-leg full of water and, if so, there is no reason to have a backpressure control valve at the outlet as cvg suggests. Non-pressurized sewer lines, for example, run at similar partially full conditions all the time. In fact, they are designed to flow at about 80% full because that is the point where the flowrate is maximized, since friction does not act between the fluid and the arc at the top of the pipe that the fluid does not touch.
Many petroleum pipelines are designed to run in cascade conditions too. Especially when they are running at "first oil" scenarios, before all the wells are producing and total field production going into the pipeline is still at a bare minimum. The BTC pipeline 42" crude pipeline from (Baku, Azerbijan to Ceyhan, Turkey) has a pressure reduction station just before going down the last slope to the marine terminal at Ceyhan which was used exactly for that purpose, to keep the high mountain head (2800 meters) from raising the pressure and accelerating the flow into the marine terminal. It was operated at cascade conditions for a number of years, before flow was increased enough to lose all (and more) to friction what it gained from its descent down the high mountain slope. During that time the pump stations only had to lift the crude to the 2800 meter head (plus some up-leg flowrate friction) to cross that last 2800 m mountain range.
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Effect of gravity on a loop