Partially Filled Gravity Pipe

[momo83]

Hi All,

I am used to calculating pressure mains which are assumed to be "flowing full" pipes.

Now, I was requested to size for a gravity line (Water Transmission) which I assume is a "partially filled" flowing pipes. Is there a standard formula to compute for the actual percentage filled of depth of water passing through the pipes? Manning Formula seems helpful but it looks like it assumes flowing full pipes or open channel system.

Thanks!!!

 

[katmar]

The Manning Formula can be used for "open channels" of circular cross section. "Open channel" just means part full and with a continuous gas phase above the liquid for the length of the channel. A Google search should find tables listing flow rates, liquid depths and slopes for each size of pipe.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[robyengIT]

American Concrete Pipe Association (

http://www.concretepipe.org)

here the link to the manual (page 220)

https://www.concretepipe.org/wp-content/uploads/2014/09/cp-manual.pdf

 

[robyengIT]

some further doc (but in italian)

 

[LittleInch]

I found this whilst looking for another post

https://www.eng.auburn.edu/~xzf0001/Handbook/Channels.html

One thing to note is that the Froude number needs to be less than 1 to avoid getting into issues of surging and glugging ( I think)

There are also designs which look at flow through vertical tubes and again once you get past some sort of critical number (probably the Froude number), your simple flowing pipe starts tog et choked with excess flow and everything changes.

however a "gravity line" is most efficient when full so maybe they just mean a full pipe which doesn't have a pump?

You may need to incorporate a back pressure control at the far end to keep it full all the time, especially if the pipe level undulates. Gravity lines running partly full need to maintain a slope down at all times.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LittleInch]

MJC - Are you sure?

It depends on what is meant by "gravity flow"?

If this means something like a sewer /drainage pipe at slope of 1% or so then it surely does.

If it means a near vertical drop then sure - you're into a different calculation which is more akin to gutter drain pipes and overflows.

The link is for a tank overflow. I don't know many tanks which have a 1:100 slope on the overflow line.

Hopefully momo83 will come back and let us know what he meant by "gravity flow"....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cvg]

mannings can absolutely be used for this

Now, I was requested to size for a gravity line (Water Transmission) which I assume is a "partially filled" flowing pipes.

I question the assumption that the pipe is not flowing full, just because it is a gravity line. You will need to do a full hydraulic analysis of the entire pipeline to determine the flow rate and it is likely that at least part of your transmission line will be flowing full. Unfortunately, this "sizing" probably cannot be done using one simple formula. You should google "Bernoulli" for more information

http://slideplayer.com/slide/483265...+of+Fluid+in+Motion:+Bernoulli´s+Equation.jpg

 

[georgeverghese]

Presume you intend to calculate pressure drop in partially filled circular cross section or rectangular pipes? This is discussed in Perry Chem Engg Handbook 7th edition, page 6-12 and 6-13 with notes on the derivation of the hydraulic diameter, Manning formula for the friction factor and roughness values for n are in table 6-52.

 

[bimr]

Mannings equation is the correct formula for gravity flow. You will have to check the profile to be assured that the pipe slope does not have any humps or dips which will interrupt the flow conditions and may make the pipe air bound.