Flow by gravity in a not full pipe

[babooman]

Dear all,

I need to perform a calculation of the flow by gravity in a not full pipe. I used in the past Manning equation for that purpose with water pipes, but this time the fluid is diesel oil. I am not sure of the correction to be done in Manning equation, since I guess that diesel viscosity must have some impact.

I appreciate any help. I'm trying to find some reference or alternative equation without success so far.

Thanks in advance.

 

[MJCronin]

Forget Manning....

Froude number is the key baboo.....

http://www.freecalc.com....

 

[babooman]

Thank you for the answer.

If I forget Manning, what equation could I use to perform the calculation? Any idea?

Thank you once again.

 

[TD2K]

Try Crane's flow of fluids, I'm pretty sure they have an example of a pipe flowing while only partially full.

 

[Latexman]

Example 4-26 in my edition of Crane.

Good luck,
Latexman

 

[ione]

babooman,
You can use the Darcy-Weisbach equation together with the Colebrook-White formulation for friction factor. This approach is more common for full pipe flow, anyway introducing the concept of hydraulic radius and making the opportune corrections it can be adopted even for open channels and so to partially full pipe flow.

 

[BigInch]

In a short section, where viscosity effects will not sum to anything of significance, Specific Weight is the important factor.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[katmar]

If it is only important to know that the pipe will have the required capacity, and it is not important to know how full the pipe is, then it is an easy calculation. You take the overall difference in height between the inlet and outlet and multiply by the fluid density and gravity to get the differential pressure which is your driving force. Then assuming that the pipe is full use the Darcy-Weisbach equation to calculate the flowrate. This requires a trial and error procedure unless you have software that does it for you. If this flowrate is greater than your required flow then the pipe will run part full.

This is the scenario where you have an existing pipe size and you want to calculate the flow capacity. Similar calculations can be done if the flow is known, but not the pipe size. Determine the pipe size with this available head for full flow and then if you use a larger pipe you know it will run less than full.

All of this assumes an evenly sloped pipe. If you have high and low points you can get slack flow and the problem is more complicated.

If it is important to know how full the pipe is then you need to do the analysis described by ione.

Katmar Software - Uconeer 3.0

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[jnich]

Since this flow is driven by gravity and your viscosity is relatively high, you may only have laminar flow within the pipe. If so, this simplifies your friction factor if using Darcy-Weisbach. Check your Reynolds Number (Re) for flow regimen. If your flow is indeed turbulent, I'd recommend the Swaimee-Jain explicit approximation for determining your friction factor (which is based on the implicit Colebrook equation and Moody Chart).

Interesting that some would use Manning's equation here...My experience is mostly in process piping where there are no slopes to speak of, only horizontal & vertical segments. I suppose long pipeline runs would have a general slope, however.

 

[jgailla]

The Manning slope is the slope of the hydraulic grade line, not the slope of the pipe. For partially full or open channel flow, however, the HGL will generally be the pipe slope.
Manning's is not the appropriate equation to use here with a heavy fluid, since it is developed for water's viscosity (sanitary or storm) and there are no adjustments in the standard Manning's equation for higher or lower viscosity. The empirical equation was also developed in the turbulent flow range.

 

[BigInch]

Viscosity of 4-5 for diesel vs 1.0 cP water will not have much effect over a short distance. In gravity flow of diesel, where I would expect any pipe lengths to be very short as the general rule, shouldn't show much of a difference in flow between such small values of viscosity. If the viscosity was several hundred cP or so, yes it would have some effect. Check the head loss for the length of pipe under consideration when flowing full for both diesel and water to get a rough idea of what error might be expected.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[MortenA]

Now you get different opinions

For a non full flow manning is fine for _near horizontal lines_

For vertival lines avoind entainment of air will prevent "gurgling" and this is the key issue and here its the Fround no=V/(sqrt(D*G) (V:m/s, D (ID, m), g grav constant (m/s²) this should be less than 0.3

Best regards

Morten

 

[MJCronin]

MortenA is on the right track....The 0.3 criteria is the key !

(TD2K: I do not believe that this information is contained in Crane 410)

http://en.wikipedia.org/wiki/Froude_number

http://www.freecalc.com

Prevent gurgling !

 

[MortenA]

Maybe i got the word wrong (gurgling)? BTW i at least got the name wrong: Its Froude

Best regards

Morten

 

[dicksewerrat]

Burping is another technical term for air trying to go upstream in a pipe with water flowing downstream.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[BigInch]

Gravity flow in a partially full pipe usually does not consider full flow in some areas and not in others. Normally it is considered as partially full along the pipe's entire length. No gurgles, burps.

Baboo, what is your situation?

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[katmar]

Its really frustrating when someone drops in to ask a question, and then never returns to check the answers and provide the missing information. I suppose these technical terms like gurgling and burping refer to air coming back out the entrance to the pipe. Is there an equivalent technical term for when the air is entrained with the liquid and passes out the tail end of the pipe?

Katmar Software - Uconeer 3.0

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[babooman]

Thank you all.

My exact situation is the lube oil drain line from a steam turbine back to the lube oil storage tank. They are short lines (20 m approx) with continuous slope (2% approximately).

 

[katmar]

Welcome back Babooman. If the entrance to your pipe is in the floor of the turbine casing such that the oil flows vertically downwards then keeping the Froude number below 0.3 will be the criterion to use for that vertical section. Once the pipe becomes gently sloped then the analysis that I described earlier (or the more complete analysis proposed by ione) will be the way to calculate that section.

Katmar Software - Uconeer 3.0

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[bimr]

Belch is probably the correct term:

http://dictionary.reference.com/browse/belch