Estimating flow depth in partially filled pipe.

[robche]

Just wondering how would I estimate the flow height of a partially filled pipe?

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Robert Markoski - Project/Mechanical Engineer

http://wwww.RobertMarkoski.net/Engineering

 

[MikeHalloran]

For horizontal pipe and very low liquid velocities and still or slow moving gas, it's just a geometry/mensuration problem, but it quickly gets out of hand with increasing flows as the flow regime transitions to bubbles, slugs, or mist.

Then it depends on the relative velocities of the liquid and gas fractions and the volume flow rates and whether the pipe is vertical or horizontal.

See "The Flow of Complex Mixtures in Pipes", authors Govier and Aziz, for many chapters worth.



Mike Halloran
Pembroke Pines, FL, USA

 

[robche]

I was thinking more of water flow down a sloping pipeline. I will definitely take a look at the book though. Thanks!

----
Robert Markoski - Project/Mechanical Engineer

http://wwww.RobertMarkoski.net/Engineering

 

[stanier]

Download freeware Flowpro 2

 

[BigInch]

If total head - vapor pressure at any point falls within the pipe diameter, its partially full flow. Just modify Bernoulli by the addition of vapor pressure, H₁ + (Z₂ + V₂²/(2g) - H_L - P_v) and calculate at each, or critical points along your flow path.

H_L, head loss between points 1 and 2, must be calculated using the hydraulic radius for partially filled pipe, which requires knowledge of the depth, so its an iterative calculation. See open channel flow for semi-circular channel cross-sections.

Depth of flow can be estimated by the V₂²/(2g) term, as it equals depth at the critical flowrate, where frictional flow loss equals energy gained by the reduction in elevation caused by the slope of the channel. Flowrates slower than that will be below D_critical, faster will be higher.

You might try this,
A 1 foot drop in elevation is roughly equivalent to 1/2 psi flow loss, so find your pipe flowrate that gives 1/2 psi loss over the pipe's length and limit the elevation drop of that run to 1 foot. That will approximate the depth at the critial flowrate which will remain constant with the slope of the pipe. Compare your flowrate with the critical flowrate at that depth to estimate the depth you'll have with your flowrate.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[robche]

@BigInch; I literally just started doing that 5 minutes a go. Thanks for reconfirming!

----
Robert Markoski - Project/Mechanical Engineer

http://wwww.RobertMarkoski.net/Engineering

 

[zdas04]

At each iteration make sure that your "slow flow" assumption is valid. Use the hyraulic radius as the length term in the Reynolds Number equation and make sure the Reynolds Number is less than 2,000. Much above 2,000 and none of your assumptions remain valid.

David

 

[CJKruger]

robche, if it is gravity flow, try the Manning equation.

 

[katmar]

I agree with CJKruger. For the formulas and tables see

http://www.cispi.org/handbook/chapter8.pdf

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com