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Buoyancy of pipe in water - with one end held in place

Buoyancy of pipe in water - with one end held in place

(OP)
I need some help with a problem that involves buoyancy of a pipe in water. We make a skimmer used in detention ponds. In some extreme cases we are having issues with overcoming buoyancy of pipe. The skimmer is attached to the outlet structure by a rigid pipe. The pipe is held in place by flexible couplings at both ends. The problem occurs when we are using a long section of 6" pipe in a deep pond. When the water depth gets over +/-6' and the pipe is over 16' long, the buoyancy of the pipe can lift the skimmer up out of the water.

I have calculated the buoyant force of the pipe, assuming it is empty, but less the weight of the pipe to be +/-12 lb/ft. What I am having trouble

RE: Buoyancy of pipe in water - with one end held in place

(OP)
Fat fingers - reposting full question

I need some help with a problem that involves buoyancy of a pipe in water. We make a skimmer used in detention ponds. In some extreme cases we are having issues with overcoming buoyancy of pipe. The skimmer is attached to the outlet structure by a rigid pipe. The pipe is held in place by flexible couplings at both ends. The problem occurs when we are using a long section of 6" pipe in a deep pond. When the water depth gets over +/-6' and the pipe is over 16' long, the buoyancy of the pipe can lift the skimmer up out of the water.

I have calculated the buoyant force of the pipe, assuming it is empty, but less the weight of the pipe to be +/-12 lb/ft. What I am having trouble with is determining the effect of the fixed end on the buoyancy. We need to hold the upper end just below the surface of the water - +/-8-10".

The skimmer itself is weighted and has a float attached. The skimmer weighs +/-100 lbs total. The portions that would add to the buoyancy are at least 1/2 full of water. I would estimate that the skimmer may add 35 lbs of buoyancy at the most for the section under water. It has a float at the water surface to keep it from sinking.

Any help would be greatly appreciated.

RE: Buoyancy of pipe in water - with one end held in place

Sounds like a problem I'd put on my first fluid mechs test back when I was adjuncting it.

First I'd neglect the weight of the PVC to make things easier and a little more conservative.

Next I'd pick an assumed design case of the pond being totally full. That will give you the angle and depth of the pipe.

Everything past that is a simple 2D Statics problem, unless I'm misinterpreting your problem statement. Consider the low end of your pipe to be a pinned connection. The top is a roller connection that goes left/right, and you're solving for the force in the up/down direction. The buoyant force on the pipe is a distributed load in the up direction, with a magnitude of 0 at the surface and a magnitude of (insert buoyancy relationship here) at the riser connection down in the pond.

Did I miss something?

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Buoyancy of pipe in water - with one end held in place

Calculations are certainly a great tool, but if I were manufacturing a product, I would want to do real world testing of the design. If one is going to do field testing anyway, it might be a lot easier to the simple calculation you have done as a starting point, and adjust weight based on testing.

RE: Buoyancy of pipe in water - with one end held in place

Specific gravity of PVC piping is around 1.2, so it should sink, somewhat slowly, if void of air being trapped in the piping. About air trapped in the skimmer giving you problems? Do a search for lead weights that can be attached to PVC piping as I remember that there are attachable weights for underground storage tanks to keep them from being buoyant.

RE: Buoyancy of pipe in water - with one end held in place

(OP)
Thanks for replies. First reply is on track. I just need to find my statics and dynamics books from 20+ years ago to actually do the problem.

We are doing testing, but challenge is a test area with sufficient depth to test the problem situations.

RE: Buoyancy of pipe in water - with one end held in place

Create a free-body diagram to identify all the forces acting on the pipe, and do not neglect the mass of the pipe.

RE: Buoyancy of pipe in water - with one end held in place

Looking at this from another angle, it sounds like your pipe is operating under inlet control. Have you thought about improving the inlet efficiency instead of weighting it? Maybe you could decrease the air volume in the pipe.

RE: Buoyancy of pipe in water - with one end held in place

(OP)
The main point of the skimmer is to control flow, so we don't want more flow in the pipe. We are looking into some other options.

Thanks for all the feedback.

RE: Buoyancy of pipe in water - with one end held in place

When you reach out you need to follow through respectfully and not just drop off ..Makes people not want to ever respond..What type of options now?

RE: Buoyancy of pipe in water - with one end held in place

(OP)
Did not mean to "drop off". Just did not want to encourage more answers if we are looking at other solutions and the answers are no longer needed.

To expand - the issue is holding one end of the pipe just slightly (4-12") below the water surface. The end tends to come to the surface and this is the problem. The other options are ways to allow the pipe to come to the surface, but still obtain the flow rates needed with less head on the pipe. We have these options under review and are affected more by design and construction issues we are working through than engineering issues.

Do appreciate the thoughts provided by all.

Thanks

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