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
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
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
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
RE: Buoyancy of pipe in water - with one end held in place
RE: Buoyancy of pipe in water - with one end held in place
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
RE: Buoyancy of pipe in water - with one end held in place
RE: Buoyancy of pipe in water - with one end held in place
Thanks for all the feedback.
RE: Buoyancy of pipe in water - with one end held in place
RE: Buoyancy of pipe in water - with one end held in place
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