[StirCrazy]

Originally Posted by Invigor

so I assume using the "shear" theory, that is why mag drives recommend you double the pipe size compared to the pump input/output? (ex: my mag 12 has 3/4" inlet/outlet so I should be using 1.5" pipe to maximize the water flow?)

I guess with less pressure my mag would be quieter too!

this is part of it but you are also incorporating laminar and turbulent flow to the equation.

now this is just off the top of my head so if I have some terms wrong for give me.

OK one assumption to make, the law of conservation of mass (meaning a given mass put in in a period of time must come out in the same period of time)

if you have a small pipe it will cause high shear stresses which will lead to turbulent flow. this is because if you look at the conservation of mass you are putting in 200 gph from a 3/4" pump outlet into a 1/2" line so the result will be a increase in pressure so it can keep the same volume of water coming out the end. this increase in pressure makes the pump work harder but it also reduces the efficiency of the pump by making it have to work harder to achieve the same result. inside the pipe as the amount of water put in increases you fine the shear stresses between the boundary layer and the rest of the water increase until the water flow becomes turbulent and requires even more work out of the pump to push the water through.

by going to a larger pipe you are slowing down the water flow and creating a laminar flow of water through the pipe. you are also dropping the back pressure allowing the pump to work more efficiently.

Steve