Shallow basin pumping

**cullyott** · 03-15-2007, 07:22 AM

I have a question concerning depth and vortexing in a water basin. My company and I are designing a system comprised of a large U-shaped basin of water (1000000L). At the end of the basin I wish to remove the water to be filtered then returned to the beginning of the tank. The dimensions of the end of the basin are 10m widex0.3m deep with the entire tank being 30cm deep. We were planning on removing water at a rate of 5.3m3/hr (~1.5L/s) but are unsure as to whether this rate will cause a vortex of air to be drawn into the pump since the water is so shallow. We have been searching for some time now and cannot find any way to confirm whether this will occur or not. If possible could you please indicate to us whether this vortexing would occur or not and/or if possible what suction velocity into the pipe would cause it to occur. We are still in the early stages and have not yet selected a pump or pipe size. Thank for your help and any input would be greatly appreciated.

**Bob NH** · 03-15-2007, 10:34 AM

With the large size of the basin and the small flow, the size of the outlet pipe is a small matter. I would make it large enough to be sure that it won't suck in air.

I would pick an outlet size so that the velocity head in the outlet does not exceed 10 percent of the depth of water above the outlet. The inlet to the outlet should be rounded to prevent formation of a vena contracta.

It is customary to make the inlet to the outlet pipe at least 50 percent larger than the inlet to the pump, but I would not make it less than the "10 percent velocity head" condition mentioned above.

I would also plan on using a screen to prevent trash from getting sucked into the pipe. That screen would contribute to preventing formation of a vortex that would suck air.

If you want to do an analysis of the problem, you can look up the analysis of a "free vortex" in an advanced reference on fluid mechanics.