Poor Tank Overflow Design

[IronMike]

A properly designed settler or clarifier has an overflow launder to distribute the upward velocity evenly around the tank. But when a cylindrical tank is installed with a single overflow outlet, say with an outlet velocity 10X the upflow velocity, what is the magnitude of the effect on upflow velocity near the outlet. I would think it would be higher and the upflow on the far side lower by the Bernoulli effect. Also, the surface velocity toward the outlet from the far side of the tank would probably be zero. Any thoughts on how much penalty on uniformity of upward flow?

 

[bimr]

It depends on many factors. If you are trying to remove heavy particles, probably not much of a penaly.

If you are trying to remove light particles like for example water or wastewater applications, you are probably looking at 75-90% penalty.

Don't forget that you should have some type of inlet flow distribution as well.

 

[Compositepro]

The inlet flow distribution is the critical issue. Flow in the tank is controlled by the momentum the flow had leaving the inlet flow distributor. Once the fluid reaches the top of the tank the flow will change due to gravity and the level gradient toward the outlet. Turbulent flow is not desired in a clarfier but is likely to affect ony a very small volume by the outlet.

 

[IronMike]

Thanks for both replies. Flow comes from bottom up through solids and probably is well distributed. Near exit, fluid has to accelerate from 1X to 10X out the outlet pipe. Without CFD to draw pretty arrows, I have to assume the flow nearest to the pipe has the highest vertical velocity and near zero on the opposite wall of the tank. Same for horizontal velocities across the tank. May have to float a ball on the tank to see where it speeds up.