More on reservoir mixing

[Whiskeytown]

At the end of his discussion in thread164-143826 on this topic, BarryEng noted that his research on tank mixing had concluded that (among other things):

"* Circular tanks - inlet thru floor & directed upwards at 45 deg in a circumferential direction. Fairly good mixing with the whole of the tank water rotating but some cross flow in elevation..."

I am working a possible mixing plan for two .5 MG circular tanks.. There is a combined 16" inflow/outflow pipe from the floor with a silt stop. I am thinking of something like the attached sketch. (Weld a tee with a inward Red Valve to the inlet/outlet. Angle an 8" or so "nozzle" with an internal Red Valve at 45 degrees up parallel to the tank wall.) Any thoughts? The tanks are undergoing some repainting, and this would be less expensive than some other schemes I have seen.

 

[BarryEng]

Looks good to me.

What surprised us on testing (of actual tanks), was that with a configuration similar to your sketch, induced the whole tank contents to move in a circular direction. That's the expected result. But after a considerable period after the pump was turned off, the contents were still moving. We had originally thought that the viscosity of the water would 'dampen out' the circular motion relatively quickly.

The 'cross flow in elevation' was the tank contents moving in a circumferential direction, & the jet flow was crossing the general flow pattern. It did not appear (from what we could see visually from the top of the tank) to interfere too much with the circular flow pattern.

The best flow pattern appeared to be when the water was introduced circumferentially above the top water level. The whole reason for this type of circulation, was 'last in, last out' for quality purposes (maintaining Cl2 levels & THM control). We did not want any stagnant (or dead) water that was not circulated.

But this location of the inlet was for a different reason - our operation people wanted an air gap between the inlet to the tank & the water level. My argument was that the pump has a head to the top of the tank. With a bottom inlet (or taking the inlet to the bottom), the static lift is less for a near empty tank allowing extra flow (depending of the H/Q curve & the total lift) virtually all the time the pump is running. I lost that argument. Remember that for a bottom inlet, any burst of the inlet pipe will drain the tank. This can be a problem is some of our really remote locations.

The only reason I would locate a reducer on the inlet, would be to reduce the cost of the valve (size). A nozzle (theoretically) will give a much greater ability to mix the water (higher velocity) but we found that this did not appear necessary - only a reasonable velocity was required. The only time we have used nozzles, are on large potable reservoirs (200 m x 200 m x 10 m deep) with a relatively slow (as a proportion of the total storage) inlet.

 

[Whiskeytown]

BE,
Thanks for coming back in on this. I think 400 gpm is a higher-end figure for the amount of inflow we will be getting. With a 16 inch pipe, that moves at 0.64 fps, but since it's the same mass of water we should get similar performance. Stands to reason, too. Why should I get fixated on nozzles, when the difference in velocities would just make for more turbulence and less macro-mixing?

Maybe I can have it about 8 feet high, so the inlet valve can be changed out without high ladders, yet be about halfway up at low alarm for optimal high/low mixing. And the tide valve would be mounted on the end of the outlet pipe in the conventional way. I like it!

 

[Artisi]

Have you considered putting a couple of baffle plates in the tank to overcome the circular rotation of the complete contents and make it more of a mixing action.

 

[BarryEng]

Whiskeytown
I accessed this site by accident the other night, so I was not waiting in anticipation for your query (it was a coincidence). Perhaps I had a premonition?

Most of our pumping mains are about 1 m/s (3 fps) & the exit velocity is sufficient for tank mixing. We did not test slower velocities. I do not know what is the 'lower bound' velocity to achieve circular flow patterns. If I was asked to design a flow pattern for a tank with a velocity of 0.64 fps, I think that I would use a reducer to 'speed up' the water a bit. BUT I do not have any test results as a basis for this conclusion.

When we did the testing, baffle plates would always result with at least one 'dead' area of water (no mixing, hence quality problems). The classic problem (of dead water, not baffles) was when we directed water across the diameter of a tank. The water direction went straight across the tank & split two ways (left & right). At the opposite side of the tank to the inlet, 'dead water' was ALWAYS the result.

Artisi
I think that we have two problems that we are discussing here - mixing of a body of water to ensure first in, first out for water quality purposes (very slow circulation, not necessarily mixing), & local mixing for the purpose of (for example) mixing of chemicals (in this case a turbulence is req'd for mixing). When I am considering mixing for a WTP chemicals (for example in the chemical mixing of a flocculting agent for a clarifier), my design of a mixing chamber is very different to what I would use for the introduction of new (incoming) water into a tank. What I am trying to do (with a tank or reservoir) is ensure that there is no dead water, with the main goal of 'first in, first out'.