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It was recently noticed by our team that after the loading state had switched to the settling state, a slow but seemingly never-ending recirculation of particles was occurring in the settling column. This recirculation is affecting our settling data, and maybe the important clue into why we haven't been seeing "great" settling curves as we would have expected in our results. To rid our setup of this recirculation problem will give us more accurate results.
It was hypothesized that this could have been a result of the elbow connections, the change in diameter tubing from the flocculator to the settling column, the valve shutoff, upflow versus downflow through the column, or the abrupt stopping of the pumps in the transition of states. All of these ideas were put to the test in simple visual experiments.
First, the pump controll shutoff was removed from the beginning of the settle state to a new state before settle state. This allowed for the pumps to be shutoff seconds before the valves were shutoff. Different times between the two shutoff points were experimented with, but each time, at the time of the valve shutoff, a jerk in motion would seem to cause a recirculation in flow. Perhaps we need to not shutoff the valve in order to prevent this recirculation.
Monroe programmed a new variable in Process Controller to ramp down the pump controls to zero instead of abruptly switching from a high pump control value to zero (shutoff) automatically. We installed this ramp down function in Process Controller and tested it on our apparatus. The ramp down would bring the flow down to an easier flow. We tried this new function with both a downward flowing and upward flowing column to detect any differences.
After careful observation, it was concluded that the recirculation was occurring even without the valve shutoff (about 20-30 seconds into settling state) and it maybe from either the pump shutoff causing a "push/pull" in the water (even though the pump is being ramped down, there is still a slight pressure change when the pump is completely shutoff) or from the diameter change from the flocculator tube to the settling column causing eddies of turbulence in the top of the column.
Next, we must determine if this second theory of differences in diameter is causing the real problem.