Orifice
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Clogging Experiment
Figure 1: Experimental Setup - Orifice Clogging
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A linear dosing meter is currently being utilized in Honduras to control the flow of alum, based on varying plant flow rates. In this system, the alum flows from the stock tank to the constant head tank through a small 0.23 cm orifice. This orifice is becoming clogged with an unknown substance, preventing the flow of alum into the entrance tank and consequently reducing the effectiveness of flocculation. The constant head tank orifice has to be cleaned multiple times per day by the operator. A more ideal system would clog at most once per day. The objective of this experiment is to determine the cause of the clogging in Honduras (precipitant or foreign material) as well as estimate the frequency of clogging in the new non-linear dosing system. If the new orifice (0.1 155 cm diameter) clogs frequently with lab grade alum, the system will need to be redesigned to incorporate a larger orifice. If the source of the clogging is determined to be foreign material (little or no clogging occurs with lab - grade alum) a strainer will be included in the overall system design to remove the foreign material.
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The experimental setup accurately mimics the conditions in the non-linear dose controller without incorporating the additional hassles of a large stock tank, constant head tank and entrance tank. As seen in Figure 1, the water circulates through a peristaltic pump to a small one liter reservoir intended to steady the pulsing input from the pump. The reservoir is connected to a tee, which connects a pressure sensor to the system just before the orifice. Water then drips through the smallest orifice, 0.1 155 cm diameter, into another one liter reservoir. The This reservoir is connected to the pump, creating a closed system. The pump will be is set to discharge water at the same rate as the water flows through the orifice to maintain steady-state operation. The experiment will was first be run with deionized water as a control. Then it will be was run with the maximum concentration of alum to be used in a plant, 125 g/L. As the experiment runs, Process Controller will record records the pressure directly upstream of the orifice. The A safety sensor is there as connected to the second reservoir to provide further validation of the presence of a clog. As the pressure from at sensor 1 increases due to a clog, the pressure from at sensor 2 decreases due to decreased inflow from the orifice.
Results
First, a control experiment, using water, was performed to allow us to see depict normal pressure readings without any clog being presentthe possibility of a clog at the orifice. These pressure readings will serve as a basis for comparison when we begin to run alum through our to the data obtained when alum is run through the system. The pressure results from pressure sensor 1 can be seen in the graph below.
Graph 1: Pressure
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Sensor 1
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Readings - Control
Next, alum was used in order to determine the time it would take for a clog to form in our .1 155 cm orifice. The experiment was run for four days but the most conclusive data was seen within the first hours. The pressure readings, from pressure sensor 1, are shown in Graph 2 below.
Graph 2: Pressure
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Sensor 1
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Readings - Alum
As can be seen from the graph there is a large spike in the pressure readings about 4 hours after beginning the test. The pressure sensor reads a higher and higher gradually increasing pressure, due to the clogging orifice, until the it builds up enough to "blow out" the clog.
Possible
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Sources of
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Error
The elevated pressure, shown in Graph 2, seems indicative of a clog, but the spike was not repeated at all for the remainder of the experiment. If the clog was "blown out" due to the increase in pressure then it should simply reform after another 4 hour time span, forming a clogging cycle. Yet this cycle of perpetual clogging and clearing of the orifice was not observed. This seems strange was unexpected and might indicate erroneous pressure readings for the spike.
Conclusions/
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Future Work
The .1 155 cm orifice appeared to clog after 4 hours, indicating that a larger dosing orifice might need to be used. Further testing will need to be performed in order to verify these results, and determine the clog time for any larger orifice we select. An ideal system would only require cleaning once a day.
Since lab grade alum was utilized in this experiment and a clog was still present, the presence of foreign material, might not be the main cause of the clogs present in the Honduran plants. An inline strainer would still be recommended for future dosing systems, yet it should be noted that this won't would not eliminate the occurrence of clogs since precipitates seem to cause them as wellthey also appear to be caused by alum precipitate.