h1. Orifice Clogging Experiment
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{center:class=myclass} h5.Figure 1: Experimental Setup{center}
h3. Overview
A linear dosing meter is being utilized in current AguaClara plants 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. The constant head tank orifice frequently needs cleaning by the operator due the accumulation of chemical precipitate that is probably a combination of calcium, aluminum hydroxide, carbonates, or some other unknown substance. The frequency of the cleaning probably varies a lot, but we should recognize the importance of keeping the orifices clear of buildup (either from precipitate or foreign material) due to its detrimental effect on dosing accuracy and thus subsequent flocculation and sedimentation processes.
The objective of this experiment is to determine the cause of the clogging in Honduras (precipitant or foreign materials) and estimate the frequency of clogging in the new non-linear dosing system. Because in our experiment we are using lab grad alum and deionized water, the chances of clogging due to foreign material is low. If the orifice clogs, it can be proposed that alum precipitate or other chemical precipitates have an effect on clogging and we will redesign the system to minimize clogging and increase the probability of effective dosing. If the orifice does not clog, then the problem of clogging in Honduras can be resolved by adding a strainer to remove foreign material. In our experiment, we chose to use a 0.1 cm diameter orifice, the smallest size orifice in our design, to test the weak point in our design with regard to clogging.
h3. Experimental Setup
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 cm diameter, into another one liter reservoir. This reservoir is connected to the pump, creating a closed system. The pump is set to discharge water at the same rate as the water flows through the orifice to maintain steady-state operation. The experiment was first run with deionized water as a control. Then it was run with the maximum concentration of alum to be used in a plant, 125 g/L. As the experiment runs, Process Controller records the pressure directly upstream of the orifice. A safety sensor is connected to the second reservoir to provide further validation of the presence of a clog. As the pressure at sensor 1 increases due to a clog, the pressure at sensor 2 decreases due to decreased inflow from the orifice.
h3. Results
A control experiment, using deionized water, was performed to depict normal pressure readings without the possibility of a clog at the orifice. These pressure readings serve as a basis for comparison to the data obtained when alum is run through the system. The results from pressure sensor 1 can be seen in the figure below.
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{center:class=myclass} h5.Figure 2: Pressure Sensor 1 Readings - Control{center}
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Next, alum was used in order to determine the time it would take for a clog to form in our 0.1 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 Figure 3 below.
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{center:class=myclass} h5.Figure 3: Trail 1 of 24 hour Alum Experiment Data Pressure Readings (11/19/09) {center}
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 gradually increasing pressure, due to the clogging orifice, until the it builds up enough to "blow out" the clog.
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{center:class=myclass} h5.Figure 3: Trail 1 of 24 hour Alum Experiment Safety Pressure Readings (11/19/09) {center}
As stated earlier, the build up of water pressure at pressure sensor 1 (data sensor), the pressure at sensor 2 (safety sensor) should decrease. However, since the supposed "clog" was short-lived there was no corresponding data from the safety sensor to confirm the clog.
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{center:class=myclass} h5.Figure 4: Trial 2 of 24 hour Alum Experiment (12/03/09){center}
Another trial with alum was done over 24 hours. As seen in Figure 4, there are spikes in the pressure readings. We are unsure whether or not these represent clogs because the maximum difference in height of the water is at most 0.2cm. This could be in the range of the pressure sensor error.
h3. Possible Sources of 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. The precipitation and blow out could possibly be more intermittent than one might be able to periodically predict. Because the second experiment had different results, more trials would have to be done to determine whether or not clogging is occurring.
h3. Conclusions/Future Work
The 0.1 cm orifice appeared to clog after 4 hours, indicating that a larger dosing orifice might need to be used. The second 24-hr experiment did not have the same results as the first and thus it is unsure whether or not clogging does occur after four hours. For the Agalteca plant, based off these experimental results and to ensure minimal clogging, we are recommending increasing the diameter of the smallest dosing orifice to 0.152 cm (0.06 inch) to minimize the occurrence of clogs. We used lab grade alum and the one in Honduras could have extraneous material that might clog the small orifices. Further experiments will have to be performed in order to verify that this will indeed increase the time till a clog forms.
In addition, we are recommending using 102 grams/liter stock tank solution, which should minimize the clogging frequency since the alum is less concentrated. The downside to using a lower concentration alum is that the stock tank will have to be changed more frequently. We will check with the operators in Honduras to ensure that the stock tank turnover rate is compatible with their schedule.
Since lab grade alum was utilized in this experiment and the results seem inconclusive, the presence of foreign material, calcium, or carbonates could be a significant cause of the clogs present in the Honduran plants. An inline strainer could be beneficial to include in future dosing systems.It would also be helpful to ensure the Honduran operators are cleaning the system regularly and ensuring no foreign matter or excessive concentrations of alum are in the system. It is also recommended that the alum stock tanks are cleaned to prevent the build up of alum precipitates. |
