Orifice
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Clogging Experiment
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Figure 1: Experimental Setup |
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Overview
A linear dosing meter is currently being utilized in Honduras 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. 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. major losses through the tubes control the head loss. In this system, there is one orifice that often gets clogged, impairing the dosing. The 0.23 cm orifice is located in the constant head tank float valve. Because the new design has multiple orfices ( in the constant head tank and the one controlling dosing), we designed an experiment to test clogging in orifices. In our experiment, we chose to use a 0.1 cm diameter orifice, the smallest size orifice in our newdesign, to test the weak point in our design with regard to clogging. If we can remove the issue of clogging with this small orifice size, it would be more likely that we would also resolve the issue in the constant head tank float valve orifices, which are larger. The clogging in the constant head tank float valve orifice due to the accumulation of chemical precipitate is probably a combination of calcium carbonate, aluminum hydroxide, 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 materialmaterials) as well as 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 new orifice (0.1 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 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, it is likely that clogging in Honduras is due to foreign material and the problem can be resolved by adding a sediment trap to remove foreign material.
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. 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 safety sensor is there as Anothersensor is 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 A control experiment, using deionized 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 figure below.
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Figure 2: Pressure Sensor 1 Readings - Control |
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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 Graph 2 Figure 3 below.
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Figure 3: Trail 1 of 24 hour Alum Experiment Data Pressure Readings (11/19/09) |
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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 pressure it builds built up enough to "blow out" the clog.
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Figure 3: Trail 1 of 24 hour Alum Experiment Safety Pressure Readings (11/19/09) |
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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Figure 4: Trial 2 of 24 hour Alum Experiment (12/03/09) |
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.
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. This seems strange and might indicate erroneous pressure readings for the spikeThe 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.
Conclusions/
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Future Work
The 0.1 cm orifice appeared to clog after 4 hours, indicating that a larger dosing orifice sediment trap might need to be used. Further testing will need 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. 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 these results, and determine the clog time for any larger orifice we select. An ideal system would only require cleaning once a daythat this will indeed increase the time till a clog forms.
Since lab grade alum was utilized in this experiment and a clog was still presentthe results seem inconclusive, the presence of foreign material, might not be the main , calcium, or carbonates could be a significant cause of the clogs present in the Honduran plants. An inline strainer would still be recommended for could be beneficial to include in future dosing systems, yet it should be noted that this won't eliminate the occurrence of clogs. It would also be helpful to ensure the operators are cleaning the system regularly and ensuring no foreign matter are in the system. It is also recommended that the sludge be removed from the alum stock tanks whenever the sludge begins to approach the level of the stock tank outlet (10 cm above the base of the tank).