Orifice Clogging Experiment
Figure 1: Experimental Setup - Orifice Clogging
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. This orifice is becoming clogged with chemical precipitate that is probably a combination of calcium, aluminum hydroxide, carbonates, or some other 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 materials) and estimate the frequency of clogging in the new non-linear dosing system. 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 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. 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.
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 graph below.
Graph 1: Pressure Sensor 1 Readings - Control
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 below.
Graph 2: Pressure Sensor 1 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 gradually increasing pressure, due to the clogging orifice, until the it builds up enough to "blow out" the clog.
Graph 3: Trial 2 of 24 hour Alum Experiment (Dec. 3. 2009)
Another trial with alum was done over 24 hours (graph 3).
As seen in graph 3, 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. 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.
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.