Orifice Clogging Experiment

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, 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 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, 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. 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. Anothersensor 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 figure below.

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.

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 pressure it built up enough to "blow out" the clog.

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.

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. 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 sediment trap 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. 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.

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 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).