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The AguaClara engineers from academic year 2009/10 not only designed, but built and installed a Non-Linear Dose Controller (NDC) in a new AguaClara plant in Algateca, Honduras. The current NDC is installed on top of the entrance tank of the plant and consists of a float connected to one end of a lever arm and the dosing orifice connected to the other end.
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Figure 1. NDC As Installed in Aglateca
The entrance tank is connected to the hydraulic flocculator by way of a rapid mix orifice. A change in flow rate through the plant is indicated by a change in the level of the entrance tank. This is transmitted to the lever arm by way of the float. For example, if the flow rate of the plant were to decrease from 100% to 80%, the entrance tank would drop by a proportional amount. Since the pivot is in the center of the lever arm, each centimeter of change in the entrance tank will translate to a corresponding change in the angle of the lever arm.
Figure 2. Entrance Tank Connection to Flocculator via Rapid Mix Tube
Alum is piped from a 120 g/L concentration stock tank to a constant head tank and from there to the metering orifice. The metering orifice is connected to the lever arm by way of a movable slide. The lever arm has an incremental scale on the top surface which corresponds to specific alum concentrations. The operator is able to set the chemical dose concentration according to the values on the scale and the lever arm will automatically adjust the flow rate of alum to maintain the correct concentration of alum as plant flow rates change.
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3. Stock Tanks and NDC
Feed back Feedback from operators and engineers in Honduras along with results from our own experimental lab-work provide us valuable information on the accuracy and ease of operation of the NDC. Additionally, brainstorming continues on how to improve and expand the function of the current design. Derived from these topics are the following recommendations for next semester's team.
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- Chlorine dosing is accomplished with 70% Calcium Hypochlorite. A 5 pound bag is mixed with water in a 55 gallon drum. This gives a chlorine concentration of 7.625 grams / Liter.
- One 55 pound bags bag of alum are is mixed with water in a 55 gallon drum providing a concentration of 120 grams / Liter.
- The CDC should be able to maintain an alum concentration of 10 to 80 mg/L in the plant. The low and high end of this scale can be manipulated slightly in order to create a uniform scale. For example, the triple-scale uses a range of 8 - 88 mg/L.
- The CDC should be functional and accurate down to a plant flow of 50%.
- 5% variance between fabricated orifices is the current goal.
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The Summer 2010 Team attempted to determine if it was reasonable to expect that a precise metering orifice could be fabricated in-house. The extent of this research is documented on the team's wiki pages, however, a few key lessons-learned are summarized below.
- Acetal and Polyamide plastics are not suitable materials for producing precisely machined orifices. These materials do not "chip" but instead heat-up and deform. This leads to a slight collapse of material around the orifice hole as the drill bit is removed and marked deformation at drill-bit break-through. We attempted to correct for this by drilling in the direction of flow so the drill-bit break-though would occur at the exit region instead of the entrance region (which has a significant effect on the coefficient of discharge). This still did not produce acceptable results; 15 - 18% variance between orifices was measured.
- As metals possess better machinability characteristics than plastics, two styles of brass caps were selected for review. Admittedly, brass may not be the best choice for chemical compatibility (particularly chlorine), it is extremely machinable. Our thesis was that if we could not machine the parts from brass in-house, it would be unlikely to find a more suitable material from which we could. We selected brass refrigeration tubing caps and SAE flare caps. Our first trial showed similar variations between orifices meaning that we were not any more successful at machining precise orifices in-house We were still drilling in the direction of flow and determined that the centering bit created a different shaped entrance region in each cap, thereby changing the K.vc for each cap. The caps were then drilled against the direction of flow so the bevel created by the centering bit occurred at the exit region. Drilled brass does not create significant burrs at drill-bit break-through so the entrance region should be the fairly consistent between orifices.
- Less than 5% variance between brass caps which were drilled against the direction of flow was recorded, proving that we could indeed fabricate precise orifices in-house. The next step is to reproduce similar results with a material that is compatible with our process chemicals. At the time of this writing, the CDC Team was planning to machine orifices from solid PVC stock. This will most likely be the starting-place for the Fall 2010 team. Note, an engineer with Kerick Valves has explained that the orifices in these the float valves are created during the injection molding process and are not machined-in separately. He claimed that the process of injection molding produced much more repeatable results.
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The importation of components, however, cannot be taken lightly. Although we have determined that it is possible to fabricate a fairly precise orifice in-house we have not yet done so with a material that is suitable for use with chlorine. The next step is to find a material that is not only suitable for use with our process chemicals but possesses the machinability characteristics that allow for repeatable fabrication of the orifice. Further, this component would then need to be readily available is the areas we service. As impossible as all this sounds, the success of an AguaClara plant would be greatly improved if an operator could easily replace any lost or damaged orifice by simply drilling an appropriately sized hole in a pipe cap.Concluding, we need to select the component that will be used for the metering orifices in AguaClara plants. Once we determine and fabricate our orifices, we can then begin to quantify accuracy and surface tension issues.
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Concluding, this sub-team faces many challenges. This team must first find a material which is not only compatible with our process chemicals but also possesses sufficient machinability to allow precise (repeatable) manufacturing of a metering orifice. This team must then determine if it would be possible to obtain this component in the areas we service. In-house fabrication versus importation of parts is the driving question. Further, surface tension and accuracy issues will need to be quantified (but only after the orifice is selected).
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Once a final decision has been made as to what material will be used for fabricating the orifice, the dosing scale will need to be developed. Again, because surface tension is directly related to material finish, fluid flow through a drilled PVC orifice is apt to behave quite differently than a laser pierced ruby. Fall 2009 developed a dual-scale, dual-orifice system. This scale does not accommodate surface tension in anyway and therefore fails at low plant flow rates and low dosing concentrations. Spring 2010 attempted to solve this problem with the development of the triple-scale, triple-orifice. This scale works well for all concentration ranges by starting the scale farther from the pivot point. However, at 50% plant flow rate, the head-loss through the plant is only 10 cm. We could easily enter the range where surface tension (depending of the material selected for the orifice) restricts flow at low concentrations of alum. The Summer 2010 team attempted to correct this by moving the float during periods of low flow so that a 4:1 relationship exists between plant flow and dose height. This solution maintains the dual-scale but does incorporate triple-orifice and requires an additional action from the operator.
One Ultimately, one of the above listed scales or a newly developed scale needs to be selected. Coding is then developed so the Automated Design Tool can generate the scale-template needed for the CDC.
Constant Head Tank Valve and Material Research (2-4 Persons)
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