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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.
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 correspond to a centimeter of change in the heights of both ends of the height of the lever arm (although, obviously, the two ends will be moving in opposite directions).
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.
Feed back 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.
Recommended Work for Fall 2010
Orifice Validation and Scale
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Development (2 - 4 Persons)
One of the priorities of AguaClara engineers is to ensure their designs are locally sustainable. It is easy to create the walls and foundations from locally manufactured materials but becomes increasingly difficult when very specific components are needed to perform very specific tasks. This proved true with the float valve used in the constant-head tank and may prove true with a metering orifice. The team that originally developed the NDC was not able to fabricate precise (e.g. repeatable) orifices for their lab tests. This team also noted that surface tension effected accuracy of a metering orifice when less than 4 cm of head was available. The Summer 2010 team concentrated on documenting the extent of the precision problem and researched various solutions. It is the belief of the Summer 2010 team that we can not properly quantify the inaccuracies associated with surface tension until the material and manufacturing techniques are finalized since material finish, or smoothness, has a strong influence on surface tension.
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- 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. Note, an engineer with Kerick Valves has explained that the orifices in these 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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One of the above listed scales or newly developed scale needs to be selected then coding developed so the Automated Design Tool can generate the scale template need for the CDC.
Constant Head Tank Valve and Material
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Research (2-4 Persons)
A key component in any CDC is the Constant-Head Tank (CH Tank). The CH Tank is connected between the stock tank and the dosing device and its purpose is to mitigate the effect that a change in level in the stock tank would have on the dosing instrument. Since our CDCs rely on head as their driving force, as the stock tank level drops, the available head drops, thereby changing the flow rate through the metering device, whether it be a tube or an orifice. The CH Tank is comprised of a small reservoir with a float valve installed. As chemical is metered to the plant, the float valve opens and replenishes reservoir, meaning a fairly constant level is maintained at all times.
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A suggested list of materials for inclusion in the database is:: *
- PVC
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- Polyethylene
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- Kynar
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- Polyamide
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- Acetal
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- Brass - Include various types
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- Stainless Steel - Include various types
At a minimum, chemical compatibility between these materials and chlorine, alum and lime needs to be established and documented. Material cost should also be discussed. Availability is more difficult to quantify but it is important to avoid specialty fittings or materials. This list should not be considered comprehensive; it should be updated as the project evolves.
In regards to fitting styles, some key points to consider: *
- Compression fitting are for use on rigid tube. They are currently in use to connect the float valve in the CH Tank. Elimination of compression fittings is ideal as they are made up of a nut and two small ferrules which must be installed the correct way in order to obtain the seal. It is too easy for one of these components to get lost, dropped etc, rendering the fitting useless.
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- Quick-connect fittings are also used on rigid tube. Their longevity is in question as is their availability outside of the US. Use of PEX rigid tubing is becoming increasingly more popular in water distribution systems and improvements on fitting styles and availability is likely to follow.
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- Barb fittings are quite stout, easily available and are fabricated from many different materials. They are used on flexible tubing (based on the ID of the tubing, not the OD.) The biggest drawback to a barbed fitting is that they can be difficult to remove from the tubing once assembled, especially after a long period of time. It is not uncommon to have to slice the tubing off from around the barbed fitting. This is not necessarily a show stopper, especially if a connection doesn't need to be undone very often.
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- PVC pipe is ubiquitous; it is currently used throughout the existing AguaClara design, is available world-wide and is suitable for use with all process chemicals involved. The current line-of-thought is to use 1/2" PVC rigid pipe and socket solvent-weld fittings for all delivery pipes (i.e. from chemical stock tanks to the CDC) and flexible tubing within the CDC instrument.. When needed, PVC threaded to barb adapters will be used to convert to flexible PVC tubing.
Once the materials and fittings are decided upon, this information needs to be incorporated into the Automated Design Tool. The goal is for the design tool to eventually include the CDC and to have a material list and AutoCAD drawing of the installation included as part of a standard design package.