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Linear Flow Orifice Meter for Application in AguaClara Drinking Water Treatment Plants (Draft)
The actual formated word version of the paper is available here, it is formatted after the papers in Water Research
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Authors: Leah Buerman
Abstract
Initial concepts for a non-electrical technique for the regulation of alum addition in a water treatment plant were tested. A riser pipe was fabricated with holes to mimic a Linear Flow Orifice Meter (LFOM) based on the concept of the sutro weir was added to the entrance tank of a pilot scale drinking water treatment plant. The entrance tank and riser pipe were evaluated on their ability to provide a linear correlation between water height in the entrance tank and the flow rate through the plant. Flow rates between 5 GPM and 36 GPM were tested in a pilot water treatment plantLFOM was evaluated for linearity and flow measurement accuracy over a range of flows from 20 to 140 L/min. The flow rate was shown to be linearly related to water height with the riser pipe implementation with a percent deviation of ten percent. This will allow creation of an alum chemical flow control module regulated by a float system in the entrance tank. The experiment shows a significant achievement in the movement to bring clean water to communities without access to electricity.
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Adaptation of water purification technology for use in developing countries is a concern of growing urgency. The lack of clean drinking water provides avenues for infection by waterborne pathogens which are currently one of the major causes of mortality for children under five years old. The Agua Clara team is working to bring clean water to communities throughout Honduras with a flocculation-sedimentation water treatment design. The design requires the addition of aluminum hydroxide as a coagulation agent. Plant operators manually adjust the alum dose based on the daily flow rate of the treatment facility. A useful innovation is necessary in the regulation of aluminum hydroxide into the system as flow changes without the use of electrical devices. Currently options for the control of chemical additions to water treatment plants are either an electric pump or manual adjustment of chemical flows. The design proposed uses an entrance tank as a way to transfer information about the flow rate to the aluminum hydroxide flow control module through a float system. The system accurately delivers consistent aluminum hydroxide dosing into the incoming water based on the height of the water in the entrance tank. Without intervention the flow rate through the entrance tank is proportional to the square root of the pressure head, height of the water in the entrance tank, as shown in equation 1.
Equation 1: The Orifice Equation. use the Latex equation technique
The relationship between water height and flow rate create a problem when transferring of data to other mechanisms. A predictable linear relationship between the height of the water in the entrance tank and the flow rate through the plant allows for a system of floats to meter the aluminum hydroxide flow rate. The sutro weir developed by Victor Sutro in 1915 can be used to create a linear correlation between head and flow rate. The configuration is shown below in figure 1.
Figure 1: The Sutro Weir Image.
The bottom width of the sutro weir is relatively large and the width gradually decreases as the height increases. As the height of the water in the tank increases the pressure created by the weight of the fluid causes the flow rate through the bottom of the weir to increase. By minimizing the weir width as height increases the change in the total flow through the weir as height increases is kept in a linear relationship. Implementation of sutro weirs in currently operating water treatment facilities is infeasible due to restrictions of shape and space. Grit chambers receive the inflow first and transmit the water to the rest of the plant through pipes. If a sutro weir shaped hole were cut into a pipe the pipe would become unstable and require skilled labor for construction. Therefore we are approximating the sutro weir with a riser pipe added onto the pipes that connect the initial grit chamber (entrance tank) and the flocculation tank. The pipe would be easily introduced into previously constructed plants and new plants at low cost. Holes would be drilled into the riser pipe that would mimic the sutro wier basics of design. The drilling of holes as specified heights doesn't require skilled labor. The water is forced to flow through the riser pipe in order to leave the entrance tank and enter the flocculation tank. Experimental tests were conducted to evaluate the accuracy of the simulated sutro weir in a pilot plant application.
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The riser pipe design's accuracy was tested at the Cornell Water Filtration Plant in Ithaca, NY. Before each trial the riser pipe is cleaned and all holes obstructions are removed. Performing the experiment requires reading the flow rate from the flow meter and measuring water height in the entrance tank with a ruler. Flow rates were varied between 5 GPM and 36 GPM by 1 GPM increments. The flow meter requires acclimation time to report accurate readings. After every manual change in flow rate there was a rest period of five minutes before water height and flow rate were measured. The recorded water height is adjusted by an offset of 3.5 inches. The offset accounts for the distance between the bottom of the bucket and the center of the first row of holes. Three trails were run on three different days.
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The experiments prove that the system of holes orifices accurately simulates the sutro weir. The height readings with the entrance tank and riser pipe show error under 10 percent when in high flow rates. The trend in the data supports even lower errors as flow rate increases to flow rates of actual water treatment plants. The new riser pipe will allow for increased autonomy of the water treatment plant. Supplies for construction of the riser pipe are low cost and addition of the pipe in the grit chambers of new and existing water treatment facilities is a low labor endeavor. The communities only need to find a length of PVC pipe to improve their water purification plant. Daily monitoring of the riser pipe is necessary to remove obstructions from the holes, at least once daily. Expectations of constant monitoring of plant flow rate by the operator are unreasonable and without correct alum dosing the water either won't be purified or the plants will waste money on overdosing. The innovation utilizes advanced theory to improve the lives of the least fortunate.
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This research was funded by the Agua Clara AguaClara project through generous donations by the Rotary Foundation and the Brown Family Sanjuan Fund. The help of Professor Weber-Shirk, Timothy Brock, Tom Cook, and Tom Thomas Rapalee was integral.