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
Authors: Leah Buerman, Monroe Weber-Shirk
Abstract
Initial concepts for a non-electrical technique for the regulation of chemical dosing in robust water treatment plants were tested. 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 LFOM 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. This will allow creation of a 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.
1. Introduction
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 weir 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.
* Add a section on the algorithm used to design the LFOM. Show the equations and add sketches that illustrate the design algorithm.
* Note that we may want to modify the algorithm slightly to make it more accurate.
* Also review the design algorithm to make sure that it is coded as simply and elegantly as possible. We will probably not include the code in this paper, but the code should be available from the wiki.
* Show some comparisons of designs (hole patterns).
* Investigate and discuss other design constraints including
** maximum flow of water through an LFOM as a function of pipe diameter
** the effect on accuracy of the ratio between vertical spacing of orifices and range of measurement
** the effect of the orifice diameter on accuracy (perhaps show the expected departure from linearity as a function of the total number of orifices in the LFOM)
* Add several LFOM designs for different flow rates.
* Include the possibility of using multiple LFOM to get a desired flow rate.
2. Materials and Methods
Experimental set-up
The Sutro weir shape is dictated by a series of equations. This doesn�t sound right. Do you have the reference to the Sutro weir design to see how it was created? Did Sutro get an analytical solution to the shape of the curve or did Sutro use a finite difference scheme? Approximating the weir for design of the LFOM was achieved through a computer program in MathCAD that simultaneously solve the equations and designs a template for orifice location and quantity on the LFOM riser pipe. The program requires input of maximum flow rate through the water treatment plant and then through iteration selects the largest drill bit size Describe this algorithm in more detail. that is able to theoretically mimic the Sutro weir with 10% this is the error you are reporting. So is this the error that you should have expected? Would it be possible to improve this accuracy simply by changing this constraint in the design algorithm? accuracy. The large orifice size is beneficial because obstructions are common in untreated water and large orifices are less likely to clog. Clogged orifices result in increased water height and an over estimate the flow rate. Using larger orfices also make construction easier since fewer holes need to be marked and drilled. A LFOM and entrance tank were designed for implementation in the AguaClara pilot plant with a maximum flow rate of 125 L/min. The LFOM is fabricated from a 3" nominal diameter schedule 40 PVC pipe with an orifice diameter of 9.525 mm (3/8"). The total vertical range of the LFOM was set to be 20 cm. This matches the vertical spacing using in AguaClara water treatment plants for regulation of chemical dosing. Every centimeter was evaluated for necessary orifices. How would accuracy be affected if the spacing were set to 2 cm or some other interval? The water from the plant flows into the entrance tank, a five gallon bucket, and through the riser pipe into the flocculation tank. In-line with the water inflow pipe a flow meter, Siemens model SITRANS F M MAG 3100, is installed. The device provides flow measurements with an accuracy of +/-0.25% of flow rate.
Measurement of Water height in Entrance Tank as Flow Rate Fluctuates
The LFOM accuracy was tested at the Cornell Water Filtration Plant in Ithaca, NY. Before each trial the LFOM pipe was cleaned. 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". The offset accounts for the distance between the bottom of the bucket and the center of the first row of holes. This isn�t a characteristic of the LFOM and thus we don�t need to report it. Three trials were run.
3. Results and Analysis
The distribution of entrance tank water height was correlated directly with the flow rate through the pilot plant. The data from the three trails was plotted in Figure 2.
Figure 2: Experimental data on the three trials showing the correlation between the Flow rate and the water height in the entrance tank. The trend line for trial one is y = 18.116x - 4.3486 with an R2 = 0.994, trial two is y = 17.527x - 2.7457 with an R2 = 0.993, trial three is y = 17.55x + 2.5598 with an R2 = 0.9974, Predicted is y = 15.814x + 0.3126 with an R2 = 0.9999.
The predicted flow based on orifice flow data set from the MathCAD This is MathCAD, correct? �program- was also plotted for comparison with the data. All of the collected data have trend lines with slopes of approximately 0.05 Units? Your data is accurate to more than one significant digit. This isn�t the value reported in Figure 2. which shows high precision among the three trials. Compare the slopes of the model and the data. There is a deviance between the predicted values and the observed values as the flow rate increase above 90 L/min. The difference between the predicted and actual values is displayed in figure 3.
Figure 3: Plotted difference between experimental data and predicted values.
Further analysis on the percent deviation of the data shows a more significant difference at low flow rates. The percent difference levels out under ten percent as displayed in figure 4.
Figure 4: Plotted percent difference between experimental data and predicted values.
The difference between the observed and predicted values is within 10 percent for the majority of the flow rate range. The high percentage ratings in the low flow range are due to the impact of small errors on small values. The data supports the accuracy of the Sutro weir approximation.
Perform a single point calibration on the LFOM and then show how well it agrees with measured values.
Describe how communities can access this design tool.
4. Conclusions
The experiments prove that the system of orifices accurately creates a linear relationship between flow rate and water height. The height readings with the entrance tank and LFOM show error under 10 percent when in high flow rates Use the single point calibrated results here. . The trend in the data supports even lower errors as flow rate increases to flow rates of actual water treatment plants. The LFOM 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 LFOM 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 robust engineering to improve the lives at the base of the pyramid.
5. Acknowledgements
This research was funded by the AguaClara project through generous donations by the Sanjuan Fund. The help of Timothy Brock, Tom Cook, and Thomas Rapalee was integral.