Flow Control Module Research and Experimentation

Current Research

This is where the proposals would go for an active research project

Cumulative FCM Research

Up-to-date documentation of the research that has been conducted on flow control modules by AguaClara.

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Abstract

The world-wide need for safe and reliable drinking water systems is the inspiration for open-source engineering work done by Cornell University's AguaClara team, which designs and builds plants in for use in rural Honduras. Modern computerized water treatment plants are not optimal in Honduras because of the lack of skilled operators and the frequent power outages, but the country's mountainous topography allows for the use of gravity-driven systems. Two gravitationally powered plants researched and designed by AguaClara are currently operational in Honduras, but chemical dosing without electric pumps is still inconsistent at the plants. Alum (aluminum sulfate) dosing must be adjusted regularly because it depends on the influent turbidity, a function of season and rainfall. Chlorine dosing must be consistent, and both chlorine and alum doses must be easily controlled. Here we describe the research and development of an effective flow control module (FCM) that can tie into AguaClara systems between the alum stock tank and the flocculator inlet at the rapid mix and between the chlorine stock tank and the system. The FCM dampens the effect of the chemical level changes in the stock tanks, using a simple manufactured float valve to maintain a near-constant chemical level in the FCM, which acts as a constant head tank. The height at which the outflow tubing from the FCM connects to the system can be adjusted to change dosing flow. The FCM is both inexpensive to produce and simple to operate.

Experiments run this fall have indicated that the float valve can hold back head of at least 8m with less than 0.5cm of change occurring in the FCM water level in the first 2m of pressure. In field tests in Honduras the FCMs have not been dosing as expected, which current lab work is aiming to correct. Data gathered in the laboratory on outflow rate has followed a linear model in the laminar flow range, but attempts to model the turbulent transition range have produced varying results. It is considered a high priority to develop a reliable model for dosing at higher flow rates, which will be used in the near future at the larger plant in Marcala, currently under construction.
Keywords: Float Control Module, turbulent flow, height of outflow, influent pressure, float valve

Introduction and Objectives

Constructing reliable and cost-effective solutions for water treatment in Honduras is challenging due to the lack of infrastructure. Electricity is unreliable, which has rendered modern computer-automated water treatment plants unusable, moreover the mountainous terrain sometimes makes access to materials needed for construction or repairs difficult. While it does isolate villages and their utilities, the rough topography has created an opportunity for the use of gravity-powered treatment plants. There has been success with the implementation of AguaClara researched and designed flocculators that are mixed by gravitationally-derived kinetic energy, but chemical dosing in these plants without the use of electric pumps is still inconsistent.

Alum, which is used in aiding flocculation, must be injected into the system during the rapid mix before influent water enters the flocculator. The concentration of alum required is dependant on the initial turbidity of incoming water, which is heavily dependant on local weather conditions and therefore subject to change. Alum is stored in a 55 gal stock tank, which feeds into the Flow Control Module (FCM) designed by AguaClara in the summer of 2007. A small plastic manufactured float valve maintains a nearly constant volume of alum in the FCM. The flow rate of alum out of the FCM is regulated by headloss in the outflow tubing (h2 in Figure 1). The outflow tubing connects directly into the rapid mix unit, with an adjustable connection so that plant operators can manipulate headloss. By changing the height at which the outflow tubing is connected to the rapid mix, alum dosing can be controlled without the use of a motorized pump or computerized control system.

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Plastic bottle FCMs have been used in Honduras for alum and chlorine dosing for several months now, and a few problems have arisen. They have been clogging and the head loss to alum dosing relationship does not appear linear. Further testing this fall has been done with more precise equipment and an extra connector was removed from the FCM to make it resemble the set up used in Honduras. The outflow to headloss relationship once again appears relatively linear in the laminar range, and it is possible that a different type of experiment should be designed to address the non-linear data from Honduras.

Other aspects of FCM performance have also been tested this fall, including maximum inlet head allowable, water level variation in the FCM, and corrosion resistance of the manufactured float valves. There have also been updates made to the materials used in the FCM design.

Research Methods

Float valve corrosion test

The first experiment conducted was a simple corrosion test for the manufactured float valves. The float valves are mostly plastic, with a stainless steel screw and pivot pin and a rubber pad that shuts off the inflow when the float rises. The pivot pin creates the joint that the float valve rises around, and if that fails then the entire float valve fails. The rubber pad is of similar importance in that if it fails then the inflow cannot be plugged and regulated. An entire float valve was placed into 120 g/L alum solution and a second entire float valve was placed in 13 g/L hypochlorite solution. These are the concentrations used in the AguaClara plant in Ojojona. The float valves were checked every week and observations were made regarding apparent wear and corrosion on the various exposed parts. The alum and chlorine solutions were replaced every week to ensure that the chemicals did not lose their reactivity. While the continuous chemical submersion is more extreme than normal operating conditions, it showed which parts of the float valves tend to fail first under exposure. Those parts can then be replaced with more corrosion-resistant counterparts to improve the life span of the float valves.

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[Corrosion Test Results]

FCM body design/selection

The earliest float valve FCM was housed in a PVC tube segment with PVC caps on each end. This design was abandoned in favor of using a manufactured plastic bottle. The manufactured bottle allows uniformity across all FCMs we assemble and use, and it also increases the ease of producing the FCMs.

At the beginning of Fall 2007, the FCM was slightly redesigned and a polystyrene jar was used instead of the previous bottle design. This container is more appropriate because it is clear, allowing us to see what is happening inside the FCM. It also has a wide mouth which makes it easier for the float valve to be assembled and maintained. In addition, the container is wider which will allow a full range of float valve positioning to take place. Originally, these containers would crack when the inlet and outlet holes were drilled. A variable bit for drilling was used instead, which solved this problem. The material was still brittle, though, so other clear, wide-mouth jars of various materials were experimented with throughout the course of the semester.

Flow versus head loss data collection in the laminar range

[Laminar Range Test Results]

Flow versus head loss data collection in the turbulent range

Previous experimentation with the FCM has focused on flows in the laminar range. This data was fitted to a theoretical model with reasonable predictive power, but there is no good data set for where the flow becomes turbulent. To determine the outflow to height relationship in the turbulent transition range, two methods were used. The first two data sets were collected by attaching the FCM to a ringstand and taping the 2m long outflow tubing onto a column with a fixed height. The headloss was then changed by raising or lowering the FCM on the ringstand, which was marked in 1cm increments. The flow rate was found by measuring the flow into the cylinder for 30 second increments.

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The equation for Reynold's number was used to find the expected flow rate at the turbulent transition with Re = 2100. The Hagen-Poiseuille equation was then used to find the expected delta h needed to put outflow from the FCM at that flow rate. To collect data on the outflow to head loss relationship in the turbulent transition range, this delta h was used as the "transition height". This is the head loss theoretically needed to create a turbulent outflow.

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This delta h was used as a ballpark value for where the expected turbulent transition would take place. Beginning with a delta h smaller than the theoretical transition value, outflow data was collected at 2 cm intervals of increasing delta h. This data was meant to encompass flows from the laminar and turbulent range, including the transition range. Delta h values were varied using centimeter increment markings on the ringstand and outflow is measured by placing a graduated cylinder at the tube outlet for 30 second intervals. Theoretically, these delta h values were to include the transition range from laminar to turbulent flow. However, the flow rate never reached that of turbulent flow.

Experimentally we found that the headloss of 12 cm is not nearly large enough to create turbulent flow, so we switched to using the transition flow rate of 430 mL/min instead. The physical experimental set up was also improved upon twice during the semester. First a series of holes was machined down the side of the column. The holes are 1cm apart from center to center, and there are 40 holes. They are also the same diameter as the out flow tubing. The FCM was placed at a fixed height with the water level even with one of the holes in the column. To adjust the headloss, the tubing was simply moved to a new hole like it is in the actual AguaClara plant rapid mix. A pressure sensor was also added at the bottom of the column. The pressure sensor was connected to Process Controller, which allowed us to gather larger data sets and calculate more precise flow rates. At this time we also realized that the FCMs used in Honduras do not have barbed connectors for the outflow tubing, while ours in the lab did. Re-calculating the Hagen- Poiseuille expected transition flow rate without the barbed connector yielded a value of 387mL/min, which was used as the target middle flow rate from here onward. These connectors were removed and two more data sets were collected with this set up.

When the column filled with water it was tipped into the sink to empty it. The final improvement to this set up was adding a plastic valve to the bottom of the column. The pressure sensor and the valve came off a T inserted into a connector near the column base. The valve connected to a tube that we rested in a roasting pan on the floor. When the FCM outflow tubing was inserted into a new hole on the column, the valve would be opened to let water drain through the column while the new flow rate equilibrated. The valve could then be shut to allow water to fill the column, while Process Controller took pressure measurements every 5 seconds. This was used to collect one final data set focusing on the turbulent transition range.

All of the Process Controller data was analyzed in a MathCAD sheet. The readings from the pressure sensor were used to find dP over the data collection at a given headloss. Process Controller took data readings every 5 seconds, which was used to find a dt. This dP/dt was used in combination with the geometry of the column (inner diameter = 2cm) to convert the change in pressure to a change in column water volume, to determine the average flow rate over that time.

[Turbulent Range Test Results]

Determining the maximum inlet shut-off pressure

The maximum inlet pressure that the float valve can shut off was investigated in order to determine how much head there can be between the chemical stock tanks and the FCMs. A peristaltic pump was connected to the inlet of the FCM, along with a pressure sensor. The outlet tube of the FCM was plugged. The peristaltic pump was then used to pump water into the FCM. The pressure of the water in the inlet tube, or the pressure the float valve could resist, was then measured using process controller. The maximum inlet pressure was determined using three orientations, with the float straight down giving the lever arm a 90 degree angle, partially down for a 45 degree angle, and straight out at zero degrees. This was to determine which orientation provided the greatest resistance. The pressure was also measured as a function of the height of the water level inside the FCM. This relationship was determined for each of the different orientations by marking the water level in the FCM at random intervals while simultaneously inserting a text flag in the Process Controller data. The text comments allowed us to show what the exact pressure was for the corresponding FCM water level.

This data was analyzed in Excel to determine maximum shut-off pressures and change in FCM water levels. The data for each float angle orientation was analyzed separately to determine an ideal angle to use in the AguaClara plants. The data was also manipulated to see if the water level in the FCM would vary greatly enough to significantly impair FCM function as a stock tank would drain in a plant.

[Inlet Shut-Off Pressure Test Results]

Results

Float valve corrosion test

The corrosion test was conducted for five weeks. There was no visible corrosion to the float valve screw, pad or pin in the alum solution. After a week, the float valve placed in the chlorine solution had severe visible corrosion of the screw, but no visible corrosion of the float valve pin or pad in the chlorine solution. Although the chlorine and alum solutions were replaced every week to maintain reactivity, we were uncertain of how reliable this data would be. The FCMs in place in Honduras are continually supplied with fresh chemicals and they are constantly monitored, so we are deferring data collection on float valve corrosion to the plant operators and engineers in Honduras.

[Corrosion Test Procedures]

Flow versus head loss data collection in the laminar range

[Laminar Range Test Procedures]

Flow versus head loss data collection in the turbulent range

Several sets of data were collected to determine the relationship between outflow and headloss in the FCM. The goal was to look at Q-h in the turbulent transition range, but initial data was collected in the laminar range due to incorrect estimation of parameters. Data from plant performance in Honduras has indicated that the FCM dosing was not varying linearly with headloss. Data from testing the FCM in Summer 2007 showed a linear relationship, but the first two data sets gathered this fall with the same experimental set up showed more variation

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The data is not linear, and it also is showing a lower outflow per headloss than theoretically expected. Small decreases of the tubing diameter (decreased by 0.053in) help fit the data to theory, which may be acceptable because of the FCM exit geometry as discussed later in this section.

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From the first two data sets collected in the expected turbulent range, we have hypothesized that a few components of the FCM should be characterized differently. The inner diameter of the outflow tubing is most likely slightly smaller than manufacturers claim. It is sold as 3/16in (0.188in), while our data best fit the theoretical model when a diameter of 0.134in was used. This smaller inner diameter would result in higher friction in the tubing, increasing headloss. We also realized that the barbed connector used to attach the outflow tubing to the FCM was creating a constriction in the flow, thereby increasing headloss. The AguaClara engineers in Honduras complained that the FCM was not dosing as expected from research in the lab, and it turned out that the FCMs in use in Honduras did not have barbed connectors in place, while those in the lab had the connectors. The barbed connector was removed from FCMs in the lab for all testing from then onward, and all modeling was done under the assumption that tubing diameter was slightly variable.

The Process Controller column with pressure sensor set up was used to take three sets of data with more precise values than the earlier semester data. The machined holes in the column allowed us to control headloss with high levels of precision, while the pressure sensor gave accurate data that was converted into flow rate information. This set was used to gather data that actually fell around the turbulent transition range, which our earlier methods had missed.

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The turbulent jump appears to occur at a flow rate of about 350mL/min, here without a barbed connector, and it can be characterized as a zone where headloss increases independently of flow rate. It is interesting to note that the data appears linear both before and after the jump, with different slopes. Below the transition the data can be made to fit the theoretical model well, while above the jump the data is generally below the theoretical curve. In the transition region it is difficult to categorize. It may be viable to use this type of FCM for both high and low flow rates, as long as the turbulent transition region is avoided. This is constrained by the maximum inlet flow rate through the float valve, which still needs to be determined. At steady state the FCM would not be able to ever dose more than this inflow rate, so any plant requiring more chemical dosing would need to modify this system.

[Turbulent Range Test Procedures]

Determining the maximum inlet shut-off pressure

The maximum inlet pressure that the float valve could resist was determined. Float orientations of 90, 45 and zero degrees all resisted a pressure of at least 8m. A float orientation of zero degrees resisted pressures of at least 11m. All of the float orientations have a maximum inlet pressure well above the 2m needed for the AguaClara plants.

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This data was further analyzed to also determine the amount of variation in FCM level with increasing pressure build up on the float valve.

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The level in the module changed by many centimeters during the experiment, but this was over the entire range of inlet pressure build-up. For the range that is pertinent for designing AguaClara plants, up to at most 2m, the level changed in the FCM much less. The table below shows the total change in FCM water level at both the highest back pressure reached, and at approximately 2m of pressure.

Positioning the float straight out from the valve caused the most variation in water level, while positioning it at an angle of 45 degrees or 90 degrees had about the same effect. Currently the floats are positioned at an angle somewhere between 45 degrees and 90 degrees to best fit inside a small FCM. Assuming this positioning is kept, the variation in water level is large enough to influence the outflow by effectively causing error in the expected headloss in the system of up to 0.4cm. While this does create a discrepancy between assumed and actual headloss in the system, it is much smaller than the uncertainty caused by a draining stock tank.

[Inlet Shut-Off Pressure Procedures]

Conclusions and Areas for Further Research

Through this semester the Flow Control Module testing apparatus has evolved to allow more precise testing to take place. The next upgrade to our testing equipment is an automated position control, which will be capable of changing the headloss in the system automatically. This will increase the precision and quantity of data collected in the future.

With the current equipment we have begun to characterize the outflow from the FCM in both laminar and turbulent flow ranges, while experimentally showing that there is too much uncertainty in the transition region to effectively set simple dosing guidelines through it. The maximum laminar flow rate for the FCM was much lower than the dosing that will be required for the new water treatment plant in Marcala, which poses a challenge for us. The easiest route may be to use several FCMs in parallel to deliver the necessary chemicals to the plant, or modeling the FCM for the turbulent range may show that one FCM alone could reliably deliver enough outflow to meet the plant's demand. Other options for increasing alum dosing with the given headloss available in the system would be to increase the strength of the alum stock, necessitating a lower flow rate, or decrease the length of the FCM outflow tubing. These options should be investigated further to compare the cost of materials and the reliability of operating in the turbulent range. This work will help to characterize the FCM technology for use in higher volume water treatment plants as well as those it was originally designed for.

To determine the amount of modification required to increase FCM flow rate, we would need accurate values for the kinematic viscosity of chlorine used in the AguaClara plants. Doing online research and contacting manufacturers provided little useful information, and the viscosity may need to be determined experimentally. This would involve running two tests: an outflow to delta h test with water and an outflow to delta h test with chlorine. The ratio of the outflows is then equal to the ratio of kinematic viscosities for water and chlorine. This testing should probably be done in the next semester.

Now that a few comprehensive data sets have been collected with the latest equipment, a new dosing chart can be made for Honduran plant operators. As this data was collected without the barbed connector in place, it should provide a better model for their systems. When the new fully automated experimental set-up is available it should be used to collect the most precise data set so far, which can be used to further refine the plant operator dosing chart from what has been established this semester. This equipment will ideally be able to produce consistent data in the turbulent range, where our data sets thus far have been showing conflicting results.

To test the FCM in a real use setting it may be useful to tie it into the pilot system at the Cornell Water Treatment Plant (CUWTP). The FCM could be tied into the system between the alum stock tank and the rapid mix, where it would be installed in a full-sized AguaClara plant. This set up could be used to gather qualitative data on the FCM performance when it is left to run for extended periods of time, not just 30 second intervals. There were various problems in Honduras with alum sediment collecting in the FCM, and occasionally clogging the float valve. While some of the original operating problems have been overcome, there are still shortcomings that we should be able to assess and observe in person.