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Theory, Design, and Application of Gravity Powered Flow Control Modules

Table of Contents

Figures and Tables

Abstract

Introduction

The lack of robust and sustainable technologies for chemical dosing and flow control that don't require electrical power continues to adversely affect the ability to reliably provide safe drinking water. Conventional municipal surface water treatment requires the addition of a coagulant solution as well as the addition chlorine. Ideally the flow control device would have the following characteristics:

  • calibrated to easily vary the flow rate
  • handle corrosive chemicals
  • incorporate a linear scale to facilitate setting the flow without need to use trial and error
  • be resistant to clogging
  • be easy to maintain and operate
  • be easily adapted for a range of flow rates
  • be economical, small, and easily used to replace existing flow control devices

Water treatment plants in industrialized nations often use variable speed peristaltic pumps or other positive displacement pumps for this purpose. Many potential water treatment plants sites in the Global South don't have ready access to electricity and frequently the electrical grid is unreliable. The AguaClara team at Cornell University recognized the need for an improved gravity powered flow control device and began evaluating the available technologies and ultimately developed and tested a flow control module with the desired characteristics.
We first recognized the need for an improved flow control module during site visits of community water supply systems in Honduras in 2004. The standard Honduran design for community water supply systems consists of a surface water source that is piped to a distribution tank and then distributed via a pipe network to homes. The only water treatment is the addition of hypochlorite. Most communities use granular calcium hypochlorite to prepare a concentrated chlorine solution in a small tank that is located on top of the distribution tank. The original design of the hypochlorinators called for a floating structure that held a flexible tube with a submerged orifice. This system theoretically provided a constant flow through the submerged orifice. In practice the orifice clogged quickly, the flow rate was very difficult to adjust, and maintenance and operation required contact with the concentrated chlorine solution. Perhaps due to these difficulties the design evolved and a 1/2" PVC valve was installed on the exit pipe at the bottom of the chlorine tank, the floating orifice was removed, and the flow is adjusted by a trial and error setting of the valve position. (add CAD drawing of hypochlorinator) This modification created a system that was easier to maintain, but the hydraulic design no longer provided a constant flow and the valve was still subject to frequent clogging.
Another design for a constant flow device is called a floating bowl (add reference). It is conceptually similar to the design used in Honduras, but the flow is adjusted by varying the submergence of the bowl. The submergence is varied by adding or removing pepples from the bowl. This technique also requires reaching into the chemical solution to adjust the pepples.

Theory

Maintaining a constant flow of chemical is difficult because of the fluctuations in the level of the chemical in the stock tank. The variable head means that any restrictions used to regulate the flow will cause a decreasing flow rate as the tank empties. One simple solution to this problem would be to use an elevated tank with a large head driving the fluid through the flow restriction. Then the small variation in the driving head as the tank emptied would not be as significant. The disadvantages of this approach are the construction and operation difficulties of the elevated tank and the clogging of the flow restriction. Thus we need a solution that isolates the flow restriction from the variable head of the stock tank and we need a flow restriction that is as large as possible to minimize clogging.

Creation of a constant flow requires a constant driving force and a constant pressure coefficient or loss coefficient. An almost constant driving force can be obtained by using a very large driving head with small variations due to level fluctuations in the stock tanks. A more practical approach is to use a float valve to regulate the chemical level in a small tank. The constant fluid level can then be used to develop a constant flow. Recent advances in small low cost chemical resistant float valves have made it possible to use float valves even with corrosive chlorine solutions.

Application

flow control module design webpage

Flow control modules will generate a linear response between head loss and chemical flow rate as long as expansion losses are small relative to shear losses and as long as the flow is laminar.
Design of the flow control module consists of choosing a maximum head loss corresponding to the maximum design flow rate, and then determining the diameter and length of the tubing.

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