Nonlinear Chemical Dose Controller

Abstract

The nonlinear chemical doser (CD) is designed to accomodate turbulent flow in the chemical doser tubing. The previous chemical doser design,[Linear Chemical Doser], requires that the alum flow be laminar. The design uses the relationship between laminar flow and major losses in the doser tube to maintain a constant chemical dose with varying plant flow rates. When flow in the dosing tube is turbulent, major losses can no longer create this linear relationship. In this case, a nonlinear CD is necessary to maintain a constant chemical dose with variations in flow rates.

The advantage to designing a nonlinear CD is that higher chemical flow rates can be used. The limit of the current flow control module is 400 ml/min. This has been adequate for the plants currently in operation (La 34, Ojojona, Tamara, Marcala and Four communities). As the demand for larger AguaClara plants becomes an issue, larger chemical dose flow rates will be required. Furthermore, head loss through the orifice entering the flow control module increases as chemical flow rate increases. This requires the chemical stock tank to be elevated to an unreasonable height.

The nonlinear doser uses an orifice (minor losses) instead of a dosing tube (major losses) to control the relationship between changing plant flow rates and chemical dose. The entrance to the rapid mix tank is also an orifice. Therefore the plant and chemical flows vary in direct proportion to each other. The relationship between chemical dose and turbidity will be changed manually as it is in the linear CD.

Methods

To reduce head loss through the flow control module orifice, a new flow controller is being used in the design. The module being used is a PT 75 LS Kerick Valve. This valve was chosen to minimize flow control module size and maximize orifice area.