Nonlinear Chemical Dose Controller

Abstract

The nonlinear chemical doser (CD) is designed for turbulent chemical does flow rates. The previous CD design for a [linear CD], requires that the chemical flow in the dosing tube beis laminar. This 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. However, when the flow in the dosing tube is turbulent, the linear relationship no logner longer exists. In this case, a nonlinear CD, one that uses minor losses to control flow rates, is necessary to maintain a constant chemical dose with the varying plant flow rates.

The advantage to designing a nonlinear CD is that higher chemical flow rates can be used. In larger water treatment plants larger chemical flow rates are necessary to dose alum and chlorine. The limit of the current flow control module is 400 ml/min. This maximum flow has been adequate for the plants currently in operation (La 34, Ojojona, Tamara, Marcala and Four communities) given their flow rates. As the demand for larger AguaClara plants grows, larger chemical dose flow rates will be required. Furthermore, at high chemical flow rates, the head loss through the flow control module orifice requires that the chemical stock tank be elevated to an unreasonable height.

Methods

The nonlinear doser uses a dose control 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, both plant and chemical flow rates are dominantly controlled by an orifice. The CD uses a lever arm to increase head in the flow control module when the plant flow rate increase. The increase in head drives the chemical flow rate to increase with the plant flow rate. The relationship between chemical dose and turbidity is changed manually in the same fashion as the linear CD.

A lever arm similar to linear CD lever arm will be used to relate plant flow rate to alum flow rate. A larger dosing tube is required to minimize major losses. This tube must be flexible to accommodate lever arm motion. Flow will be channeled through the dose control orifice into a channel. This channel will deposit chemical flow in to the appropriate location in the entrance tank regardless of the corresponding dosing location on the lever arm.

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. The flow controller will housed in a square tub.