Controlling the addition of Chlorine into a Process

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Courtesy of Walchem, Iwaki America Inc.

There are several methods for controlling the addition of chlorine into a process. These methods include flow proportional injection, ORP (oxidation reduction potential) or a residual chlorine analyzer. Each method measures a different process characteristic. Depending on the specific type of process being controlled, there generally is a best choice for controlling the injection of either chlorine gas or sodium hypochlorite (liquid bleach, NaOCl).

In order to better understand why one method is preferred over another, it is best to define exactly what the measurement is for each method, what the desired control situation is, the limitations of each system and what routine maintenance is required.

Flow Proportional Injection
In flow proportional control, a volume of chlorine is injected relative to the flow rate (of water) through the system. In most applications, there is a desired resultant concentration of chlorine (i.e., five ppm), and the system is configured to enable the dosing of the chlorine (generally bleach through a metering pump) to achieve this value. Table 1 lists the volumes required per gallon to achieve a one-ppm level of chlorine. To achieve a concentration of five ppm, simply multiply these values by five.

The flow proportional control system uses a flow sensor to determine the flow rate of the carrier solution (generally water). Many types of flow sensors are designed to produce a pulse output (per volume of flow) directly from the sensor. These include multi-jet meters, paddlewheel, turbine, magnetic, vortex and others. The signal may be taken through an intermediate flow controller (normally required when the pulse rate off the sensor is not adjustable) or directly into a metering pump. The function of the intermediate meter is to ratio a pulse output to that received from the flow sensor. Other functionality such as flow rate monitoring and flow totalization also may be achieved.

Many metering pumps feature an on-board microprocessor-based controller and are capable of multiplying or dividing the pulse signal directly. This eliminates the need for an intermediate controller for flow sensors with non-adjustable pulse outputs and can simplify the overall situation.

The pulse rate required for the application is determined by the amount of bleach required per gallon (generally in ml) and output per stroke of the selected metering pump. Adjustment of the stroke length on the pump can enable the pump output (per stroke) to be trimmed to exactly the dose per stroke needed. For example, if five ml of sodium hypochlorite is required for each gallon of water, a pump with an output of six ml per stroke can be set at an 80-percent stroke length and programmed to stroke once for each pulse received from a flowmeter generating one pulse per gallon. As the flow rate increases, the pulse output from the flowmeter increases proportionally, achieving a consistent dosing per volume of flow through the system. When selecting a metering pump, using a smaller pump (in ml/stroke) pulsing at a higher rate (i.e., ten strokes per gallon vs. one stroke per gallon) can reduce 'slugging' of the chemical into the flow stream. Metering pumps on the market today can stroke at 300 to 360 strokes per minute, virtually eliminating slug feed of the hypochlorite.

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