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Accurately Sizing Your Chemical Metering Pump

This is the second article in the series Sizing & Selecting the Right Chemical Metering Pump. Read the previous article titled "Planning for your Application."

A metering pump is utilized to inject chemicals precisely into a process to achieve a desired outcome. This outcome might include injecting a specific volume or mass of fertilizer, a set amount of acid to balance pH levels in water, or a precise amount of disinfectant to obtain a predetermined PPM (parts per million).

The rate of chemical injection required depends on the application, the desired result, and the concentration of the chemical. For example, when determining the amount of chlorine needed, the chemical injection rate can be calculated using the formula:

IR = 0.006xQxC / S

Where Q represents the system flow rate in GPM, C is the desired PPM for the chemical, and S stands for the concentration of the chlorine bleach in percentage.

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Engineers often use various methods to determine the required chemical injection rate based on the application. Once the injection rate, typically given in Gallons Per Hour (GPH), is known, the next step is sizing the pump appropriately.

An essential piece of information is the pressure the pump must overcome to inject the chemical. In scenarios where the chemical is injected into a pipe, the pump must overcome both the pipe's pressure and the pressure drop within the chemical lines from the pump to the injection point. For instance, a pump injecting into a 150 psi pipe located 250 feet away must handle significant line losses. Therefore, the pump should deliver the required flow rate at a pressure well above 150 or even 200 psi. The pipe's diameter also impacts the pressure drop; a smaller diameter increases drag and pressure drop. If the pump cannot overcome the combined pressure, the chemical will not enter the process. Conversely, any pump's flow decreases at high pressure, potentially leading to insufficient chemical injection. It's crucial to follow the pump performance curve against pressure to ensure flow requirements are met at the necessary pressure.

The injection rate in an application often varies due to factors like water flow and quality. Typically, you're working with a minimum and maximum flow capacity for the injection pump. It is recommended not to exceed 90% of the pump's capacity for maximum flow and not go below 10% for minimum requirements. Methods like using VFD and stroke adjustment can extend the pump's effective range, which will be detailed in the controls section of this article.

Another important consideration in pump sizing is potential future changes in requirements. If an increase in treated water is anticipated, it is better to select a pump with a higher flow capacity, provided the minimum requirement doesn't fall below 10% of the pump's capacity. This allows you to operate the pump at lower capacity until the demand increases, preventing the need for purchasing another pump.

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