How to Evaluate a Metering Pump’s Turndown Accuracy

Chemical dosing applications require precision accuracy. This is true whether you are dealing with chemical manufacturing, oil and gas processing or water treatment. The standard acceptable turndown ratio is generally within ±1.0%. That does not give you a lot of margin for error. 

Adjusting for Changing Operating Conditions

In addition, you have to account for changing operating conditions. This requires a metering pump with the ability to alter flow rates without losing any of its accuracy. With multi-state chemical dosing processes, different volumes of chemicals should be accurately released at specific times. When you don’t have exceptional turndown accuracy, you will not be able to achieve the desired dosing results. 

What is Turndown?

Turndown is the ratio of a metering pump’s output. For example, if a pump has a turndown ratio of 1000:1, that means it should be able to move anywhere from 1,000 GPH or 1 GPH within the same accuracy rating. It allows the user to adjust for changing operating conditions or production rates in a seamless manner, without ever losing the precision chemical dosing performance required.

The Importance of Steady-State Turndown Accuracy

In other words, turn down ratings must be repeatable and sustainable with steady-state accuracy. The first step toward achieving this important objective is to answer these three questions:

  1. What turndown does the process require?
  2. If turndown exceeds 100-to-1, what is the best and most efficient way to address it?
  3. Will the pump selected provide the turndown required with the accuracy expected, regardless of variable conditions?

Let’s take a look at the three industries where turndown accuracy is most important with metering pump systems:

Municipal Water Treatment

Chemical dosing is required in water treatment. It is done at specific intervals to prevent coagulation and flocculation, manage pH levels, de-chlorinate, disinfect and maintain desired taste and odor control. Proper chemical dosing is absolutely critical to create the desired quality of municipal water. At the same time, you can’t afford to over-treat the water. Under-dosing can be harmful to people using the water. Over-treating can also be hazardous while also hurting your operating budget. Incoming water can be different on a daily basis. This makes turndown and flexible chemical dosing important to account for changing conditions.

Chemical Processing

Chemical processing is all about creating unique chemical bonds to manufacture various products. Catalysts and polymers need to be add in precise quantities at specified intervals. The production process may scale up or scale back, and thus turndown accuracy is required for the metering pump system.

Oil & Gas

Demand will spike and decline daily in the oil and gas industry—perhaps more than an in any other industry. Flow assurance chemicals that keep well-heads flowing are very expensive, and you cannot afford to waste them when demand is low. Turndown accuracy and adjustability are absolutely vital in oil and gas production.

Steady-state accuracy and repeatability will ensure accurate dosages. This will minimize chemical usage while maintaining proper chemical treatment throughout the processing. You must select and configure a metering pump system that achieves these desired performance characteristics. Milton Roy is a leading producer of metering pumps. We highly recommend you download this great white paper they put together:

Criteria for Evaluating a Metering Pump’s Turndown with Steady-State Accuracy

To learn more about Milton Roy metering pumps and for configuration, installation and maintenance guidance for your chemical dosing system, contact DXP Cortech today.