Properly Measure Duct Flow

Getting accurate readings can pose challenges.

By Dave Winters and Amy K. Johnson, Emerson Process Management

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Figure 4 shows a graphical dashboard summary of the results of performing the inline calibration procedure. It led to a change in K factor to 0.6819868 from 0.5569, and thus better measurement accuracy.

However, don't assume a revised K factor from a single inline calibration procedure will accurately calculate flow under any operating condition. Most process plants change operating conditions to match marketplace demand. The air flow pattern in the duct and the K factor will vary depending on the load.

You must take several steps to achieve improved measurement accuracy under varying load conditions:

1. Obtain an initial measurement of K factor for a duct when the process is operating in one of multiple steady-state process conditions.

2. Change the process condition and measure the corresponding K factor for that condition using the pitot traverse method.

3. Repeat this procedure for each of the other steady-state conditions to obtain their K factors.

4. Apply the correct K factor for the particular process condition to calculate flow.

Best practices automatically select the K factor corresponding to current conditions.

Correcting for distortions can improve accuracy by as much as 1.5 –2%. In some situations, even 0.5% better accuracy potentially can contribute tens of thousands of dollars to the bottom line. For instance, in a boiler application, a more accurate measurement can significantly raise fuel-use efficiency and reduce energy consumption. In addition, proactive monitoring may help predict maintenance issues and extend the lifecycle of plant equipment.

As monitoring flow in ducts becomes more essential, so too does a systematic approach to meet the regulatory challenges, avoid surprises and reduce risks. Product application expertise and improved methodologies to design and install measurement devices can maximize the benefits to plant operations and the bottom line.

DAVE WINTERS is a Boulder, Colo.-based field service engineer for Emerson Process Management. AMY K. JOHNSON is director, marketing/business development, for Emerson Process Management, Chanhassen, Minn. E-mail them at and

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