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 2 shows a cross-sectional view of an averaging pitot tube having a T-shaped design. The high pressure sensing port is upstream on the flat face and uses slots instead of holes to average the signal. The low pressure ports face downstream and are in a quiet zone behind the sensor face. This design also features an integral thermowell for housing a temperature sensor. The impact and stagnation zones help keep particulate matter from entering sensor ports. The T-shape controls the separation point and stabilizes the low pressure measurement. Rounded edges reduce wear on the instrument. Devices with averaging slots instead of holes measure the flow profile more accurately.

You can mount an averaging pitot tube in any number of ways, such as with a combination compression/packing type fitting, "field fitted" duct flanges or standard piping flanges.

Stacks — vertical ducts having a cross-sectional area more than about 500 in.2 and a height greater than around 30 feet — can present special challenges because of particulates or condensable matter. They may require use of a "severe service" averaging pitot tube. This features large internal plenums and passages, multiple clean-out ports, and extra ports for addition of purging and draining components, if required. In applications where periodically shutting air flow to allow sensor cleaning isn't practical, a purge system can automatically clean out particulate or condensable matter. Figure 3 illustrates a severe service unit with a purge system.

DEALING WITH NONLINEAR FLOWS
An inline calibration technique called pitot traverse enables customization of the K factor to improve measurement accuracy under nonlinear flow conditions. The technique is EPA approved for conducting RATA (relative accuracy test audit) to demonstrate compliance.

Such an inline calibration system includes an averaging pitot tube permanently mounted in a duct, its secondary components, and an S-type (or Stausscheibe) pitot tube, preferably mounted in the duct about 8–16 in. upstream. You change the insertion depth of the S-type pitot tube to measure dP and temperature at various points located in a number of equal areas (referred to as centroids of equal areas) in the cross-section. Typically, you perform two traverses at right angles to each other across the diameter of the duct. For rectangular ducts, you should divide the cross-section into a number of equal rectangular areas and measure dP at the center of each. You then compare these dP reference measurements to the measurements obtained from the averaging pitot tube during the traverses to calculate a calibrated value.

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.

GO WITH THE FLOW
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 Dave.Winters@emerson.com and Amy-K.Johnson@emerson.com.

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