Properly Measure Duct Flow

Getting accurate readings can pose challenges.

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

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T-shaped Design
Figure 2. This device features two low-pressure ports on the downstream side.

Often the best choice is an averaging pitot tube. It measures differential pressures (dPs) created by an obstruction in the flow path across multiple points within a duct, to come up with an average flow rate. (In contrast, a standard pitot-tube-based device senses pressure at a single point selected to represent the average flow velocity.) This flow rate is a function of K, a flow coefficient, and the square root of dP. The K factor is the ratio of the actual flow rate to the calculated (theoretical) flow rate. The dP is proportional to the square of the fluid velocity through the duct. Averaging-pitot-tube-based measurement provides the highest level of reliability and accuracy as well as least amount of obstruction and energy loss, and is less expensive than other technologies. Also, dP technology is widely used and understood, and requires no special service or maintenance equipment. In addition, an averaging pitot tube can be easily modified or custom tailored (e.g., in mounting hardware, sensing port locations, or multiple elements) to the specific duct application.

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.

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.

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