Properly Measure Duct Flow

Getting accurate readings can pose challenges.

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

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Regulatory requirements related to greenhouse gas (GHG) emissions are mandating additional measurements at plants — for example, boiler air flow, fuel flow and their ratio. Adding a device to measure air flow may sound simple and straightforward. However, experience reveals that getting accurate readings of flow through a duct may pose challenges and surprises. So, here we'll provide some tips for success.

While we'll focus on measuring air flow in large ducts, you may apply the techniques to any fluid flowing through large ducts or pipes, i.e., those having a cross-sectional area of about 500 in.2 or greater.

One challenge that older plants often face is lack of documentation about process equipment. In addition, their ducts, fans, dampers, etc., may not have been designed to accommodate installation of additional measurement devices. Such issues can increase project risk and result in higher costs.

A measurement and automation supplier commonly will conduct an initial site survey to document the existing setup, including whether ductwork has geometries that may affect measurement reliability and accuracy. As a result of this survey, the supplier typically recommends a customized solution that reduces project risks, identifies emissions' reductions, and estimates potential cost savings.

To maintain reliability and accuracy and mitigate project risks, installation and calibration of the measurement device should adhere to its manufacturer's guidelines. Because air flow sensors work best with uniform (or linear), fully developed flow, virtually all recommended installation practices suggest straight sections of duct before and after the meter. However, this may not be practical at many plants.

Try to identify specific potential installation location(s) with a fully developed turbulent flow profile. Unfortunately, unique duct geometries that can introduce distortion (or non-linearity) in the flow pattern and hence compromise measurement reliability and accuracy often exist. These geometries typically include elbows or bends, presence of fans and dampers in the duct, placement of manway doors and service access ports, and internal structural restrictions such as (often undocumented) support members (Figure 1). In such situations, customized solutions can compensate for the distortions.

In addition, thin-wall and fiber-glass ducts and transitional shapes (round-to-rectangle, etc.) can complicate installation.

A variety of devices measure flow. Options include meters using pitot tubes, orifice plates, venturis, Coriolis, vortex shedding, turbine, magnetic, ultrasonic, positive displacement and thermal dispersion. However, most of these don't come in configurations suitable for rectangular ducts. The large size of some ducts compounds the challenge. For example, it wouldn't be practical to use an orifice plate or venturi tube to measure flow in a 60 in. × 48 in. duct. On the other hand, the space and size requirements for installing turbine meters may limit their use in smaller ducts. Positive displacement devices may not make sense due to their moving parts' maintenance requirements. Thermal-dispersion meters may not provide adequate reliability and accuracy because build-up of particulate matter or corrosion may affect their sensors.

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.

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