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Facts at your Fingertips: Flow Measurement in Large Lines, Ducts and Stacks

| By Scott Jenkins

Process industry plants install sophisticated air-pollution control systems, but they can be ineffective if the flowmeters on which they rely deliver inaccurate or unreliable data. This one-page reference provides information on measuring the flow of gaseous combustion products in large pipes, ducts and stacks.

Flue products

A flue is typically a large pipe, duct, stack, chimney or other venting apparatus attached to an industrial plant system, such as a boiler, furnace, steam generator or oven, through which waste gases are exhausted. Depending on a number of factors, including the type of industrial plant, process characteristics, fuel used and efficiency, fluegases can include the following compounds:

  • CO2
  • CO
  • CH4
  • N2
  • NOx
  • O2 and H2O
  • Ozone
  • Sulfur oxides
  • Particulate matter

Environmental standards

For large stack-monitoring applications, the U.S. Environmental Protection Agency (EPA) requires a continuous emissions monitoring system (CEMS) or rate monitoring system (CERMS). For CERMS, the flowmeters must perform an automated and on-demand self-checking of calibration drift (CD) at both a low-range and a high-range flow point.

In the E.U., these systems are referred to as automated measuring systems (AMS). The flowmeters that support them also need to meet the quality assurance level 1 standard, confirming compliance to EN 15267-1,-2,-3 and EN 14181 standards.


Figure 1. The composition of fluegas is mixed and the volume varies depending on products, workload and weather

Figure 1. The composition of fluegas is mixed and the volume varies depending on products, workload and weather


Measuring the flow of stack- or fluegas is a challenge (Figure 1). Fluegases are generally of mixed composition, and the volume emitted varies based on the products of the process, workload schedules and seasonal temperature and humidity fluctuations. This variability can lead to irregular swirling flows that are difficult to measure without multipoint sensing.

Large-diameter pipes, stacks and ducts present unique physical challenges to successful flowmeter installation and performance. Installations are complicated by difficult-to-reach access points, single-plane platforms, long cable runs, extra mechanical support and exposure to weather extremes.

Lack of pipe straight-run, distorted flow profiles, low flowrates and wide turndown rates are common performance challenges for many flow-metering technologies. Furthermore, the gas can be dirty and at high temperatures, which can result in measurement degradation, clogging and fouling that lead to excessive maintenance procedures or premature flowmeter technology failures.

Gas flow measurement is increasingly a multipurpose endeavor: objectives are to ensure compliance with government regulations and to provide data on process gases for scrubbers and flare systems. The combination of these factors results in the need for flowmeters that operate accurately and reliably over a wide flow range in rugged environments with distorted and swirling flow profiles.


Evaluating technologies

In considering flowmeters for gas monitoring, the first step is to choose the appropriate flow technology. Several flow-sensing technologies are available, including the following: Coriolis (mass); differential pressure; electromagnetic; positive displacement; thermal (mass); turbine; ultrasonic; and vortex shedding. These technologies all have advantages and disadvantages depending on the type of process fluid (air/gas or liquid). Process engineers must consider many factors, such as limited straight-run challenges, dirt and particulate matter, mechanical installation, high temperatures and moisture entrained in the flow stream, in addition to cost-benefit considerations in meeting accuracy requirements, maintenance and life expectancy of the equipment.

Looking at these factors, along with the plant’s layout, environmental conditions, maintenance schedules, energy cost and return on investment (ROI), will narrow the field. The most common flow-sensing technologies chosen for fluegas measurement are differential pressure (averaging Pitot tubes) flowmeters and thermal dispersion mass flowmeters. Both technologies have similar accuracy levels when configured with multiple sensing points within the large cross-sectional area of a fluegas line. For swirling flows of hot fluegases, multipoint sensing generally provides more accurate flow measurement than single-point technologies.

Maintenance requirements, which raise operating and lifecycle costs while reducing return on investment, are different for the two technologies. Most averaging Pitot tube flow sensors require a daily manual or compressed-air backpurge system to keep the inlets from clogging. Thermal flowmeters, with no inlets or moving parts, can require virtually no maintenance for years.

Examining the accuracy, installed cost and lifecycle cost of the various flow-sensing technologies available for fluegas monitoring allows an informed selection of equipment. Unique problems may benefit by consulting flowmeter suppliers.

Editor’s note: The content for this edition of “Facts at your Fingertips” column was initially prepared by Steve Craig, a senior member of the technical staff at Fluid Components International (FCI; San Marcos, Calif.;