Flue Gas Flow and Velocity Measurement: Pitot Tubes and CEMS Flow Systems
Gas velocity and flow measurement in stacks and ducts: S-type pitot tubes, QAL1-certified differential pressure systems and isokinetic probe sets.
Why is flue gas flow measured?
The velocity and volumetric flow of gas discharged from a stack are core inputs for calculating emissions by mass. An analyzer only gives concentration (mg/Nm³ or ppm); to determine an annual or hourly emission load, this concentration must be multiplied by the gas flow. In isokinetic dust sampling, pump flow is also adjusted according to stack gas velocity, so velocity measurement affects sampling accuracy too. In continuous emission monitoring systems (CEMS), flow measurement is often a standard channel reported together with the other parameters.
Where is flow measured?
- Stacks and ducts: By measuring gas velocity across the section.
- In CEMS: As a continuous flow channel.
- In periodic emission measurements: Together with isokinetic sampling.
The method used for flue gas flow
The most common approach is the differential pressure method. A pitot tube senses the difference between total pressure in the flow direction and static pressure perpendicular to the flow; this difference is converted to velocity and, with the duct section, to flow. S-type pitot tubes are preferred for stack applications because they resist clogging in dusty, dirty flue gases. Gas temperature and pressure are also taken into account in the calculation, so probe sets generally include a thermocouple. In continuous systems, the pitot probe, pressure transmitter and calculation unit work together; some systems are offered with certificates such as QAL1.
Choosing a flue gas flow system
- Duct diameter, temperature and expected velocity range
- Dust and moisture load; probe material
- Certification requirements for continuous monitoring
- Mounting point of the probe on the duct and its position within the section
Flue gas flow measurement with Tetra
Tetra offers its own Darcy S-type pitot tube system, the SKI AccuFlo QAL1-certified flow system and Mega System S-Probe and Isocheck isokinetic equipment. It also provides mounting, commissioning, calibration and service itself.
Frequently asked questions
How is flue gas velocity calculated with a pitot tube?
Gas velocity is found by multiplying the pitot coefficient by the square root of twice the measured differential pressure divided by the gas density. Gas density is calculated from temperature, pressure and gas composition. Volumetric flow is obtained by multiplying the average velocity by the duct cross-sectional area.
What unit is flue gas flow reported in?
Velocity is measured in m/s, and flow in m³/h at operating conditions. For emission reporting, flow is converted to normal conditions (0 °C and 101.325 kPa), mostly on a dry basis, and given in Nm³/h. This correction requires gas temperature, pressure and moisture content to be known.
Why are measurements made at more than one point across the section?
The velocity profile in a stack is low near the walls and high at the center, and it can be asymmetric because of disturbances. A measurement from a single point may not represent this profile. In periodic measurements, the section is therefore divided into the points defined by the standard, and the average velocity is calculated from measurements at those points.
How is a continuous flow measurement system verified?
A continuous flow measurement system is calibrated by comparison with reference flow measurements made at the same section; in emission monitoring systems, this is part of the QAL2 process under EN 14181. In operation, clogging of the pitot tips is the most common problem, so automatic back-purging and zero checks of the pressure transmitters should be included in the maintenance plan.
Products measuring this parameter
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