『Flue Gas Flow Meter Sizing: Thermal Dispersion and Pitot Tube Systems for High-Temp Stack Monitoring』Related information(clamp on meter|electromagnetic meter|venturi meterrotameter|orifice meter|ultrasonic flow meter|mass flow meter|coriolis mass flow meter|coriolis flow meter|magnetic flow meter|magmeter flow meter|magflow flow meter|mag meter flow meter|electromagnetic flow meter|vortex flow meter|turbine flow meter|thermal mass flow meter|thermal flow meter|rotameter flow meter)
Flue Gas Flow Meter Sizing: Thermal Dispersion and Pitot Tube Systems for High-Temp Stack Monitoring
Quick Answer: Flue gas flow meter sizing starts with gas temperature, duct internal diameter, normal velocity, and dust load. Use a thermal dispersion flow meter for stacks up to 400 °C with low to moderate dust. Use a pitot tube system for large ducts, high velocity, or gas above 400 °C. Send your stack ID in mm, gas composition, temperature in °C, pressure in bar, and flow range in kg/h to Silver Automation Instruments for a size check.
Stack gas measurement is not a simple insertion sensor job. A power plant in Vietnam had a 1600 mm stack with 9 m/s average velocity and periodic soot blowing. The first insertion thermal probe worked until dust covered the sensor. The correct sizing step was to check velocity, probe length, and purge air before ordering.
Thermal Dispersion Flow Meter Sizing for Stack Gas
Thermal dispersion flow meters measure gas mass flow by sensing heat removal. They work well on clean flue gas, combustion air, and biogas. For stack use, check the gas temperature at the probe location. A standard thermal dispersion flow meter from Silver Instruments, model SIL-TMF-400, handles up to 400 °C with a PT100 reference sensor and 4-20 mA HART output.
Sizing inputs matter more than the meter brand. You need the duct internal diameter in mm. You need normal flow in kg/h or Nm3/h. You need minimum and maximum velocity. A 1200 mm duct at 300 °C and 5 m/s may need a 450 mm insertion probe. If the duct is 2000 mm, a short probe reads only one point. Most engineers skip this part until the flow reading drifts.
Dust and moisture cause drift. A cement plant in Southeast Asia measured kiln exhaust at 180 °C. The gas carried fine dust and 12 percent water vapor. The thermal dispersion probe worked after we added a sintered filter and a longer insertion length. The same meter failed on a coal boiler with sticky ash at 42 percent relative humidity and no purge air.
For sizes, a thermal dispersion flow meter for flue gas from Silver Instruments comes in DN50 to DN300 in-line versions and insertion versions for ducts from 300 mm to 6000 mm. Output is 4-20 mA HART plus Modbus RTU. The meter body is 316L stainless steel. For corrosive gas, specify Hastelloy C276 wetted parts.
Pitot Tube Flow Meter Sizing for High-Temp Ducts
Averaging pitot tube flow meters measure differential pressure across multiple sensing points. They handle high stack temperatures up to 800 °C, large ducts, and dust with purge systems. Sizing is different from thermal dispersion because you need the square root relationship between differential pressure and flow.
For a 2200 mm stack at 450 °C and 15 m/s, a pitot tube system with a 0 to 250 Pa differential pressure transmitter is typical. The probe material depends on the gas. A waste incinerator in the Middle East ordered a 316L pitot tube with PTFE impulse lines. A refinery in Mexico needed Hastelloy C276 because the flue gas contained 180 ppm HCl and 60 percent relative humidity.
A pitot tube flow meter from Silver Instruments, model SIL-APT-800, works with gas temperatures from -40 °C to 800 °C. The probe length is cut to order from 300 mm to 6000 mm. The differential pressure transmitter can be 4-20 mA HART, Profibus PA, or Modbus. For high dust, add a blowback purge system. The purge air pressure should be 0.5 bar to 1 bar above stack pressure.
In practice, pitot tube sizing requires the density of the gas at operating conditions. You need gas composition, moisture content, static pressure, and temperature. A common mistake is using standard air density for w
Key Sizing Inputs for Flue Gas Flow Meter Selection
Send these values to Silver Instruments when you ask for a quote.
Stack inner diameter in mm. Gas composition or fuel type. Normal flow in kg/h or Nm3/h. Minimum and maximum flow. Gas temperature in °C. Static pressure in bar. Dust concentration in mg/Nm3. Moisture content in percent volume. Probe insertion length from duct wall to center.
We use these to calculate velocity at normal flow. For thermal dispersion, we check if the minimum velocity stays above 0.5 m/s and if the maximum velocity stays below 45 m/s. For pitot tubes, we check if the differential pressure at minimum flow stays above the transmitter accuracy limit. A 0 to 100 Pa transmitter loses accuracy below 4 Pa. In that case we recommend a 0 to 40 Pa range.
Common Mistakes in Stack Flow Meter Selection
One mistake is ordering a short insertion probe for a large duct. The sensor sees only the wall boundary layer. The reading reads low. Another mistake is ignoring the acid dew point. If the gas cools below the acid dew point, sulfuric acid condenses on the probe. The sensor drifts and the transmitter card corrodes.
Thermal dispersion does not work well in high dust above 1 gram per Nm3. A pitot tube with constant purge works better. But a pitot tube has a low turndown ratio because the differential pressure follows the square of velocity. If the turndown is 20 to 1, a 0 to 250 Pa transmitter is too coarse at minimum flow. A dual range transmitter or a thermal dispersion meter may be a better fit.
For ATEX Zone 1 stacks, specify the appropriate transmitter housing and probe certification. Silver Instruments can supply thermal dispersion meters with Ex d IIC T6 gas group certification and pitot tube transmitters with Ex ia IIC T4 safety parameters.
FAQ: Flue Gas Flow Meter Sizing
What is the maximum temperature for thermal dispersion flow meters?
Standard thermal dispersion flow meters from Silver Instruments handle up to 400 °C. High temperature versions with ceramic probe materials can go to 550 °C. Above that we recommend a pitot tube system.
Can a pitot tube flow meter measure wet flue gas?
Yes if you add a blowback purge system and heat trace the impulse lines. The gas temperature must stay above the dew point at the transmitter. Wet gas without purge plugs the sensing holes within days.
Which flow meter is better for a 300 mm stack?
For small stacks, a thermal dispersion insertion probe is simpler. A pitot tube may fit but the insertion length and averaging ports matter less in a small duct. In-line thermal mass flow meters are also an option for DN50 to DN300 pipes.
How do I size the probe length?
Probe length equals the stack internal diameter divided by two plus the nozzle height plus 100 mm for the compression fitting. For a 1600 mm stack, that is 800 plus 200 plus 100 equals 1100 mm.
What output signal is common for flue gas flow meters?
4-20 mA HART is the most common for plant control. Modbus RTU is useful for remote monitoring and data logging. Both are available from Silver Automation Instruments.
Need a flue gas flow meter size check? Contact Silver Automation Instruments. Tel: +86-25-68650347. Whatsapp: +86-25-52155837. WeChat: +86 15365082610. Website: flow-meter.com.au. Send your stack ID, gas temperature, pressure, and flow range. We reply with a model and quote within 24 hours.
Hot Product
Related AD
