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Low Flow Gas Meter Selection: Thermal Dispersion Transmitters for Precise Low-Velocity Vapor Pipelines
Quick Answer
For low gas flow in vapor pipelines, a thermal dispersion transmitter is often the best choice. It reads mass flow directly from 0.5 Nm/s to 100 Nm/s without pressure or temperature compensation. Silver Instruments TMF series works well for DN15 to DN300 lines at low pressure.
Why Low Flow Gas Measurement Fails with Standard Meters
Most engineers skip this part. In practice, a vortex meter loses signal below about 2 Nm/s. A differential pressure device needs a beta ratio that is too small for low energy gas. A turbine meter may stop spinning at low velocity. Thermal dispersion fills this gap.
The TMF sensor uses two PT100 elements. One element is heated. The gas flow cools the heated element. The temperature difference relates to mass flow. Because the measuring principle is thermal, the meter does not need pressure and temperature compensation. Direct mass measurement is the main benefit for low pressure vapor pipelines.
Key Parameters to Specify for Low Flow Gas Meters
You need gas composition, pressure, temperature, pipe size, and flow range. Do not send a request with only pipe size. Gas type matters for thermal conductivity. Air, nitrogen, natural gas, and vapor have different cooling rates.
Silver Instruments TMF series outputs 4-20 mA HART. It has an accuracy of plus or minus 1 percent of reading from 0.5 Nm/s to 60 Nm/s. It works at temperatures up to 400 °C for an insertion model. Process pressure can be up to 40 bar. Inline models fit DN15 to DN50. Insertion models fit DN80 to DN300.
For ATEX Zone 1 areas, request the Ex d version. The flow meter electronics can be remote mounted up to 50 meters away.
Where Thermal Dispersion Transmitters Fit in Real Plants
We have seen this on customer sites many times. A paint manufacturer in Southeast Asia uses a TMF inline meter on a DN40 solvent vapor recovery line. Flow velocity is only 1.2 Nm/s at 0.5 bar. The meter holds a steady signal and reports total mass flow every second.
Last year a customer in Vietnam asked us about a DN25 nitrogen purge line. Their existing rotameter needed cleaning every month. We supplied a TMF-DN25 inline thermal mass flow meter. It has run 14 months without maintenance. The plant uses the 4-20 mA output to control a blower.
In desalination plants,
Recommended Silver Instruments Models
TMF-100 inline thermal mass flow meter fits DN15 to DN50. Process connection can be G thread, NPT, or flange. It handles gas flow from 0.5 Nm/s to 100 Nm/s. Power supply is 24 VDC. Output is 4-20 mA HART plus Modbus RTU.
TMF-200 insertion thermal mass flow meter fits DN80 to DN300. It has a retractable probe. Hot tapping is possible under pressure. The probe length is calculated based on pipe inside diameter.
Both models use PT100 sensors. The mass flow output is in kg/h or Nm3/h. The sensor material is 316L stainless steel. Ex d and non-Ex versions are available.
How to Get a Quote from Silver Instruments
Send us your pressure (bar), temperature (°C), pipe size (DN), and flow range. Add the gas composition if it is a mixture. We will reply within 24 hours with a model and price. Tel: +86-25-68650347. WhatsApp: +86-25-52155837. WeChat: +86 15365082610.
FAQ
Does a thermal dispersion flow meter work for wet gas or saturated vapor? Wet gas and saturated vapor can cause drift on the sensor. We recommend a condensate trap and a filter upstream. For saturated steam, use a different meter type such as vortex with pressure compensation.
What is the lowest velocity a TMF series can measure? The standard calibrated range starts at 0.5 Nm/s. For some clean gas lines we can calibrate down to 0.1 Nm/s. Ask for a low flow calibration report when ordering.
Can I use a thermal mass flow meter for gas mixtures? Yes, if the gas composition is stable. You must give us the component percentages. The thermal conductivity of the mixture affects the calibration. A large change in gas composition reduces accuracy.
Does Silver Instruments provide local service in Southeast Asia? We ship to Vietnam, Indonesia, Thailand, Malaysia, and the Philippines. We have local distributors in several countries. We also support remote commissioning through WhatsApp video.
What maintenance does a TMF series need? The sensor needs periodic cleaning if the gas contains oil or dust. A simple alcohol wipe on the PT100 elements is enough. The electronics have no moving parts.
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