In Part II of this series on flare gas measurement, we discuss the challenges for monitoring or measuring flare gas, including extreme flow variation, the potential for changing gas composition, and working in hazardous locations.

Flow Meter Challenges

There are inherent challenges for operations or measurement engineers in many flare gas applications to select the correct flow meter. When considering the appropriate instrument, the user must consider the following:

Mixed Gases and Calibration

Thermal mass flow meters measure heat transfer and associate heat transfer to mass flow based on the calibration. Since various gases have different heat transfer properties, the thermal mass flow meter must be calibrated with the specified gas to prove an accurate flow rate. The Sage Metering flow meters are calibrated using the actual gas composition (or a combination as close as possible to the specified composition). This calibration method is more accurate than using air with correction factors for different gases, which is another commonly used calibration procedure.

Whenever there is a combination of gases, it is unlikely that the composition will remain constant. Often the variations in the gas mixture occur on a seasonal basis. SAGE can predict the variations in performance based on the different gas compositions, if known.

Wide Turndown Ratio Required

The device must be able to measure extremely low flow associated with normal venting conditions. Simultaneously, be able to measure extremely high flow accurately during an emergency or unsettled state.

Challenges for Thermal Mass Flow Meter when Measuring Flare Gas
Flow meters must meet the accuracy and calibration requirements established by the EPA and/or emissions trading regulations.

Environmental Regulations

Flow meters must meet the accuracy and calibration requirements established by the EPA and/or emissions trading regulations. Some environmental agencies and emissions trading regulations require flow meters to be accurate to ±5% or better, as referenced in 40 CFR 98.3.

Hazardous Location

Because of the flare gas’s combustible and flammable properties, the flow meter must be approved for use in a hazardous location by the appropriate agency. The approval should not rely on an enclosure-only classification but instead on the entire flow meter.

Pipe Access Limited

Access to areas where flow meters need to be installed, serviced, and maintained can be challenging in flare gas applications. Ultrasonic spool-type meters typically require lengthy shut-down time and high labor costs to install and maintain these meters.

Large Pipes

Effective flow meter options decrease as pipe sizes increase.

Limited Straight Run

Some facilities have limited space, such as offshore platforms, and obtaining good straight run to achieve repeatable flow accuracy can be challenging to achieve.

If you want to see the paper in its entirety, visit Flare Gas Measurement Using Sage Thermal Mass Flow Meters. In Part III of this series, we consider one of the most challenging obstacles in flare gas measurement, Changes in Gas Composition, and Hydrogen.

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