What is Gas Flaring?
A gas flare is a safety device found in many industrial plants for the combustion of waste or excess flammable gas. Flares are most commonly found in the oil and gas industries including refineries and natural gas production and processing. However, flares can also be found in other industries such as waste water treatment plants, biogas and landfill operations. The measurement of flare gas flow is frequently required for environmental reporting or for obtaining a material balance for the operation.
Flare Gas Measurement Challenges
Flare gas measurement has several challenges which must be considered for any flare gas meter. While some of these difficulties are industry specific, Sage Metering addresses these issues as follows:
Flow variations. During normal operation in a flare system, the amount of gas flowing is quite low, while in the event of an upset or other non-normal operation the flow rate may increase considerably. Sage Thermal Mass Flow Meters provide the sensitivity to measure low flow rates during normal operation, and because of its high rangeability, it can measure high flow rates during upset conditions.
Gas mixtures. Generally the flow to the flare will consist of a mixture of different gases. The flare from refineries and other process operations will commonly contain a mixture of hydrocarbons and in some cases hydrogen. The composition will depend upon the source of the gas going to the flare. Biogas, digester gas, and landfill gas is a mixture of methane (CH4) and carbon dioxide (CO2) along with small amounts of other inert gases. Natural gas is typically 90%+ methane with ethane and a small amount of other hydrocarbons; inert gases such as nitrogen (N2) and carbon dioxide (CO2) may also be present. Sage Metering calibrates the thermal flow meter using the actual gas mixture specified providing the highest possible accuracy, while other manufacturers may perform an “air equivalency” calibration.
Changing gas composition. Changing gas composition will affect the heat transfer capabilities of the gas and affect the performance of the flow meter. Sage can model how varying gas mixture will affect performance. A minor change in composition, such as variations in the methane/ carbon dioxide percentages of biogases will have a minor change in performance which is within the accuracy requirements of the application. However, some applications such as the main flare header in a refinery, gas plant or chemical plant may have significant changes in gas composition. Sage Metering will review the variations of gas composition and will not recommend the use of thermal flow meters in any application where significant changes in gas composition can occur.
Flare Gas Meter
The use of thermal mass flow meters in flare gas service must be thoroughly evaluated. Using thermal flow meters in flare applications for the main flare header in refineries and other process operations should be avoided. Whereas, Sage Prime has demonstrated success in flare applications for biogas, digester gas, landfill gas and natural gas.