In Part VII of this flare gas series, we consider flare gas flow meter technologies (ultrasonic meters, averaging pitot tube, and thermal flow meters) and expand on thermal mass flow measurement principles. If you are interested in reading the Sage Metering white paper in its entirety, visit measuring flare gas.
Flare Gas Flow Metering Technologies
Ultrasonic Flow Meters
Measuring flare gas becomes a challenge for most flow meters.
Ultrasonic Flow Meters are highly accurate and are a useful tool to measure flare gas. They tolerate some condensed liquid, are not affected by gas composition, and endure fluctuations in pressure and temperature. With this type of performance, however, come high costs ranging from $50,000-$100,000 per installation. The ultrasonic meter does require pressure and temperature measurement to achieve mass flow. Ultrasonic flow meters measure the difference in transit time of pulses that travel from a downstream transducer to the upstream transducer, compared to the upstream transducer’s time back to the downstream transducer. It also can determine the molecular weight of the gas by measuring the speed of sound. This data is then used by the flow meter to reach a mass flow measurement in real-time when variations in gas mixture occur.
Averaging Pitot Tube
Averaging Pitot Tube is a differential pressure flow measurement device. The instrument has limitations with gas flow measurement, exceptionally low flow sensitivity, and has limited turndown. The measurement is contingent upon achieving velocity pressure. At low flow rates, it is most likely that there is insufficient velocity to achieve an acceptable signal. Additionally, if there are changes in gas specific gravity, the pressure drop is impacted, creating flow measurement error.
Thermal Mass Flow Meters
Thermal mass flow meters are suitable for measuring flare gas when the gas composition is consistent and known, and there is no condensation. Additionally, in some applications, when the operator is willing to trade off lower accuracy, the thermal mass flow meter can save money versus the expense of an ultrasonic flow meter.
Principles of Thermal Mass Flow Measurement
Thermal flow meters measure gas flow based upon the principle of convective heat transfer. Either insertion style probes or in-line flow bodies support two sensors that are in contact with the gas. The sensors are resistance temperature detectors (RTDs). The SAGE sensors consist of highly stable reference-grade precision-matched platinum windings clad in a protective 316 SS sheath for industrial environments. One sensor is heated by the circuitry and serves as the flow sensor, while a second RTD acts as a reference sensor and measures the gas temperature.
The SAGE proprietary sensor drive circuitry maintains a constant overheat between the flow sensor and the reference sensor. As gas flows by the heated sensor (flow sensor), molecules of the flowing gas transfer heat away from this sensor, the sensor cools, and energy is lost. The circuit equilibrium is disturbed, and the temperature difference (ΔT) between the heated sensor and the reference sensor has changed. Within one second, the circuit will restore the lost energy by heating the flow sensor, so the desired overheat temperature is restored. The power required to maintain this overheat represents the mass flow signal. There is no need for external temperature or pressure devices.
One of the advantages of thermal mass flow meters is that they have no moving parts, which reduce maintenance and enable their use in demanding application areas. They also do not require temperature or pressure corrections to achieve mass flow and maintain good overall accuracy and repeatability over a wide range of flow rates. This meter style measures mass flow rather than volume and is one of the few categories of meters that can measure flow in large pipes and ducts. In our next post of this series, we will discuss the Sage Metering difference and how thermal mass flow meters can measure flare gas flow.
Perhaps you may find this video on an ultrasonic flare gas meter of interest: