Flare gas meters are an indispensable part of flare systems. They provide plant operators and engineers with vital data to ensure legal compliance and an insight into how much gas is being flared to improve processes. Flare meters also help close the gap on mass balance, reduce emissions, and protect the environment by accounting for the volumes of natural gas flared.
So how can operators ensure their flare meters meet operational needs and comply with legal, social, and environmental demands?
1) Choosing the right technology
Calculating and metering the volumes of natural gas expelled through flare systems is arguably the most challenging form of gas flow measurement.
Flare gas is subject to wildly fluctuating velocity ranges, varying atmospheric conditions, extreme temperatures, and mixed compositions which makes it especially difficult to measure its flow accurately.
The challenge is increased further in very large pipe sizes and when installations lack straight run pipelines or have insufficient flow conditioning, for example on offshore platforms.
Operators typically review various technologies when looking for a solution to measure flare gas. These include differential pressure, thermal mass, optical-photo measurement, and ultrasonic measurement.
Let’s explore their differences.
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Differential Pressure (DP) Measurement:
DP meters were used in the early days of flare measurement. They have a limited turndown ratio and often need regular re-calibration and long, straight pipe run to be effective, limiting their usefulness. They are subject to fouling and their mostly intrusive nature can lead to pipes being obstructed by dirt and fouling.
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Thermal Mass Measurement:
Thermal mass meters rely on the concept of convective heat transfer to measure flare gas flow. The meter uses data from a heated element and a temperature sensor that are mounted on a rod and inserted into the pipe. When the temperature sensor comes into contact with flowing flare gas, it starts to cool down as the gas molecules transport some of the heat away.
The thermal mass meter then determines the flare gas flow by measuring the amount of heat the sensor has lost compared to the element. The faster the flow, the cooler the element will be.
Thermal mass meters are ideal for conditions where the gas composition is constant, otherwise, it requires regular correction when the composition changes, a likely event in real conditions – where gas composition change drastically without notice. There is a workaround to use a process gas analyser, but the latency of measurement means the flaring event may be over before the results and recalibrations can be supplied to the meter.
Additionally, the intrusive measuring rod is subject to fouling due to the significant amounts of sand, wax, oil, carbon, and other solid matters present in the gas flow. When the sensors become coated by fouling, then the amount of heat dissipation changes, resulting in significant measurement errors. The solution is then to remove the rod, clean it, or replace it which requires a process shutdown to complete.
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Optical-Photo Measurement:
Optical-photo meters use the optical scintillation technique to measure the movement of turbulence found in a gaseous flow stream. Said more simply, the meter sends laser beams back and forth across the width of a pipe to measure flare gas flow.
The non-intrusive meters measure across the entire pipe diameter and provide highly accurate path-averaged air velocity measurements.
Although measurement is unaffected by straight pipe length, extreme temperatures or pressure, the meters are highly susceptible to fouling that develops inside the pipeline.
Fouling can block the laser beams from travelling across the pipe, affecting the accuracy of measurement. The only workaround is a complete shutdown of the installation to clean the fouled pipeline when this happens – which is a highly undesirable action for most operators.
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Ultrasonic Measurement:
Ultrasonic flare meters measure the time that sound takes to travel between two points to determine the gas velocity among other useful parameters. This is
known as the ‘time of flight’ method of measurement.
For example, a typical configuration of Fluenta’s ultrasonic flare measurement system uses two ultrasonic transducers placed at an angle flush to the Inner Diameter of the pipe (pipe ID). Both transducers send and receive ultrasonic signals, one with the flow of gas, and the other against it.
When gas flows through the pipe, the signal travelling against the flow takes longer to reach the opposite sensor than the one travelling with the flow of gas. The time difference is then used to calculate the flow in the flare line.
Having the sensors flush to the pipe ID ensures non-intrusive measurement. This shields them from the impact of particles and gas during high flow velocities and allows signals to travel across the whole diameter of the pipe.
Ultrasonic flare meters are generally better at coping with fouling deposits. A layer of deposits may attenuate the ultrasound signals but they’re unlikely to block them completely and they can continue to measure flow at an acceptable level of accuracy.
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Summary: the best technology for flare gas flow measurement
This brief review of the various technologies available to measure flare gas flow concludes that the use of ultrasonic meters is advantageous. Ultrasonic meters are less impacted by fouling and don’t require a plant shutdown to maintain compared to other technologies. The use of ultrasound is also the most accurate and repeatable form of measurement to ensure legal compliance among other benefits.
Additionally, the API’s Manual of Petroleum Measurement Standards, chapter 14, section 10, commonly referred to as API 14.10, scientifically outlines the advantages of ultrasonic flare meters compared to other technologies.
However, not all ultrasonic flare meters are made equal. So, how can operators choose the right solution?
2) Choosing the right ultrasonic meter for your application
Narrowing down the choice to the right technology is only half the battle. The performance and features of ultrasonic flare meters vary depending on the manufacturer.
At Fluenta we encourage our customers to take into consideration the various flare meter specifications when selecting a product. These include:
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Technical suitability
Temperature and acoustic attenuation are the primary considerations when selecting ultrasonic technology.
For example, Fluenta recently launched the Fluenta FlarePhaseTM range of transducers that can handle the widest temperature range on the market (-200°C to +350°C). These were developed to suit the requirements of chemical, LNG, and oil and gas plants both operationally, legally, and to future proof against tightening regulations to come.
Acoustic attenuation is a function of the design and frequency of the transducer. To minimise acoustic losses in gasses such as CO2 and Methane, Fluenta utilises a narrow band device (the FlarePhaseTM transducer) with targeted frequencies and unique signal processing. This ensures performance in these challenging conditions.
Other technical aspects to consider include turndown ratio, velocity range and pipe size capabilities. For instance, Fluenta’s transducers offer a turndown ratio of 4000:1, velocity ranges from 0.03m/s to 120 m/s and measurement in pipes 6” to 72” as standard.
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Reliability
Flare meters are subject to harsh conditions and variable factors such as gas composition, which can affect the consistency of obtaining accurate results over a prolonged period of time.
To meet this requirement, Fluenta offers non-intrusive ultrasonic sensors as standard, which have no moving parts and aren’t affected by the dirt, debris, and pollutants that travel through pipelines.
Additional steps can be taken to prolong the reliability of the meter. Where intrusive measurement is necessary, Fluenta offers high-performance coatings to protect against fouling. Operators can also benefit from in-situ verification with Fluenta’s Health Check analysis with the UFM Manager software.
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System flexibility and compatibility
Plants often have varying requirements that dictate the setup and configuration of the flare meter. It is important to consider the flexibility of the chosen solution to ensure it meets your measurement criteria.
Fluenta’s ultrasonic flare systems can be installed in a single path, dual-path, or bias-90 configuration depending on a facility’s accuracy, redundancy, and space demands, to name a few.
The FGM 160 from Fluenta doesn’t require conditioning plates or other pipework inserts. The field computer can also be mounted up to 50 meters from the transducers allowing easy, compact, and flexible installation and positioning.
For optimal operation, the transducers only require 10 pipe diameters of straight pipe upstream and 5 diameters downstream of the metering point. For facilities with limited straight length pipeline available, Fluenta also offers CFD (Computational Fluid Dynamics).
Fluenta’s flare measurement systems also make it easy for operators and engineers to obtain several measurement parameters beyond the standard and actual flow volumes. These include mass flow, molecular weight, standard and actual density, pressure, temperature, speed of sound, gas velocity, and more.
Conclusion
Many of the world’s leading testing and certification institutes as well as the largest LNG, chemical, and oil and gas companies attest that ultrasonic flare meters are the best technology for flare gas metrology.
Fluenta’s innovative product range has been developed in response to considerable flaring challenges in the energy industry from accuracy, flexibility, and repeatability, to access to support, spares, and limiting the need for interruptive maintenance.
So, if you’re an operator looking to upgrade your existing flare measurement setup or install a new one – the choice is easy – because we’re fluent in flare gas.
