Recently, the gas emitted during oil production has garnered significant attention. Often, due to the lack of extensive gas collection systems, this gas must be burned off. Numerous regions mandate monitoring such flare gas to help minimize emissions and assess taxes, penalties, and additional charges. Finding the optimal instruments for measuring flare gas presents a challenge, yet the market offers a diverse range of potential solutions.
This article examines flare gas and the challenges in choosing an appropriate measurement solution for oil and gas companies. Available flare gas monitoring technologies and the advantages and disadvantages of each option are explored.
Image Credit: Sierra Instruments
Understanding Flare Gas Monitoring
Hydraulic fracturing and other oil extraction processes produce excess natural gases. Those gases require careful handling to avoid dangerous increases in pressure at extraction sites. It is only sometimes possible to harvest the gases for other uses due to infrastructure challenges and cost.
To remove the excess gas safely and efficiently, oil and gas companies use “flaring”, a process that burns it off. As the gases combust, flaring creates soot and carbon dioxide, byproducts with environmental implications when entering the atmosphere.
Many government organizations require oil and gas companies to measure how much gas is being flared. Investing in accurate, cost-effective measurement solutions is essential for monitoring greenhouse gas contributions and avoiding expensive fines.
Measuring the flow rate of flare gas can be challenging as it is not a pure gas but a combination of several different gases. Correct measurement of the total flare flow rate requires knowing the composition of the flare. This is usually determined by processing a small sample (a cut) through a gas chromatograph (GC).
Operators should account for the associated gas as soon as the gas begins to flow from a well. This means a flow meter calibrated to measure the flow rate of the flare gas is required.
Almost all current flow measurement technology needs a known composition for calibration. This offers a “chicken or egg” issue as the composition may not be known initially and will alter over time. In many situations, the flow meter may become inaccurate at best and inoperable at worst.
Below are some commonly used solutions that oil and gas companies utilize to overcome that challenge.
Flare Gas Measurement Tools
Ultrasonic Flow Meters
A commonly used solution for applications with variable gas compositions is multi-path ultrasonic flow meters. Built into in-line pipe sections, these meters measure the speed of sound through the flare gas to derive its density.
Ultrasonic flow meters work despite the variable gas composition characteristic of flare gas. They are also less prone to clogging or getting dirty as their exposure to the flare gases is indirect, resulting in reduced maintenance costs.
Ultrasonic flow meters measure mass flow versus volumetric flow, offering greater accuracy in the readings through factors like temperature changes. The disadvantage is that the whole meter must be removed to clean pipe sections. This can cause accuracy problems over time. Swirl and other flow profile effects can also affect the accuracy of ultrasonic flow meters.
Finally, ultrasonic flow meters are a costly solution. They calculate flow rates utilizing multiple paths, since more paths means better measurements. However, each added path pushes up the cost of the meter.
Averaging Pitot Tubes
Another tool for flare gas monitoring is averaging pitot tubes. These work by direct placement of an obstruction in the gas flow, then measuring the differential flow pressure on either side of the obstruction.
The benefit is that they measure the average across the pipe rather than at a single point. They are also a largely inexpensive option.
Averaging pitot tubes are less than ideal for flare gas measurement because they measure volumetric flow instead of mass flow. This pivotal difference can affect the accuracy needed to meet essential government regulations. These tubes are also known to have clogging and other issues as they are directly exposed to the gas flow. Their high and low flow (turndown) capabilities are poor, and they cannot measure variations in gas composition changes.
Thermal Gas Flow Meters
Historically, thermal gas flow meters have not been ideal for flare monitoring because of their requirement for factory recalibration when gas composition changes. Recent advancements in thermal gas flow meter technology have surpassed this challenge, and thermal gas flow meters have become a viable solution for flare gas measurement.
An important advantage of these meters is their ability to meet or exceed EPA CFR 40. Advanced thermal gas flow meters maintain high accuracy even through gas composition changes. Similar to ultrasonic flow meters, they measure mass flow for better accuracy but have further benefits for flare gas measurement.
Sierra’s QuadraTherm® Flow Meters for Flare Gas Monitoring
Modern immersible thermal flow meter technology innovations offer an answer to the “chicken or egg” issue outlined above. Sierra’s QuadraTherm flow meter accurately measures the mass flow of gases over a very wide turn-down.
An industry-first algorithm called qMix™offers users the ability to alter the meter’s calibration in the field to reflect the actual gas composition. This technology uses the NIST RefProp database of gases to derive the heat transfer properties of complex gas mixtures and maintain accurate metering.
qMix™ Mathematical Model – How It Works
QuadraTherm’s advanced mathematical model operates a microprocessor-based system that offers the basis for in-the-field compositional compensation. In a thermal mass flow meter, a velocity sensor measures heat loss from a heated sensor by the flowing gas. Further sensors measure other heat losses, including those resulting from natural convection, stem, radiation, and end loss. The heat removed by the flowing gas is in proportion to the mass flow.
The four-temperature microprocessor-based system measures the resistance of four RTD sensors as well as the current in the velocity sensor. The resistance values are converted to their four corresponding temperature values. The current to the velocity sensor is also converted to electrical power or wattage.
The inputs to the system consist of the four temperatures, the wattage, and the gas composition. The gas property algorithms compute the updated properties of the gas (mass density, thermal conductivity, dynamic viscosity, and heat capacity). The system then calculates the desired output or the total mass flow rate in the pipeline.
QuadraTherm meters can be stocked and ready for service when a spare or new meter is required. They are easy to install and the sample gas composition is programmed at the start-up of the well, which ensures flow measurement accuracy from day one.
Selecting a Flare Gas Measurement Solution
Sierra’s QuadraTherm® flow meters have become a clear leader in flare gas measurement. Wide turndown, direct mass flow measurement, and advanced accuracy from day one make QuadraTherm® meters the most cost effective and highly accurate flare gas measurement tool for oil and gas applications.
For more on Sierra’s flare gas flow meters, download “New Developments in Thermal Dispersion Mass Flow Meters: In-The-Field Compensation for Changes in Natural Gas Composition”.
Reference and Further Reading
Blog copy reprinted from Olin, J. G. 2014. New Developments in Thermal Dispersion Mass Flow Meters: In-The-Field Compensation for Changes in Natural Gas Composition. Presented at 2014 American Gas Association Operations Conference, Pittsburgh, PA, May 20-23, 2014
This information has been sourced, reviewed and adapted from materials provided by Sierra Instruments.
For more information on this source, please visit Sierra Instruments.