In the oil and gas industry regulations and requirements to measure, monitor and report flared gases continue to expand and extend. The U.S. Environmental Protection Agency continues to focus on enhancing regulations aimed at reducing emissions of methane and volatile organic compounds into the environment.

The recently held multinational Paris Climate Conference and its resulting agreement have even broader global implications for still more attention on measuring flared gases. As environmental and climate change-driven regulations in the U.S., Canada, the EU and elsewhere around the globe continue to become more stringent, companies in the oil and gas sector will be continuously challenged to invest in accurate and reliable flare gas measurement across their operations.

Typical challenges
Flare gas flowmeters provide a tool to signal abnormal process changes and early leak detection and provide a flow measurement that is used for environmental agency reporting. Flare gas applications present several challenges to plant, process and instrument engineers when selecting a flowmeter solution.

Low flow sensitivity is critical to identify and measure leaking valves and the normal low flow associated in day-to-day operations. The capability to measure very high flows is needed during system upset conditions, requiring a meter that needs to accurately measure flow over an extremely wide turndown range.

Flowmeter calibration specifically for hydrocarbon composition gases and matched to actual process conditions is essential. Gas composition changes and wide turndown lead to relatively poor measurement accuracy in flare applications.

The use of large-diameter pipe can be challenging in that as the line sizes increase, the number of effective and suitable flowmeter sensing technologies decrease, and costs increase. In addition, the lack of straight runs in the line also are a challenge. All velocity-based flowmeter technologies have straight-run requirements upstream and downstream from the meter to achieve repeatable flow measurements that are not always available.

Pipe access and re-access for installation, maintenance or servicing is frequently difficult. For example, spool-piece flowmeters can require prolonged process shutdown and extensive onsite labor costs to install and continuously maintain the system as opposed to insertion-style meters that can be easily inserted or retracted into or out of the process through a ball valve.

For installation in hazardous locations, the entire flowmeter instrument should carry agency approval credentials for installation in environments with potential hazardous gases; enclosure-only ratings are inadequate. Corrosive saltwater environments such as those on offshore platforms, floating production vessels and LNG tankers can require the use of stainless steel on all exposed instrument materials, including the enclosures. In nearly every application the flowmeter must meet performance and calibration requirements mandated within government-issued regulations.

Flare gas sensing technologies
The three flowmeter sensing technologies regularly considered in flare gas measurement are ultrasonic, optical and thermal. While all flowmeter technologies have their advantages and disadvantages, some are generally better than others depending on the specific media (liquids [volume] and air/gas [mass]) and the application environment.

Ultrasonic flow sensing technology relies on ultrasound and the Doppler effect to measure volumetric flow rate. In ultrasonic flowmeters a transducer emits a beam of ultrasound to a receiving transducer. The transmitted frequency of the beam is altered linearly by particles or bubbles in the fluid stream. The shift in frequencies between the transmitter and receiver can be used to generate a signal proportional to the flow rate.

Optical flowmetering relies on laser technology and photo detectors. This technology requires the presence of tiny droplets or particles of condensation, lubricants, dust and other particles in the gas stream. These particles scatter the light beam, and the time it takes for these particles to travel from one laser beam to the other laser beam can be used to calculate the gas velocity and volumetric flow rate.

Thermal flow sensing provides direct mass flow measurement and is ideal for measuring gas flow. Two platinum resistance temperature detector (RTD) temperature thermowells are used as the sensor. One RTD is heated, while the other measures the process temperature. The temperature difference between the RTDs results in an analog output and is proportional to the media cooling effect and compensates for changes in pressure and temperature to give a direct mass flow output without additional instrumentation.

Flare gas flowmeters
Whether the flare systems is a single flare line or a large header with a complex array of feeder lines, thermal flowmeters provide the lowest installed cost and the lowest life-cycle cost solution. From detecting the smallest gas flows of 0.25 standard feet per second (sfps) to 1,000 sfps to measure major upset conditions at high flows, thermal flowmeters offer an accurate, dependable, low-maintenance and long-life flaring solution.

For example, the ST100 Series Thermal Flowmeter (Figure 1) from Fluid Components International consists of an easy-to-install insertion flow element with rugged electronics and transmitter. With specific calibrations for mixed gas compositions, if needed the split-range dual calibration feature with three 4-milliamp to 20-milliamp analog outputs or digital bus communications make it ideal for flare applications.

Many oil and gas operators, refineries and chemical plants have flare applications uniquely challenged with two diverse operating flow conditions: very low flow under normal conditions or very high flow during a blowdown condition (Figure 2). Site or plant operators are then further challenged to comply with environmental and emissions regulations that stipulate meter accuracy of ± 5% of reading over the entire measuring range. The meter’s split-range dual calibration feature supports both requirements.

In pipe sizes larger than 16 in., a dual-probe averaging flow sensor system can provide improved accuracy compared to a single-probe system. Dual-probe sensors are connected via a single flow transmitter to provide an averaged output.

Calibration verification
With the newest environmental regulations requiring routine flowmeter calibration verification, a simple-to-use tool to validate the meter’s calibration is an option. VeriCal is a “wet” verification where gas is pressure-controlled through a sonic nozzle on to the sensor. Flow vs. pressure is used to verify the performance of the flowmeter. This can be done without removing the flowmeter from the process.

With the growing importance of reducing carbon emissions and monitoring waste gas in the petrochemical industry, an accurate, dependable and low-maintenance flare gas flowmeter is a must-do site or plant requirement. There are multiple flare gas flowmeter technologies available on the market, but thermal flowmeters provide direct gas mass flow measurement without the addition of pressure or temperature sensors. They feature a no-moving-parts design with low pressure drop, which is virtually maintenance-free for a low installed cost and a long life cycle.

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