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Optical flow metering is an emerging technology for natural gas and flare measurement. It uses optical laser technology to measure the actual speed of particles rather than the speed of sound through a medium (as in ultrasonic flowmeters) and conductivity (as in mass thermal flowmeters).

Commercial instruments are essentially optical transit time velocimeters, which rely on optical scintillation technology. They measure the speed of particles that accompany natural and industrial gases flow through a flare pipe. As they pass through a laser beam, these particles cause laser light to scatter and generate a pulse signal. The speed of the particles is determined by measuring the time between signals from the known distance between two photodetectors.

Optical flowmeter (OFM) companies claim their technology offers unmatched effectiveness for flare measurement.  This is good news for the environment and the oil & gas industry, which sees potential profits go out the pipe every year. The amount of flared gas in 2006 was valued around $40 billion, up 14% from 2004, according to a World Bank-financed study. Flaring dropped significantly in Norway and Canada, which have developed flare metering initiatives.

Optical flowmeters are designed for flare and biogas applications with highly variable flow rates and a wide range of pipe diameters, including smokestacks, ducts, vents, and other confined spaces. Global construction of pipelines is expected to place a high demand worldwide as China begins a rapid program of gas pipeline infrastructure construction.

Photon Control, a leader in the field, is also developing a line of optical flow meters to measure flow rates of gases and clear liquids in process control and custody transfer applications. This market worldwide is estimated at 7 billion dollars per year and growing. Optical Scientific Inc. is also a leader in optical flow.

Advantages of optical flowmeters vs. ultrasonic flowmeters:

• Better turndown ratio, even under less-than-ideal environmental conditions – 1000:1 for OFM vs. 5000:1 for ultrasonic

• Measures any process temperature, while conventional ultrasonic meters cannot measure flare gases above 150^o

• Accuracy and performance unaffected by pipe vibration and acoustic noise

• Direct measurement of the particle speed, rather than the speed of sound through a material, so measurement is less affected by variable flow conditions

• Single installation point for simplicity of installation and maintenance

Advantages of optical flowmeters vs. thermal mass flowmeters

• More accurate data because measurement is independent of the gas composition. (With thermal mass meters measure the flare gas thermal conductivity, accuracy can be poor in flares that contain a mixture of gases, flares with unknown moisture or CO2 content and flares that have been contaminated by other substances

•  Inherently more resistant to contamination and fouling

• Outward error signaling makes sure problems are immediately detected

Flare Metering with Optics: From Blue-Sky Technology to the Real World (review of the development, testing and deployment of the Photon Control optical flowmeter, including the technology) This paper was presented at the 25^th International North Sea Flow Measurement Workshop, October 16-19, 2007, Oslo, Norway.

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