Fluorescence Quenching Eclipses Conventional O2 Measurements in Natural Gas Pipelines

Jennifer Soto — Wed, 03 Feb 2010 22:00:00 GMT

For as long as anyone can remember, Oxygen (O₂) has been a problem in natural gas pipelines, especially when high levels of H₂S are present.

Measurement of O₂ has always come with its own set of issues: maintenance costs, inaccuracies, and reliability. But all that is changing with the introduction of a new technology…Fluorescent Quenching.

Based on a proven technology in the bio-tech world, it uses a sensor (optical probe) that contains an indicator dye sensitive to O₂. The dye is optically excited by an LED light source. The fluorescence emission of the indicator dye decays at a known rate, but if oxygen is present an energy transfer occurs and “quenches” the fluorescence, hence the name, Fluorescent Quenching.

Advantages over traditional measurement include greater accuracy, much faster response, simplified calibration, and virtually no maintenance. There is also no need for a scrubber and the net result is lower overall operating costs.

The sensor is not affected by even high levels of H₂S and there is no cross sensitivity to contaminants or other gases found in natural gas.

Larry Ewing, the owner of Ewing Energy Consulting of Tulsa, Oklahoma, is an expert in gas quality measurement technology. He see’s a wide range of potential applications in natural gas pipelines for Fluorescent Quenching. “It is ideal technology for any area that has H₂S concentration such as sour gas monitoring, production areas, and pumping stations.”

SpectraSensors of Houston, Texas, recently introduced the OXY4400 optical oxygen analyzer for natural gas pipelines using this technology. SpectraSensors has pioneered the use of optical analyzers with its Tunable Diode Laser units that are used world-wide in the industrial process and environmental monitoring markets.

The OXY4400 oxygen analyzer is a compact, stand-alone, one-channel meter with LCD display and data logger. The unit uses a light source (LED), an optical sensor probe and a photo detector. Pulsed light from the LED is sent down a fiber optic cable to the sensor probe (optode) where the energy from the light is absorbed by an indicator dye. The light (fluorescence emission) is sent back through the cable to the photo detector, where it is converted to an electrical signal that can be read.

The amount of “quenching” is determined by the amount of O₂ in the stream. The result is an exact and almost instant measurement down to 0.5 ppm.

Accuracy

Conventional membrane sensors (Clark Cells and Galvanic Cells) can be destroyed by high levels of hydrogen sulfide in the stream. For this reason, they require a scrubber to absorb the H₂S. But when the scrubber becomes saturated the sensor loses it accuracy. In addition, traditional sensors use an electrolyte solution that is “self-consuming” caused by an electrochemical reaction to the oxygen concentration. But that consumption degrades the sensor (similar to a battery) over time forcing users to perform constant calibration. Once these trace oxygen sensors are exposed to high levels of O₂ it can shorten the life or destroy the sensor which must be purged and that can take a long time.

All of this adds up to high maintenance costs and replacement parts to keep membrane sensors accurate.

Fluorescent Quenching offers new levels of accuracy. It does not consume O₂ and needs no scrubber, in fact, it reads the O₂ without being affected by the hydrocarbons or sulfur content. Optical response time is measured in seconds, not minutes.

Since there are no moving parts or electrolytes, there is no need for constant calibration once it is installed. The optical sensor is also unaffected by EMI, shock, or vibration.

Cost Comparison

Initial costs of the OXY4400 are higher than the cost of a traditional system. But the total cost of ownership over the first year is much less than a membrane system. The difference comes from the elimination of constant maintenance and replacement parts for Galvanic cell or Clark cell units. In addition, the cost savings of the optical measurement increases over the lifetime of the instrument.

“It’s a ten-fold savings in cost of ownership compared to membrane technology,” adds Ewing, “and the reliability is much higher. Just think of the late night maintenance calls when an O₂ spike sets off an alarm. Membranes take hours to recover, the optical sensor recovers immediately. There is no need for a maintenance operator to stay on task for hours testing for a gas that isn’t there just because his antiquated sensor has not recovered yet to give him an accurate reading.”

Sam Miller of SpectraSensors sees a host of applications for the OXY4400. “This technology transforms oxygen measurement in the natural gas industry due to its long-term stability. This is especially important for remote locations.”

3-Feb-10 4:00 PM

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Fluorescence Quenching Eclipses Conventional O2 Measurements in Natural Gas Pipelines