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Infrared Detector Sniffs Out Gas Leaks

Photonics Spectra
Nov 2000
Daniel C. McCarthy

About 7000 miles of distributed gas pipelines wend their way beneath the Binghamton, N.Y., metropolitan area, home to more than 200,000 residents. New York State Electric and Gas Corp., also known as NYSEG, supplies gas to the city. As a safety precaution, the company routinely patrols the streets to seek out leaks in the pipelines. Optical sensing technology from Heath Consultants Inc. is helping to speed these patrols.

Most of NYSEG's patrol trucks carry a flame ionization unit, which uses sampling cones mounted in front of the vehicle's bumper to draw air into the flame ionization detector via a pump. If the air sample carries traces of any combustible hydrocarbons, it causes a chemical reaction in the detector chamber and signals the possibility of a leak. The technique has several disadvantages, including the need to carry several fuel bottles and a sampling pump. If water gets into the sample detector's chamber, performance suffers. Consequently, on rainy days, technicians must continually change the filters. Another disadvantage is that flame ionization can operate only while traveling below 7 mph. More importantly, it detects other combustible hydrocarbons including vehicle exhaust, resulting in frequent false-positive readings.

Truck-mounted optical methane detectors are speeding routine patrols for gas leaks in Binghamton, N.Y. Courtesy of NYSEG.

Sounds the alarm

Heath Consultants' Optical Methane Detector is providing an alternative solution. Earlier this year, NYSEG mounted a single unit beneath the front bumper of one of its trucks. The unit comprises an infrared light source aimed at an optical filter and a photodetector. The filter transmits only the wavelengths at which methane is absorbed, so if traces of gas pass through the beam path, the detector signals an alarm in the truck's cab.

Both methods deliver sensitivity to 1 ppm, according to Mayra Castorina, marketing coordinator for Heath. However, the optical detector is specific to methane and detects propane and ethane at one-third the sensitivity, reducing false positives.

Unlike flame ionization, Heath's unit signals a leak immediately and, because it is digital, data can be stored for later analysis.

Also, the optical sensor allows the NYSEG truck to travel at 25 to 35 mph. "We did over 30 miles in under four hours with the [optical methane detector]," said Mark Cole, NYSEG's manager of gas engineering and project management. "We've never had that kind of success or production with flame ionization units."

Heath's optical detector complements ionization methods, which currently are the only portable option.

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