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Laser-Based Detector Puts the Freeze on Water

Photonics Spectra
Sep 2000
Daniel C. McCarthy, Senior Editor/Special Projects

There is a spot in New Mexico where six natural gas pipelines converge, carrying the energy source from producers in New Mexico and Colorado to customers in California. At that junction, 2.8 billion cubic feet of gas pass each day, monitored closely by El Paso Natural Gas, the company that runs the pipeline. Also, at that point, the gas is user-ready -- meaning water, among other things, should be absent. Moisture in the gas will do worse than make flames flicker on customers' stoves. If it goes unchecked, ice can form in the pipe and inhibit regulation of gas flow.

El Paso traditionally had relied only on capacitance sensors to monitor moisture. These sensors incorporate a bead coated with aluminum or platinum. A higher electrical capacitance across the bead signals higher water content. These sensors, however, are susceptible to contamination by glycols or amines in the gas, which can cause exaggerated readings or a failure to detect. An erroneously high reading could cause El Paso to temporarily shut down the pipeline.

A shutdown can cost as much as $5000 to $10,000 an hour and, if prolonged, could halt gas suppliers' production operations. "That makes them mad," said Russ Pyeatt, operations manager for El Paso Natural Gas. On the other end, El Paso's customers can start losing gas pressure. "It's like a domino effect through the whole line," he added.

The company's search for faster and more refined detection led it to use a compact laser-absorption spectrometer from SpectraSensors. The breadbox-size instrument incorporates a distributed feedback diode laser tuned to 1.37 µm to better detect water amid strong concentrations of methane -- the main component of natural gas. Software compares the laser's emitted power against the light returned from a mirror and calculates water content by the amount of absorbed optical energy.

SpectraSensors' technology, originally developed for NASA to measure atmospheric moisture, can detect moisture in the parts-per-billion range, far in excess of what El Paso needed. "When we approached them and told them what we wanted, they said, 'I guess we can detect levels that high,' " Pyeatt recounted.

El Paso purchased the first sensor last year and has since ordered four more. The two companies have also started applying the sensors to detect CO2 in the pipeline.


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