Control Engineering Online Update for February 13, 2004

02/19/2004



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TVA opts for vibrating element method over thermal conductivity for hydrogen gas purity measurement at its Kingston fossil plant.

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TVA switches hydrogen purity measurement instrumentation

By Jim Scott

It is an important event when power plant engineers and managers select new instrumentation for hydrogen gas purity measurement for a facility's generators, especially considering that the generators are a critical system from both a safety and economic point of view.

The TVA (Tennessee Valley Authority) Kingston fossil plant decided to make such a change and upgrade the facility's hydrogen gas purity measurement less than a year ago. Kingston replaced nine thermal conductivity analyzers with Yokogawa GD402 hydrogen purity monitor (HPM) systems. The HPM systems are used to measure hydrogen gas purity on five GE 200MW and four Westinghouse 150MW generators.

According to Craig Self, senior instrument mechanical foreman at the TVA plant, Kingston's management team evaluated his recommendations and made a decision to replace the existing hydrogen purity meters due to excessive maintenance and support issues with the thermal conductivity analyzers. Self says, "They [thermal analyzers] experienced unstable readings and inconsistent analyzer support and were the cause of excessive maintenance time and manpower expenditure."



Yokogawa GD402 hydrogen purity monitor.


Kingston ordered and installed the nine HPM systems in a 12-day period. The Kingston installation and commissioning team installed the first three units at a one-unit-per-day rate. After gaining confidence with the installation procedure, the team stepped up the installation rate to two units per day for the next three days, installing and commissioning all nine systems within a six-day period.

"The installations used the existing generator gas sample ports," Self says, "and the similarity to other Yokogawa equipment already in the plant made the operation nearly plug and play, which greatly reduced installation and startup time."

Initially, Kingston was using a single Yokogawa GD402 as a portable unit to check the performance of the thermal conductivity systems and to monitor the generator purge gas sequence. TVA reports that the Yokogawa product has improved repeatability and lack of measurement drift. The thermal conductivity systems were experiencing drift on the order of 1% hydrogen between calibration intervals, requiring biweekly calibrations to stay within the generator operational tolerances for gas purity. The calibration of the thermal conductivity system required a minimum of eight man-hours. The new Yokogawa products require less than three hours to calibrate all nine. For the first five months following the installation, engineers at Kingston conducted drift and calibration checks using the old preventative maintenance schedule. Self says they are now looking to modify the schedule based on the lack of measurement drifts.

Unlike thermal conductivity, the Yokogawa GD402 uses vibrating element measurement method; commonly referred to as the tuning fork principle. The GD402's detector directs a continuous gas sample through a chamber containing a resonating tube. Different sample gas purities allow the resonating tube to vibrate faster or slower. The measurement of gas purity is determined by the frequency of the resonating tube. In comparison to most thermal conductivity systems, vibrating element technology has superior measurement drift performance, better response times, shorter and less expensive calibration procedures, and better resistance to the common problem of oil misting. Hundreds of power utilities across the North America have welcomed this measurement method and its solution to many of the issues created by older, thermal conductivity and fan-differential type measurement methods.

Jim Scott is product manager at Yokogawa . This article is approved for use by TVA.

 





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