Hydrogen cooled generators need to undergo carbon dioxide (CO2) purging before being placed into service and when taken offline. This process typically takes 4 to 12 hours, and can take as long as 36 hours in extreme cases, to fully and safely purge a generator. Reducing the volume of hydrogen gas in these generators is essential for reducing the risks of explosions. If these purge times could be shortened, improvements in safety, shorter outages, and increased production could be realized. This paper describes plant testing of a CO2 Fast Degas purging system for hydrogen cooled generators. Results from eight test runs at two different plants are presented in tabular and graphical form. Mean reduction from pure hydrogen to less than 4% hydrogen was 39.8 minutes, while maintaining CO2 temperatures above 80°F (27°C). This eliminates the possibility of CO2 freeze up, and reduces the stress on the piping and the detrimental effects on the generator from extreme temperature swings that occur when CO2 is de-pressurized. These rapid purge rates were accomplished while maintaining the generator pressure within a set range. In order to achieve the minimum purge time, it is critical that mixing of the two gases be minimized during the purge operation. By utilizing the slope of the graphs provided, the system was optimized to minimize purge times to reach safe levels. Tests were performed on both purging operations, replacing hydrogen with CO2 and replacing air with CO2. Samples to analyze the generator gas purity were taken from the vent line using multiple thermal conductivity purity instruments to assure accurate results. The system was tested in both automatic and semi-automatic modes of operation. The fast degas system was found to significantly reduce generator purge times, reducing down time, and improve operator efficiency, positively affecting the overall safety profile of the plant.

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