GE Frame 7FA Generator - Hydrogen Purity Sensor


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Is this a bad idea, or do we need three purity monitors. (casing, CE and TE Enlargement) or do we need to monitor something else. My understanding of Hydrogen Purity is Casing Purity. So how do you corrolate CE and TE readings to Casing Purity?

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Engineer Teresa [email protected],

The lowest concentrations are always in the bearing seal drain enlargement areas since air entrained in the seal oil is the source of air in the generator (including the casing, and a problem there usually predates a problem in the casing. Scavenging is done from the seal drain enlargement tanks (under normal valving/operation), not from the generator casing.

With a system such as you described, it's advisable to periodically switch between TE (Turbine End) and CE (Collector End) with sufficient time to purge the lines to get a proper representative reading from each area to try to detect a possible impending problem. This can be automated with a couple of solenoids and some tubing.

Just about ANY casing purity problem can be traced to seal oil flow-rate changes (presuming proper casing purging and charging, and other than improper purging of hydrogen dryer systems or problems with hydrogen purity from the supply source), and so careful monitoring of seal oil flow-rates is crucial to anticipating as well as diagnosing hydrogen purity problems.

The OEM usually only provides one (1) seal oil flow-rate sensing device for BOTH ends of the generator--which can make diagnosis of purity problems difficult. (If I worked or advised any site with hydrogen-cooled generators and a single oil flow-rate sensor, I would strongly suggest installing seal oil flow-rate sensors at both ends of the generator and make sure they are regularly monitored and the values recorded and trended by operations personnel.)

Having said that, the other critical issue which should always be logged and trended is any change in hydrogen scavenging rates--which should only be done when monitoring the TE/CE areas. Everything must be taken as a whole and monitored and analyzed. Certainly, sensors continuously monitoring both the TC and CE are best, but with a standard operating procedure for periodically checking each end's seal drain enlargement purity AND monitoring seal oil flow-rate for any change AND recording and trending (that's the IMPORTANT thing--trending any change in scavenging rate at both ends of the generator because just recording any change in flow-rate without checking how the flow-rate has to be changed over time is useless, until someone notices after a purge shutdown event there have been changes...) to anticipate any purity issue.

With only a single sensor, it's either going to require a standard operating procedure for manual periodic monitoring of each end of the generator, or automating the periodic checking of each of the generator. Again, the correlation is that air can only get into a generator casing under two bar (30 psi) pressure is from the seal oil. So, the highest concentration of air is at the seal drain enlargement tanks at either end of the generator.

Higher seal oil flow-rates will release more air increasing the amount of air and decreasing the purity in the enlargement tank(s), which, left unchecked by increasing the hydrogen scavenging rate(s) will cause the seal drain enlargement purity to decrease, and eventually decrease the casing purity. But, casing purity--in the majority of circumstances--will lag seal drain enlargement purity, which will coincide with some difference in seal oil flow-rate.

Hope this helps!