Finding Data Diamonds

Aug. 13, 2009
Detecting Problems Early Mean Having the Time and Ability to Make Informed Decisions
This article was printed in CONTROL's August 2009 edition.
By Jim Montague, Executive Editor

If knowledge is power, then detailed and sophisticated information that arrives early enough to prevent problems and even disasters is super-duper power. For instance, Entergy Corp.'s ( goal for more than 60 years has been to produce electricity safely. Located in Woodlands, Texas, outside Houston, Entergy provides electricity though its Fossil Energy divisional headquarters to 2.8 million customers in Arkansas, Michigan, Louisiana and eight counties in southeast Texas.

To give it advanced pattern-recognition capabilities, Entergy's seven-year-old Performance Monitoring and Diagnostics Center (PMDC) began beta testing SmartSignal's ( EPI*Center software in 2006 and now uses it on 36 units at about 16 plants.

In fact, shortly after installing EPI*Center on its 411-megawatt Waterford Unit 2 gas/oil generator, Entergy's engineers and operators got an alert from the software that vibration levels on the exciter end of the unit's turbine were slightly elevated and starting to steadily increase. Gary Barnes, PMDC's superintendent, says this unit usually runs at 3,600 rpm, and that an ideal vibration level is 1 mil (a unit of vibration equivalent to 0.001 of an inch), though usually operators are satisfied with 2 mils.

"The first notification at 12:30 p.m. was that the unit was running at 3-3.5 mils, which is elevated, but not a lot, because there's not usually a problem until you reach 5-7 mils," explains Barnes. "So, we weren't too concerned and notified the plant and continued to watch the situation. However, the vibration level continued to increase until at 5:30 p.m. it reached 6-7 mils, and the decision was made to shut the unit down. Because of the early notification from EPI*Center, we learned about the elevated vibrations very early, watched them increase and then jointly consulted with the plant and dispatch center about taking the unit offline. We were able to proceed very calmly and shut down safely. If we hadn't had this information from the PMDC, then the plant wouldn't have known there was a problem until the vibration level reached 6-7 mils, and an alarm went off. As a result, we would have had a lot more critical situation, maybe not enough time to shut down safely, and possibly have had a catastrophic failure."

After the safe shutdown, Barnes says Entergy's engineers diagnosed and disassembled the unit and found a 2.5-inch deep crack running 180° around the end of the rotator shaft. Analysis revealed that the new crack was due to stress fatigue from a previous arc gouge that had been repaired several years earlier.

"The consensus of our rotating equipment and generator experts was that if we hadn't taken this unit offline, then the shaft would have cracked through, and we would have had a catastrophic failure. The 3,600 rpm section would have hit the stationary area, which would have completely wrecked the generator, done serious damage to the turbine, and probably caused hydrogen coolant and oil fires. An incident like this would have caused $40 million to $50 million or more in damage, and this unit would have been down for months, if not years. Because of our safe shutdown, the repair cost only $5 million, and the unit was only down six weeks." 

Barnes adds that having OSIsoft's PI and past failure data makes it easier to do failure-mode- and-effect analysis, and build in improved safety features. "For us, the real, day-to-day benefit of detecting problems early is simply that we can have time and ability to make informed decisions," explains Barnes. "So, instead of running a process until we have to shut down a unit right away, now we can know enough about a problem to know how long we can continue to safely run its process. This lets us decide when to shut down and make repairs. In general, unplanned outages are more costly than planned ones, and so it's especially useful that we can now turn unplanned outages into planned ones." Super-duper. No question.