Fault Catching and Cost Savings
Personnel savings were just the beginning. Within two months, the catching of one major fault and two smaller faults saved Exelon over $500,000. The following descriptions of these faults were provided by Demars.
First Catch. We discovered that a condensate pump motor’s bearing oil temperatures were not within the allowable range as defined by the multidimensional PRiSM model. The cause was found to be an improperly assembled coupling that was seizing and approaching mechanical failure. Had this gone undetected, the coupling would have resulted in damage to both the motor and the pump, requiring a replacement time of four to six weeks.
Replacement cost, expediting fees, and craft overtime were estimated at $700,000. The probability of this failure was estimated as 0.70 or $490,000. Online loss of the pump with a failure of the standby pump to start would have resulted in a power reduction of 34% for 12 hours or about $100,000. The probability of this failure scenario was estimated as 0.10 or $10,000. Additionally, potential fatalities or injuries resulting from the ejection of coupling material were completely avoided.
Second Catch. The second significant catch was a service water temperature controller failure that would have resulted in a $30,000 loss. The main turbine vibration model was alerted by a small step change on the number 11 bearing. The vibration level itself was not significant enough to cause an alarm in any of the normal plant-monitoring systems. The step change was caused by a change in the generator hydrogen temperature, which is controlled by stator water cooling and then by service water. The stator water turbine trip function had not been blocked, as had been done in other plants. The temperature/flow control valve was gagged at most plants to limit travel and not induce huge swings in temperature and potentially cause a turbine trip.
Although staff were planning to do that in the upcoming refuel outage, Peach Bottom Atomic Power Station was still susceptible to that type of turbine trip. In other words, the conditions identified by the software may have prevented a turbine trip, which results in a reactor plant shutdown as well as a couple of days of lost revenue. Trip of the main turbine would have resulted in a loss of generation for 24 hours or $600,000. The probability of a turbine trip was estimated as 0.050 or $30,000.
Third Catch. The third catch was a reactor feed pump (RFP) lube oil cooler temperature controller failure that would have been a $20,000 loss. A nuclear unit was recovering from the effects of a transformer failure – induced voltage transient that caused some system isolations and momentary power losses. Shortly after the transient, the RFP bearing the models for all three pumps went into alert. The plant was notified the following day that one of the controllers did not recover from the initial transient and was continuing to cycle significantly. The station determined that the controller for the RFP oil cooler had failed; staff were able to stabilize temperatures manually until the controller was replaced.
The worst case scenario is bearing damage due to rapid overheating and loss of the RFP. The physical damage was estimated at $100,000 with a probability of 0.10 and lost generation of 33% for 24 hours, or $200,000 with a probability of 0.05.
Avoided Costs Add Up
According to Demars, "The total avoided costs for the two-month period was $540,000. If detected failures of a similar magnitude continue to be revealed by the InStep CPM solution, we expect an annualized avoidance of $3.3 million. Avoidance of a failure of a generation critical component could also easily exceed this amount, but the cost avoidance calculation methods are conservative."
The PRiSM software application has now become a critical part of Exelon’s fleet-monitoring solution. According to Demars, "The use of this intelligent monitoring technology within a centralized group monitoring a fleet of generating stations would apply across the industry."
— Contributed by Steve Lundin (slundin@bigfrontier.org) BIGfrontier Communications Group.
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