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Saving the company money is a celebratory event, especially when a group of dedicated team players serves management the lubrication savings.
River Bend Station, located near St. Francisville, La., deploys a commercial nuclear electric power generation facility owned and operated by Entergy subsidiaries. River Bend is a boiling water reactor design producing 1,065 megawatts of electricity. Feed water is supplied to the reactor by three motor-driven pumps, each with independent lubrication subsystems.
The motor and pump share a common oil reservoir and supply feed system. Coupled between the motor and pump is a geared speed increaser, which has an independent oil system. Both oil reservoirs utilize a top-of-the-line, turbine-grade, mineral-based oil from two suppliers. The speed increaser is lubricated with a premium mineral-based oil that is formulated with a mild EP additive to protect gear mesh surfaces.
When these units operate at full load, all three pumps are in operation, making online maintenance difficult and costly.
Excessive oil leakage had long been a problem at River Bend from several locations throughout the lubrication subsystem (Figure 2).
The pumps were placed on the plant’s top-ten list of targets for reliability improvement and to reduce cost-consuming problems. A multidiscipline team was established to address all the identified issues concerning feed pump performance, which included correcting oil leakage sources.
The team set a goal of zero oil leakage on these components to resolve this long-standing issue. Part of the plan to eliminate leakage included identifying and repairing all existing leaks and making any modifications needed to improve oil system integrity based on the results of the leak detection efforts.
To assist in tracking oil consumption on these components, an electronic database was developed in 1996. As maintenance and operations personnel make their daily rounds, they log oil additions into this database, which is periodically reviewed by the lubrication engineer to identify any adverse trends.
These pumps are located in low-level radiation fields. Despite the low levels, accumulation of radiation exposure is unavoidable during leak identification and corrective activities while the plant is operational. Minimizing time in the area reduces radiation exposure to plant personnel.
Due to the potential for increased health risk from exposure to low levels of radiation over time, the federal government and nuclear power utilities have established limits that are low enough to minimize this risk. It is the company’s responsibility to keep radiation exposure as low as reasonably achievable.
The source of radiation levels near the feed water pumps comes from the piping and associated equipment. It is impractical to shield the piping or increase personnel distance from the pumps during leak detection efforts. The only viable exposure reduction tool was to reduce exposure time near the pumps.
This constraint hindered conventional leak identification techniques due to the time intervals required to clean oil-coated surfaces and perform extensive visual inspections. It was estimated that it would take two to three craft personnel at least two weeks (approximately 150 to 250 man-hours) to remove existing oil from pump/piping surfaces and perform visual inspections to identify leakage sources with doubtful certainty of identifying the source of all leakage.
The use of a fluorescent oil dye additive was considered to assist maintenance in prompt comprehensive leak identification with the goal of lubrication savings. The critical characteristics for the selection of the dye included base oil and additive compatibility. The dye had to be oil soluble to eliminate additional particulate contamination and/or increased filter loading. Other considerations included compatibility with elastomers used in the systems. The final consideration was ease of use.
Both lubricant manufacturers were asked to approve a list of products that are suitable for use with their oil. Although not specifically endorsed for use, both companies did approve oil soluble dyes not known to present any incompatibility problems. A fluorescent dye product was selected from this list and evaluated by station engineering for use in the feed water pump lubrication systems.
The product selected was a ready-to-use mineral oil-based concentrate that could be added directly to the various component reservoirs. Based on reservoir volumes and recommended concentrations, it was determined that the testing would require approximately 1.5 gallons of dye. The cost of the selected product was $410 per gallon or a total of $615. Total engineering man-hour cost was estimated to be $750 to properly evaluate and document the use of the product.
The leak detection procedure involved adding the oil dye into the oil reservoir and waiting about 15 minutes for mixing and circulation before inspecting the system for gross leakage using a UV lamp (Figure 3).
The equipment was inspected again approximately four hours later and the next day for identification of minute leakage sources. The total inspection time for dye addition and leak detection was less than 40 man-hours. As leaks were identified, they were marked, photographed and documented to ensure the leaks would be repaired during the next available maintenance window.
System operating parameters including bearing temperatures, pump operating pressures, oil temperature, differential pressure across all oil filters and oil levels were measured before and after each dye addition and during follow-up leak detection inspections (Figure 4).
No system parameters changed during the leak detection process. Routine oil analysis on the motor/pump and speed increaser did not indicate any adverse effects from adding the dye. Lubricant oil analysis parameters and wear metal trends remained constant and within specification.
Although engineering personnel are not necessarily proponents of lubricant additives in general, the use of fluorescent dye in this application facilitated prompt, comprehensive lubricant leak detection for these pumps while reducing radiation exposure to personnel.
Man-hour savings were estimated to be approximately 210 hours and radiation exposure for the leak identification efforts is estimated to have reduced by a factor of five compared to conventional leak detection methods (Figure 5).
Evaluation of the identified leakage sources was also instrumental in the identification of potential system modifications to eliminate recurring oil leak sources. Prior to this project, it was believed that most of the leaks were coming from threaded flange connections.
Planning was underway to implement a design change while the dye project was in progress to replace all accessible threaded connections with a welded end flex coupling design. The leakage sources identified very few leaks at threaded connections, which enabled the plant to avoid the unnecessary cost of the planned corrective actions.
The final cost for the project was approximately $3,200. Short-term cost savings over using conventional oil leak detection techniques ($11,250) were approximately $8,050. The long-term savings for lost oil due to leakage will be more than $4,500 each year based on previous leakage. Likewise, the plant now benefits by avoiding leakage-related failures and the unnecessary cost to replace all accessories threaded connections.
This approach performed well for the facility. All of the minor, more accessible leaks have been repaired, with a few remaining leaks to be completed during the next planned plant outage.
Table 2 summarizes oil consumption/leakage on the three feed pumps since 1996.
Another direct expense is the contaminated waste oil processing fee of $1.50 per pound prior to disposal. Based on 861 gallons at $7.20 lb/gal, the station has outsourced processing of a minimum of about 6,200 pounds of oil totaling approximately $9,300 during that period.
Indirect expenses include unplanned man-hour expenditures to make up the oil level in the pump systems. Since 1996, there were 153 documented oil additions to the feed water pumps. At an estimated two man-hours per addition, this equates to approximately 306 unplanned man-hours total, or approximately $13,770 since 1996.
Total estimated expense due to the oil leaks on these three components since 1996 was more than $28,000. This estimate does not include one gear increaser rebuild prior to that period, before oil consumption was tracked, where the failure attributed to loss of oil level. The replacement cost for the gearset was approximately $100,000.
The delayed effects of radiation exposure, such as cancer, are not a certainty but are expressed in terms of increased risk. Risk can also be expressed in terms of life expectancy. To keep radiation exposure in perspective, Table 1 compares common industrial/health risks.
Methods to reduce radiation exposure include the following: