Increasing Maintainability and Operability of Emergency Diesel Generators with Thermographic Inspections,

Richard N. Wurzbach Jeffrey E. Hart

Philadelphia Electric Company Peach Bottom Atomic Power Station

Delta, PA 17314

ABSTRACT

In the Nuclear Power Generation Industry, Emergency Diesel Generators are a critical component in ensuring the ability to safely operate the plant. Relegated to a standby role, this equipment must be immediately available to provide power to important plant equipment needed to safely shutdown the reactor, in the event of a loss of normal off-site sources of power. Consequently, maintenance, operation, and surveillance of these generators is performed under close scrutiny, and any deviations from established parameters can potentially lead to the mandatory shutdown of the reactor, and subsequent loss of power generation revenue.

At Peach Bottom Atomic Power Station, Thermographic Inspection has become an integral part of the operation of the Diesel Generators. Generators are operated according to required surveillance tests, and a Thermographic Inspection is made at least twice a year. Some of the applications include checking for exhaust leaks, generator end bearing and cylinder exhaust temperatures, observing the effects of thermal mixing between crosstied cooling systems, and other mechanical and electrical troubleshooting.

14 ! SPIE Vol. 1933 Thermosense XV (1993) @8 194- 1169-d/93/$4.00

Proc. of SPIE Vol. 1933, Thermosense XV: An International Conference on Thermal Sensing and Imaging Diagnostic Applications, ed. L R Allen (Apr1993) Copyright SPIE

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I. Introduction

Emergency Diesel Generators are vital to the safe operation of all nuclear power plants. The Nuclear Regulatory Commisision (NRC) and the Institute of Nuclear Power Operations (INPO) place a great deal of importance on maintaining high reliability and availability of Emergency Diesel Generators. If a station loses its normal offsite power supply, the emergency AC power provided by the Diesel Generators is the only source of power available to emergency cooling pumps necessary to prevent overheating and meltdown of the reactor.

Florida Power and Light Company's Turkey Point Nuclear Power Plant displayed the necessity for emergency AC power when Hurricane Andrew struck in August, 1992,and knocked out primary sources of electricity to the station.' It was then that the Diesel Generators started and continued to provide power to emergency safety systems required to maintain the plant in a safe shutdown condition.

Because of the importance of Emergency Diesel Generators, the NRC and INPO have set standards for reliability and availability. Reliability refers to a generatorts ability to start and maintain operation when attempted from a standby condition. Availability is a measure of the time a generator is not out of service for a maintenance outage, It is the responsibility of the station to maintain reliability and availability as high as possible to minimize operational challenges.

When a Diesel Generator becomes inoperable, it places the plant in a Limiting Condition for Operation (LCO) a When a LCO is in effect, there are additional actions and tests that must be performed to compensate for the loss of the Diesel Generator. The restrictions placed on the plant by a LCO increase the probability that the reactor would be required to be manually shut down to comply with safety procedures.

The NRC requires all plants to have a reliability program. Phildelphia Electric Company (PECo) has chosen to use a reliability program outlined by the Nuclear Management and Resources Council (NUMARC). When system failures occur, numerous labor intensive actions must be performed.

Regulatory requirements, labor intensive testing requirements, and the overall importance of emergency diesel generators in providing safe nuclear power operation dictate that every effort must be made to improve their operability and maintainability. In addition to regulations and prescribed actions, it is important, as well as beneficial, in showing initiative in utilizing innovative methods to address the maintenance and operational difficulties that are

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encountered. This paper will outline the thermographic techniques utilized by PECoss Peach Bottom Atomic Power Station to aid in increasing the reliability and availability of its Emergency Diesel Generators.

II. Exhaust Temperature Indication

Emergency Diesel Generators at Peach Bottom are tested about once'every two weeks. For these tests, the engines are run at full load for at least an hour to reach steady-state conditions. Many parameters are logged as the engine reaches steady-state, and with multiple tests occurringp the parameters are trended over time. Performance monitoring depends heavily on individual cylinder exhaust temperatures. These temperatures can indicate that a cylinder is firing incorrectly, is overpowering or is unloaded. A thermocouple located within the exhaust port of each cylinder provides temperature indication.

Individual cylinder exhaust temperatures typically maintain a temperature differential of less than 250°F. A greater differential temperature may be indicative of a bad cylinder. Faulty timing, clogged fuel injector or excessive 'coking' of the cylinder are possible causes. 'Coking' is a term that describes combustion product accumulation. One cylinder may be running at a low temperature for one of these reasons while the higher temperature cylinder is running hotter to compensate for the first cylinder. Operating in such a manner is very undesirable. The reliability of the engine is degraded. While Peach Bottom has experienced high cylinder exhaust differential temperatures on many occasions, it has rarely been for the reasons described above. In most cases, the source of the high differential temperature is faulty indication from the thermocouple.

Since the thermocouples are located directly in the cylinder exhaust, they are susceptible to coking. Engine vibration may also cause a thermocouple malfunction. The problem with determining if a thermocouple has gone bad is that it is inaccessible with the engine intact. The junction box in which all the thermocouples are terminated cannot be accessed without disabling the engine because of safety considerations. Proper troubleshooting requires that the diesel generator be taken out of service, rendering it unavailable.

The station realized the need for another method for troubleshooting cylinder exhaust temperatues short of an expensive modification. While the thermographs are unable to provide exact exhaust temperatures, they can show a proportional representation of the cylinder internal temperature at a dummy port located on the cylinder. On a

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typical engine run, the temperture differential for individual cylinder exhaust thermocouple readings was 180°F. The thermographic readings of the cylinders on the same run provided a temperature differential of 163°F. Both differential temperatures are well below the 250°F alert value mentioned previously. The thermographic readings provide a comparable set of temperatures that are used in the same manner as the individual cylinder exhaust temperatures. The correlation that exists between thermocouple readings and thermography data allows identification of failed thermocouples.

III. Jacket Cooling and Air Cooling Systems

Arkansas Nuclear One (ANO) reported a problem with thermal mixing of their.Jacket Coolant and Air Coolant in a cross-tie line.2 Temperatures on the Air Coolant system were higher than the manufacturer specifications for that service, falling in the same range as the normally hotter Jacket Coolant system. Configuration was such that the Air Coolant was being heated up by the Jacket Coolant and engine performance was affected. AN0 used thermography to identify the point of mixing, Minor piping modifications or valve manipulations were deemed necessary to compensate for a poor design.

Figure 1

Expansion Tank, Jacket Cooling, and Air Cooling Piping

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Peach Bottom and the rest of the industry were informed of the possible problem through regulatory information notices. Although engine running temperatures for Jacket Coolant and Air Coolant were within manufacturer recommendations, at Peach Bottom a thermographic inspection was performed to determine the thermal profile of the two systems, particularly any gradients existing at the crosstie. An expansion tank common to both coolant systems was identified as a heat sink to dissipate any possible cross- heating effects, thus eliminating the thermal mixing concern.

IV. Turbocharger Performance

Most emergency diesel generators have twin turbochargers to compress combustion air and increase cycle efficiency. Any imbalance in the shared load between them can result in engine

Figure 2

Diesel Turbocharger

(See color plate, p. 124)

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performance problems. A sluggish turbine due to "coking' worn bearings, or other mechanical resistance will produce backpressure, reducing exhaust gas flow, and thus, heat flow through the turbine. Flow reduction will produce a temperature differential between the two turbochargers, since they are not mechanically linked. Figure accessible portion of a turbocharger which comparison to its twin.

3 shows the is used for

A performance problem in spring 1992 at Peach Bottom pointed to the possibility of a clogged or mechanically impeded turbocharger. Typically, this would be confirmed by disassembly and inspection of the turbochargers., An interruption which takes an emergency diesel generator out of service requiring extensive testing of other plant equipment, as described in the introduction concerning emergency diesel generator availability. Thermography showed in this case that the turbochargers were operating at identical temperatures, and, therefore, disassembly was not required. A minor adjustment was made to a backpressure check valve which addressed the concern.

V. Bearings

The high temperatures of the engine block prevent thermography from being performed on the engine journals. Bearings can be scanned on associated equipment. Lube oil pumps, fuel oil pumps, air compressors, and other auxiliary pumps, fans and motors are candidates for scanning.

Figure 3

Generator End Bearing

(See color plate, p. 124)

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The generator end bearing may be inspected to verify thermocouple readings, and can reveal the level of the oil in the housing. With the proper quantity of oil present, the level actually slopes due to the rotation of the roller bearing.

The low pressure blower bearing is not monitored by a thermocouple, so thermography provides the only indication of the bearing's condition. In Figure 4, the location of the bearing is indicated by the boxed number 1. Baseline data should be obtained for this point and comparisons made during initial start when the blower is most heavily loaded.

Figure 4

Low Pressure Blower

(See color plate, p. 124)

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VI. Line Blockages

Detecting line blockages, valve mispositioning, and line obstructions can be importantwhentroubleshootingperformance problems. Numerous support systems supply or circulate air, fuel, or water to the engine. Any interruption or restriction can affect, limit or interrupt operation of the Diesel Engine. Figure 4 illustrates the air flow pattern through the blower as it travels from the entrance end near the fan bearing (indicated by boxed number 1) and curves down toward its entrance to the air coolers at the right side of the image.

Fuel supply, cooling water, and lube oil supply lines can all be scanned to provide confirmation of proper flow when performance problems raise questions about these flowpaths.

VII. Exhaust Leaks

Typically, the high transmissibility, and thus low emissivity, of gases make inspections extremely difficult without special considerations, such as using special filters or lasers tuned to the gas wavelength to cause excitation. However, the very high differential temperature of diesel engine exhaust gases above ambient allows leak detection without camera modifications or aids.

Thermography was used as a tool in this application to diagnose the source of excessive exhaust leaks which could be smelled during operation. Likely areas were scanned both with normal camera configuration and with a CO, filter installed. This filter excludes IR energy outside of a band that approximates peak absorbance for CO, gas. The transmitted wavelength band is quite narrow in relation to the full shortwave spectral band detected by the system used for this inspection. Because most of the infrared energy is attenuated by this filter, only high temperature sources in the CO, wavelength ban provide adecyate resolution. While the exhaust gases were visible through the CO, filter in this application,it did not provide a resolution significantly better than the unfiltered image, and therefore, the inspection was made without the filter to provide better background contrast.

The areas scanned to pinpoint the leak were flanged or gasketed piping connections from the manifold/header flange to the exhaust pipe roof penetration. The infrared image of the gas was observed at a flanged connection at the side of the engine.

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Figure 5 Control Photo of Exhaust Header

Figure 6 shows a still thermal image of the connection. While readily visible when viewed in real-time due to the fluctuations of the exhaust gas plume, it is difficult to observe in the still thermal image.

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Figure 6

Thermal Image of Exhaust Header

(See color plate, p. 125)

Figure 7

Subtractive Image of Exhaust Header

(See color plate, p. 125)

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Figure 7 is a differential image produced by subtracting consecutive images from the videotaped motion. The resulting enhancement of the plume is indicated by the box, showing its location at the flanged connection.

Investigation revealed that the flange bolts adjacent to the engine were difficult to access and provided a challenge to obtain the required torque. It was brought to the attention of the maintenance foreman responsible for this work. By modifying work practices, the flange bolts were properly tightened in a subsequent repair outage. Post- maintenance infrared inspections revealed that the improved work practices were successful.

While the improvement in maintenance practices had given immediate results, the station found that, over the long haul, with repeated starting and stopping of the engine, the gaskets wear thin and exhaust leaks reappear. The gaskets undergo cyclic heating which greatly decreases their compressibility. As a result, the industry is awaiting the development of a better gasket material to be manufactured. In the meantime, thermography is being used to detect this degredation and the resulting exhaust gas leaks.

VIII. MCCs and Electrical Cabinets

Motor control centers (MCCs) and other electrical equipment are routine thermographic fare, and they apply to diesel inspections as well. MCCs of associated equipment includes standby oil pumps, starting air compressors, cooling pumps, auxiliary oil pumps and ventilation fans. In order to cover all this equipment, inspections are required with the diesel in standby, as well as while the diesel is running. Starting air compressors must be checked immediately after engine start or while initiating a compressor start by slowly bleeding air from receiving tanks. Care must be taken not to overbleed the tank and exceed operability limits.

output control cabinets are scanned for typical electrical component anomalies. Hot fuse clips due to loose mounting screws are an example of findings in these cabinets. Initial inspections accompanied by technical personnel cognizant of the function and anticipated loading is necessary for successful inspections. Fuses and terminations are the focus of the inspection.

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Figure 8

Lube Oil Heater Fuses - Hot Fuse Clips

IX. Conclusion

Because of their consequences of failure, is among the most tested

operational significance and the the Emergency Diesel Generator System and scrutinized systems in a nuclear _

power plant. It is incumbent upon the plant to maximize the availability of those units to ensure safe operation. At PECo's Peach Bottom Atomic Power Station, thermographic monitoring has filled that role, and continues to improve the performance and reliability of this essential equipment.

X. Acknowledgements

The authors wish for document review preparation.

XI. References

to thank Frank Ruddy and Trish DuBois and Tracy Barlok for manuscript

1. Plunkett, T., "Stormy Challenge, Quiet Resolve", Journal of the National Academy for Nuclear Traininq, Vol. 7, No. 4, Fall, 1992.

2. Moriarty, J. M. to U.S. Nuclear Regulatory Commission, "Part 21 Notification for Fairbanks Morse Engine Division Model 38TD8 - l/8 Cooling System Investigation - Arkansas Nuclear One Unit #2”, Colt Industries Memo, September 16, 1991.

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