ML20212B032

Emergency Diesel Generator Component Failures Due to Vibration, Engineering Evaluation Rept

ML20212B032
Person / Time
Site:	Browns Ferry, Grand Gulf, Cooper, Prairie Island, Turkey Point, Duane Arnold, Farley, San Onofre, Zion, 05000000
Issue date:	12/17/1986
From:	NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD)
To:
Shared Package
ML20212B028	List: ML20212B023 ML20212B032
References
TASK-AE, TASK-E612 AEOD-E612, NUDOCS 8612290135
Download: ML20212B032 (9)

Text

- _ . . __

AE00 ENGINEERING EVALUATION REPORT

• UNIT: Zion 2 EE REPORT N0: AE0D/E612 DOCKET N0.: 50-304 DATE: December 17, 1986 LICENSEE: Commonwealth Edison EVALUATOR / CONTACT: C. Hsu NSSS/AE: Westinghouse /Sargent & Lundy

SUBJECT:

EMERGENCY DIESEL GENERATOR COMP 0NENT FAILURES DUE TO VIBRATION EVENT DATE: February 15, 1985 (LER 85-002-00)

SUMMARY

Licensee Event Report (LER)85-002, dated February 15, 1985, for Zion 2 describes an event in which a lube oil leak occurred during a loading test of the "0" emergency diesel generator. The diesel generator was then shutdown and declared inoperable. This occurred when the unit was at 99% power with the "2A" emergency diesel generator out of service. An unusual event was declared for the unit since two of the unit's three emergency diesel generators were inoperable. The lube oil leak was traced to a cracked copper line which supplies oil from the turbocharger lube oil header to the starting air-distributor of the diesel engine. The crack in the copper line was determined to be due to fatigue failure that resulted from engine-induced vibration. This was a recurrent event. Zion station has experienced problems with copper tubing on diesel generators in the past. The majority of it has been replaced with steel tubing.

A search of the Sequence Coding and Search System LER data base found seven additional events involving similar small bore piping failure that resulted in a loss of required onsite emergency ac power system redundancy. In one of the events, a fire occurred as lube oil spraying from a cracked lube line was ignited by the hot exhaust piping of the diesel engine. These additional events occurred at Turkey Point 3, Grand Gulf 1, Duane Arnold, Farley 2, San Onofre 1, Cooper and Browns Ferry 1. The failed lines were used in the lube oil, fuel oil and coolant water systems of emergency diesel generators. The

, piping failures were caused by vibration fatigue cracks that resulted from

engine-induced vibration. In four of the events identified, the cracked lines were made of either copper or brass material, which are considered to be vulnerable to vibrational loads. In each case the licensee replaced the piping with steel components.

The study found that the forced vibration induced from engine operation produces complex and interacting vibration modes that can affect all components connected to the engine. Lube, fuel and water lines are often subject to relatively high amplitude pulsating loads that may cause rapid degradation.

The pulsating loads are very difficult to define at the piping system design stage. Additionally, there are no specific requirements for an evaluation of such conditions in the applicable codes or the regulatory review guides.

Generally, the loading conditions for this type of vibration have not been

analyzed in the original piping design. Although design contingency for vibration loads has been considered in some design practice, these identified 8612E90135 861217 PDR ADOCK 05000282-S PDR

• This document supports ongoing AE00 and NRC activities and does not represent the position or requirements of the resconsible NRC program office.

events indicate that the design contingencies included for vibratory conditions in the original design were inadequate. The piping fatigue failures were detected neither in preoperational vibration tests nor in inservice inspections.

It was detected only after a leak or a fire resulting from the leak had already developed. Such leaks could result in sudden disabling of the emergency diesel generators when needed and could adversely affect a safe plant shutdown in the event of a loss of offsite power. In view of this safety concern, the following action is suggested:

IE should consider issuing an information notice to inform licensees of the potential safety problem concerning fatigue cracks in the fuel oil, lube oil and coolant lines of the emergency diesel generator. The information notice should make licensees aware that copper material used in these lines are particularly vulnerable to failure due to cyclic loading during generator operation.

INTRODUCTION A total of eight events involving piping failure of the emergency diesel generator support systems that resulted in loss of an emergency generator were found in a search of the Sequence Coding and Search System (SCSS) LER data base file. The search was confined to events that occurred for the period covering from 1981 to early 1986. The piping failures are due to fatigue cracking. The failed piping identified in the events are small bore and are used in the lube oil, fuel oil and cooling water systems of the emergency diesel generators.

Anong these events, four of the piping failures involved a copper line cracking. In each case, the cracked component was replaced with a steel component. These events occurred at Zion 2, Turkey Point 3, Grand Gulf 1,

-Duane Arnold, Farley 2, San Onofre 1, Cooper and Browns Ferry 1. The EDGs involved in these plants are from different manufacturers. The EDGs at Zion 2 and Cooper, Turkey Point 3 and Browns Ferry 1, Duane Arnold and Farley 2, and Grand Gulf 1 and San Onofre 1 were manufactured by Cooper-Bessemer, General Motors, Fairbanks-Morse and Transamerica Delaval, respectively. These operating experiences, their underlying causes, the corrective actions taken and the need for follow-up actions are discussed in the following sections.

DISCUSSION

Operating Experience Zion

! LER 85-002 for Zion 2 describes an event involving a lube oil leak that resulted in declaring an emergency diesel generator (EDG) inoperable. The leak was found during a loading test of an EDG and was traced to a cracked copper i line which supplies oil from the turbocharger lube oil header to the starting air distributor of the "0" diesel generator engine. The crack in the copper line was believed to have been caused by fatigue failure that resulted from engine-induced vibration. This occurred when the EDG unit was operating at rated power with the "2A" emergency diesel generator out of service. An unusual event was declared as two of the three diesel generators were, therefore, inoperable. This was a recurrent event. Zion station has experienced fatigue failure problems with copper tubing on the EDGs in the past. The majority of such tubes had been replaced with steel tubing.

l

Turkey Point On December 22, 1984, the "B" EDG at Turkey Point 3 was declared out of service to replace an approximately 12" long section of vent line from the engine ccolant discharge to the surge tank. (The "A" EDG had been taken out of service earlier.) The vent line was discovered to be leaking during a daily operability test of the EDG. The event was reported in LER 84-038. The line was leaking because. vibration of the engine coolant discharge line during generator operation caused the crown holding the vent line in the coolant line to break. The approximately 12" section of leaking vent line and crown were replaced. The solid copper vent line was replaced with flexible piping to eliminate the vibration effect on the crown.

Grand Gulf The event at Grand Gulf 1 was reported in LER 83-167 dated February 7, 1984.

On October 22, 1983, during a 7-day surveillance run, the Division 2 diesel generator developed a leak in the fuel oil filter differential pressure instru-ment line after only about 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br /> of operation. The diesel generator was declared inoperable. The failure was due to vibration which caused the line to rub against an air box on the diesel generator eventually causing a rupture.

The tubing was replaced and rerouted to prevent rubbing. Subsequent inspection of all fuel oil lines on both the Division 1 and the Division 2 diesel generators found some additional damage on the lines of both generators. It was planned that the lines would be replaced and rerouted as necessary prior to the next startup.

Duane Arnold The event reported in LER 82-40 at Duane Arnold occurred on June 10, 1982.

During cold shutdown surveillance testing, the jacket coolant system on EDG IG-21 was found leaking. The leakage occurred at the brass nipple connecting a pressure indicator to the coolant header. The nipple was cracked as a result of fatigue due to normal engine vibration. The generator was then declared inoperable. The licensee's corrective action was to replace the cracked nipple with a steel component and to shorten the associated piping in order to reduce the effect of vibration. The corresponding nipple and piping on the other EDG was also to be replaced with a steel component, and the piping was to be made as short as po:sible during the subsequent annual inspection.

Farley Farley 2 LER 81-013 reported that on May 15, 1981, during plant startup testing, the diesel generatcr 2B was declared inoperable due to a fuel leak. The fuel leak was due to fatigue failure of the compression fitting on a copper line. The fatigue failure was caused by engine-induced vibration. The copper line was replaced with stainless steel and rerouted to reduce vibration.

San Onofre San Onofre 1 LER 81-017, dated July 14, 1981, reported ar. event involving diesel generator lube oil spraying from a cracked instrument line. The oil spray was ignited by the hot exhaust piping above the diesel engine. The fire was extinguished in 8 minutes by the station fire brigade. The event occurred i

4 while conducting a diesel generator test during normal plant operation. The cracked instrument line was a 1/2" lube oil gauge line fabricated with brass.

Metallurgical investigation of the failed component revealed indications of fatigue cracking. Markings on the fracture indicated that the crack propagated rapidly. It was concluded that the direct cause of the line failure was fatigue cracking of the brass fitting caused by vibration during engine

, operations. All equipment within the fire damage boundary was inspected and repaired or replaced as necessary. Operational tests were conducted on affected systems to verify proper diesel generator performance.

Cooper Cooper LER 81-021, dated July 28, 1981, stated that a fuel injection line failed during a surveillance operability test of EDG 2 (EDG 1 was inoperable).

The EDG undergoing the test was shut down and declared inoperable. The cause of failure of the fuel injection supply line was believed to be metal fatigue and vibration. The line, a 1/2" heavy wall stainless steel tubing, severed completely near an injector compression fitting. The injection line was immediately replaced. Although this was the first failure of this type, the licensee ordered some more components for replacement when needed.

Browns Ferry Browns Ferry 1 LER 81-089/03 describes a diesel generator failure event that occurred in 1981. During normal plant operation, while performing a surveillance test, diesel generator "D" tripped on overspeed after receiving a fast start signal. The cause of trip was determined to be a result of a cracked nipple in the control fluid line from the booster pump to the governor.

The cracked nipple led to an oil leak. The nipple cracking was due to cyclic fatigue that resulted from vibration. The cracked nipple was replaced and administrative controls were added to inspect the governor oil level daily and to initiate corrective action immediately when needed.

Analysis and Evaluation Cause Analysis I

The eight events discussed in the previous section illustrate a common cause failure of small bore piping that can be a potentially serious safety problem.

The failed piping identified in these events are used in the lube oil, fuel oil and cooling water systems of the emergency diesel generators. The failures were caused by fatigue cracking that resulted from engine-induced vibration and were not identified until a leak had occurred. The pipe cracks have caused fluid leaks that ra ulted in inoperability of the associated EDGs. In one case a fire was caused by lube oil leaking from a cracked pipe.

The forced vibration induced from engine operation produces complex and inter-l acting vibration modes that can affect all components which are attached or connected to the engine, generator or surrounding structures. Lube, fuel and water lines of the engine and generator are often subject to pulsating loads

that may cause rapid degradation during the engine operation. Pulsating loads

of this nature are very difficult to define at the piping system design stage.

Because of the complexities involved and the high uncertainty and potential variability of loading, the vibratory loads from steady state operation have not normally been analyzed as part of the design stress analysis prepared to l

1

qualify the design adequacy of the piping. Additionally,Section III of the ASME design code requires that an evaluation for fatigue be made for Class I components, but not for Class 2 and 3 components. It appears that none of the design requirements provided in the applicable design code addresses the need to consider vibratory loads generated from steady state vibration.

Vibratory loads have been considered important for evaluating the integrity and functionality of piping systems. Since 1981, NRC has had detailed requirements for vibration testing and analysis of nuclear power plant safety-related piping under the dynamic testing requirement which are evaluated by the staff under Standard Review Plan (SRP) Section 3.9.2 (Ref. 1). The vibratory loadings during startup testing (defined in this SRP for licensing review) include only the loads due to fluid flow transient phenomena and postulated seismic events.

The loads due to engine or plant equipment-induced vibration are not included.

It appears that due to lack of specific requirements in the applicable design codes and the regulatory review guidance, the impact of fatigue loading on small bore piping due to steady state vibration has been inadequately analyzed or overlooked by the design engineer. A more complete design review is typically delayed until related problems are identified during plant operation. The solution to these problems often requires a change of tubing material, and the addition of supports and/or piping rerouting. However, many times the identification of this type of problem is not possible until a leak occurs.

It appears there may be additional reasons why the vibration problem has not been treated as a source of concern for piping systems. Those reasons are (1) conservative allowable stresses are used for sustained loads; (2) effect of vibration on aging is considered in the qualification of the component; and (3) seismic qualification of the equipment is sufficient to cover the effects of other undefined vibratory loads that may occur during the life of the plant.

Vibrating load of steady state vibration is a separate loading component on piping system and the loading conditions vary from case to case. The three above-mentioned design considerations may not be sufficient to assure adequacy of the design qualification for the piping system subject to steady state vibration. The events of vibratory fatigue failure of small bore piping identified in this study indicate that the contingency provided is not i

sufficient and could not be provided to substitute for the testing or analytical assurance of structural integrity for piping system which is required under Appendix A, " General Design Criteria for Nuclear Power Plants,"

to 10 CFR Part 50.

In the past, it has been generally accepted that steady state vibration will not pose a serious concern. This has been the case even though this type of vibration is generally not considered analytically, since piping systems are subject to preoperational and startup tests. In these tests structural integrity and functionality of piping system under the operational vibration are verified. Preoperational and startup tests are initial test programs under the provision of Section XI, " Test Control," of 10 CFR 50 Appendix B, " Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing Plants."

Section XI requires that a test program be established to ensure that structures, systems and components will perform satisfactorily in service.

1

In conjunction with this requirement NRC Regulatory Guide 1.68 (Ref. 2) provides specific guidance for implementing the above requirements related to initial test for light water nuclear power plants. This guide describes the general scope and depth of initial test programs acceptable to the NRC and provides a representative listing of the plant structures, systems, components, design features and performance capbility tests that should be demonstrated during the initial test program. However, the guide does not provide a specific requirement with regard to acceptable methods of measuring the damage or failure potential of piping subject to high frequency vibration, such as steady state plant or engine vibration. Moreover, it is stated that not all the items be tested to the same stringent requirement even though it is required that all structures, systems, and components important to safety be tested. For this reasen, it appears likely that small bore pipings are items that are neither included nor tested to the extent necessary during the testing program.

Since 1982, the ASME has had ANSI /ASME OM3 standard, " Requirements for Preoperational and Initial Start-up Vibration Testing of Nuclear Power Plant Piping Systems," (Ref. 3) available to be used in preoperational and initial start-up vibration testing qualification of nuclear power plant piping systems.

The vibration testing standard includes steady state and transient vibration testing and corresponding acceptance criteria, instrumentation and measurement techniques, and recommendations for corrective action when required. This standard may also serve as a guide for the assessment of vibration levels of applicable piping systems during plant operation. Although we do not have specific evidence that the standard would have solved or prevented the piping fatigue failure due to +

steady state vibration, it seems plausible that some of the problems which have developed in the service would have been detected if the standard had been used in the vibration testing qualification for piping systems. It is not known whether the standard has been widely implemented by the industry and to what extent with regard to piping system qualification for operational high frequency vibration, since the standard has not been endorsed by the NRC regulatory requirements.

Fatigue failure of small bore piping due to steady state vibration has been a long standing problem. The problem can be attributed to an industry-wide lack of attention to small bore piping, especially regarding its capability to withstand the effects of machine or plant equipment induced vibration, during the design of piping system. The lack of industry attention was due to inadequate design code requirements, lack of specific regulatory guidance and difficulty in defining the loading condition for steady state vibration such as machine or plant equipment induced vibration.

Safety Evaluation Operability of EDG units is important for safe plant shutdown following a loss of offsite power. Considerable effort has been spent in monitoring, repairing, and testing these units. However, little attention has been focused on small bore piping of the associated supporting systems with regard to fatigue failure due to engine-induced vibration. In each of these identified events, the piping failure was caused by fatigue cracking that resulted from

engine-induced vibration and was not detected by the inservice inspection for piping nor by the vibration program during preoperational testing. The failures were only discovered when the cracks eventually propagated through-wall and began leaking. Consequently, the associated EDG became inoperable and the plant lost some of its required onsite emergency ac power redundancy. The inoperable EDGs remained out of service for periods up to several days. There is the potential that such piping failure could result in a total loss of emergency onsite ac power if the other redundant EDG becomes inoperable prior to the recovery of the failed unit. Inoperability of these back-up power sources significantly increase the risk of plant damage should a loss of offsite ac power occur. In addition, the oil leak from the piping failure could be ignited resulting in a fire that can be a major distraction to the operators trying to recover from a transient (e.g., a LOOP). This suggests the need for including the inspection of small bore piping susceptible to vibration in the inspection and surveillance procedures of EDGs.

FINDINGS AND CONCLUSIONS Based on the preceding discussion and related follow-up activities conducted for this evaluation, the following findings and conclusions are provided.

1. Failures of small bore piping associated with EDG engines and structures have occurred that are attributed to cyclic fatigue cracking due to engine-induced vibration.

Several events have occurred involving small bore piping failure in cracking that resulted in loss of required onsite emergency ac power redundancy at eight operating plants. The pipe cracking was attributed to cyclic fatigue due to engine-induced vibration during operation of the emergency diesel generator. The failed small bore pipings were used in the lube oil, fuel oil and cooling water system of the emergency diesel generators.

2. The pipe cracks were not detected until a leak had developed.

In each of the identified events, the piping failure was caused by fatigue cracking that resulted from engine-induced vibration and was not detected by the inservice inspection for piping nor by the vibration program during preoperational testing. The failure was only discovered when the crack eventually propagated through-wall and began leaking.

l 3. Pipings made of copper or brass have increased susceptibility to this cause of failure.

The cracked pipes in four of the events identified were made of copper. The licensees determined that the material is very vulnerable to vibratory loading condition and have replaced the affected copper fittings with stainless steel components. The Zion station has experienced cyclic fatigue failure of copper tubing on the emergency generators and the majority of it has been replaced with steel tubing.

4. Problems involving pipe fatigue cracking have occurred with EDGs manufactured by at least four different manufacturers and at different plants.

Similar pipe damage has occurred to piping systems of EDGs at eight plants:

Zion 2, Turkey Point 3, Grand Gulf 1, Duane Arnold, Farley 2, San Onofre 1, Cooper and Browns Ferry 1. The EDGs involved were manufactured by four different manufacturers: Cooper-Bessemer, General Motors, Fairbanks-Morse and Transamerica Delaval.

5. Vibratory loads from steady state operation are very difficult to define at the time of. piping system design. Additionally, there are no specific recuirements for consideration of such conditions in the applicable design coces or the regulatory review guidance. Therefore, the load conditions have not normally been analyzed in the original piping design.

A review of the design standards for nuclear power piping ASME Section III as well as the other applicable design standards indicated that the various design requirements in these standards would not have prevented nor reduced many such failures because there are no specific requirements for an evaluation of vibratory loads due to steady state vibration, such as plant or engine-induced vibration. Additionally, past and current NRC regulatory guidance does not provide definite requirements for qualification of piping system subject to plant or engine-induced vibration. Since vibratory loads of this nature are very difficult to define during the design stage, the loading conditions of this type may not have been accounted for in the original system design analysis, and, consecuently, vibration fixes are typically delayed until the problems are identified during plant operation.

Many times this identification is not possible until a leak already occurs.

This is a situation in which the associated emergency diesel generator will become inoperable. Replacement or repair of a damaged piping system .

normally takes time. Accordingly, there is the potential that a total loss of emergency onsite ac power could occur if the other redundant EDG becomes inoperable prior to the recovery of the failed system,

6. To minimize fatigue and other vibration-related failures, pipes and components subject to engine-induced vibration can be examined under operating conditions, and mounting methods can be modified, as necessary, by either line rerouting and/or the addition of supports.

High frequency vibration leads to rapid failure of piping system and indication of the failure may not be detected at initiation of fatigue cracking without performing a close inspection. Therefore, the inspection of the pipe lines should be included in the procedure of EDG surveillance testing.

SUGGESTION IE should issue an IE information notice which addresses the identified events l and inform licensees of the potential problem concerning fatigue cracking in the emergency diesel generator fuel oil, lube oil and cooling water pipings.

This information notice should also indicate that copper pipes used in these systems are particularly vulnerable to vibration fatigue resulting from engine-induced vibration, and several plants have replaced the affected copper fittings with steel components. Further, the information notice should note that the inspection of the pipe lines should be included in the procedure of EDG surveillance testing so as to minimize fatigue and other vibration-related piping failures.

REFERENCES

1. USNRC, " Standard Review Plan - LWR Edition," USNRC Report NUREG-0800, Section 3.9.2, " Dynamic Testing and Analysis of Systems, Components, and Equipment," Rev. 2, July 1981.

2. USNRC, Regulatory Guide 1.68, " Initial Test Programs for Water-Cooled Nuclear Power Plants," Rev. 2, August 1978.

3. ANSI /ASME-0M3-1982, " Requirements for Preoperational and Initial Start-up Vibration Testing of Nuclear Power Plant Piping Systems," ASME, September 1982.