ML19332F796

Final Deficiency Rept CP-89-024 Re Broken Emergency Diesel Generator Turbocharger Holddown Bolt.Initially Reported on 891002.Unit 1 Bolts Replaced W/Higher Strength,Longer Bolts. Bolts Will Be Replaced Prior to Unit 2 Fuel Load

ML19332F796
Person / Time
Site:	Comanche Peak
Issue date:	12/15/1989
From:	William Cahill TEXAS UTILITIES ELECTRIC CO. (TU ELECTRIC)
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
CP-89-024, CP-89-24, TXX-89826, NUDOCS 8912190047
Download: ML19332F796 (3)

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Log # TXX-89826

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File # 10110  :

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- - 909.5 C C Ref. # 10CFR50.55(e)

RIELECTRIC December 15, 1989 f i9 Y kt l U. S. Nuclear Regulatory Commission Attn: Document Control Desk .

Washington, D. C. ~20555 ,

SUBJECT:

COMANCHE PEAK STEAM ELECTRIC STATION (CPSES)

DOCKET NOS. 50-445 AND 50-446

. . _ . EMERGENCY DIESEL GENERATOR (EDG) TURBOCHARGER 80LT V' SDAR: CP-89-024 (FINAL REPORT)

Gentlemen: -

On October 2,1989, TU Electric notified the NRC by report logged TXX-89728 of a deficiency involving a broken bolt on the Emergency Diesel Generator (EDG) turbocharger. After further evaluation, this condition has been determined.to be reportable under the provisions of 10CFR50.55(e). The required information follows.

Description

! On August 2,1989, one holddown bolt on the Unit 1. Train "A" EDG left bank-I turbocharger base was found broken during the post-run visual inspection required by the appropriate project procedure. Subsequently, the remaining

- holddown bolts from the four Unit 1 EDG turbochargers were removed for L metallurgical analysis. During the' removal process, an additional bolt broke.

All of the bolts, including the broken bolts, had apnroximately 270 hours0.00313 days <br />0.075 hours <br />4.464286e-4 weeks <br />1.02735e-4 months <br /> of engine operation.

A generic concern with these bolts was identified by the TransAmerica Delaval, Inc.,.(TDI) owner's group program. The TDI owner's group evaluation stated that the existing SAE J429 Grade 5 bolts were acceptable but also called for L periodic inspections of the bolts due to the high vibration loads.

l During an unrelated test conducted at CPSES and witnessed by a vendor representative, it had been observed that normal vibration loads on these bolts were amplified because the turbocharger auxiliary support frame is in a resonance with the engine. A seismic qualification analysis utilizing data

from this test was in progress at the time of the original bolt failure.

Subsequent.results of the seismic analysis indicated that, due to the higher

' loads imposed by resonant vibrations, the bolted connection design was inadequate.

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TXX-8982f, December 15, 1989 Page 2 of 3 A metallurgical analysis was performed on all of the turbocharger holddown bolts removed from the Unit 1 EDG's. This analysis confirmed that the two bolt failures were due to cyclic fatigue loading. The analysis also indicated -

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- that each of the unfailed Unit I holddown bolts met SAE.J429 Grade 5 bolt '

chemical and physical specifications. The two bolts that failed also met the l required chemical properties. Tensile testing of the failed bolts could not be performed due to the configuration of the broken pieces but hardness testing was performed and based on an approximate correlation to material hardness, it indicated that the two failed bolts met up to 80 to 90% of their specified strength.

The bolt failures are attributed to high vibrational loads with originally L specified bolt pre-load torque. Additionally,.a decrease in bolt strength could have decreased the bolt life.

Based on the above, it was concluded that this issue is limited to safety-related installations involving rotating equipment which may cause significant I vibrational loads. Safety-related equipment (including rotating equipment) is '

I- included in the seismic equipment qualification program and has been I demonstrated to be acceptable. Additionally, safety related Unit I rotating l equipment installations were reviewed to determine if SAE J429 Grade 5 I holddown bolts are currently installed. The review revealed that only the I centrifugal charging pump motors and gear cases have SAE J429 Grade 5 holddown I bolts. Westinghouse has confirmed that SAE J429 Grade 5 bolts are acceptable I in this installation, even if the bolts are only 80% of rated strength. Unit l

. 2 rotating equipment installations will be reviewed and any identified SAE J429 Grade 5 bolts will be evaluated for adequacy and, if necessary, replaced prior to Unit 2. fuel load.

Safety Sionificance Failure of these bolts could have resulted in performance degradation or damage to the EDG. This represents a significant deficiency in design which, had it remained uncorrected, could have adversely affected the safety of plant I I

operations.

l Corrective Action I l

The deficient Unit 1 EDG turbocharger bolts have been replaced with higher I strength, longer bolts, and the pre-load torque was increased. Unit 2 EDG turbocharger bolts will be replaced prior to Unit 2 fuel load.

In order to ensure the integrity of the EDG turbochargers, the bolts will be l- replaced periodically after approximately 92 hours0.00106 days <br />0.0256 hours <br />1.521164e-4 weeks <br />3.5006e-5 months <br /> of operation. This replacement rate-is based on a conservatively determined bolt life and EDG

operating time, and includes sufficient margin to ensure operation of the EDG L for seven days after a postulated accident.

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TXX-89826 December 15, 1989 Page 3 of 3 Prior to completion of the first refueling outage of Unit 1, additional testing may be performed to more accurately determine the vibration loads experienced by the turbocharger holddown bolts. Data obtained from the performance of any additional testing would be used to analyze whether the bolt life can be increased. If necessary, the EDG turbocharger support frame will be modified to change-its resonant frequency and reduce the bolt-vibration loads.

Sincerely, ,

t William J. Ca ill, Jr.

l MCP/smp 4 l

l- c - Mr. R. D. Martin, Region IV ,

! Resident Inspectors, CPSES (3) l l

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