Information Notice 1994-01, Turbine Blade Failures Caused by Torsional Excitation from Electrical System Disturbance

Turbine Blade Failures Caused by Torsional Excitation from Electrical System Disturbance

ML031070067
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	01/07/1994
From:	Grimes B Office of Nuclear Reactor Regulation
To:
References
IN-94-001, NUDOCS 9401030084
Download: ML031070067 (10)

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UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON D.C. 20555 January 7, 1994 NRC INFORMATION NOTICE 94-01: TURBINE BLADE FAILURES CAUSED BY TORSIONAL

EXCITATION FROM ELECTRICAL SYSTEM DISTURBANCE

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees to potential blade failures in low-pressure

turbines reportedly caused by torsional vibration resulting from disturbance

in the electrical power system. It is expected that recipients will review

the information for applicability to their facilities and consider actions, as

appropriate, to avoid similar problems. However, suggestions contained in

this information notice are not NRC requirements; therefore, no specific

action or written response is required.

Description of Circumstances

On July 12, 1993, at Susquehanna Unit 1, automatically shut down as a result

of high turbine vibration, which was felt in the control room (Reference 1).

The maximum amplitude recorded was about 0.038 cm 115 mils] at bearing No. 8.

Two blades from the L-1 row on the governor end of the low-pressure C turbine

separated from the rotor. The failed blades gouged diaphragms (stationary

rotors) and damaged other blades in the L-0 and L-1 rows. Cracks were found

at the roots of several blades in the L-1 row. The fragments of the blades

damaged 50 to 100 condenser tubes.

The plant was operating at 100 percent power at the time of the event.

Control room operators responded to the event by entering the appropriate

procedures. The emergency core cooling and engineered safety feature systems

were not initiated. The licensee concluded that primary containment integrity

was maintained.

Susquehanna Unit 1 uses General Electric low-pressure turbines, model number

TB 170X592. Each turbine operates at 1800 revolutions per minute. The L-1 blades are about 96 cm [38 inches] long. The root (finger dovetails) of a

blade consists of six plates, each plate being about 6 cm [2 1/2 inches] wide,

10 cm [4 inches] long, and 1 cm [3/8 inch] thick. Each plate has six pin

holes arranged in three rows of two holes each. The blade is attached to the

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IN 94-01 January 7, 1994 rotor by pins staked in the holes. The failed blades were severed at the top

rows of holes, as indicated in Figure 1.

Previous Events

Between 1973 and 1976, at Prairie Island Units 1 and 2, the licensee, Northern

States Power Company, had sustained a total of five blade failures or cracking

events in its Westinghouse low-pressure turbines. All failed blades were from

the L-2 or L-1 row of the low pressure rotor. The licensee attributed these

failures to torsional vibration resulting from electrical system excitation.

To resolve the problem, the licensee (1)modified the L-2 blades and the

L-1 disc to be unresponsive to 120-Hz excitation, (2)performed a torsional

test before returning the system to service.

In 1985, eight blades from the last stage of a General Electric low-pressure

turbine failed and separated from the rotor at one of the units of a foreign

nuclear plant. The blade failures were attributed to high cycle fatigue as a

result of the electrical disturbance which excited the 19th torsional mode of

rotor vibration at or near the resonant frequency of 120.5 Hz. The failed

blades caused unbalanced mass and produced a large vibration on the turbine- generator unit, which in turn failed the alternator shaft. The failed

alternator shaft caused leakage of hydrogen and lubrication oil from the seal

system and led to a fire in the generator-alternator area. A subsequent

torsional test at the other unit of that plant showed that a 0.05 Hz change in

the electrical frequency contributed to a three fold increase in stress

response in the L-0 row blades.

Discussion

Since the 1970's, some blade failures of low-pressure turbines at nuclear

power plants have been attributed to torsional excitation of the turbine- generator shaft as a result of an electrical system disturbance. When the

frequency of the excitation coincides with the natural torsional frequency of

the turbine rotor, the rotor and blade responses are highly magnified. The

blade eventually fails by fatigue.

The electrical system disturbances could be caused by (1)negative-phase

sequence current resulting from unbalanced load among the three electric

phases, caused by the load imbalance creating a current in the generator

stator opposite to the main current; (2)electric faults resulting from high

speed reclosing of field relay contacts; (3)load rejection; and (4)switching

operations on the transmission line (Reference 2).

IN 94-01 January 7, 1994 This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nu ear

Reactor Regulation (NRR) project manager.

an K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contact: John Tsao, NRR

(301) 504-2702 References:

1. Letter from E. C. Wenzinger of the

U.S. Nuclear Regulatory Commission to

R.G. Byram of Pennsylvania Power & Light

Company, Subject: NRC Region I Combined

Inspection 50-387/93-11; 50-388/93-11, August 31, 1993.

2. Technical Information Letter (TIL-1012-2),

"Effect of Electrical System Vibration

on Turbine-Generator Torsional Response,"

General Electric Company, Schenectady, New York, September 8, 1987.

Attachments:

1. Figure 1. The Finger Dovetail of a Typical Blade

2. List of Recently Issued NRC Information Notices

Atta 48t 1 IN ry-,1 January 7, 1994 Failure Surface

Steam

Finger No.- 1 2 3 4 5 6 Side View

No Failures

in Pins (Not Shown)

Figure 1 The Finger Dovetail of a Typical Blade

At'rchment 2 IN 94-01 January 7, 1994 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

93-101 Jet Pump Hold-Down Beam 12/17/93 All holders of OLs or CPs

Failure for boiling-water reactors93-100 Reporting Requirements 12/22/93 All U.S. Nuclear Regulatory

for Bankruptcy Commission licensees.

91-29, Potential Deficiencies 12/22/93 All holders of OLs or CPs

Supp. 2 Found During Electrical for nuclear power reactors.

Distribution System

Functional Inspections

93-99 Undervoltage Relay and 12/21/93 All holders of OLs and CPs

Thermal Overload Setpoint for nuclear power reactors.

Probl ems

93-98 Motor Brakes on Valve 12/20/93 All holders of OLs and CPs

Actuator Motors for nuclear power reactors.

93-97 Failures of Yokes 12/17/93 All holders of OLs or Cls

Installed on Walworth for nuclear power reactors.

Gate and Globe Valves

93-96 Improper Reset Causes 12/14/93 All holders of OLs or CPs

Emergency Diesel for nuclear power reactors.

Generator Failures

93-95 Storm-Related Loss of 12/13/93 All holders of OLs or CPs

Offsite Power Events for nuclear power reactors

due to Salt Buildup located close to a large

on Switchyard Insulators body of salt water.

93-94 Unauthorized Forced 12/09/93 All holders of OLs or CPs

Entry into the Pro- for nuclear power reactors.

tected Area at Three

Mile Island Unit 1 on

February 7, 1993

93-93 Inadequate Control of 12/08/93 All holders of OLs or CPs

Reactor Coolant System for nuclear power reactors.

Conditions During Shutdown

OL - Operating License

CP - Construction Permit

IN 94-01 January 7, 1994 This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

WOWgnafigned by

Brian K.Grimes

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contact: John Tsao, NRR

(301) 504-2702 References:

1. Letter from E. C. Wenzinger of the

U.S. Nuclear Regulatory Commission to

R.G. Byram of Pennsylvania Power & Light

Company, Subject: NRC Region I Combined

Inspection 50-387/93-11; 50-388/93-11, August 31, 1993.

2. Technical Information Letter (TIL-1012-2),

"Effect of Electrical System Vibration

on Turbine-Generator Torsional Response,"

General Electric Company, Schenectady, New York, September 8, 1987.

Attachments:

1. Figure 1. The Finger Dovetail of a Typical Bucket

2. List of Recently Issued NRC Information Notices

• SEE PREVIOUS CONCURRENCES

OFFICE l*OGCB/DOR *TECH ED *DE/EMCB *DE/EMCB/NRR *D:DE:EMCB

NAME lCVHodge DGable JTsao JStrosnider KWichman

DATE l11/18/93 I_10/20/93 10/20/93 10/25/93 M1/22/93

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IN 94-01 January 7, 1994 This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

Uigintat Stgned by

Brian K.Grimes

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contact: John Tsao, NRR

(301) 504-2702 References:

1. Letter from E. C. Wenzinger of the

U.S. Nuclear Regulatory Commission to

R.G. Byram of Pennsylvania Power & Light

Company, Subject: NRC Region I Combined

Inspection 50-387/93-11; 50-388/93-11, August 31, 1993.

2. Technical Information Letter (TIL-1012-2),

"Effect of Electrical System Vibration

on Turbine-Generator Torsional Response,"

General Electric Company, Schenectady, New York, September 8, 1987.

Attachments:

1. Figure 1. The Finger Dovetail of a Typical Bucket

2. List of Recently Issued NRC Information Notices

• SEE PREVIOUS CONCURRENCES

OFFICE *OGCB/DOR *TECH ED *DE/EMCB *DE/EMCB/NRR *D:DE:EMCB

NAME CVHodge DGable JTsao JStrosnider KWichman

DATE 11/18/93 10/20/93 10/20/93 10/25/93 R/22 93

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December xx, 1993 This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

Brian K. Grimes, director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

John Tsao, NRR

(301) 504-2702 References:

1. Letter from E. C. Wenzinger of the U.S. Nuclear Regulatory Commission to

R.G. Byram of Pennsylvania Power & Light Company, Subject: NRC Region I

Combined Inspection 50-387/93-11; 50-388/93-11, August 31, 1993.

2. Technical Information Letter (TIL-1012-2), "Effect of Electrical System

Vibration on Turbine-Generator Torsional Response,' General Electric

Company, Schenectady, New York, September 8, 1987.

Attachments:

1. Figure 1. The Finger Dovetail of a Typical Blade

2. List of Recently Issued NRC Information Notices

This information notice was coordinated with OIP because it mentions a foreign

plant.

-

• SEE PREVIOUS CONCURRENCE

OFFICE l*OGCB/DORSj *TECH ED. *DE/EMCB *DE/EMCB/NRR *D:DE/EMCB

NAME CVHodge DGable JTsao JStrosnider KWichman

DATE 11/18/93 , 10/20/93 10/20/93 , 10/25/93 1 10/22/93

• C: EELB/DE *D:DE/NRR *C:OGCB/DORS D:DORS/NRR

CBerlinger JMiggins GHMarcus BKGrimes _

10/27/93 11/24/93 111/30/93 112/ /93 DOCUMENT NAME: TURBUORS.INR

IN 93-XX

November xx, 1993 electrical disturbance which excited the 19th torsional mode of rotor

vibration at or near the resonant frequency of 120.5 Hz. The failed buckets

caused unbalanced mass and produced a large vibration on the turbine-generator

unit, which in turn failed the alternator shaft. The failed alternator shaft

caused leak of hydrogen and lubrication oil from the seal system and led to a

fire in the generator-alternator area. A subsequent torsional test of the

turbine of the sister unit at the plant showed that a 0.05 Hz change in the

electrical frequency contributed to a three fold increase in stress response

in the L-O row buckets.

References 7 through 12 in Attachment 1 are included to provide additional

information.

This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

Brian K. Grimes, director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

John C. Tsao, NRR

(301) 504-2702 Attachments:

1. References

2. Figure 1. The Finger Dovetail of a Typical Bucket

3. List of Recently Issued NRC Information Notices

• SEE PREVIOUS CONCURRENCE

OFFICE OGCB/DORS *TECH ED *DE/EMCB *DE/EMCB/NRR D:DE:EMCB

NAME CVHodge tDGablo--JTsao JStrosnider KWichman

DATE J11/1/93 10 10/20/93 10/25/93 10/22/93 C: EELB/DE* D-DEW\. C: OGCB/DORS D:DORS/NRR

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IN 93-XX

November xx, 1993 result of the electrical disturbance which excited the 19th torsional mode of

rotor vibration at or near the resonant frequency of 120.5 Hz. The failed

buckets caused unbalanced mass and produced a large vibration on the turbine- generator unit, which in turn failed the alternator shaft. The failed

alternator shaft caused leak of hydrogen and lubrication oil from the seal

system and led to a fire in the generator-alternator area. A subsequent

torsional test at the other unit of that plant showed that a 0.05 Hz change in

the electrical frequency contributed to a three fold increase in stress

response in the L-O row buckets.

References 7 through 12 in Attachment 1 are included to provide additional

information.

This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

Brian K. Grimes, director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

John Tsao, NRR

(301) 504-2702 Attachments:

1. References

2. Figure 1. The Finger Dovetail of a Typical Bucket

3. List of Recent'1lv Issued NRC Information Notices

OFFICE 7OGCB/DORSi *TECH ED *DE/EMCB *DE/EMCB/NRR *D:DE:EMCB

NAME CVHodge K DGable JTsao JStrosnider KWichman

DATE 11/1ff/93 10/20/93 10/20/93 1 10/25/93 10/22/93

• C:EELB/DE D: DE: NRR C:OGCB/DORS D:DORS/NRR

CBerlinger JWiggins l GHMarcus BKGrimes

10/27/93 11/ /93 11/ /93 11/ /93 DOCUMENT NAME: TURBTORS.INF