Information Notice 1991-83, Solenoid-Operated Valve Failures Resulted in Turbine Overspeed

Solenoid-Operated Valve Failures Resulted in Turbine Overspeed
	ML031190391
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:	12/20/1991
From:	Rossi C; Office of Nuclear Reactor Regulation
To:	;
References
	IN-91-083, NUDOCS 9112160318
	Download: ML031190391 (11)
	v • d • e

- s

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 December 20, 1991 NRC INFORMATION NOTICE 91-83: SOLENOID-OPERATED VALVE FAILURES RESULTED

IN TURBINE OVERSPEED

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 turbine overspeed problems and turbine trip failures

resulting from the failures of multiple solenoid-operated valves (SOVs). 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 require- ments; therefore, no specific action or written response is required.

Background

Westinghouse turbines are tripped using a combination of electro-hydraulic

control (EHC) fluid and auto-stop oil (Attachment 1). In the electro-hydraulic

portion, SOVs dump the fluid to the EHC sump, causing the throttle valves, reheat stop valves, intercept valves, and the governor valves to shut, stopping

the flow of steam to the turbine. Failure of the SOVs in the EHC system to

open could lead to the turbine overspeeding. Several events have occurred in

the industry that involved the failure of SOVs that must operate for turbine

emergency trip and overspeed protection. The consequences of these failures

have varied according to the initiating event and the type of SOV failure.

These failures suggest that there are weaknesses in the preventive maintenance

and testing of these SOVs.

Description of Circumstances

On November 9, 1991, Unit 2 of the Salem Nuclear Generating'Station sustained

severe damage to its turbine and generator. The event occurred while'the

Public Service Electric and Gas Company (the licensee) was conducting routine

turbine testing at 100-percent power. The licensee bypassed the turbine's

auto-stop oil trip mechanism in accordance with the test procedure. During the

test, an oil pressure perturbation occurred in the auto-stop trip system, the

exact cause of which is yet to be determined. This caused the interface valve

to open and thereby depressurized the EHC fluid. Both the turbine and the

reactor tripped and, as expected, all turbine stop valves closed. However, the

emergency trip solenoid valve failed to open upon receiving the trip signal.

9112160318

-e \\\xt ar - //c,

,IN91-83

-,December 20, 1991 When the auto-stop oil repressurized, the interface valve closed, and all

turbine stop valves reopened allowing steam flow into the turbine. As

designed, the generator output breakers had opened upon receiving the reactor

trip signal, so the steam flow through the unloaded turbine caused the turbine

to overspeed. Both overspeed protection controller (OPC) solenoid valves

failed to open preventing the governor valves and intercept valves from closing

properly. The turbine continued to overspeed to an estimated 160-percent of

rated speed. Operators observed increasing noise and vibration from the

turbine and a fire at the generator. The low pressure turbine blades penetra- ted the turbine shroud. The overspeed resulted in severe damage to the low

pressure turbine, the generator exciter unit, the condenser, and associated

support structures, systems, and components.

Discussion

The staff sent an augmented inspection team (AIT) to the site to investigate

the event. The AIT concluded that the proximate cause of the event was the

failure of the emergency trip solenoid valve and both overspeed protection

controller solenoid valves to open when energized. The specific failure

mechanisms of the Parker-Hannifin SOVs are yet to be determined, but prelimi- nary analysis indicated that the pilot valve assembly in each solenoid unit was

mechanically bound sufficiently to prevent movement. Other previous failures

of these valves in the industry have also been attributed to mechanical bind- ing, corrosion, and worn or pinched elastomeric parts.

Several precursory factors contributed to the event. The licensee for the

Salem Nuclear Generating Station has no preventive maintenance program for any

of these three SOVs. The surveillance and operational testing of the turbine

trip and overspeed circuits does not specifically verify the proper hydraulic

functioning of each SOY independently. Further, information concerning previ- ous SOV failures has not been well disseminated.

The licensee had two earlier indications of problems with these SOVs. Similar

valves on Salem Unit 1 required replacement, yet the licensee had not effec- tively verified the operability of the SOYs in Unit 2. Another indication

occurred during a startup in October 1991. The OPC solenoid valves failed to

open when a test of the system was performed. The licensee was required to

verify proper OPC operation by closing the intercept valves when the OPC test

switch was activated. The intercept valves did not close, indicating a possi- ble malfunction of both of the OPC solenoid valves. The results were

apparently misinterpreted, leading management to believe a procedure problem

existed, rather than an equipment problem. The licensee continued the startup

without further diagnosis and resolution. The staff will include more details

of this event in NRC Inspection Report 50-311/91-81 when issued.

Previous Events

The following previous events involved failures of SOVs in the turbine trip

system during reactor trip events:

On April 6, 1985, at the R. E. Ginna Nuclear Power Plant, the turbine failed to

trip automatically following a reactor trip because of mechanical binding of

the emergency trip solenoid valve. -

IN 91-83 December 20, 1991 On February 28, 1988, at the Crystal River Plant, Unit 3, the turbine failed to

trip automatically following a reactor trip because of a faulty emergency trip

solenoid valve.

On September 10, 1990, at Salem, Unit 1, the reactor tripped because of a steam

generator water level transient caused by a spurious overspeed signal. Mechan- ical binding prevented the OPC solenoid valves from functioning.

On September 29, 1990, at Ginna, the reactor tripped because of personnel

error, but mechanical binding prevented the turbine emergency trip solenoid

valve from functioning.

These events indicate that proper maintenance and operability testing of both

the emergency trip solenoid valve and the OPC solenoid valves is prudent.

Related Generic Communications

The staff issued Generic Letter 91-15, "Operating Experience Feedback Report, Solenoid-Operated Valve Problems at U.S. Reactors," to distribute NUREG-1275, Volume 6, to industry. This document provided the staff's analysis of recent

U.S. light water reactor experience (primarily 1984-1989) with SOYs.

Appendix A of NUREG-1275, Volume 6, cited similar SOV failures at four other

plants. The staff noted the susceptibility of SOVs to common-mode failures.

The staff found that most SOVs cannot tolerate contaminants, need preventative

maintenance or periodic replacement, and have a propensity for rapid aging and

deterioration when subjected to elevated temperatures. The staff provided

recommendations to aid in preventing common-mode SOV failures.

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

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

of the technical contacts listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

Charles E. Rossi, Dir or

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical contacts: John White, Region I

(215) 337-5114 David Gamberoni, NRR

(301) 504-1171 Attachments:

1. Westinghouse Electro-Hydraulic Control System

2. List of Recently Issued NRC Information Notices

II

EMERGENCY TRIP

SOLENOID VALVE

C

TO EHC

SUMP

...... I................ AUTO-STOP OIL SYSTEM

INTERFA'

VALVE

TO En-IC

SUIMP

c

ELECTRO-HYORAULIC CONTROL FLUID

................ AUTO-STOP OIL

TO EHC

SUAJP

OVERSPEED PROTECTION (D Z r'

CONTROLLER SOLENOID Gm n rt

VALVES (D 9 I' 03

0oD11 co B

a

LO (D

WESTINGHOUSE ELECTRO-HYDRAULIC CONTROL SYSTEM Ft M

o

o_ _

'\Attachment2 IN 91-83 December 20, 1991 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

91-18, High-Energy Piping Failures 12/18/91 All holders of OLs or CPs

Supp. 1 Caused by Wall Thinning for nuclear power reactors.

91-82 Problems with Diaphragms 12/18/91 All holders of OLs or CPs

in Safety-Related Tanks for nuclear power reactors.

91-81 Switchyard Problems that 12/16/91 All holders of OLs or CPs

Contribute to Loss of for nuclear power reactors.

Offsite Power

91-80 Failure of Anchor 12/11/91 All holders of OLs or CPs

Head Threads on Post- for nuclear power reactors.

Tensioning System During

Surveillance Inspection

91-79 Deficiencies in the 12/06/91 All holders of OLs or CPs

Procedures for Instal- for nuclear power reactors.

ling Thermo-Lag Fire

Barrier Materials

88-92, Potential for Spent 11/29/91 All holders of OLs or CPs

Supp. 1 Fuel Pool Draindown for nuclear power reactors.

91-78 Status Indication of 11/28/91 All holders of OLs or CPs

Control Power for for nuclear power reactors.

Circuit Breakers Used

in Safety-Related Appli- cations

90-57, Substandard, Refur- 11/27/91 All holders of OLs or CPs

Supp. 1 bished Potter & Brum- for nuclear power reactors.

field Relays Repre- sented as New

91-77 Shift Staffing at 11/26/91 All holders of OLs or CPs

Nuclear Power Plants for nuclear power reactors.

OL = Operating License

CP = Construction Permit

HE

UNITED STATES

NUCLEAR REGULATORY COMMISSION FIRST CLASS MAIL

POSTAGE 6 FEES PAID

WASHINGTON, D.C. 20555 USNRC

PERMIT No 0-57 OFFICIAL BUSINESS

PENALTY FOR PRIVATE USE, $300

I

II