United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 91-83: Solenoid-Operated Valve Failures Resulted in Turbine Overspeed

                                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 requirements; 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
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                                                       IN 91-83
                                                       December 20, 1991
                                                       Page 2 of 3

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 penetrated 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 
preliminary 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 binding, 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 SOV independently.  Further, information 
concerning previous 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 effectively verified the operability of the SOVs 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 possible 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.
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                                                       IN 91-83
                                                       December 20, 1991
                                                       Page 3 of 3


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.  
Mechanical 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 SOVs.  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, Director
                                   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
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