Information Notice No. 91-83: Solenoid-Operated Valve Failures Resulted in Turbine Overspeed
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
All holders of operating licenses or construction permits for nuclear power
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.
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.
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
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
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.
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.
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
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
David Gamberoni, NRR
1. Westinghouse Electro-Hydraulic Control System
2. List of Recently Issued NRC Information Notices
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