Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
August 22, 1988
Information Notice No. 88-67: PWR AUXILIARY FEEDWATER PUMP TURBINE
OVERSPEED TRIP FAILURE
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is being provided to alert addressees to potential
problems affecting the reliability of turbine-driven auxiliary feedwater (AFW)
pumps to trip during overspeed events. Similar overspeed trip mechanisms are
employed on turbine-driven high-pressure coolant injection (HPCI) and reactor
core isolation cooling (RCIC) pumps. 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 do not constitute NRC requirements;
therefore, no specific action or written response is required.
Description of Circumstances:
The steam-driven AFW pumps at San Onofre Units 2 and 3 are powered by Terry
steam turbines, which are provided with a mechanical overspeed trip assembly.
Actuation of the trip assembly causes the steam inlet valve to the turbine to
close by releasing the stop valve closure spring.
On July 3, 1988, personnel at San Onofre Unit 2 were performing low-voltage
testing of the steam inlet valve to the AFW turbine. During the test, the
steam inlet valve failed to close within the required period of time. In
accordance with procedures, the valve technician in the pump room removed
power from the steam inlet valve, which also removed power from the governor
valve to the AFW turbine (fails open). Loss of power to the governor resulted
in the turbine reaching an overspeed condition. Control room indication
showed the pump discharge pressure to be at full scale (2000 psig), and
maximum rotational speed was approximately 5000 rpm (the overspeed trip
setpoint is 3500 rpm). An attempt
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August 22, 1988
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was made to close the steam inlet valve from the control room, but this was
unsuccessful since power had been removed. The control room operator then
closed the turbine inlet isolation valve, terminating the event.
Discussion:
The licensee's investigation into this event identified the root cause to be
failure of the mechanical overspeed trip device because of damage to the
tappet ball located in the mechanical trip linkage. The licensee made visual
inspections for damage from overpressurization of piping, valves, and
instrumentation. There was no evidence of cracks, leaks, or deformation of
the overpressurized piping. Analysis performed after the event revealed that
piping code stresses were not exceeded.
The mechanical overspeed trip linkage consists of an emergency governor weight
(located in the shaft of the turbine), an emergency tappet (see attachment 1),
and linkage (not shown in attachment 1) to the trip (steam inlet) valve.
During an overspeed condition, centrifugal force causes the emergency governor
weight to overcome a restraining spring located in the shaft. The weight then
strikes the emergency tappet, which, in turn, rises against a spring allowing
the trip linkage to release the stop valve spring closure mechanism closing
the steam inlet valve. Initial inspection and test results failed to
duplicate the overspeed event. Upon disassembly of the mechanical trip
device, visual inspection showed excessive wear on the polyurethane tappet
ball. The tappet ball fits into the emergency tappet as shown in attachment
1, with the ball free to rotate. Approximately half the polyurethane was
missing from the tappet ball so that the emergency governor weight could not
contact the tappet ball with sufficient force to actuate the trip mechanism.
The ball was also covered with a gummy substance of unknown origin.
The licensee implemented several corrective actions and plans to upgrade the
mechanical trip linkage to the current vendor design (the new design has no
tappet ball arrangement). The vendor recommended overspeed testing every
refueling outage. Licensees may wish to consider a visual examination of the
tappet ball arrangement during the refueling outage.
Information Notice 86-14, "PWR Auxiliary Feedwater Pump Turbine Control
Problems," Supplement 1 to Information Notice 86-14, "Overspeed Trips of AFW,
HPCI, and RCIC Turbines," and Information Notice 88-09, "Reduced Reliability
of Steam-Driven Auxiliary Feedwater Pumps Caused by Instability of Woodward
PG-PL Type Governors," discuss problems related to the Terry turbine overspeed
trip systems.
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August 22, 1988
Page 3 of 3
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the
technical contacts listed below or the Regional Administrator of the
appropriate regional office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: J. Thompson, NRR
(301) 492-1175
T. Silko, NRR
(301) 492-9059
Attachments:
1. Terry Turbine Mechanical Trip Device Tappet and Ball Linkage
2. List of Recently Issued NRC Information Notices
. Attachment 2
IN 88-67
August 22, 1988
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
88-66 Industrial Radiography 8/22/88 All NRC industrial
Inspection and Enforcement radiography
licensees.
88-65 Inadvertent Drainages of 8/18/88 All holders of OLs
Spent Fuel Pools or CPs for nuclear
power reactors and
fuel storage
facilities.
88-64 Reporting Fires in Nuclear 8/18/88 All holders of OLs
Process Systems at Nuclear or CPs for nuclear
Power Plants power reactors.
88-63 High Radiation Hazards 8/15/88 All holders of OLs
from Irradiated Incore or CPs for nuclear
Detectors and Cables power reactors,
research reactors
and test reactors.
88-62 Recent Findings Concerning 8/12/88 All holders of NRC
Implementation of Quality quality assurance
Assurance Programs by program approval
Suppliers of Transport for radioactive
Packages material packages.
88-61 Control Room Habitability - 8/11/88 All holders of OLs
Recent Reviews of Operating or CPs for nuclear
Experience power reactors.
88-60 Inadequate Design and 8/11/88 All holders of OLs
Installation of Watertight or CPs for nuclear
Penetration Seals power reactors.
88-04, Inadequate Qualification 8/9/88 All holders of OLs
Supplement 1 and Documentation of Fire or CPs for nuclear
Barrier Penetration Seals power reactors.
88-59 Main Steam Isolation Valve 8/9/88 All holders of OLs
Guide Rail Failure at or CPs for nuclear
Waterford Unit 3 power reactors.
88-58 Potential Problems with 8/8/88 All holders of OLs
ASEA Brown Boveri ITE-51L or CPs for nuclear
Time-Overcurrent Relays power reactors.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
