United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-67: PWR Auxiliary Feedwater Pump Turbine Overspeed Trip Failure

                                  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 








8808160414 
.                                                            IN 88-67 
                                                            August 22, 1988 
                                                            Page 2 of 3 


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. 

.                                                            IN 88-67 
                                                            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 
Page Last Reviewed/Updated Tuesday, November 12, 2013