United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-76: Failure of Turbine Overspeed Trip Mechanism Because of Inadequate Spring Tension

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555
 
                              December 7, 1990
 
 
Information Notice No. 90-76:  FAILURE OF TURBINE OVERSPEED TRIP 
                                   MECHANISM BECAUSE OF INADEQUATE SPRING 
                                   TENSION
 
 
Addressees:
 
All holders of operating licenses or construction permits for nuclear power 
reactors.
 
Purpose:
 
This information notice is intended to alert addressees to the potential 
failure of the overspeed trip mechanism on turbine-driven pumps as the 
result of inadequate tension in the emergency trip spring.  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:
 
On March 17, 1990, and on August 14, 1990, during two separate monthly 
surveillance tests of the Terry turbine-driven auxiliary feedwater pump at 
Arkansas Nuclear One, actuation of the local manual trip lever on the 
overspeed trip mechanism failed to close the turbine trip and throttle 
valve.  If a malfunction causes the turbine to overspeed, the overspeed trip 
mechanism (shown in Figure 1) normally will rapidly close the turbine trip 
and throttle valve, stopping steam flow to the turbine.  Excessive turbine 
speeds could damage the turbine and pump and overpressurize the auxiliary 
feedwater system.
 
In response to an overspeed, a weight in the turbine shaft is forced outward 
by the excessive centrifugal force, striking a tappet, which moves upward, 
releasing the emergency head lever (see Figure 1).  This lever normally 
allows the emergency trip spring to move a rod connected to the turbine trip 
and throttle valve trip mechanism.  The movement of the connecting rod 
should then disengage a trip hook lever on the trip and throttle valve 
allowing the valve to close.  However, during the two failed tests, the 
emergency trip spring failed to move the connecting rod.  During the first 
of these tests, the plant personnel were able to actuate the trip mechanism 
by tapping on the emergency trip spring holder, which produced an erratic 
response, indicating that the spring force and the frictional forces were 
evenly matched.  Arkansas Nuclear One personnel lubricated the critical 
mating surfaces and achieved good trip response.  
 
 
 
9012030076 
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                                                            Page 2 of 3
 
 
However, the second failure clearly indicated that insufficient spring force 
was the root cause of the problem.  The original emergency trip spring, 
which had been in service approximately nine years, had stretched 1/8 inch 
beyond its normal at rest position.  This spring was replaced with a new one 
of identical design.  After this change, plant personnel successfully 
completed a number of trip tests.  
 
Discussion:
 
Inspection of the original emergency trip spring provided neither evidence 
of a manufacturing defect nor that the spring had ever been stretched beyond 
its elastic limit.  The observed elongation was apparently the result of a 
long-term relaxation process, which could develop on this or any other 
similar trip mechanism.  The tension in the spring, and thus the force 
applied to the connecting rod, can be controlled by adjusting the position 
of the trip spring holder on a threaded portion of the connecting rod.  This 
adjustment is made during turbine installation and normally is not changed 
or checked thereafter.  The Arkansas Nuclear One staff intends to revise the 
maintenance procedures to specify adjustment of the trip spring holder 
position in the event of any future problems attributable to insufficient 
spring force.
 
The failure of overspeed trip mechanisms could result in severe damage to
turbines and pumps in boiling water reactor high-pressure coolant injection 
and reactor core isolation cooling systems and in pressurized water reactor 
auxiliary feedwater systems.  The resulting overpressurization can also 
damage other components.  NRC Information Notice 90-45 discussed two 
overspeed events involving turbine-driven auxiliary feedwater pumps.  One 
occurred at Rancho Seco, where the turbine, rated at 3600 rpm, reached 6020 
rpm a few seconds after start.  According to calculations, the auxiliary 
feedwater system pressure reached 3850 psig, significantly exceeding the 
system design pressure of 1325 psig.  This overpressurization also affected 
the motor-driven auxiliary feedwater train because the two trains were tied 
together.  The other overspeed event occurred at San Onofre, where the 
turbine-driven auxiliary feedwater pump reached 5000 rpm and the auxiliary 
feedwater system pressure was calculated to have reached 2420 psig.  
 
The event at Arkansas Nuclear One revealed an additional failure process for 
overspeed trip mechanisms.  It serves as a reminder that the characteristics 
of mechanical components, such as springs, are subject to change.  Such 
variability highlights the importance of periodic checking and adjustment, 
inherent to adequate maintenance and testing practices. 
 
Related Generic Communications:
 
1.   NRC Information Notice 90-45, "Overspeed of the Turbine-Driven 
     Auxiliary Feedwater Pumps and Overpressurization of the Associated 
     Piping Systems", dated July 6, 1990.  
     
2.   NRC Information Notice 88-67, "PWR Auxiliary Feedwater Pump Turbine 
     Overspeed Trip Failure", dated August 22, 1988. 
 
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                                                            IN 90-76
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                                                            Page 3 of 3
 
 
3.   NRC Information Notice 88-09, "Reduced Reliability of Steam-Driven 
     Auxiliary Feedwater Pumps Caused by Instability of Woodward PG-PL Type 
     Governors", dated March 18, 1988.
 
4.   Information Notice No. 86-14, "PWR Auxiliary Feedwater Pump Turbine 
     Control Problems", dated March 10, 1986.
 
5.   Information Notice No. 86-14, Supplement 1, "Overspeed Trips of AFW, 
     HPCI, and RCIC Turbines", dated December 17, 1986.
 
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 NRR project 
manager.
 
 
 
 
                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation
 
 
Technical Contacts:  Thomas F. Stetka, Region IV
                     (817) 860-8247
 
                     Michael F. Runyan, Region IV
                     (817) 860-8142 
 
 
Attachments:
1.  Figure 1.  Emergency Feedwater Pump Overspeed Trip Mechanism
2.  List of Recently Issued NRC Information Notices
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Page Last Reviewed/Updated Tuesday, November 12, 2013