United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 86-14, Supplement 2: Overspeed Trips of AFW, HPCI and RCIC Turbines

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, DC  20555

                               August 26, 1991


NRC INFORMATION NOTICE NO 86-14, SUPPLEMENT 2:  OVERSPEED TRIPS OF AFW, 
                                                 HPCI AND RCIC TURBINES


Addressees:    

All holders of operating licenses or construction permits for nuclear power 
reactors  

Purpose: 

This information notice supplement is intended to alert addressees to a 
recently identified condition in which turbine-driven pumps may trip on 
overspeed because of the sluggish response of the turbine speed governor 
caused by an accumulation of dirt and grit in the governor's control oil 
system  Recent overspeed trips of turbine-driven pumps have also prompted 
the staff to issue this supplement to reemphasize previously identified 
causes of overspeed trips  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 supplement do not constitute NRC requirements; 
therefore, no specific action or written response is required 

Background:

On March 10, 1986, the NRC issued Information Notice (IN) 86-14, "PWR 
Auxiliary Feedwater Pump Turbine Control Problems," to alert addressees to 
certain conditions that could cause turbine-driven pumps to trip on 
overspeed  In August 1986, the NRC Office for Analysis and Evaluation of 
Operational Data (AEOD) issued study AEOD/C602, "Operational Experience 
Involving Turbine Overspeed Trips"  On December 17, 1986, the NRC issued 
IN 86-14, Supplement 1, "Overspeed Trips of AFW, HPCI and RCIC Turbines," 
which summarized the results of the AEOD study 

Recent operating experience has shown that overspeed trips of turbine-driven 
pumps continue to occur from the same basic causes identified in the AEOD 
report  A description of recent operating experience and a newly identified 
condition follows

Description of Circumstances:  

On November 13, November 29, and December 6, 1990, during three separate 
operational tests of the turbine-driven auxiliary feedwater (AFW) pump at 
Arkansas Nuclear One (ANO), Unit 2, the turbine tripped on overspeed during 

9108200172 


                                                  IN 86-14, Supplement 2
                                                  August 26, 1991
                                                  Page 2 of 5


initial acceleration  After each of the three overspeed trips, the licensee 
manually reset and successfully started the turbine several times  After 
the third overspeed trip, the licensee determined that fouling of components 
in the control oil system had caused the governor's response to be too slow 
to control the turbine's initial acceleration  Subsequent turbine starts 
were successful because the governor's components had been sufficiently 
exercised and loosened to permit faster response  

On June 18, 1990, during fast start surveillance testing at the LaSalle 
County Nuclear Station, Unit 1, the reactor core isolation cooling (RCIC) 
turbine tripped on overspeed and the licensee declared the system 
inoperable  The licensee determined that contaminated oil had fouled the 
components of the governor, slowing the governor's response and causing the 
turbine to trip on overspeed

In October 1990, the licensee at the Millstone Nuclear Power Station, Unit 
3, declared the turbine-driven AFW pump inoperable when the turbine tripped 
repeatedly on overspeed during testing  The licensee noted that, before 
each test start, the turbine was rolling because of steam leaking past the 
steam admission valves  The licensee determined that the turbine rolling 
caused oil to be admitted into the governor's speed setting cylinder which 
resulted in the overspeed trips 

Discussion:  

ANO-2 has one AFW pump powered by a motor and one powered by a steam 
turbine  Upon initiation of a start signal to the turbine-driven pump, a 
bypass valve around the normally closed isolation valve in the steam supply 
line to the turbine opens and the turbine accelerates to a minimum idle 
speed  Following a preset time delay, the isolation valve opens and the 
turbine governor valve positions to allow the turbine to accelerate to rated 
speed  The governor valve is positioned by an electronic governor type-R 
(EG-R) hydraulic actuator in conjunction with a remote servo valve  The 
EG-R actuator converts the electrical speed demand signal to a hydraulic 
signal which is then sent to the servo to adjust the governor valve's 
position  The hydraulic medium for the governor control system is filtered 
oil taken from the turbine lube oil system

After the December 6, 1990, overspeed trip, the licensee for ANO-2 brought a 
field representative of the Woodward Governor Company onsite to help 
determine the cause of the recurring overspeed trips  Upon examination, the 
control oil was found to be contaminated with dirt and grit  A thick 
gelatinous coating of dirt and hardened oil was observed on some governor 
components including the EG-R actuator and remote servo  The three 
overspeed trips resulted from contaminated oil that slowed the response of 
the governor  To correct this condition, the licensee changed the turbine 
lube oil, replaced the filter, cleaned the remote servo and control tubing, 
and replaced the EG-R actuator  The licensee tuned the governor to ensure 
proper response and successfully tested the turbine  The licensee declared 
the pump operable and returned it to service 

The ANO preventive maintenance (PM) program provided for sampling the 
turbine lube oil each month and for changing the lube oil and filter every 
six months  


                                                  IN 86-14, Supplement 2
                                                  August 26, 1991
                                                  Page 3 of 5


Maintenance records showed that the licensee had changed the lube oil and 
filter on September 2, 1990, approximately two months before the overspeed 
trip on November 13, 1990  However, the PM program did not provide for 
periodic inspections of the oil sump and other components of the governor 
control oil system  The vendor manual for the Terry Corporation turbine 
contained a note stating that oil used to fill the turbine lube oil system 
should be filtered through a 5-micron filter  The licensee had overlooked 
this note and had not performed this step when filling the system  Since 
the inline filter in the lube oil system is a 25-micron filter, the lube oil 
system contained a large quantity of particles of approximately 5 to 
25 microns  This condition and the low flow rate of oil through the 
governor resulted in a heavy accumulation of impurities in the governor  
Because the accumulation occurred over a period of years, the periodic oil 
sampling and changing of the oil and filter in the turbine lube oil system 
failed to control or detect the accumulation of particles inside the 
governor  The licensee revised the PM program to include periodic cleaning 
or replacement of the EG-R actuator and its associated remote servo valve  
The licensee plans to clean the turbine lube oil system during the next 
refueling outage  

On June 18, 1990, the licensee at LaSalle County Nuclear Station, Unit 1, 
identified a similar problem  During fast start testing, the RCIC turbine 
tripped on overspeed  The licensee's investigation included the removal and 
inspection of the EG-R actuator  The licensee found sediment inside the 
actuator and on the actuator's components  The licensee tested the oil for 
particles between 5 and 250 microns and found that the amount of these 
particles greatly exceeded allowable limits  To prevent the problem from 
recurring, the licensee revised the plant procedure to require the oil to be 
filtered before filling the turbine lube oil system  Also, the licensee 
will flush the oil system and disassemble, inspect, and clean the EG-R 
actuator during each outage

In October 1990, during testing of the turbine-driven AFW pump at Millstone 
Unit 3, the turbine tripped repeatedly on overspeed  The licensee noted 
that the turbine shaft was rotating before each of the test starts caused by 
steam leaking past the steam admission valve  The turbine rolling caused 
oil to be admitted into the governor's speed setting cylinder  The 
combination of the turbine's initial rolling and the position of the speed 
setting bushing was sufficient to cause the turbine to trip on overspeed 
during the turbine's initial acceleration  The licensee developed a 
maintenance program to eliminate the steam leaking past the admission valve 
and also to periodically check if the turbine is rolling

In AEOD report C602, the staff identified several turbine overspeed events 
related to oil contamination  The events at ANO and LaSalle have revealed 
an additional mechanism by which contaminated oil can cause turbine-driven 
pump overspeed trips  These events demonstrate that turbine governor 
control oil systems are sensitive to the accumulation of impurities in the 
oil or on surfaces exposed to the oil medium  To compensate for this 
sensitivity, licensees may wish to periodically examine and clean these 
critical components in addition to the traditional practice of periodically 
changing the lube oil and filters 



                                                  IN 86-14, Supplement 2
                                                  August 26, 1991
                                                  Page 4 of 5


In the AEOD report, the staff also noted that steam valve leakage and 
undrained condensate can cause overspeed trips  The staff listed three 
events of turbine overspeed caused by steam valve leakage  Those events 
occurred at the St Lucie Plant, Unit 2, the Crystal River Plant, Unit 3 and 
the Virgil C Summer Nuclear Station  The turbines at these three plants 
are equipped with Woodward PG-PL governors which are set to control turbine 
acceleration properly when the turbine starts from rest  These turbines 
tripped on overspeed because the turbines were rolling before being started 
which increased the oil pressure and caused oil to flow into the governors' 
speed setting cylinder  The oil pressure in the cylinder prevented the 
governor from responding fast enough to close the governor valve and control 
the initial turbine acceleration  This overspeed problem is not limited to 
the PG-PL type governor  Other types of Woodward governors that use a ramp 
bushing to control acceleration may also trip on overspeed  In addition, 
the increased oil pressure in the speed setting cylinder does not decrease 
immediately and must be released by locally exercising and resetting the 
speed setting knob  This characteristic may cause the turbine-driven pump 
to be unavailable for immediate starts or quick restarts 

At Crystal River, the licensee installed a modified governor with an 
automatic bleed feature to relieve oil pressure in the speed setting 
cylinder  This modification should prevent the turbine from tripping on 
overspeed as a result of the turbine rolling before the pump is started  

In the AEOD report, the staff identified nine turbine overspeed trip events 
that occurred as a result of undrained condensate in the turbine steam 
supply lines  Although steam lines are usually designed to separate and 
remove condensate, it is possible that during a cold start the condensate 
may not be separated or removed fast enough to prevent it from reaching the 
turbine  Because this condensate contains significantly less energy than an 
equivalent mass of steam, the turbine's initial acceleration is slower than 
expected  In response to the slower acceleration, the governor opens the 
governor valve further to allow more steam to enter  However, once the 
condensate clears, the governor cannot respond fast enough to prevent the 
turbine from tripping on overspeed  

The actual condition causing such an overspeed trip is often not determined 
because subsequent restarts are usually successful as the steam line has 
been heated and the condensate removed  To prevent similar trips, these 
plants increased the capacity of the condensate removal process or minimized 
the condensate formation by keeping the steam supply line in a hot and 
pressurized condition 

Previous Similar Occurrences:

The staff reviewed LERs received since the middle of 1985 and identified two 
turbine overspeed trips caused by undrained condensate  It is likely that 
other turbine overspeed trips have occurred but were not reported  The 
overspeed trips caused by undrained condensate occurred at San Onofre 
Nuclear Generating Station, Unit 2, in August 1990, and at the Crystal River 
Plant, Unit 3, in November 1986  The turbine overspeed trip at San Onofre 
occurred during testing  Initially, the licensee could not determine the 
cause of the 


                                                  IN 86-14, Supplement 2
                                                  August 26, 1991
                                                  Page 5 of 5


trip because subsequent restarts were successful  However, during a 
followup investigation, the licensee found that a procedural deficiency had 
resulted in  an isolation valve for a steam trap remaining closed after a 
previous outage  The licensee modified the procedure to ensure that the 
valve was properly aligned and added a program to check the steam drain 
system periodically  At Crystal River, the AFW system actuated 
automatically  The turbine-driven AFW pump started as required but 
immediately tripped on overspeed  The motor-driven AFW pump started 
normally and supplied feedwater to the steam generators  The licensee later 
found that the warmup line for the turbine throttle valve had been isolated 
for unknown reasons and had allowed the steam supply line to cool  
Condensate formed in the steam supply line and caused the turbine to trip on 
overspeed  The licensee revised procedures to ensure that the warmup line 
was not inadvertently isolated  

The NRC issued IN 86-14 and its supplement to alert addressees to the 
possibility that turbine-driven pumps could trip on overspeed and to 
summarize the results of AEOD report C602  However, the staff believes some 
licensees are not fully aware of the problem or may have inadequate programs 
to control the problem  AEOD is continuing to study the reliability of 
safety-related steam turbine-driven standby pumps to address the continuing 
repetitive failures of turbine assemblies  Further information will be 
issued to addressees if appropriate  

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 

                     William T LeFave, NRR 
                     (301) 492-3285 

Attachment:
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