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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
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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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Page Last Reviewed/Updated Thursday, March 29, 2012
