Information Notice 1986-14, Overspeed Trips of AFW, HPCI & RCIC Turbines
| ML031220596 | |
| Person / Time | |
|---|---|
| Issue date: | 08/26/1991 |
| From: | Rossi C Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| IN-86-014, Suppl 2, NUDOCS 9108200172 | |
| Download: ML031220596 (6) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 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 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
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
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 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 parti- cles 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 parti- cles 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 elimi- nate 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 overspedd 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 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
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 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 possibil- ity 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. Ifone
you have any questions about the information in this notice, please contact
of the technical contacts listed below or the appropriate NRR project manager.
Arles`E~.os~sei, 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:
List of Recently Issued NRC Information Notices
- ,81 Attachment
IN 86-14, Supplement 2 August 26, 1991 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
Information Date of
Notice No. Subject Issuance Issued to
91-51 Inadequate Fuse Control 08/20/91 All holders of OLs or CPs
Programs for nuclear power reactors.
91-50 A Review of Water Hammer 08/20/91 All holders of OLs or CPs
Events After 1985 for nuclear power reactors.
91-49 Enforcement of Safety 08/15/91 All Nuclear Regulatory Com- Requirements for Radiog- mission (NRC) licensees
raphers authorized to use sealed
sources for industrial
radiography.
91-48 False Certificates of Con- 08/09/91 All holders of OLs or CPs
formance Provided by West- for nuclear power reactors.
inghouse Electric Supply
Company for Refurbished Com- mercial-Grade Circuit
Breakers
91-47 Failure of Thermo-Lag 08/06/91 All holders of OLs or CPs
Fire Barrier Material to for nuclear power reactors.
Pass Fire Endurance Test
89-56, Questionable Certification 07/19/91 All holders of OLs or CPs
Supp. 2 of Material Supplied to for nuclear power reactors.
the Defense Department by
Nuclear Suppliers
91-46 Degradation of Emergency 07/18/91 All holders of OLs or CPs
Diesel Generator Fuel Oil for nuclear power reactors.
Delivery Systems
91-45 Possible Malfunction of 07/05/91 All holders of OLs or CPs
Westinghouse ARD, BFD, and for nuclear power reactors.
NBFD Relays, and A200 DC
and DPC 250 Magnetic Con- tactors
OL = Operating License
CP = Construction Permit