Information Notice No. 88-09: Reduced Reliability of Steam-Driven Auxiliary Feedwater Pumps Caused by Instability of Woodward PG-PL Type Governors
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
March 18, 1988
Information Notice No. 88-09: REDUCED RELIABILITY OF STEAM-DRIVEN
AUXILIARY FEEDWATER PUMPS CAUSED BY
INSTABILITY OF WOODWARD PG-PL TYPE
All holders of operating licenses or construction permits for nuclear power
This information notice is provided to alert addressees to continuing problems
affecting the reliability of steam-driven auxiliary feedwater pumps caused by
instability problems with Woodward governors. It is expected that recipi-
ents will review the information for applicability to their facilities and
consider actions, if appropriate, to avoid similar problems. However, sugges-
tions 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 auxiliary feedwater pumps at Calvert Cliffs are powered by
Terry steam turbines (GS-2N) with Woodward PG-PL type governors. Before
July 23, 1987, periodic surveillance testing of the steam-driven auxiliary
feedwater pumps (AFWPs) was preceded by a warmup of the turbines before
initiation of the quick startup tests. On July 23, 1987, during a loss-
of-offsite-power event, the number 11 AFWP on Unit 1 tripped on its initial
demand as a result of turbine overspeed. To ensure that future periodic
(monthly) surveillance testing of the turbine-driven AFWPs would be conducted
under more realistic conditions, the test procedures were modified to require
quick starts from cold conditions. During subsequent tests in July through
October 1987, a number of trips of the steam-driven AFWPs occurred at Calvert
Cliffs. On July 30, 1987, during rapid cold startup testing, both of the Unit
2 steam-driven pumps tripped. On September 26, 1987, the number 11 AFWP on
Unit 1 began oscillating after the initial startup attempt and subsequently
tripped on overspeed. On October 23, 1987, the number 12 AFWP on Unit 1
tripped on overspeed.
The licensee conducted an intensive testing and troubleshooting program to
determine the causes of the failures. During these tests, a number of test
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March 18, 1988
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failures were experienced because of turbine governor oscillation and
overspeed. The most frequent failure sequences were either rapid initial
acceleration of the turbine to the overspeed trip point, or large undamped
speed oscillations that increased in magnitude to the overspeed trip point.
Less frequently, trips occurred when the mechanical latch mechanism holding
the trip valve open (which appeared to be overly sensitive) tripped.
Subsequent attempts to test the pumps immediately after initial steam-driven
AFWP failures were normally successful.
Several factors were identified which appear to have contributed to the
reduced reliability of the AFWPs. These include:
1. Use of governor buffer springs of less than optimal stiffness, resulting
in the inability of the governor to dampen out upset conditions. One of
the installed governors and all three of the spare governors had buffer
springs of a lower stiffness than that listed in the procurement specifi-
cation on file at the Woodward company.
2. Excessive condensate trapped in the steam supply lines, resulting in
governor valve damage, governor linkage damage, and throttle control
instabilities as slugs of water hit the governor valve and turbine wheel.
3. Improperly adjusted and degraded governor linkage, resulting in excessive
4. Governor valve binding, resulting in governor actuator over-reaction to
small feedback signals.
5. A failed governor on the Unit 2, number 22 AFWP.
6. Damaged and misaligned overspeed trip mechanisms, resulting in
oversensitivity to vibration, jarring, and waterhammer.
The licensee implemented several corrective actions and plans additional
upgrades. These are described below.
1. Stiffer buffer springs were installed in the governors of all AFWPs to
increase control system dampening at the expense of increased control
system response time.
2. Upgrading of both the procedures and the systems was initiated, which
included more thorough drain procedures and drain lineup verification.
The interval for manually draining the steamlines and turbine casings was
decreased from every 8 to every 4 hours. Additional manual drains were
installed in the system low points to eliminate water from the
3. Various parts of the governor valves, governor linkages, and trip
linkages were overhauled, adjusted, and replaced. Trip linkages
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March 18, 1988
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the overspeed mechanisms and the trip throttle valves were adjusted to
increase trip latch engagement and thereby reduce sensitivity to physical
shock. For some parts, such as linkage plates (cams), it was necessary
for the utility to obtain the special materials involved and fabricate
replacement parts in house.
4. Further steamline drain improvements are being evaluated.
The Calvert Cliffs problems highlight the importance of optimally sizing
buffer springs, since the single, most effective short-term corrective action
appeared to be installation of the stiffer buffer springs. However, changes
in spring stiffness for the purpose of improving stability can adversely
affect other governor response characteristics. Therefore, the selection of
optimal spring stiffness should be carefully considered. In addition, it is
important to ensure maintenance of proper spring stiffness following initial
determination of optimal stiffness. In the case of Calvert Cliffs, the
addition of stiffer springs appeared to provide an extra margin of stability.
This temporarily compensated for other auxiliary feedwater system deficiencies
that also required correction.
Reliability problems were much more evident when the auxiliary feedwater pumps
were periodically tested using quick starts from cold conditions. This demon-
strates the importance of surveillance testing which, in so far as practical,
duplicates the service conditions that would exist if the equipment were
called on to operate.
Information Notice 86-14, "PWR Auxiliary Feedwater Pump Turbine Control Prob-
lems," and 86-14, Supplement 1, "Overspeed Trips of AFW, HPCI, and RCIC Tur-
bines," discuss problems closely related to those discussed in this
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical
contact listed below or the Regional Administrator of the appropriate NRC
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: D. Limroth, RI
Attachment: List of Recently Issued NRC Information Notices
March 18, 1988
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
88-08 Chemical Reactions with 3/14/88 All NRC licensees
Radioactive Waste generating or pro-
Solidification Agents cessing low level
88-07 Inadvertent Transfer of 3/7/88 All NRC broad
Licensed Material to licensees and
Uncontrolled Locations licensees
material as sealed
88-06 Foreign Objects in Steam 2/29/88 All holders of OLs
Generators or CPs for PWRs.
88-05 Fire in Annunciator Control 2/11/88 All holders of OLs
Cabinets or CPs for nuclear
88-04 Inadequate Qualification 2/5/88 All holders of OLs
and Documentation of Fire or CPs for nuclear
Barrier Penetration Seals power reactors.
88-03 Cracks in Shroud Support 2/2/88 All holders of OLs
Access Hole Cover Welds or CPs for BWRs.
88-02 Lost or Stolen Gauges 2/2/88 All NRC licensees
under a specific
88-01 Safety Injection Pipe 1/27/88 All holders of OLs
Failure or CPs for nuclear
86-81, Broken External Closure 1/11/88 All holders of OLs
Supp. 1 Springs on Atwood & Morrill or CPs for nuclear
Main Steam Isolation Valves power reactors.
OL = Operating License
CP = Construction Permit
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