Information Notice No. 96-48: Motor-Operated Valve Performance Issues
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
August 21, 1996
NRC INFORMATION NOTICE 96-48: MOTOR-OPERATED VALVE PERFORMANCE ISSUES
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to (1) lessons learned from the Electric Power
Research Institute (EPRI) Motor-Operated Valve (MOV) Performance Prediction
Program, (2) performance problems with MOV key failures described in a recent
NRC Office for Analysis and Evaluation of Operational Data (AEOD) study, and
(3) the potential for torque output from MOV actuators to be less than
predicted by Limitorque Corporation. 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 are not NRC requirements; therefore, no
specific action or written response is required.
Background
In the 1980s, continuing problems with the performance of MOVs at nuclear
power plants raised concerns regarding MOV design, testing, and maintenance.
In response to these problems, both the nuclear industry and NRC initiated
efforts to improve the performance of MOVs at nuclear plants. In 1989, the
NRC staff issued Generic Letter (GL) 89-10, "Safety-Related Motor-Operated
Valve Testing and Surveillance," requesting that nuclear power plant licensees
and construction permit holders verify the design-basis capability of their
safety-related MOVs. In response to GL 89-10, the nuclear industry has
studied the performance of MOVs through testing and analyses. As a result of
these activities, some weaknesses in the design and manufacture of MOVs were
discovered through evaluation of the performance history of MOVs. In this
information notice, the staff discusses three issues involving MOV performance
that have been identified.
Description of Circumstances
1. Lessons Learned from EPRI MOV Performance Prediction Program
As part of the industry effort regarding the MOV issue, EPRI initiated an MOV
Performance Prediction Program to develop a methodology to be used by
licensees in demonstrating the design-basis capability of MOVs when valve-
specific design-basis test data are not available. The program included
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development of improved methods for prediction or evaluation of system flow
parameters; gate, globe, and butterfly valve performance; and motor actuator
rate-of-loading effects (load-sensitive behavior). Further, EPRI performed
testing to provide information for refining the gate valve model and rate-of-
loading methods and conducted numerous MOV tests to provide data for model and
method development and validation, including flow loop testing, parametric
flow loop testing of butterfly valve disk designs, and in situ MOV testing.
In November 1994, the Nuclear Energy Institute (NEI) submitted the proprietary
EPRI Topical Report TR-103237, "EPRI MOV Performance Prediction Program -
Topical Report," for review by the NRC staff. EPRI prepared 25 additional
reports to support the topical report. On March 15, 1996, the NRC staff
issued a Safety Evaluation (SE) documenting the staff review of the topical
report. With the conditions and limitations described in the SE, the staff
stated that the EPRI program provides an acceptable methodology to predict the
thrust or torque required to operate gate, globe, and butterfly valves within
the scope of the EPRI program and to bound the effects of load-sensitive
behavior on motor actuator thrust output.
In a letter dated September 27, 1995, NEI forwarded a summary of important
contributions and findings resulting from the EPRI MOV Performance Prediction
Program. As described in an enclosure to the NEI letter, important findings
(or confirmatory information) from the EPRI MOV program include the following:
a. The traditional methods for predicting gate valve performance might be
nonconservative for many applications because of incomplete equations,
design features, manufacturing controls, and wide-ranging friction
coefficients.
b. The edge radii on disk seats and guide slots are critical to gate valve
performance and predictability.
c. Stellite friction coefficients increase with differential-pressure valve
strokes in cold water to a plateau level, stabilize quickly in hot
water, and decrease as differential pressure increases.
d. Gate valves with carbon steel guides and disk guide slots with tight
clearances might fail to close under blowdown conditions.
e. Many existing gate valve manufacturing and design processes and
controls, and plant maintenance practices, might contribute to poor
valve performance.
f. Traditional methods for predicting globe valve performance for
incompressible flow conditions are nonconservative for globe valves in
which differential pressure acts across the plug guide.
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g. Globe valve thrust requirements for some designs can be excessive under
compressible flow and blowdown conditions because of the potential for
plug-side loading.
h. Rate-of-loading effects (load-sensitive behavior) can reduce the static
thrust output by up to 30 percent under dynamic conditions.
i. Hydrodynamic torque coefficients used by some butterfly valve
manufacturers might be nonconservative for certain applications, with
valves located near piping elbows especially vulnerable.
j. Butterfly valve seats should be periodically replaced to avoid hardening
or degradation.
In addition to these reported important findings, EPRI confirmed that thrust
requirements to unwedge a gate valve can be higher under dynamic conditions
than under static conditions.
2. MOV Key Failures
On March 29, 1996, AEOD issued report AEOD/E96-01, entitled, "Engineering
Evaluation - Motor-Operated Valve Key Failures," on the continuing occurrence
of problems with keys in MOVs at nuclear power plants. A significant number
of MOV key failures have been identified that involved (a) anti-rotation keys,
(b) valve operator-to-valve stem keys, and (c) motor pinion gear keys. A
total of 73 reports were written involving MOV key failures between January
1990 through September 1995. Many of these key failures were not detected
during surveillance tests but were detected on demand, during valve
operations, or during maintenance activities and had existed for some time
before they were discovered. A number of key failures were discovered during
maintenance activities even though the valves had been operated satisfactorily
and passed all previous surveillance tests.
3. Limitorque Motor Actuator Performance
In 1977, Limitorque Corporation established guidelines (referred to as the SEL
documents) for sizing ac-powered motor actuators used in MOVs. Those
guidelines predicted the motor actuator output torque as a product of the
nominal motor-rated start torque, pullout efficiency, application factor
(typically 0.9), overall actuator gear ratio, and a degraded voltage factor.
Over the past few years, Limitorque has accepted the use of run efficiency for
closing valves powered by ac-powered motor actuators. Limitorque has also
stated that licensees may eliminate the application factor when voltage
supplied to the motor is less than 90 percent of its rated voltage. (See
Limitorque Technical Update 93-03 [Accession 9608120083].) Recent industry .
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and NRC-sponsored test information has raised questions regarding the accuracy
of the Limitorque assumptions for actuator efficiency.
Discussion
1. Lessons Learned from the EPRI MOV Performance Prediction Program
As discussed in the staff SE on the EPRI topical report, the EPRI program
provided important information on the design, testing, and maintenance of MOVs
in nuclear power plants. Some of the EPRI information is applicable to gate,
globe, and butterfly valves regardless of the type of actuator operating the
valve. Examples of such information are given below:
Gate Valves
Almost all flow testing by licensees in response to GL 89-10 was conducted
under pumped-flow conditions. Several gate valves tested by EPRI under
blowdown conditions demonstrated unpredictable performance and internal
damage. Extrapolation of test data from pumped-flow conditions to blowdown
conditions may not be sufficient to ensure that a gate valve can operate under
its design-basis conditions.
Valve aging conditions can influence gate valve performance. The thrust
requirements to operate gate valves under normal flow conditions can increase
with time and valve stroking.
Thrust requirements to unwedge gate valves under dynamic conditions may be
greater than under static conditions.
Globe Valves
Limited testing by EPRI of globe valves under blowdown or high-temperature
flow conditions suggested that higher thrust than typically predicted may be
required to operate these valves.
Thrust requirements for globe valves are influenced by the area of the valve
seat or guide, depending on the valve design.
The EPRI test database is not sufficient to justify modifying the Limitorque
guidelines for sizing and setting globe valves to lower the typical valve
factor of 1.1 assumed in the guidelines.
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Butterfly Valves
Several areas of the EPRI Butterfly Valve Application Guide need improvement
or correction. EPRI is currently revising the application guide and plans to
include new information on flow and torque coefficients; system analysis
techniques; treatment of bearing, packing, and hub-seal torque; upstream elbow
modeling; and rated and survivable torque calculations.
2. MOV Key Failures
The MOV key failures may involve a common-cause failure that could render
redundant trains of certain safety-related systems inoperable if they had
remained undetected. The MOV key failures can be attributed to (a) instal-
lation and design deficiencies for anti-rotation keys, (b) loosening or
slipping, wear or normal aging, excessive force or overtorque, and discre-
pancies in material or size for valve operator-to-valve stem keys, and (c)
high-impact loads, improper materials, installation deficiency, wear or normal
aging, and vibration for motor pinion gear keys.
The anti-rotation key failures involving installation deficiencies were
generally associated with inadequate staking and securing of setscrews during
installation of the keys. It appeared that the installation instructions
provided by the vendors were not always included in licensee maintenance
procedures.
Many motor pinion gear key failures involving an installation deficiency were
due to failure to stake the keys following replacement of the motors or the
pinion gears. Although licensees revised their MOV maintenance procedures to
include restaking the pinion key or motor shaft as recommended by Limitorque
Maintenance Update 89-1 (Accession 9608120068), many licensees did not
investigate the potential problems of maintenance activities that were
conducted before their procedure changes.
The motor pinion gear key failures attributable to high-impact loads or
improper material appear to involve AISI (American Iron and Steel Institute)
type 1018 keys in high-speed and high-inertia configurations. The replacement
of 1018 keys with harder 4140 keys in some cases may lead to keyway deforma-
tion or damage, depending on impact loads and the shaft material. The situa-
tion may present a complex stress problem that is not completely considered in
design and could produce a severe and complex stress concentration on the key,
as well as the keyway. This situation could lead to cracking and failure of
the shaft.
The potential for these key problems to render safety systems inoperable empa-
sizes (a) the importance of plant maintenance programs in assuring that MOV
keys are staked and secured as required, (b) the importance of plant MOV
surveillance and maintenance activities in the early detection of key .
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degradation, and (c) the possibility of shaft cracking as a result of
replacement of 1018 keys with harder material when the replacement will
involve a relatively soft shaft and high impact loads.
3. Limitorque Motor Actuator Performance
The NRC staff conducted an inspection at Limitorque in May 1993 and reviewed
the basis for its motor actuator sizing guidelines. As discussed in NRC
Inspection Report 99900100/93-01, the staff found that the values for
individual parameters assumed in the Limitorque sizing equation were not
determined by testing but were founded primarily on engineering judgment. The
lack of significant failure history of motor actuators when using the
Limitorque sizing equation has been the primary basis for confidence in the
equation. Licensee modifications of the parameters in the Limitorque sizing
guidelines has the potential to influence performance.
Tests of MOVs under differential pressure and flow conditions performed by
licensees in response to GL 89-10 have revealed that significantly more torque
and thrust are required to open and close many gate valves than predicted by
the valve vendors. This need for more torque than originally believed has led
licensees to evaluate the Limitorque motor actuator sizing guidelines to
determine whether more torque output is available from the motor actuators
than was predicted by the guidelines. The Limitorque sizing guidelines have
typically been assumed to underestimate the output torque capability of motor
actuators. Therefore, some licensees eliminate the application factor from
the output torque equation and use run efficiency for ac-powered MOVs in the
closing direction. Further, some licensees have asserted that motor torque
greater than the nominal start rating may be assumed in the sizing guidelines
because motors typically deliver more torque than their rating before they
stall.
In response to the questions surrounding the Limitorque sizing equation, the
NRC Office of Nuclear Regulatory Research evaluated the performance of
Limitorque motor actuators through testing at the Idaho National Engineering
Laboratory (INEL). Preliminary results of the INEL tests suggest that (1)
motor output is greater than the nominal rating for many motors, (2) the
actual output efficiency may not reach "run" efficiency for some Limitorque
actuators and may drop below "pullout" efficiency under high loads, (3) the
torque loss under degraded voltage conditions can be more severe for some ac
motors than the typically assumed square of the ratio of actual voltage to
rated voltage, and (4) the torque loss under degraded voltage conditions can
be more severe for some dc motors than the typically assumed linear ratio.
Preliminary results of this testing are documented in NUREG/CR-6100, "Gate
Valve and Motor-Operator Research Findings" (September 1995). INEL is
preparing a report, NUREG/CR-6478, to document its recent findings in this
area. This report is scheduled to be issued by the end of 1996.
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At meetings of the Motor-Operated Valve Users' Group of nuclear power plant
licensees in February and July 1995, Commonwealth Edison (ComEd) presented the
results of its motor and actuator output testing program. The testing
conducted by ComEd was more extensive than the NRC-sponsored testing and
revealed similar results. Previously, in NUREG/CP-0137, "Proceedings of the
Third NRC/ASME Symposium on Valve and Pump Testing" (July 1994), motor
actuator testing by Texas Utilities raised questions regarding Limitorque
motor actuator output. Texas Utilities also found lower output during in situ
motor actuator testing compared to torque stand testing.
This information raises concerns regarding the basis for Limitorque acceptance
of licensee assumptions that the torque output of its actuators is greater
than predicted by the original Limitorque SEL guidelines. The NRC staff has
been discussing with Limitorque the discrepancy between guidance relaxing the
original motor actuator sizing criteria and the recent motor actuator test
results. The manufacturer has stated that updated information for the
industry on the sizing of its motor actuators is being developed.
Related Generic Communications
. NRC IN 81-08, "Repetitive Failures of Limitorque Operator SMB-4
Motor-to-Shaft Key," March 20, 1981 (Accession 8011040272).
. NRC IN 88-84, "Defective Motor Shaft Keys in Limitorque Motor
Actuators," October 20, 1988 (Accession 8810140018).
. NRC GL 89-10, "Safety-Related Motor-Operated Valve Testing and
Surveillance," June 28, 1989 (Accession 8906290082).
. NRC IN 90-37, "Sheared Pinion Gear-to-Shaft Keys in Limitorque Motor
Actuators," May 24, 1990 (Accession 9005180095).
. NRC IN 90-40, "Results of NRC-Sponsored Testing of Motor-Operated
Valves," June 5, 1990 (Accession 9005290270).
. NRC IN 93-42, "Failure of Anti-Rotation Keys in Motor-Operated Valves
Manufactured by Velan," June 9, 1993 (Accession 9306030147).
. NRC IN 93-88, "Status of Motor-Operated Valve Performance Prediction
Program by the Electric Power Research Institute," November 30, 1993
(Accession 93111904527).
. NRC IN 94-10, "Failures of Motor-Operated Valve Electric Power Train Due
to Sheared or Dislodged Motor Pinion Gear Key," February 4, 1994
(Accession 9402010052).
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. NRC IN 94-69, "Potential Inadequacies in the Prediction of Torque
Requirements for and Torque Output of Motor-Operated Butterfly Valves,"
September 28, 1994 (Accession 9409210211).
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 Office of
Nuclear Reactor Regulation (NRR) project manager.
signed by B.K. Grimes
Thomas T. Martin, Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Thomas G. Scarbrough, NRR Chuck Hsu, AEOD
(301) 415-2794 (301) 415-6356
Email: tgs@nrc.gov Email: cch1@nrc.gov
Michael T. Bugg, RIII
(630) 829-9500
Email: mtb@nrc.gov
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