Bulletin 85-03: Motor-Operated Valve Common Mode Failures During Plant Transients Due to Improper Switch Settings
SSINS No.: 6820
OMB No.: 3150-0011
Expiration Date: 08/01/88
IEB 85-03
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
November 15, 1985
IE BULLETIN NO. 85 03: MOTOR-OPERATED VALVE COMMON MODE FAILURES DURING
PLANT TRANSIENTS DUE TO IMPROPER SWITCH SETTINGS
Addressees:
All holders of nuclear power reactor operating licenses (OLs) or
construction permits (CPs) for action.
Purpose:
The purpose of this bulletin is to request licensees to develop and
implement a program to ensure that switch settings on certain safety-related
motor-operated valves are selected, set and maintained correctly to
accommodate the maximum differential pressures expected on these valves
during both normal and abnormal events within the design basis.
Description of Circumstances:
There have been two recent events, and a number of earlier events, during
which motor-operated valves failed on demand, in a common mode, due to
improper switch settings.
Event 1, Davis-Besse Plant - On June 9, 1985, the Davis-Besse Plant
experienced a complete loss of main and auxiliary feedwater. This event was
described previously in IE Information Notice No. 85-50, "Complete Loss of
Main and Auxiliary Feedwater at a PWR Designed by Babcock & Wilcox," and in
NUREG-1154, "Loss of Main and Auxiliary Feedwater Event at the Davis-Besse
Plant on June 9, 1985.'" Normally open, Limitorque motor-operated auxiliary
feedwater (AFW) gate valves failed to reopen on either an automatic or
manual signal from the main control room after they were inadvertently
closed during the event. While other failures also occurred in the AFW
system, the failure of these two valves was itself enough to prevent AFW
from reaching either steam generator. During the recovery from this event,
the valves were opened with the handwheels.
The results of licensee troubleshooting activities after the event led to
the conclusion that the setting for the torque switch bypass limit (torque
bypass) switch in each valve's control circuit had not been set to remain
closed long enough to provide the necessary bypass function on valve opening
with differential pressure conditions across the valve. During the event,
the valves experienced a high differential pressure after closing. The
torque bypass
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IEB 85-03
November 15, 1985
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switch on both valves was improperly set, causing the torque switches to
become operable prematurely. This condition stopped valve travel before the
valve discs were fully off their seats.
The torque bypass switches were set to drop out after the valves opened to
5% full-stroke. The 5% full-stroke setting was based on a number of
handwheel turns. In a 10 CFR 21 report, submitted subsequent to the event,
Toledo Edison Company identified two reasons why the torque bypass switch
settings were not adequate: (1) the 5% full-stroke settings were not
adequate for unseating the AFW system discharge valves with large
differential pressures across the valves and (2) the procedure for setting
this switch was defective in that the 5% full-stroke was not specified to be
in addition to the handwheel turns required to take up the motor-operator
coast and backlash. The torque bypass switch setting errors were revealed
only when high differential pressure conditions across the valves caused
higher loadings. The valve failures were reproduced during tests performed
by the licensee with differential pressures applied across the valves.
During the tests, the valves operated properly when low differential
pressures were applied across them, but failed to open when high
differential pressures were applied. The valves were instrumented during
these tests to obtain signature traces of critical parameters.
Event 2, Sequoyah Plant Unit 2 - An event involving partial loss of main
feedwater occurred on May 2, 1985, at Sequoyah Nuclear Plant Unit 2 while in
Mode 2 and returning to power after a reactor trip. Feedwater was being
supplied through the main feedwater (MFW) system isolation valve bypass
lines. Operators attempted to open the MFW system isolation valves to supply
water to the steam generators; however, two of the four MFW isolation valves
would not open. The startup was discontinued and the unit was returned to
hot shutdown.
During examination to determine the reason for the valve failures, the
licensee discovered that both valve stems had sheared from their discs. The
discs were found in the closed positioning within the valve seats. The stems
had suffered fracture failures through approximately three-quarters of the
diameter of the shafts, in addition to stress failures of the remaining
quarter. The Limitorque motor-operators on the valves use limit switches to
control valve motion in the open direction. These MFW system isolation
valves are large (18 inch diameter), fast acting (154 inches per minute
travel speed) valves. Because of the high speed of these valves and the
large mass of the discs, the selection of the limit switch setpoint needs to
account for the large momentum of the disc and its continued motion after
the limit switch deenergizes the valve motor-operator. The set point was not
correctly established and the disc impacted the backseat during opening. The
failure mechanism of these valves was identified by the licensee to be
impact loading of the stem on the opening stroke as a result of the disc
impacting the backseat, combined with a stress failure of the remaining
portion of the stem on the opening stroke. Main feedwater valves are not
included in the actions requested by this bulletin. The NRC is, however,
continuing to evaluate the Sequoyah event.
NRC Field Evaluation - As a part of the resolution of Generic Issue II.E.6
"In-Situ Testing of Valves," the NRC contracted with the Oak Ridge National
Laboratory in 1984 to perform a limited study to determine the effectiveness
of signature tracing techniques in determining the operational readiness of
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IEB 85-03
November 15, 1985
Page 3 of 6
safety-related motor-operated valves. It was hoped also that this study
could provide some insight as to current conditions of valve switch settings
at nuclear power plants. Signature traces of motor current, torque and limit
switch actuations and axial motion of the worm gear (an indication of
operator torque) were obtained from 36 motor-operated valves at 4 nuclear
plant sites. Although the formal technical report [NUREG/CR-4380 "Evaluation
of the Motor-Operated Valve Analysis and Test System (MOVATS) to Detect
Degradation, Incorrect Adjustments, and Other Abnormalities in
Motor-Operated Valves"] has not been issued, the current draft of the report
indicates that (1) this inspection method can be used to improve current
ASME methods and (2) there were abnormalities with nearly every valve
tested.
Table 1 contains a summary of the study's findings with respect to switch
setting abnormalities. Of particular interest with respect to the events
described above is the finding that 75% of the valves had improperly set
torque bypass switches (56% of the valves had the close-to-open torque
bypass switch set so that it was opening before the valve fully unseated)
and 8% of the valves were unintentionally backseating. The abnormalities in
Table 1 have not been fully evaluated at this time, and they should not be
interpreted to mean that any abnormality resulted in an inoperable valve.
Background:
The NRC has previously identified common mode failures, on demand, of
valves.
IE Circular No. 77-01, "Malfunction of Limitorque Valve Operators," reported
that on October 28, 1976, two motor-operated (Limitorque) valves located
between the refueling water storage tank and the charging pump suction at
the Trojan Nuclear Plant failed to open in response to a spurious safety
injection (SI) signal. The malfunction in both valves resulted from the
torque switch in the opening circuit becoming activated before the valves
were fully off their seats. The valves also were equipped with a torque
bypass switch. Each of the valves that malfunctioned was found to have its
torque bypass switch adjusted such that it allowed the torque switch to be
operable in the circuit before the valve was moved from its seat. The
licensee's investigation revealed that in each case the valve had been
manually closed hard on its seat following a maintenance operation.
Examination by the licensee revealed similar improper adjustments of the
torque bypass switches on several other motor-operated valves in
safety-related systems.
IE Information Notice No. 81-31, "Failure of Safety Injection Valves To
Operate Against Differential Pressure," reported on September 3, 1981, that
both trains of the San Onofre Unit 1 safety injection system were found to
be inoperable when challenged to operate against differential pressure.
Improperly set switches were the principal cause of these failures. There
were no adverse consequences in this particular event because there was no
accident that required safety injection. The reactor pressure remained above
the safety injection pump's shutoff head; therefore, no actual injection of
later would have occurred if the valves had opened. However, had reactor
pressure decreased and actual injection been required, injection flow would
not have been automatically available as designed. These valves had been
regularly tested at each
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IEB 85-03
November 15, 1985
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refueling outage, but the tests were not required to be performed with
differential pressure across these valves.
Florida Power Corporation reported an event at Crystal River Unit 3 in LER
77-9. During plant cooldown with the unit in hot shutdown, decay heat
removal valves in the decay heat removal pump suction would not open with
remote actuation. These failures were caused by pressure acting on the gate
valve discs. The valves were opened manually with the handwheels. The torque
switches were reset.
In addition to common mode valve failures on demand, there have been
numerous common mode failures discovered during testing or as a result of
investigating a single failure. NUREG/CR-2270, "Common Cause Fault Rates for
Valves," February 1983, contains reports of 99 common cause valve fault
events from 1976 through 1980.
The NRC has previously identified other problems with motor-operated valve
switches in Bulletin No. 72-3, "Limitorque Valve Operator Failures"; IE
Information Notice No. 79-03, "Limitorque Valve Geared Limit Switch
Lubricant; Circular No. 81-13, "Torque Switch Electrical Bypass Circuit for
Safeguard Service Valve Motors"; Information Notice No. 82-10, "Following Up
Symptomatic Repairs To Assure Resolution of the Problem"; and Information
Notice No.84-10, "Motor-operated Valve Torque Switches Set Below the
Manufacturer's Recommended Value."
The failure and potential failure of Westinghouse Electro-Mechanical
Division motor-operated gate valves to close are discussed in IE Bulletin
No. 81-02 and IE Bulletin No. 81-02, Supplement 1, "Failure of Gate Type
Valves To Close Against Differential Pressure."
Copies of the above referenced NRC bulletins, circulars and information
notices can be obtained from your local public document room.
Actions for All Holders of Operating Licenses or Construction Permits:
For motor-operated valves in the high pressure coolant injection/core spray
and emergency feedwater systems (RCIC for BWRs) that are required to be
tested for operational readiness in accordance with 10 CFR 50.55a(g),
develop and implement a program to ensure that valve operator switches are
selected, set and maintained properly. This should include the following
components:
a. Review and document the design basis for the operation of each valve.
This documentation should include the maximum differential pressure
expected during both opening and closing the valve for both normal and
abnormal events to the extent that these valve operations and events
are included in the existing, approved design basis, (i.e., the design
basis documented in pertinent licensee submittals such as FSAR analyses
and fully-approved operating and emergency procedures, etc). When
determining the maximum differential pressure, those single equipment
failures and inadvertent equipment operations (such as inadvertent
valve closures or openings) that are within the plant design basis
should be assumed.
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IEB 85-03
November 15, 1985
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b. Using the results from item a above, establish the correct switch
settings. This shall include a program to review and revise, as
necessary, the methods for selecting and setting all switches (i.e.,
torque, torque bypass, position limit, overload) for each valve
operation (opening and closing).
If the licensee determines that a valve is inoperable, the licensee
shall also make an appropriate justification for continued operation in
accordance with the applicable technical specification.
c. Individual valve settings shall be changed, as appropriate, to those
established in item b, above. Whether the valve setting is changed or
not, the valve will be demonstrated to be operable by testing the valve
at the maximum differential pressure determined in item a above with
the exception that testing motor-operated valves under conditions
simulating a break in the line containing the valve is not required.
Otherwise, justification should be provided for any cases where testing
with the maximum differential pressure cannot practicably be performed.
This justification should include the alternative to maximum
differential pressure testing which will be used to verify the correct
settings.
Note: This bulletin is not intended to establish a requirement for
valve testing for the condition simulating a break in the line
containing the valve. However, to the extent that such valve operation
is relied upon in the design basis, a break in the line containing the
valve should be considered in the analyses prescribed in items a and b
above. The resulting switch settings for pipe break conditions should
be verified, to the extent practical, by the same methods that would be
used to verify other settings (if any) that are not tested at the
maximum differential pressure.
Each valve shall be stroke tested, to the extent practical, to
verify,that the settings defined in item b above have been properly
implemented even if testing with differential pressure can not be
performed.
d. Prepare or revise procedures to ensure that correct switch settings are
determined and maintained throughout the life of the plant. Ensure
that applicable industry recommendations are considered in the
preparation of these procedures.
e. Within 180 days of the date of this bulletin, submit a written report
to the NRC that: (1) reports the results of item a and (2) contains the
program to accomplish items b through d above including a schedule for
completion of these items.
This item s intended to be completely consistent with action item 3.2,
"Post-Maintenance Testing (All Other Safety-Related Components)," of
Generic Letter 83-28, "Required Actions Based on Generic Implications
of Salem ATWS Events." These procedures should include provisions to
monitor valve performance to ensure the switch settings are correct.
This is particularly important if the torque or torque bypass switch
setting has been significantly raised above that required.
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IEB 85-03
November 15, 1985
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1. For plants with an OL, the schedule shall ensure that these items
are completed as soon as practical and within two years from the
date of this bulletin.
2. For plants with a CP, this schedule shall ensure that these items
are completed before the scheduled date for OL issuance or within
two years from the date of this bulletin, whichever is later.
f. Provide a written report on completion of the above program. This
report should provide (1) a verification of completion of the requested
program, (2) a summary of the findings as to valve operability prior to
any adjustments as a result of this bulletin, and (3) a summary of data
in accordance with Table 2, Suggested Data Summary Format. The NRC
staff intends to use this data to assist in the resolution of Generic
Issue II.E.6.1. This report shall be submitted to the NRC within 60
days of completion of the program. Table 2 should be expanded, if
appropriate, to include a summary of all data required to evaluate the
response to this bulletin.
The written reports shall be submitted to the appropriate Regional
Administrator under oath or affirmation under provisions of Section 182a,
Atomic Energy Act of 1954, as amended. Also, the original copy of the cover
letters and a copy of the reports shall be transmitted to the U.S. Nuclear
Regulatory Commission, Document Control Desk, Washington, DC 20555 for
reproduction and distribution.
This request for information was approved by the Office of Management and
Budget under a blanket clearance number 3150-0011. Comments on burden and
duplication may be directed to the Office of Management and Budget, Reports
Management, Room 3208, New Executive Office Building, Washington, DC 20503.
Although no specific request or requirement is intended, the time required
to complete each action item above would be helpful to the NRC in evaluating
the cost of this bulletin.
If you have any questions regarding this matter, please contact the Regional
Administrator of the appropriate NRC regional office or the technical
contact listed below.
James M. Taylor, Director
Office of Inspection and Enforcement
Technical Contacts: H. A. Bailey, IE R. J. Kiessel, IE
(301) 492-9006 (301) 492-8119
Attachments:
1. Table 1
2. Table 2
3. List of Recently Issued IE Bulletins
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Attachment 1
IEB 85-03
November 15, 1985
TABLE 1
Summary of Significant MOV Abnormalities
Bypass switch improperly set 75
Incorrect thrust 50
Unbalanced torque switch 33
Valve backseating 8
High motor current 3
Torque switch abnormalities 2
Miscellaneous abnormalities 33
Percent of valves experiencing abnormality. The total does not equal
100 percent as most valves had more than one abnormality.
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