ML20044C453
| ML20044C453 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 03/16/1993 |
| From: | Westerman T NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV) |
| To: | |
| Shared Package | |
| ML20044C448 | List: |
| References | |
| 50-498-93-08, 50-498-93-8, 50-499-93-08, 50-499-93-8, NUDOCS 9303230053 | |
| Download: ML20044C453 (16) | |
See also: IR 05000498/1993008
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APPENDIX
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U.S. NUCLEAR' REGULATORY C0fMISSION
REGION IV
Inspection Report: 50-498/93-08
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50-499/93-08
.0perating Licenses: NPF-76-
Licensee: Houston Lighting & Power Company
P.O. Box 1700
Houston, Texas 77251
Facility Name: South Texas Project Electric Generating Station-(STPEGS),-
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Units 1 and 2
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Inspection At: Matagorda County, Texas
Inspection Conducted: -February 17-19 and 23-26, 1993
Inspector:
M. F. Runyan, Reactor Inspector,' Engineering Section
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Division of Reactor Safety
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Approved:
7b
$7-- / 6-18 '
T.F. Westerman, Chief. Engineering Section.
Date
Division of Reactor' Safety'
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Inspection Summary
Areas Inspected: Nonroutine, announced, special! inspection'of. technical. issues
associated with the failure of motor-operated valve SI-31A; Unit 2, and thel
licensee's identification of five Unit 1: residual heat removal system-
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. motor-operated valves that were experiencing excessive torque. .
Results:
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The licensee identified that Unit 2.had operated from April:1989 to-
October 1990 with valve SI-31A inoperable due to a burned:out motor.
During that' time period, .the. licensee would have been unable to initiate
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hot leg. recirculation on the "A"
train of low head safety injection.
This condition was in. violation of Technical. Specification 3.5.2.
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item was identified as an apparent violation.(SectionJ1.1).
The licensee ~did'not undertake corrective actions following a 1989
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failure of valve SI-31A, Unit 2, to prevent recurrence' of the event.
The same valve failed under similar circumstances -in February 1993.
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9303230053 930317
PDR. ADOCK 05000498
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This item was identified as an apparent violation of 10 CFR 50,
Appendix B, Criterion XVI (Section 1.1).
Preliminary findings indicated that the failure of Valve SI-31A may have
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been due to hydraulic lock of the actuator springpack, thermal binding,
or wedging of the valve stem bearing block,- or a combination of these
causes (Section 1.1).
During the sequence of events following the valve failure, SI-31A may
have been torqued in excess of its actuator rating by application of
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excessive force to the manual handwheel. At the time of the inspection,
the licensee had not addressed this potential problem (Section 1.1).
The inspection frequency of actuator springpacks may not be sufficient ~
to anticipate conditions leading to hydraulic lock (Section 1.1).
A future NRC inspection of the adequacy of the licensee's. lubrication
program will be tracked by an inspection followup item (Section 1.1).
The licensee identified that five Unit I residual heat removal suction
isolation valves had been torqued to levels exceeding'110' percent of the
nominal actuator rating for approximately 50 cycles (Section 1.2).
The apparent failure to provide a proper operability determination for -
the five residual heat removal valves was identified as-an apparent.
violation of 10 CFR 50, Appendix B, Criterion XVI (Section 1.2). This
judgment was based on the fact that there are no vendor or industry
rerating programs providing for the acceptance of motor-operated valves
in an overtorqued condition.
The apparent unacceptable operability determination of the overtorque-
condition was similar to a previous violation issued for unacceptable
determinations of operability for valves that were subject to excessive
thrust (Section 1.2).
In the five affected residual heat removal motor-operated valves, the
licensee replaced several major components that are generally considered
susceptible to the application of large amounts of torque (Section 1.2).
The licensee lowered the maximum expected differential pressure of the
subject residual heat removal valves; this action was considered
temporarily acceptable but will eventually require further justification
(Section 1.2).
The licensee retested the residual' heat removal valves and determined
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that they were no_ longer in an overtorqued condition. The valves were
declared operable (Section 1.2).
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Concerns were identified in the areas of stem factor considerations,
degraded voltage capability, underthrusting, and excessive packing
loads. These issues were identified as an inspection followup item
(Sections 1.2.1 through 1.2.4).
Summary of Inspection Findinos:
Apparent Violation 499/9308-01 was opened (Section 1.1);
Apparent Violation 499/9308-02 was opened (Section 1.1);
Inspection Followup item 498;499/9308-03 was opened (Section'l.1)
Apparent Violation 498/9308-04 was opened (Section 1.2); and
Inspection Followup Item 498;499/9308-05 was opened (Section 1.2).
Attachment:
Attachment - Persons Contacted and Exit Meeting
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DETAILS
1 FOLLOWUP AND MAINTENANCE PROGRAM IMPLEMENTATION (92701,62700)
1.1 Valve A2SIMOV0031A Motor Failung
On February 9,1993, Unit 2 Motor-Operated Valve (MOV) A2SIMOV0031A (SI-31A),
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failed to open on demand from the control room. Subsequently, oil and grease
were observed to be leaking from the gear casing and the valve motor was
emitting smoke. The licensee determined that the valve motor had burned up in
a failed attempt to lift the valve disc off the closed seat.
Valve SI-31A functions as the cold leg injection isolation for the A train
residual heat removal (RHR) and low head safety injection (LHSI) pumps.
The
valve is normally open with power removed at the breaker. The primary safety
function of Valve SI-31A is to close approximately 13 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br /> following a
loss-of-coolant accident to establish hot leg recirculation.
Valve SI-31A is
an 8-inch flexible wedge gate valve driven by a Limitorque SB-1 actuator and a
60 foot-pound Reliance motor.
Just prior to this event, RHR Pump 2A had developed a leak of 0.5 gallons per
minute (gpm) from the pump casing flange. The licensee established shutdown
cooling.on an alternate RHR train and closed Valve _ SI-31A from the control
room. The valve closed successfully with no unusual indications. RHR Pump 2A
was secured. The A train RHR suction valves remained open during this
sequence of events. To place A train RHR in the normal standby mode, the
licensee attempted to open Valve SI-31A from the control room. The attempt to
open the valve occurred approximately 10 minutes after it had been closed.
The control room operator did not receive the normal dual ir.dication (open and
closed) lights on the control board. An operator was dispatched to the valve
and reported that the motor was smoking and that oil'and grease were leaking
from the gear casing. Another operator went to the valve breaker and noted
that the breakor had tripped open. The breaker had tripped on a thermal
overload sized to protect the electrical penetration. The thermal overloads
associated with protection of the motor were bypassed and only provided
indication in the control room. The control room operators noted that the
motor thermal overload lights had illuminated; however, these warning lights
did not provide an opportunity for the operators to protect the motor from
overheating. The configuration of overload protection associated with
Valve SI-31A is typical of all MOVs at STP. After these events, plant
operators attempted to open Valve SI-31A manually and noted that excessive
force was needed to dislodge the disc from the valve seat.
In fact, this
effort required two operators in order to apply sufficient force to the valve
actuator handwheel. Once the disc was off the seat, the valve moved freely.
After verifying that the motor had failed, the licensee replaced it with an
1dentical motor.
For post-maintenance testing, the licensee performed a
H0 VATS diagnostic test and verified that valve operation was in specification.
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The valve was declared operable. The licensee initiated Station Problem
Report (SPR) 93-0443 to investigate the cause of the motor failure. During
the M0 VATS test ag, the licensee noted that the area between t% actuator
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springpack washe,s was thoroughly packed with partially hardeneo and slightly
discolored grease. The springpack was cleaned and retested. Other than the
burned motor and greased springpack, no other discrepancies were noted.
However, the licensee did not perform a detailed inspection of the actuator
prior to returning the valve to service.
Valve SI-31A in Unit 2 had failed previously under what may have been
identical circumstances. The motor for this valve burned up in April 1989, in
a failed attempt to open the valve. At that time, Work Request.SI-72428 was
issued to replace the motor. No station problem report was issued;
consequently, no effort was undertaken to determine the root cause.
For
reasons not fully understood by the licensee, the valve motor was not replaced
until October 1990, under the first revision of the original work request.
Unit 2, therefore, operated for a complete fuel cycle with Valve SI-31A '
inoperable. The valve was left in its normally open position with power
removed during this time. Had a loss-of-coolant accident occurred while
VaIve SI-31A was in this condition, the licensee would have been unable to
place A train LHSI in ~ hot leg recirculation after the accident because
Valve SI-31A would not have been accessible in a post-accident environment.
Since this condition would have prevented the LHSI pump from performing one of
its intended safety functions, the licensee reported under 10 CFR 50.73 that
the LHSI system was inoperable for a period in excess of the 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />
permitted by Technical Specification 3.5.2.
SPR 93-0469, initiated to
investigate this incident, was still open at the conclusion of the inspection.
A review of the licensee's disposition of the Valve SI-31A valve motor failure
that occurred in April 1989 identified two apparent violations, as follows:
(1)
The licensee operated in violation of Technical Specification 3.5.2 for
an entire fuel cycle. (apparent violation 499/9308-01)
(2)
The licensee failed to acceptably investigate the April 1989 valve
failure, correct the condition in a timely manner, determine the root
cause of the event, and take actions to prevent recurrence, contrary to
10 CFR 50, Appendix B, Criterion XVI, " Corrective Action."
Consequently, the same valve failed under similar circumstances in
February 1993. (apparent violation 499/9308-02)
In January 1988, Valve SI-31A in Unit I experienced a motor burn-up during an
opening cycle.
In this case, however, the valve was reported to have lifted
off its closed seat for a short distance anJ both indicating lights in-the
control room had come on indicating such travel. The investigation
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(SPR 88-0030) revealed the most likely cause to be overheating due to
excessive stroking over a short period of time. Because the valve disc
successfully dislodged from the closed seat, it is not likely that the Unit i
Valve SI-31A motor burn-up was analogous to the two subsequent events in
Unit 2.
The inspector questioned the licensee whether any other MOVs had experienced
motor failure during an attempted or actual cpening stroke. The licensee
stated that the only other such incidents (other than Valve SI-31A in Unit I
and the two Valve SI-31A events in Unit 2) occurred in 1988 and 1990 when the
motors for Valve CICVMOV0006 '(CV-06) in Unit I and Valve C2CVMOV0006 (CV-06)
in Unit 2, respectively, failed while attempting.to open the valve. The root
cause of these two valve failures was determined to be pressure locking of the
valve bonnet area, which had been subjected to an increase in temperature
after the valve was closed. Both of these valves were modified by drilling
holes to equalize pressure between the bonnet area and the downstream piping.
During the inspection, the licensee was in the process of determining the root
cause of the more recent failure of Valve SI-31A. This effort was to be
documented in SPR 93-0443. The licensee had retained the services of.a
consultant to aid in the investigation. A representative' of the consultant
informed the inspector that preliminary findings had all but ruled out
pressure locking as the cause of the Valve SI-31A motor failure. . This initial
conclusion was based on a review of temperature and pressure profiles which
did not suggest the potential for valve bonnet pressurization. The consultant
stated that the following three factors separately or in combination may have
caused the event:
(1)
Hydraulic lock of the actuator springpack. Based on the as-found
condition of the springpack (completely filled with hardened and .
discolored grease), the force of closure may have been greater than
normal and the motor may have been partially damaged or overheated
during the previous closing cycle. This would have been caused by the
extra power the motor would have been required to generate to trip the
torque switch. The valve was modified shortly following this event to
close on the limit switch. NRC will conduct a future inspection to
review the adequacy of the licensee's lubrication program. This
inspection will be tracked by an inspection followup item
(498;499/9308-03).
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(2)
Thermal binding. The valve cooled very rapidly after being closed. As
a result, the disc may have become bound in the seat.
(3)
Wedging of the valve stem bearing block on the valve disc. This was
only a speculation based on an unusual indication on the valve
diagnostic trace.
The inspector did not identify any matters of immediate concern regarding the
licensee's handling of the most recent failure of Valve SI-31A. However, one
issue was identified to the licensee as a potential operability concern. This
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hydraulic lock of the springpack) and/or when the valve was manually opened by.
the two operators.
Based on rough calculations and.Limitorque vendor
information, the inspector concluded that the more likely of the two scenarios
to cause an overtorque condition was the manual handwheel opening stroke.
Depending on the handwheel configuration and the force applied by the two
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operators, torque in excess of 200 percent of the nominal actuator rating
(850 foot-pounds) may have been applied in the effort to unseat the valve. Of
additional concern, the licensee had not addressed this consideration in the
scope of its initial corrective actions. The valve internals were not-
inspected prior to returning the valve to an operable status. The licensee
stated that the potential overtorquing of Valve SI-31A would be investigated
and all necessary corrective actions would be taken during the ongoing Unit.2
refueling outage.
The inspector reviewed the licensee's response to information disseminated by
Limitorque discussing the subject of hydraulic lock of the springpack. The
licensee determined in their review process that every one of their
s,)ringpacks was of the newer design that included an internal grease relief
from the springpack back to the gear casing.
Limitorque had stated that
hydraulic lock should not occur in a springpack of this design, but that if
the grease were permitted to harden, the relief function would be defeated.
In this case the grease appeared to be of sufficient hardness to prevent flow -
through the relief valve.
The licensee stated that this was the first
apparent incident on springpack hydraulic lock at this site. The grease that
had migrated into the springpack of Valve SI-31A was approximately 5 years in
age.
In the licensee's MOV program, each springpack is inspected
approximately once every 5 years. The inspector discussed with the licensee
the possibility that the 5-year inspection frequency of actuator springpacks
may not be sufficient to detect hardening of grease in time to prevent
additional occurrences of hydraulic lock. The licensee stated that this issue
would be reviewed within the context of the corrective action plan developed
in response to this event.
1.2
Overtoroue of Unit 1 RHR Suction Valves
On February 2,1993, while closing out the review of MOV diagnostic test data
from the recently concluded 1RE04 refueling outage, the licensee discovered
that all six Unit 1 RHR suction isolation valves were set' in the closing
direction at torque levels that exceeded the manufacturer's rated torque
capacity of the valve actuator. These valves are 12-inch flexible wedge gate
valves and are driven _ by a Limitorque SB-2 actuator and a 60 foot-pound
Reliance motor, which together operate at an overall actuator ratio of 150:1.
The nominal torque structural rating of the actuator is 1250 foot-pounds.
Limitorque has authorized operation of its actuators up to 110 percent of the
nominal rating to account for inertial loads that develop after motor cutoff.
The RHR suction isolation valves are normally closed. When the unit is shut'
down, these valves are opened to establish the lineup for long-term shutdown
cooling. The primary active safety function of these valves is to close to
isolate a downstream line break in the RHR system during shutdown conditions.
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The primary passive safety function is to prevent the RHR system from
overpressurization during normal operations.
The licensee calculated the applied torque levels, including allowance for
diagnostic system measurement inaccuracy, to be as follows:
Valve Number
Closino Toroue (ft.-lbs.)
Percent of Ratina
AIRHMOY0060A
1772.2
142
BIRHMOV0060B
1788.6
143
CIRHMOV0060C
1262.7
101
BIRHMOV0061A
3204.0
256
CIRHMOV0061B
2869.1
230
AIRHMOV0061C
2802.1
224
in a similar manner, the licensee checked the torque levels on the
corresponding valves in Unit 2 and found that only one of the six valves had
been overtorquel, but that this valve was within 110 percent of nominal torque
as permitted by Limitorque. Therefore, the licensee concluded that the Unit 2
RHR suction isolation valves were acceptable.
The licensee initiated SPR 93-0365 to investigate the Unit-1 overtorque
situation and to determine whether the valves should be considered operable.
Based on the results of the inspections of valve internals performed during
the IRE 04 refueling outage, the licensee determined that the valves could
perform their design function, but would be limited to a design duty of less
than 2000 cycles as originally designed.
Based on previous test results and
records'of maintenance and operations, the licensee determined'that the six
RHR valves had been overtorqued to the levels shown above for approximately
50 cycles. The licensee contacted Westinghouse, the valve supplier, and
received a calculation predicting the torque capability of the valves. The
licensee declared the valves operable with the intention of continuing
operation until the next refueling outage (Spring 1994) before taking actions
to reduce the torque applied to the actuators. Unit I was shut down on
February 4,1993, due to unrelated problems, and the licensee decided to take
advantage of this opportunity to correct the overtorque condition.
The inspector focused on the validity of the licensee's initial determination
that the valves were operable. Of concern, was the fact that no industry
testing has been performed to uprate the torque structural limits of
Limitorque actuators. Several qualification testing programs have increased
the qualified thrust carrying capability of the actuators, but these programs
have consistently stipulated that the torque-levels should be maintained at or
below the vendor torque ratings (including a 10 percent allowance for inertial
loads). The inspector reviewed the calculation Westinghouse supplied in
response to this event.
Based on a scaling model of the cross-sectional areas
of the worm gears of previously tested MOVs to the valves in question,
Westinghouse calculated that the actuator worm gears could withstand a torque
load of 1880 foot-pounds for a total of 10 cycles. The inspector questioned
how a calculation increasing the qualified actuator torque to 1880 foot-pounds
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of the worm gears of previously tested MOVs to the valves in question,
Westinghouse calculated that the actuator worm gears could withstand a torque
load of 1880 foot-pounds for a total of 10 cycles. The inspector questioned
how a calculation increasing the qualified actuator torque to 1880 foot-pounds
for 10 cycles could be used to justify continued operation of actuators that
had been torqued to as much as 3204 foot-pounds for 50 cycles. The licensee
stated that the Westinghouse calculation was used only on a supplementary
informational basis to lend additional support to the inspection results of
the valve internals performed during the previous outage. The valve internal
inspections, therefore, provided the actual basis for the operability
determination.
The inspector reviewed several of the work packages from the IRE 04 refueling
outage associated with the valves in question and noted that several
observations were recorded of " excessive" wear on the worm gear, a component
highly vulnerable to the application of large amounts of torque. Despite
these work package comments, none of the worm gears were replaced. After the
ensuing forced outage of February 4, the worm gears of five of the six valves
were removed and at least three of them were observed to show signs of
accelerated wear and improper alignment to the worm (the inspection of two of
the worm gears occurred after the conclusion of the inspection).
In consideration of the issues discussed above, the inspector concluded that
the licensee had apparently failed to make a proper operability determination
of February 2, 1993.
Even had the. actuator inspection ~ results fro::; the
IRE 04 refueling outage shown no signs of unusual wear and had the Westinghouse
calculation encompassed the measured torque values, the basis for considering
the valves operable would still have been challenged due to the long-term,
consistent, and well-documented position of Limitorque, the NRC, and the
industry that Limitorque actuators should not be overtorqued.
stated in Limitorque Maintenance Update 92-01 that their actuators can be
torqued on a one-time basis up to 200 percent of the nominal rating without
damage or sacrifice to the actuator qualification.
In this same document,
limitorque recommended that if the 200 percent torque limit is exceeded or if
an overload occurred more than once, that an inspection of the actuator should
be performed to determine if any damage occurred and.if the actuator is
suitable for continued service.
In this case, three of the actuators had been
torqued over 200 percent of the nominal rating for approximately 50 cycles,
and no deliberately focused evaluation of the torque-carrying components had
ever been performed (i.e., only general inspections had been performed, none
with the expressed intent of evaluating the effects of overtorque).
The operability determination of February 2 and the failure to take_immediate
corrective actions to address the overtorque problem of the RHR suction
isolation valves is considered to be an apparent violation of 10 CFR 50,
Appendix B, Criterion XVI, " Corrective Action" (apparent violation
498/9308-04).
Based on a review of a previous enforcement issue, the inspector determined
that the apparent violation discussed above is similar to a violation of
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was based on the licensee's unacceptable justification for continuing
operations with MOVs left in a condition that would result in delivered thrust
exceeding the actuator thrust structural rating. At the time, there were two-
preliminary third-party test results indicating that the licensee's measured
thrust levels' would eventually fall within an expanded qualified thrust range.
However, the relevant test studies were still under evaluation and had not
been officially promulgated. Therefore, it was determined that the licensee's.
operability determination was invalid at the time that it was made. The
overtorque issue is analogous to the overthrust issue of a year earlier, in.
that both involved placing stress on the actuators in excess of the' qualified
ratings. Of special note, the current issue of overtorque it considered more
serious than the previous overthrust issue because there does not exist even
preliminary test results enveloping the observed conditions.
Further, while
it was recognized that the overthrust situation existed at the time the
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affected actuators were visually inspected (suggesting that the inspection
would be appropriately focused to address that concern), the overtorque
problem was not recognized until after the inspections performed during the
last refueling outage had been accomplished, thus,' rendering those inspections
less effective in the evaluation of operability.
After Unit 1 entered a forced outage on February 4,1993, for reasons
unrelated to the RHR MOV prob 1_ ems, the licensee decided to undertake actions
to address the overtorque situation. Beginning with the "B"
train valves,
five of the six actuators were disassembled and inspected. Valve CIRHMOV0060C
(RH-60C) was not included in the scope of work because its calculated torque
of 1262.7 foot-pounds only slightly exceeded the actuator rating of
1250 foot-pounds and was well within the margin allowed for inertial' loading
as previously discussed. For each of the remaining five valves, the licensee
replaced the wormshaft, worm, and worm gear, which comprise some of the
components most susceptible to excessive torque loads.. In soms of the valves,
including at least _the two B train RHR valves,- the stem nuts were replaced.
The stem nuts in each of-these valves exhibited an unusual groove running down
the middle of the thread face, perhaps caused by a foreign particle lodged
between the stem nut and the. stem.
In Valve CIRHMOV0061B (RH-61B), an
inspection of the disassembled parts revealed a misalignment of the worm and
the worm gear.
The worm was riding high on the worm gear and causing
accelerated wear of the worm gear.
In Valve BIRHM0V0060B (RH-608), a
misalignment of the stem to the actuator was observed. Each of the conditions
discussed above could have contributed to the high: torque values that were
observed.
The calculated stem factors (torque divided by' thrust) were as high as 0.045
(for Valve RH-61A, corresponding to a stem friction coefficient of 0.28) as
compared to the assumed value of 0.029) (corresponding to a stem friction
coefficient of 0.15). As a result, the actuator had to apply additional
torque to deliver the same amount of thrust to the valve stem. Since the
valves were set up to close on a limit switch (which the licensee attempts to
set at' a valve _ disc position that corresponds to the calculated thrust
requirement)-and the-torque switch was removed from the motor circuit, there
was no backup protection against overtorquing the actuators. Often, high stem
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valves were set up to close on a limit switch (which the licensee attempts to
set at a valve disc position that corresponds to the calculated thrust
requirement) and the torque switch was removed from the motor circuit, there
was no backup protection against overtorquing the actuators. Often, high stem
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factors are associated with inadequate stem lubrication, but for these valves
that did not appear to be the case. After a fresh lubrication, which preceded
the replacement of parts, the valves were retested with very little change
noted in the measured torque levels.
In addition to the replacement of some of the torque-bearing components in the
actuators, the licensee took additional action to lower the applied torque to
the valves. This step was to reduce the maximum expected differential
pressure (hcDP) under which the valves would be expected to operate. The
original MEDP was 700 psid based on the setpoint of the previously-installed
auto closure interlock. The interlock had been removed and no longer
represented a basis for the MEDP. The licensee established a new MEDP of
450 psid based on plant administrative procedures that require the RHR suction
isolation valves to be closed any time the reactor coolant system (RCS)
pressure is above 450 pounds per square inch gage (psig). By lowering the
MEDP, the required stem thrust was likewise lowered, allowing for the limit
switch to be set earlier in the valve stroke. This in turn would result in
less torque being applied to the valve actuators. The inspector questioned
the 450 psid limit because both the design pressure of the RHR piping and the
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setpoint for the RHR relief valve is 600 psig and the lower setpoint of the
cold overpressure mitigation system is 485 psig. The licensee stated that the
revised MEDP calculation would be revisited and confirmed in a more rigorous
manner. The inspector informed the licensee that use of the 450 psid.MEDP
would be considered acceptable for the current fuel cycle based on the
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proceduralized administrative' controls in place to prevent 'pressurizino the
RHR system above that level. However, for periods of operation beyond that
time frame, a more fully justified basis would be considered necessary.
With the use of the lowered MEDP and the refurbishment of the actuators
(replaced and realigned components), the licensee was successful in lowering
the as-left torque levels of Valves RH-608, RH-61B, and RH-61C to less than
110 percent of the nominal actuator rating. Test results from Valves RH-60A
and RH-61A were not available at the conclusion of the inspection. As stated
previously, no actions were taken for Valve RH-60C. After successful
diagnostic tests, the valves were returned to an operable status.
The inspector reviewed the final test results of Valves RH-60B, RH-61B, and
RH-610. The as-left total torque levels (including instrument inaccuracies)
for Valves RH-60B and RH-61B were less than the nominal rating of
1250 foot-pounds. The as-left torque for Valve RH-61C was 1275 foot-pounds or
102 percent of the rating. This was well within the Limitorque limit of 110
percent.
For consideration of immediate operability, the inspector determined
that the licensee's actions and test results were sufficient to establish an
acceptable basis for declaring the valves operable. However, the inspector
identified four concerns that need to be addressed to establish that the
valves will remain operable over time. These issues have generic significance
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and should be addressed as part of the licensee's Generic Letter 89-10
program. Collectively, these concerns have been identified as inspection
followupitem(498;499/9308-05). The four concerns are discussed in the
following sections.
1.2.1
Stem Factor Considerations
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Based on a review of the test results discussed above, the inspector
determined that, even with the use of the lowered MEDP, the licensee would
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have to maintain lower stem factors than those originally assumed in order to
keep the RHR suction valves within the vendor torque limits. The assumed stem
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factor of 0.0291, which is based on a stem friction coefficient of 0.15, is
not sufficiently low enough to ensure that the valves will not be overtorqued.
Even though the most recent tests confirmed that the current stem factors were
sufficiently low enough to avert the overtorque situation, there is a
possibility that the stem factors will increase over the next fuel cycle (due
to heat and age degradation of the stem lubricant) to an extent that the
torque limits will once again be exceeded. Under the STP Generic Letter 89-10
program, the licensee would not perform diagnostic tests on these valves until
Refueling Outage IRE 07, approximately 4 years in the future. Though the
licensee's program stipulated that the valve stems are relubricated every
outage, testing is performed only every third outage. The licensee will need
to consider the effects of stem factor degradation in its long-term
disposition of the subject valves.
1.2.2 Degraded Voltage Capability.
The inspector determined that the licensee did not consider the degraded
voltage capability of the six RHR suction valves in making the operability
determination of February 2, 1993. The inspector determined with a rough
calculation that all of the 6 MOVs still had sufficient margin (as of
February 2 and using the then-current MEDP of 700 psid) to be able to close
the valves under design basis conditions. However, the calculated margin for
Valve RH-61A was only 2 percent. The inspector's calculation did not include
adjustments for measurement inaccuracy, a practice consistent with the
licensee's method for calculating stem factors. The licensee had not
justified this methodology, which clearly does not give a conservative
estimate of stem factor. Another factor which may have further decreased the
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degraded voltage capability of the valves is what is known as the load
sensitive behavior effect. The RHR MOVs were tested under static conditions.
Under dynamic conditions, which correlate to the calculated MEDP, the stem'
factor may increase as has been observed on many MOVs tested under both static
and dynamic conditions. -For torque-controlled valves, this effect is of
considerable concern because the delivered torque is. limited by the torque
switch and the higher stem factor would, thus, result in less thrust being
delivered to the valve stem.
This may result in the failure of the valve to
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close. For limit-controlled MOVs such as the RHR valves, the reduced thrust
at torque switch trip is not of concern because the motor will generate
additional torque to deliver the same required thrust. However, the degraded
voltage capability remains a concern since the ability of the motor and
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actuator to deliver the thrust necessary to trip the limit switch is lessened
by the higher stem factor. Therefore, a more conservative calculation of stem
factor combined with an allowance for load sensitive behavior would likely
have shown that some of the RHR suction valves may have been unable to close
under the design basis conditions at degraded voltage. With the lowering of
the MEDP from 700 to 450 psid and_the improved stem factors following
refurbishment, the immediate concern with degraded voltage does not pose a
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problem at this time. The inspector considered this issue to be a weakness
because degraded voltage conditions were not addressed in the itcensee's
operability determination process.
1.2.3 Underthrusting of MOVs
The inspector discovered upon review of the test data from the IRE 04 refueling
outage that RHR Valves RH-60C, RH-61A, and RH-61C had measured stem thrusts
corrected for diagnostic system inaccuracy that were less than the calculated
thrust required to close the valves under the design basis conditions. This
would be unacceptable for a torque-closed valve since it would imply that the
motor would cut off before the valve was closed.
In a limit-closed valve, the
motor would not cut off until the limit switch gearing had turned to its
setpoint, which would ^mean that the valve would go closed as long as the motor .
could supply the necessary power.. A concern that could result from this
scenario would be that the valve may not fully close thereby resulting in
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excessive process fluid leakage. The licensee expressed _ confidence that as
long as the limit switches for limit-closed valves were set during ' static
tests at thrust levels close to the required closing thrust, the applicable
leakage requirements would be met. The inspector informed the licensee that
this practice would be considered acceptable as long as it could be assured
that the motor was capable of bringing about a trip of the limit switch and
that the valve could meet its leakage requirement, if applicable, when closing
against the design basis differential pressure (a condition under which.the
valve seating forces would be potentially less than those measured when
performing tests under a static condition).
1.2.4
Excess Packing Loads
In the review of test data packages and diagnostic traces associated with the
six RHR suction isolation valves, the inspector noted that the measured
packing loads were consistently greater than the packing loads that had been
assumed and used in the calculation determining the thrust required to close
the valve under MEDP conditions.
For these valves, the assumed packing load
was 5000 pounds, whereas the measured packing loads were typically in the
range of 7500 pounds. The 2500 pounds necessary to overcome the packing load
would not be available to close the valve and thus the calculated required
thrust would have to be increased by that amount to ensure that the valve
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would.close under design conditions. The inspector noted that. the licensee
had not taken this iterative step.to correct the thrust calculation. The
licensee stated that this deficiency had been recently recognized-and that'
steps were being made to correct the process. The inspector did not find any.
examples in which the consideration of the additional packing loads would have
affected valve operability.
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ATTACHMENT
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1 PERSONS CDNTACTED
1.1 Licensee Personnel
- J. Calloway, Maintenance Assessor
- M. Chakravorty, Executive Director, Nuclear Safety Review Board
- R. Dally-Piggott, Engineering Specialist, Licensing
- D. Denver, General Manager, Nuclear Assurance
- D. Hall, Group _ Vice President
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- S. Head, Department Licensing Manager
- H. Hesidence, Acting Director, Independent Safety Engineering Group
- R. Kersey, Design Engineering
- W. Kinsey, Vice President, Nuclear Generation
- H. McBurnett, Manager, Integrated Planning and Scheduling
- M. McGehearty, Staff Engineer
- H. Pacy, Manager, Design Engineering
- G. Parkey, Plant Manager
- ll. Patil, Supervising Engineer
- 2. Rehkugler, Director, Quality Assurance
- C. Rowland, Design Engineering
- E. Stansel, Manager, Plant Engineering Division
- T. Underwood, Manager, Maintenance
- C. Walker, Manager, Public Information
1.2 NRC Personnel
R. Evans, Resident Inspector
- J. Tapia, Senior Resident Inspector
In addition to the personnel listed above, the inspector contacted other
personnel during this inspection.
- Denotes personnel that attended the exit meeting.
2 EXIT MEETING
Ar exit meeting was conducted on February 26, 1993. During this meeting, the
itspector reviewed the scope and findings of the inspection. The licensee did
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not identify as proprietary any information provided to or reviewed by. the
inspector.
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