Information Notice 1988-72, Inadequacies in the Design of DC Motor-Operated Valves
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
September 2, 1988 NRC INFORMATION NOTICE NO. 88-72:
INADEQUACIES IN THE DESIGN OF DC MOTOR-
OPERATED VALVES
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
This information notice is being provided to alert addressees to potential
problems in the design specifications of dc motor-operated valves, especially
for conditions that may involve reduced or degraded dc voltage and/or elevated
temperatures. 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 do not constitute NRC requirements; therefore, no specific action or
written response is required.
Description of Circumstances
On July 1, 1988, a high pressure coolant injection (HPCI) steam admission valve
failed to open during a post-maintenance test at the Brunswick nuclear power
plant, Unit 1. The same valve had failed in December 1987 and on May 28, 1988.
The licensee, Carolina Power and Light Company, established a team to investi- gate the cause of failure, and the team identified the most probable cause as
a dc motor failure due to a shunt-winding to series-winding short circuit. The
team believed that this condition was precipitated by thermal binding of the
valve internals. The previous failure in May was also diagnosed as having been
caused by thermal binding. As a result of these failures, the licensee reviewed
the design of the dc motor-operated valves for both the HPCI and the reactor
core isolation cooling (RCIC) systems. This review identified a number of
significant design deficiencies going well beyond the problems with thermal
binding.
The deficiencies constitute a potential common cause failure mechanism
for safety-system valves. Unit 1 was shut down on July 14, 1988 to replace the
failed HPCI valve motor and to implement design modifications to other motor- operated valves.
Discussion:
Pressure locking and thermal binding of gate valves were identified as poten- tially important valve failure mechanisms within the nuclear industry several
880830018t
A"Py-
. I
IN 88-72 September 2, 1988 years ago.
Pressure locking occurs when a gate valve is closed under full
system pressure and fluid is trapped in either the bonnet cavity or between
the disks of a double disk valve.
When the valve is subsequently heated, the
trapped fluid expands or flashes to steam and causes pressure to increase in
the valve bonnet area and between the wedges of the valve disk. The pressure
increase inhibits opening of the valve by causing the wedges to press tightly
against the valve seats, resulting In binding of the valve. This phenomenon
contributed to the May 1988 HPCI valve failure.
To prevent recurrence-of this
failure, the licensee drilled a small drain hole in the upstream disk to provide
a pressure relief path.
This remedy was only partially successful because the
valve also underwent thermal binding, which was not recognized at that time.
Thermal binding occurs when a valve is seated in a hot condition and, during
subsequent temperature changes, the valve body contracts a proportionally
greater amount than the valve internals because of the different expansion
and contraction characteristics of the valve body and the disk.
This is
particularly true for valves with internals which have reduced clearances
due to improper maintenance or alterations.
Several potential remedies have
been suggested to alleviate this situation, including slightly opening and
reclosing a valve during cooldown, limiting valve actuator closing forces, and using compensating spring packs to reduce valve inertial closing forces.
In general, neither ac nor dc valve motor operator sizing analyses account
for the extra torque needed to unseat a valve when it is thermally bound.
It should be noted that certain valves may become functionally inoperable
when conditions induce thermal binding.
The dc motor operator design problems discovered at Brunswick as a result of
the review following the July 1 HPCI valve failure were attributed by the
licensee to a lack of design coordination and inadequate consideration of
the valves' functional operability requirements by the architect engineering
firm, United Engineers and Constructors. The licensee found that inadequate
torque was available to open the valves, particularly under reduced dc bus
voltage conditions, and when MOVs were installed in locations that are-normally
at elevated ambient temperatures.
Reduced or degraded dc bus voltage conditions
could occur during accidents in which battery charging capability is lost (for
example, during a station blackout or failure of the chargers).
Four design
flaws were identified in the licensee's review.
(1) The specified motor operator torque was found to be deficient. Also, the design temperatures-used for sizing the motors were found to be
below the actual ambient temperature in which some valves were oper- ating. Thus, the motors were unable to develop the torque required
to unseat (open) the valves under reduced dc voltage conditions. Ad- ditionally, the operability of some of the valves during accident
conditions, such as high-energy line breaks, was questionable.
IN 88-72 September 2, 1988 (2) The presence of starting resistors and their impact on starting
torque were not considered in motor sizing.
The resistors were
installed to limit the dc current until the motor starts and ac- celerates toward rated speed. However, they were found to reduce
the "hammer blow" effect needed to ensure valve opening. They also
increased the potential for motor stalling against a seated valve.
(3) Cable resistance contributed to reduced motor terminal voltage and
starting current. As a result, actual torque supplied by the motor
was further reduced.
(4) High-voltage transients were induced in the dc motor's shunt windings
whenever the motor's power supply circuit breaker was opened.
(In
some cases, high voltage transients may also result when motor starter
contacts are opened in installations in which the shunt field is set
up for intermittent service; i.e. energized only when the armature is
energized.) This process led to degradation of the insulation on the
shunt and field windings.
These dc motor-related design flaws were not uncovered during routine surveil- lance testing nor during post-maintenance testing. In part, this was because
dc voltage was normally maintained at or above nominal values by battery chargers
operating "in float" with the batteries when the tests were conducted. Thus, design flaws related to reduced dc voltage performance would not be readily
detected.
Additionally, Motor Actuator Characterizer (MAC) traces were made from valve
tests performed during certain maintenance testing activities. These-MAC
traces indicated anomalies in valve performance. However, the deviations
from expected performance were subtle. Careful engineering review was neces- sary to properly interpret these traces.
The corrective actions taken by the licensee included removal of starting re- sistors, replacement of certain motors and cable, and the addition of metal
oxide varistors to dissipate induced voltage transients during power supply
interruptions.
(Discussions with Limitorque have revealed that, for the past
five years, the company has recognized the need to control voltage transients
in their motor operators.
Therefore, they have included depictions of voltage
surge suppression devices in their electrical drawings for their motor-operated
valves.)
It is important to note that the removal of dc motor operator starting resistors
may cause a significant increase in the 1-minute load on the station's batteries.
A battery performance assessment was conducted by the Brunswick licensee to
ensure that the dc power system continued to meet plant safety analysis require- ments following removal of the resistors.
IN 88-72 September 2, 1988 No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the technical
contacts listed below or the Regional Administrator of the appropriate regional
office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: P. W. Baranowsky, NRR
(301) 492-1157
E. N. Fields, NRR
(301) 492-1173 Attachment: List of Recently Issued NRC Information Notices
Attachment
September 2, 1988 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
Information
Date of
Notice No.
Subject
Issuance
Issued to
88-71
88-70
88-69
Possible Environmental
Effect of the Reentry
of COSMOS 1900 and
Request for Collection
of Licensee Radioactivity
Measurements Attributed
to That Event
Check Valve Inservice
Testing Program
Deficiencies
Movable Contact Finger
Binding in HFA Relays
Manufactured by General
Electric (GE)
9/1/88
8/29/88
8/19/88
All holders of OLs
or CPs for nuclear
power reactors, fuel
cycle licensees, and
Priority 1 material
licensees.
-
All holders of OLs
or CPs for nuclear
power reactors.
All holders of OLs
or CPs for nuclear
power reactors.
88-48,
Supplement 1
Licensee Report of Defective
Refurbished Valves
8/24/88
All holders of OLs
or CPs for nuclear
power reactors.
88-68
88-67
88-66 Setpolnt Testing of Pres- surizer Safety Valves with
Filled Loop Seals Using
Hydraulic Assist Devices
PWR Auxiliary Feedwater Pump
Turbine Overspeed Trip
Failure
Industrial Radiography
Inspection and Enforcement
Inadvertent Drainages of
Spent Fuel Pools
Reporting Fires in Nuclear
Process Systems at Nuclear
Power Plants
8/22/88
8/22/88
8/22/88
8/18/88
8/18/88
All holders of OLs
or CPs for nuclear
power reactors.
-
All holders of OLs
or CPs for nuclear
power reactors.
All NRC industrial
radiography licensees.
All holders of OLs
or CPs for nuclear
power reactors and
fuel storage facilities.
All holders of OLs
or CPs for nuclear
power reactors.
88-65
88-64 OL = Operating License
CP = Construction Permit
September 2, 1988
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the technical
contacts listed below or the Regional Administrator of the appropriate regional
office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts:
P. W. Baranowsky, NRR
(301) 492-1157 E. N.
(301)
Fields, NRR
492-1173 Attachment:
List of Recently Issued NRC Information Notices
- see previous concurrence
- OEAB:NRR *OEAB:NRR *RPB:ARM
PBaranowsky NFields
TechEd
8/12/88
8/12/88
8/04/88
- C:OEAB:NRR
WLanning
8/12/88
- D:DEST:NRR
LShao
8/17/88
- C: OGCB: NRR&!~VC
CHBerlinger CERossi
8/25/88
8/3//88
IN 88- August , 1988 (
some cases, high voltage transients may also result when motor
sta er contacts are opened in installations in which the shunt
field is set up for intermittent service, i.e. energized only when
the arm ture is energized.) This process led to degradation of the
insulati
on the shunt and field windings.
These dc motor-relate design flaws were not uncovered during routine
surveillance testing no0during post-maintenance testing.
In part this was
because dc voltage was no ally maintained at or above nominal values by
battery chargers operating 'in float" with the batteries when the tests were
conducted.
Thus, design fla
related to reduced dc voltage performance would
not be readily detected.
Additionally, Motor Actuator Cha
terizer (MAC) traces were made from valve
tests performed during certain main enance testing activities.
These MAC
traces indicated anomalies in valve
rformance.
However, the deviations from
expected performance were subtle.
Car ful engineering review was necessary to
properly interpret these traces.
The corrective actions taken by the licens
included removal of starting
resistors, replacement of certain motors and able, and the addition of metal
oxide varistors to dissipate induced voltage t ansients during power supply
interruptions.
(Discussions with Limitorque ha
revealed that, for the past
five years, the company has recognized the need t control voltage transients in
their motor operators. Therefore they have include
depictions of voltage
surge suppression devices in their electrical drawin
for their motor
operated valves.
It is important to note that the removal of dc motor oper tor starting resistors
may cause a significant increase in the 1-minute load on t station's
batteries. A battery performance assessment was conducted t ensure that the
dc power system continued to meet plant safety analysis requi ments following
removal of the resistors.
No specific action or written response is required by this informa ion notice.
If you have any questions about this matter, please contact the tech ical
contact listed below or the Regional Administrator of the appropriate egional
office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact:
P.W. Baranowsky
(301) 492-1157 E.N. Fields
(301) 492-1173 p
Attachment: List of Recently Issued NRC Information Notices
- see previous concurrence
M145
- OEAB:NRR *OEAB:NRR *RPB:ARM C:OEAB:NRR D:DEST:NRR C:OGCB:NRR D:DOEA:NRR
PBaranowsky NFields
TechEd
WLanning
LShao
CHBerlinger CERossi
/
/88
/
/88
/
/88
/
/88
/
/88 J'/,S88
/
/88
IN 88- August , 1988 (In me cases, high voltage transients may also result when motor
starte
contacts are opened in installations in which the shunt
field i set up for intermittent service, i.e. energized only when
the armat e is energized.)
This process led to degradation of the
insulation
the shunt and field windings.
These dc motor-related d ign flaws were not uncovered during routine
surveillance testing nor
ing post-maintenance testing.
In part this was
because dc voltage was norma ly maintained at or above nominal values by
battery chargers operating in
oat with the batteries when the tests were
conducted.
Thus, design flaws elated to reduced dc voltage performance would
not be readily detected.
Additionally, Motor actuator charac rizer (MAC)
traces were made from valve
tests performed during certain mainte nce testing activities. These MAC
traces indicated anomalies in valve pe ormance.
However, the deviations from
expected performance were subtle. Caref 1 engineering review was necessary to
properly detect and interpret these trace
The corrective actions taken by
the licensee included removal of starting r sistors, replacement of certain
motors and cable, and the addition of metal
ide varistors to dissipate
induced voltage transients during power supply interruptions.
(Discussions
with Limitorque have revealed that for the past five years the company has
recognized the need to control voltage transients in their MOVs.
Therefore
they have included depictions of voltage surge sup ression devices in their
electrical drawings for their MOVs.)
It is important to note that the removal of dc motor o erator starting resistors
may cause a significant increase in the 1-minute load
batteries.
A battery
performance assessment was required to ensure that the d power system
continued to meet plant safety analysis requirements folbo ing removal of the
resistors.
No specific action or written response is required by this inf mation notice.
If you have any questions about this matter, please contact the echnical
contact listed below or the Regional Administrator of the appropr ate regional
office.
Charles E. Rossi, Director
Division of Operational Events Assessm t
Office of Nuclear Reactor Regulation
Technical Contact:
P.W. Baranowsky
(301) 492-1157 E.N. Fields
(301) 492-1173 Attachment: List of Recently Issued NRC Information Notices
- See previous concurrence
- OEAB:NRR *RPB:ARM C:;
P
NRR D:D W
R C:OGCB:NRR
D:DOEA:NRR
IB ranowsky NFields
Tech.Ed. WLanning
I, o
CHBerlinger
CERossi
M
17/88 g.//i8/
/8/
8
//(V
88
/17/88
/
/88
/
/88
+ W4 YOU
IN 88-
<AX
August
, 1988 Motor act
tor characterizer (MAC) traces were made f om valve tests performed
during certa
maintenance testing activities. These traces indicated anomalies
in valve perfo ance. However, the deviations from expected performance were
subtle. Careful
gineering review was necessary to properly detect and
interpret these tra s.
The corrective actions taken by the licensee included
removal of starting re istors, replacement of certain motors and cable, and the
addition of metal oxides ristors to dissipate induced voltage transients during
power supply interruptions
I
It is important to note that t
removal of dc motor operator starting resistors
may cause a significant increase n the 1-minute load on batteries. A battery
performance assessment was require to ensure that the dc power system
continued to meet plant safety analy s requirements following removal of the
resistors.
No specific action or written response is equired by this information notice.
If you have any questions about this matter please contact the technical
contact listed below or the regional/dminist tor of the appropriate regional
office.
Technical Contact:
P.W. Baranowsky
(301) 492-1157 Charles E. Rossi, rector
Division of Operatio _
Event
Office of Nuclear Reac r Re(
LtA MAJ
1o
fNorhcSt
E.N. Fields
(301) 492-1173 A1"^,o+: *u5+- ,
9t
.CAOY TES&
- 0EAB:NRR
10EAB:NRR TV)OEs C:4EAB:NRR
PBaranowsky 8 Fields
Wianning
/o5/88
£/of/88
'i/O9I/88
/
/88 D:DEST:NRR
LShao
/
/88 C:OGCB:NRR
CHBerlinger
/
/88 D:DOEA:NRR
CERossi
/
/88