Information Notice No. 88-72: 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
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 investigate 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
8808300018
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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
operating. Thus, the motors were unable to develop the torque
required to unseat (open) the valves under reduced dc voltage
conditions. Additionally, the operability of some of the valves
during accident conditions, such as high-energy line breaks, was
questionable.
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September 2, 1988
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(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 necessary 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 requirements following removal of the resistors.
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September 2, 1988
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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
IN 88-72
September 2, 1988
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
88-71 Possible Environmental 9/1/88 All holders of OLs
Effect of the Reentry or CPs for nuclear
of COSMOS 1900 and power reactors,
Request for Collection fuel cycle
of Licensee Radioactivity licensees, and
Measurements Attributed Priority 1
to That Event material
licensees.
88-70 Check Valve Inservice 8/29/88 All holders of OLs
Testing Program or CPs for nuclear
Deficiencies power reactors.
88-69 Movable Contact Finger 8/19/88 All holders of OLs
Binding in HFA Relays or CPs for nuclear
Manufactured by General power reactors.
Electric (GE)
88-48, Licensee Report of Defective 8/24/88 All holders of OLs
Supplement 1 Refurbished Valves or CPs for nuclear
power reactors.
88-68 Setpoint Testing of Pres- 8/22/88 All holders of OLs
surizer Safety Valves with or CPs for nuclear
Filled Loop Seals Using power reactors.
Hydraulic Assist Devices
88-67 PWR Auxiliary Feedwater Pump 8/22/88 All holders of OLs
Turbine Overspeed Trip or CPs for nuclear
Failure power reactors.
88-66 Industrial Radiography 8/22/88 All NRC industrial
Inspection and Enforcement radiography
licensees.
88-65 Inadvertent Drainages of 8/18/88 All holders of OLs
Spent Fuel Pools or CPs for nuclear
power reactors and
fuel storage
facilities.
88-64 Reporting Fires in Nuclear 8/18/88 All holders of OLs
Process Systems at Nuclear or CPs for nuclear
Power Plants power reactors.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
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