United States Nuclear Regulatory Commission - Protecting People and the Environment

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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                                                            Page 3 of 4 


     (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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                                                            Page 4 of 4 


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 
Page Last Reviewed/Updated Tuesday, November 12, 2013