United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 89-43: Permanent Deformation of Torque Switch Helical Springs in Limitorque SMA-Type Motor Operators

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555

                                   May 1, 1989

                                   HELICAL SPRINGS IN LIMITORQUE SMA-TYPE
                                   MOTOR OPERATORS 


All holders of operating licenses or construction permits for nuclear power 


This information notice is being provided to alert addressees to potential 
problems resulting from permanent deformation of the torque switch helical 
springs in Limitorque SMA-type motor operators.  It is expected that re-
cipients 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: 

Problems with the helical springs were discovered during a series of dynamic 
tests that were conducted with a motor-operated wedge-gate valve from the 
decommissioned Shippingport Atomic Power Station (Shippingport).  The valve 
was installed in a portion of the piping system that had been modified to 
simulate the stiffness of a typical United States piping system.  The valve 
was 30 years old and had its original Limitorque SMA-type motor operator.  

One of the main objectives of these tests was to determine the operating 
capability of the valve when subjected to simultaneous internal hydraulic 
and seismic loadings.  This was typically accomplished by operating the 
valve to achieve maximum hydraulic loading during maximum seismic loading.  
During testing, an operability problem with the valve motor operator occurred.  

Disassembly and inspection of the motor operator revealed that the torque 
switch helical spring had a free length of 3.99 inches.  Review of the 
Shippingport records for the valve indicated that the torque switch spring 
had not been changed while the valve had been in service at Shippingport.  
Limitorque's records indicated that the original free length of the spring 
was 4.46 inches.  Thus, the spring had developed a permanent deformation 
of 0.47 inches during the motor operator's 30-year life.  Since the spring 

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                                                                 May 1, 1989
                                                                 Page 2 of 2

had been installed in a space 4.06 inches in length, a gap of approximately 
0.07 inch had existed during the tests.  This gap appears to indicate that 
there was no preload on the helical spring during the tests.  

The torque switch helical spring from a second Shippingport valve with a 
Limitorque SMA-type motor operator was also found to have a permanent defor-
mation.  For this second spring, the original free length was 3.84 inches; 
the current free length was 3.74 inches; and it was installed in a space 3.37 
inches in length.  However, even though this spring was subjected to a preload 
and there was no gap, the amount of preload was less than expected because of 
the permanent deformation.  


Permanent deformation of the torque spring without compensating for it will 
reduce the torque required to open the torque switch.  The switch setting 
that is selected corresponds to the closing torque that the motor operator 
is to deliver.  If the permanent deformation of the spring sufficiently 
reduces this closing torque, it may cause the motor operator to stop before 
the valve is fully seated.  In an extreme case, the motor operator may stop 
with the valve partially open.  

The design of the SMA-type motor operator is such that the torque switch 
helical spring is in its most compressed condition when the valve is closed.  
Thus, a normally closed valve is more likely to experience permanent 
deformation of the helical torque spring.  A review of the Shippingport 
records indicated that both of these valves had been used as normally closed 
valves at Shippingport.  Thus, normally closed, safety-related valves with 
Limitorque SMA-type motor operators may not accomplish their intended 
safety-related function because the original torque switch setting may result 
in lower output torque caused by the permanent deformation of their torque 
switch helical springs.  

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 techni-
cal 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:  Gerald H. Weidenhamer, RES
                     (301) 492-3839

                     Richard J. Kiessel, NRR
                     (301) 492-1154

Attachment:  List of Recently Issued NRC Information Notices

..                                                            Attachment 
                                                            IN 89-43
                                                            May 1, 1989
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                             NRC INFORMATION NOTICES
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

88-97,         Potentially Substandard       4/28/89        All holders of OLs
Supp. 1        Valve Replacement Parts                      or CPs for nuclear
                                                            power reactors.

89-42          Failure of Rosemount          4/21/89        All holders of OLs
               Models 1153 and 1154                         or CPs for nuclear
               Transmitters                                 power reactors.

89-41          Operator Response to          4/20/89        All holders of OLs
               Pressurization of Low-                       or CPs for nuclear
               Pressure Interfacing                         power reactors.

88-75,         Disabling of Diesel           4/17/89        All holders of OLs
Supplement 1   Generator Output Circuit                     or CPs for nuclear
               Breakers by Anti-Pump                        power reactors.

89-40          Unsatisfactory Operator Test  4/14/89        All holders of OLs
               Results and Their Effect on                  or CPs for nuclear
               the Requalification Program                  power reactors.

89-39          List of Parties Excluded      4/5/89         All holders of OLs
               from Federal Procurement                     or CPs for nuclear
               or Non-Procurement Programs                  power reactors.

89-38          Atmospheric Dump Valve        4/5/89         All holders of OLs
               Failures at Palo Verde                       or CPs for nuclear
               Units 1, 2, and 3                            power reactors.

89-37          Proposed Amendments to        4/4/89         All U.S. NRC 
               40 CFR Part 61, Air                          licensees. 
               Emission Standards 
               for Radionuclides

89-36          Excessive Temperatures        4/4/89         All holders of OLs
               in Emergency Core Cooling                    or CPs for nuclear
               System Piping Located                        power reactors.
               Outside Containment

OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Friday, May 22, 2015