United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 92-83: Thrust Limits for Limitorque Actuators and Potential Overstressing of Motor-Operated Valves

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                              December 17, 1992


NRC INFORMATION NOTICE 92-83:  THRUST LIMITS FOR LIMITORQUE ACTUATORS 
                               AND POTENTIAL OVERSTRESSING OF MOTOR-
                               OPERATED VALVES


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to (1) possible overstressing of motor-operated
valves (MOVs) during operation and testing and (2) reviews by the NRC staff of
programs by two nuclear industry organizations to justify increased limits on
the thrust that Limitorque motor actuators for MOVs can withstand.  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 are not
NRC requirements; therefore, no specific action or written response is
required.

Description of Circumstances

During inspections of programs developed by licensees in response to Generic
Letter (GL) 89-10, "Safety-Related Motor-Operated Valve Testing and Surveil-
lance," and its supplements, the NRC staff has found instances in which
licensees have subjected MOVs to thrust or torque that exceeded their limits.
In June 1992, during an inspection at the Three Mile Island Nuclear Station,
Unit 1 (TMI-1), the NRC staff discovered an 8-inch crack in the main housing
of a Limitorque actuator for a safety-related MOV in the steam line to an
emergency feedwater pump.  The crack began at a bolt hole for the upper
bearing housing cover of the actuator.  Licensee test records indicated that
the actuator had been subjected to thrust above its rating on numerous
occasions over the past few years.  Upon disassembling the actuator in late
June 1992, the licensee found two other cracks starting at other bolt holes in
the housing cover.  The licensee replaced the actuator.  Other licensees have
also informed the NRC staff of actuators that have been damaged at nuclear
power facilities.

Upon testing MOVs, nuclear power plant licensees have found the need to
increase torque switch settings for some MOVs because the torque or thrust
required to operate the valves under differential pressure and flow conditions



9212110089
.

                                                            IN 92-83
                                                            December 17, 1992
                                                            Page 2 of 4


was greater than predicted.  In certain cases, licensees have set the MOV
torque switch near its maximum allowable setting, which is based on the
structural and electrical capability of the MOV.  At these high settings, MOVs
can exceed their torque or thrust ratings because of uncertainties such as the
amount of inertia exhibited by the actuator during operation, MOV diagnostic
equipment accuracy, and torque switch repeatability.  As a consequence, some
licensees have been attempting to justify thrust and torque limits for MOV
actuators greater than the ratings established by the manufacturer, Limitorque
Corporation (Limitorque).  The following describes two programs to increase
thrust limits of Limitorque actuators.

Kalsi Engineering

On April 15, 1992, the NRC staff held a public meeting with representatives of
Duke Power Company and Kalsi Engineering (Kalsi) to discuss a study by Kalsi
of the capability of MOV actuators to withstand thrust greater than the
ratings published by Limitorque.  In August 1992, the staff reviewed the
report of the Kalsi study while inspecting the GL 89-10 program at the Wolf
Creek Generating Station. 

In its testing program, Kalsi tested one actuator each of sizes SMB-000, -00,
-0, and -1 for 4000 cycles in both the open and close directions at 200
percent of the Limitorque thrust ratings of each actuator.  The Kalsi testing
included seismic loading conditions for several cycles.  In its Technical
Update 92-01, Limitorque has endorsed the Kalsi study, with certain
conditions, for up to 140 percent of the thrust ratings.  In a letter to the
utilities that participated in the study, and hence that can obtain the
proprietary report, Limitorque has allowed those utilities to rely on the
actuators to withstand thrust of up to 162 percent of the rating for 2000
cycles.

The staff documented a number of concerns about the design of the Kalsi study
in a letter to Duke Power Company on June 10, 1992, and NRC Inspection Report
50-482/92-15, September 30, 1992, for the Wolf Creek Generating Station.  The
deficiencies identified may affect the reliability of the test results.  A
summary of the staff's comments on the Kalsi study can be found in
Attachment 1.
.

                                                            IN 92-83
                                                            December 17, 1992
                                                            Page 3 of 4


Westinghouse Corporation

On July 7-9, 1992, the NRC Vendor Inspection Branch conducted an inspection at
the Westinghouse Corporation (Westinghouse) to review a program intended to
demonstrate that Limitorque actuators can withstand thrust greater than their
published ratings.  

After comparing actuator parameters, Westinghouse placed the SMB-000, -00, and
-0 actuators in one group and the SMB-1 and -2 actuators in another group.
Westinghouse selected an SMB-00 actuator to represent the actuators of size
SMB-000, -00, and -0 and selected an SMB-2 actuator to represent the actuators
of size SMB-1 and -2.  The plan for each tested SMB actuator consisted of an
initial torque test, mechanical aging tests, a seismic test, a final thrust 
overload test, and a final torque test.  The test plan required the tested
actuators to be operated in the open and close directions under various load
conditions for many cycles. 

In NRC Inspection Report 99900404/92-01, August 14, 1992, the NRC staff
concluded that the Westinghouse study did not adequately address certain
issues and, therefore, that utilities should not currently be using the
results from the Westinghouse program.  However, the staff stated that some of
the data in the test reports could be useful for evaluating temporary
justifications for the operability of actuators in individual cases.  A
summary of the staff's principal concerns with the Westinghouse program can be
found in Attachment 2.

Related Generic Communications

The NRC has issued other generic communications on MOV actuators.  For
example, on September 25, 1992, the NRC issued Information Notice 92-70,
"Westinghouse Motor-Operated Valve Performance Data Supplied to Nuclear Power
Plant Licensees," to alert licensees to the possibility of overestimating the
thrust output capability of actuators when using performance data from valve
manufacturers.
.

                                                            IN 92-83
                                                            December 17, 1992
                                                            Page 4 of 4


This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation project manager.


                                      ORIGINAL SIGNED BY


                                   Brian K. Grimes, Director
                                   Division of Operating Reactor Support
                                   Office of Nuclear Reactor Regulation

Technical contacts:  Jeffrey B. Jacobson, NRR
                     (301) 504-2996

                     Thomas G. Scarbrough, NRR
                     (301) 504-2794

                     P. K. Eapen, Region I
                     (215) 337-5150

                     Michael F. Runyan, Region IV
                     (817) 860-8142

                     Leonard Prividy, Region I
                     (215) 337-5140

Attachments:  
1.  Summary of NRC Staff Comments on
      the Kalsi Study.
2.  Summary of NRC Staff Comments on
      the Westinghouse Program
3.  List of Recently Issued NRC Information Notices
.

                                                            Attachment 1
                                                            IN 92-83
                                                            December 17, 1992
                                                            Page 1 of 1


                        Summary of NRC Staff Comments
                             on the Kalsi Study


1.   Kalsi lubricated the stem more frequently than it would be lubricated in 
     a normal application.  As the lubricant deteriorates over time, the
     internal parts may exhibit greater wear than indicated in the Kalsi
     results. 

2.   Kalsi found certain internal components that experienced torque to have
     failed or have significant wear. 

3.   In Technical Update 92-01, Limitorque indicated that the actuator bolts 
     must be tightened to a certain torque before applying the increased
     allowable percentage thrust above the ratings.  The Arkansas Power and
     Light Company, the licensee for Arkansas Nuclear One, informed the staff
     that, after tightening the bolts, the thrust delivered when the torque
     switch tripped was up to 50 percent less than the thrust delivered before
     tightening the bolts.  The licensee stated that it had determined that
     the probable cause was the overcompression of the housing cover gasket
     caused by the increased torque requirements of the housing cover bolts.
     Kalsi is reassessing the need to tighten the bolts to a specific torque
     value.

4.   Kalsi used new actuators in its testing program, and did not consider
     possible differences in manufacturing over the years or the effects of
     aging.

5.   Kalsi had not ensured that tightening of the actuator bolts after seismic
     testing did not affect reliance on the results of the tests for the
     remaining cycles.

6.   During the April 1992 presentation, Dr. Kalsi stated that the total
     number of cycles that an actuator has operated from the beginning of its
     life must be counted in the 2000 cycle limit stated in the study. 

7.   In the Kalsi study, the motor pinion key on some MOVs failed, and the
     manual declutch lever spuriously engaged during tests of an actuator.

8.   The Kalsi study did not ensure that the actuator bolts, stem, or stem nut
     had adequate strength for specific plant applications.
.

                                                            Attachment 2
                                                            IN 92-83
                                                            December 17, 1992
                                                            Page 1 of 1


                        Summary of NRC Staff Comments
                         on the Westinghouse Program


1.   Westinghouse did not adequately establish similarity in grouping actua- 
     tors for testing.

2.   Westinghouse did not adequately address the margin necessary to account
     for statistical variations among actuators within each group of actuators
     when sampling only one actuator from each group for testing.

3.   Westinghouse did not clearly indicate that actuator bolts, stem, or stem
     nuts were not included in its study.

4.   Westinghouse stated that its program may only be applied to actuators for
     which the total number of operating cycles and thrust conditions are
     known.

5.   Westinghouse did not address any margin provided to account for the
     inaccuracy of test equipment used in the Westinghouse program or used by
     licensees applying thrust allowable limits from the program.

6.   Westinghouse had not resolved an issue of spurious engagement of the
     manual declutch lever during testing of an actuator.
.
Page Last Reviewed/Updated Tuesday, November 12, 2013