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UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555-0001

July 24, 1998

**Addressees**

All holders of operating licenses for nuclear power reactors except those who have permanently ceased operation and havecertified that fuel has been permanently removed from the reactor vessel.

**Purpose**

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to recent guidancefrom Limitorque Corporation for predicting torque output from its motor actuators used to open and close motor-operatedvalves (MOVs). This supplement to NRC Information Notice (IN) 96-48 (August 21, 1996), "Motor-Operated ValvePerformance Issues," revises the information on motor actuator output presented earlier. The information in IN 96-48 onthe Electric Power Research Institute (EPRI) MOV Performance Prediction Program and on the failure of MOV keys is notaffected by this supplement's updated information on motor actuator output. 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.

**Background**

As discussed in IN 96-48, Limitorque Corporation established a standard practice for determining the appropriate size ofmotor actuators as described in its "SEL" documents in the 1970s. According to those documents, Limitorque predicts thetorque output of MOV actuators through the use of the following equation:

Limitorque provided additional information on the sizing of motor actuators in its Technical Update 92-02 (dated October9, 1992).

In the SEL documents and Technical Update 92-02, Limitorque specified the following to be assumed in the equation forsizing ac-powered motor actuators:

The NRC staff reviewed the criteria for sizing motor actuators established by Limitorque during an inspection in 1993. TheNRC staff found Limitorque's assumptions for the individual parameters in the sizing criteria to be based primarily onengineering judgment without independent justification. However, the NRC staff retained confidence in the prediction ofoutput torque based on the success of the Limitorque criteria in sizing motor actuators over the previous years. For moreinformation on the inspection findings, see NRC Inspection Report 99900100/93-01, dated June 28, 1993 (Accession Nos.9308180192, 9308180201, and 9308180277).

In the early 1990s, dynamic tests of MOVs by nuclear power plant licensees in response to Generic Letter (GL) 89-10,"Safety-Related Motor-Operated Valve Testing and Surveillance," revealed that valve vendors had significantlyunderpredicted the operating requirements for many safety-related valves under their design-basis conditions. This findingled licensees to request that Limitorque relax certain criteria used in the sizing equation for ac-powered motor actuatorswhich would result in predicting higher actuator output torque. For example, Limitorque allowed particular licensees toassume 110 percent of the rated motor starting torque and a "run" efficiency (up to 20 to 30 percent greater than the"pullout" efficiency) for specific MOVs and their application. Further, Limitorque Technical Update 93-03 (datedSeptember 1993) allowed licensees to assume an application factor of 1.0 (rather than 0.9) where the motor voltage wasless than 90 percent of the motor's rated voltage. As a result of such relaxations of Limitorque's motor actuator sizingcriteria, many licensees revised their MOV calculations to incorporate the relaxed criteria for all ac-powered motoractuators.

**Description of Circumstances**

In 1994, concerns were raised about the adequacy of Limitorque's relaxed criteria in reliably predicting the torque output ofac-powered motor actuators. In a paper presented by Texas Utilities Electric Company at an NRC/ASME Symposium onValve and Pump Testing in July 1994, it generally appeared that the actuator sizing method would only be adequate if theoriginal sizing criteria were used with "pullout" efficiency and a 0.9 application factor assumed. (See NUREG/CP-0137(July 1994), "Proceedings of the Third NRC/ASME Symposium on Valve and Pump Testing.") At another NRC/ASMEsymposium in July 1996, Commonwealth Edison Company reported that its review of available test data indicated that useof "pullout" efficiency combined with a 0.9 application factor was needed to establish a conservative lower bound foractual actuator gear efficiencies. (See NUREG/CP-0152 (July 1996), "Proceedings of the Fourth NRC/ASME Symposiumon Valve and Pump Testing.") Also at the July 1996 symposium, the Idaho National Engineering and EnvironmentalLaboratory (INEEL) reported that preliminary results of tests sponsored by the NRC Office of Nuclear RegulatoryResearch similarly suggested the need to combine "pullout" efficiency with a 0.9 application factor to reliably predict theactuator gear efficiency. In IN 96-48, the NRC staff alerted licensees to these results of NRC-sponsored and industry testsof motor actuator output.

In July 1997, the NRC issued NUREG/CR-6478, "Motor Operated Valve (MOV) Actuator Motor and Gearbox Testing,"which described in detail the results of the NRC-sponsored tests of motor actuator performance. The testing programrevealed that motor output was typically greater than nominal starting torque. However, the program indicated that actuatorefficiency was normally less than "run" efficiency (and in some cases even less than "pullout" efficiency), and that motoroutput under degraded voltage conditions was less than predicted by the square of the ratio of actual to rated motor voltage.

In IN 96-48, the NRC staff stated that it was discussing with Limitorque the discrepancy between guidance relaxing theoriginal motor actuator sizing criteria and the recent motor actuator test results. The NRC staff also stated that Limitorquewas developing updated information for the industry on the sizing of its motor actuators. At an MOV Users Group meetingof nuclear power plant licensees in December 1997, Limitorque informed the licensees that it was considering retracting itsprevious relaxation of the sizing criteria and that it would issue a documented notification. In April 1998, the NRC staffconducted an inspection at Limitorque of the manufacturer's response to the information on the adequacy of its relaxationof the ac-powered motor actuator sizing criteria. For information on the specific inspection findings, see NRC InspectionReport 99900100/98-01, dated July 13, 1998.

**Discussion**

On May 15, 1998, Limitorque prepared Technical Update 98-01 to provide updated guidance for determining the outputtorque capability of an ac-powered Limitorque motor actuator. In that update, Limitorque specifies that, in the sizingequation, licensees should use nominal motor starting torque, "pullout" efficiency, overall actuator gear ratio based on theparticular actuator and an application factor (typically 0.9), and should apply an exponent of 2 to the ratio of actual to ratedmotor voltage. Where voltage exceeds 90 percent of the motor's rated voltage, Limitorque allows customers to disregardthe degraded voltage factor because of standard motor design characteristics. Limitorque also allows customers to use moreoptimistic assumptions than are specified in Technical Update 98-01 whenever actual test data or certain engineering dataare available. In light of available test data, Limitorque alerts customers to specific motor actuators that require specialattention in determining output torque capability. Limitorque guidelines for considering the effects of elevated temperatureon ac-powered motor starting torque continue to be provided in Limitorque Technical Update 93-03.

With respect to dc-powered motor actuators, the sizing equation developed by Limitorque in the SEL documents is similarto the one used for ac-powered motor actuators. The assumptions in the sizing equation for dc-powered motor actuatorsinclude nominal motor starting torque, application factor, "pullout" efficiency, overall actuator gear ratio based on theparticular actuator and a degraded voltage factor where an exponent of 1 is applied to the ratio of actual to rated motorvoltage. The NRC Office of Nuclear Regulatory Research is presently sponsoring a program at the INEEL to study theperformance of dc-powered motor actuators and to provide a basis for re-evaluating initial test results for a dc-poweredmotor actuator described in NUREG/CR-6478. Limitorque will review the information on the output of dc-powered motoractuators to determine whether the sizing guidance needs to be updated.

In summary, in light of recent tests and studies of motor-actuator output, Limitorque has retracted its relaxation of thesizing criteria for ac-powered motor actuators through issuance of Limitorque Technical Update 98-01. Limitorque hasconducted workshops with nuclear power plant licensees to discuss its updated guidance on sizing ac-powered motoractuators. Limitorque is continuing to evaluate its sizing guidance for dc-powered motor actuators. The NRC staff will beevaluating licensee consideration of the updated Limitorque guidance as part of its review of MOV programs developed inresponse to GL 96-05, "Periodic Verification of Design-Basis Capability of Safety-Related Motor-Operated Valves."

**Related Generic Communications**

NRC Generic Letter 89-10, "Safety-Related Motor-Operated Valve Testing and Surveillance," June 28, 1989.

NRC Generic Letter 96-05, "Periodic Verification of Design-Basis Capability of Safety-Related Motor-Operated Valves,"September 18, 1996.

NRC Information Notice 96-48, "Motor-Operated Valve Performance Issues," August 21, 1996.

NRC Information Notice 97-07, "Problems Identified During Generic Letter 89-10 Closeout Inspections," March 6, 1997.

This information notice requires no specific action or written response. However, recipients are reminded that they arerequired by 10 CFR 50.65 to take industry-wide operating experience (including information presented in NRCInformation Notices) into consideration, where practical, when setting goals and performing periodic evaluations. 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 (NRR) project manager.

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