Information Notice No. 84-36, Supplement 1: Loosening of Locking Nut on Limitorque Operator
SSINS No. 6835 IN 84-36 SUPP. 1 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 SEPTEMBER 11, 1984 INFORMATION NOTICE NO. 84-36 SUPPLEMENT 1: LOOSENING OF LOCKING NUT ON LIMITORQUE OPERATOR Addressees: All nuclear power reactor facilities holding an operating license (OL) or construction permit (CP). Purpose: This information notice is being provided to supplement Information Notice 84-36 for the purpose of assuring that loosening of locking nuts does not occur in Limitorque valve operators. The NRC expects that recipients will review this notice for applicability to their facilities. Suggestions contained in this notice do not constitute NRC requirements; therefore, no specific action or written response is required. Description of Circumstances: Information Notice 84-36 described events at the Hatch Nuclear Power Plant where loosening of a set screw on a locking nut on the worm gear shaft of a Limitorque operator led to backing out of a locknut and subsequent inoperability of a safety-related valve. Recent information received from the vendor indicates that the specific part affected on the SMB-4 valve actuator was Part #83 (bearing locknut, see Figure 1), which is located on the actuator worm shaft--not the worm gear shaft. An enlarged drawing showing more details of the bearing lock nut is shown in Fig. 2. The bearing lock nut performs the function of transmitting loads from the worm gear (Part #16) to the worm shaft causing axial movement of the worm shaft followed by compression of the belleville spring and activation of the torque switch. The vendor indicates that the operators at Hatch contain a unique feature that is not found in most other valve applications that resulted in the originally noted defect. These units use a ball screw valve stem instead of the valve manufacturer's conventional Acme threaded valve stem. The ball screw has the opposite hand (right-hand) thread to the standard Acme (left-hand) thread. The significance of this difference is that unthreading of the bearing locknut could affect either the closing torque switch function when the valve is equipped with a right-hand stem thread or the opening torque switch function 8409070254 . IN 84-36 SUPP. 1 September 11, 1984 Page 2 of 2 (with obstructed or overloaded disk) when the valve is equipped with a standard Acme left-hand stem thread. This is because the torque control mechanism is activated as a result of axial movement of the worm shaft due to forces generated as the valve comes to a full-close position, full-open position, or some intermediate obstructed position. Figure 2 shows the direction of movement of the worm shaft at the Hatch facility when the valve comes to a closed position. It is recommended that holders of CPs and OLs consider this additional information for applicability in their ongoing efforts to resolve the concerns addressed in Information Notice 84-36. No written response to this information notice is required. If there are any questions regarding this matter, please contact the Regional Administrator of the appropriate NRC regional office or this office. Edward L. Jordan Director Division of Emergency Preparedness and Engineering Response Office of Inspection and Enforcement Technical Contact: John J. Zudans, IE (301) 492-8030 Attachments: 1. Figure 1, "SMB-0 to SMB-4 & SMB-4T" 2. Figure 2, "Worm & Torque Spring Assembly" 3. List of Recently Issued IE Information Notices
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021