ESTIMATION OF MANUALLY RECOVERABLE FRACTION OF COMMON CAUSE FAILURES OF MOTOR OPERATED VALVES

Transcription

1 ESTIMATION OF MANUALLY RECOVERABLE FRACTION OF COMMON CAUSE FAILURES OF MOTOR OPERATED VALVES Young G Jo Southern Nuclear Operating Company PSA 17 9/24-9/28/2017 Pittsburgh PA USA 1

2 Table of Contents I. INTRODUCTION II. ESTIMATION OF THE FRACTION OF MOV CCFs WHICH MAY BE RECOVERED BY MANUAL HAND WHEEL OPERATION II.A. Details of MOV Actuator and Its Operation II.B. MOV CCF Experience Events II.C. Determination of the Recoverability of MOV CCF by Manual Handwheel Operation II.D. Calculation of the Manually Recoverable Fraction of MOV CCF III. CONCLUSIONS 2

3 I. INTRODUCTION Some motor operated valve (MOV) common cause failures (CCFs) may be recovered by local manual handwheel operation It is desirable to credit such manual recovery action for more realistic Probabilistic Risk Assessment (PRA) results. In this paper, The details of typical MOV actuator subparts and their operations were investigated first in order to figure out which subpart failures may be recovered by the manual handwheel operation. Then MOV CCF experiences events were reviewed to determine the recoverability by manual actions and The fraction of MOV CCF events which may be recoverable by local manual actions was estimated 3

4 II. ESTIMATION OF THE FRACTION OF MOV CCFs WHICH MAY BE RECOVERED BY MANUAL HAND WHEEL OPERATION 4

5 II.A. Details of MOV Actuator and Its Operation Limitorque MOV operator 5

6 II.A. Details of MOV Actuator and Its Operation Electrical Train Motor -> Motor pinion gear -> Worm shaft clutch gear -> Worm shaft clutch -> Worm shaft -> Worm-> Worm gear -> Drive sleeve -> Stem nut -> Stem Manual Train Handwheel -> Handwheel shaft -> Handwheel shaft gear -> Hand wheel clutch pinion -> Worm shaft clutch -> Worm shaft -> Worm-> Worm gear -> Drive sleeve -> Stem nut -> Stem 6

7 II.A. Details of MOV Actuator and Its Operation Electrical Train Motor -> Motor pinion gear -> Worm shaft clutch gear -> Worm shaft clutch -> Worm shaft -> Worm-> Worm gear-> Drive sleeve -> Stem nut -> Stem In the electric mode operation, the valve will continue to be repositioned until the limit switch, torque switch, a manual stop, or the motor overload protection device activates to stop motor rotation. If the above stopping mechanisms do not work, the valve only stops when the motor burns out or actuator breaks. 7

8 II.A. Details of MOV Actuator and Its Operation Electrical Train Limit switch: Determines when the actuator has operated sufficiently to reposition the valve by counting turns of drive sleeve assembly. Is operated by a gear on the drive sleeve. When enough drive sleeve rotations occur, the limit switch stops the motor. Also provides valve position indications before, during, and after MOV operation. 8

9 II.A. Details of MOV Actuator and Its Operation Electrical Train Torque Switch Torque switch s operation is based on the compression of spring pack as it responds to the increasing torque load experienced by the actuator. When the stem resistance increases, either by the valve being on it main seat or back seat or due to some valve damage, it is felt by the drive sleeve. And in turn the worm gear feels the resistance and feeds it to the worm. The worm, although being rotated by the motor, is free to move in a linear direction axially on the splines located on the worm shaft. It is held in place by the spring pack. The resistance of the worm gear allows the worm to overcome the spring resistance and move axially on the shaft. The displacement of the worm against the spring pack is used to operate the torque switch to stop the motor. 9

10 II.A. Details of MOV Actuator and Its Operation Electrical Train Spring Pack: The spring tension of the spring pack (called preload ) is set by the stop nut the bearing cartridge stem. Tightening the nut increases the preload. Loosening the nut reduces the preload. The proper position of the nut is determined by the manufacturer and should be maintained in order to keep the torque switch calibration accurate. 10

11 II.A. Details of MOV Actuator and Its Operation Manual Train Handwheel -> Handwheel shaft -> Handwheel shaft gear -> Hand wheel clutch pinion -> Worm shaft clutch -> Worm shaft -> Worm-> Worm gear-> Drive sleeve-> Stem nut -> Stem For handwheel operation, the declutch lever must be pressed first to make declutch mechanism disengage the motor from the worm shaft and engage handwheel shaft to the worm shaft. When the actuator is engaged in manual, the worm shaft clutch and its manual lugs are moved to engage the handwheel clutch pinon rings, the clutch drives the worm shaft through internal splines. 11

12 II.A. Details of MOV Actuator and Its Operation Manual Train Handwheel -> Handwheel shaft -> Handwheel shaft gear -> Hand wheel clutch pinion -> Worm shaft clutch -> Worm shaft -> Worm-> Worm gear-> Drive sleeve-> Stem nut -> Stem The worm/worm gear and handwheel gear set ratios allow the operator to provide more torque than the motor. And the manual power train does not have any protection schemes built in to protect the actuator or valve. Therefore, operating personnel must be extremely careful to prevent damage which can be caused by exerting excessive on the handwheel. 12

13 II.A. Details of MOV Actuator and Its Operation 13

14 II.B. MOV CCF Experience Events MOV CCF (failure to operate) experience events during 1980 and 1995 Collected in the NRC CCF Analysis and Database System (*) MOV failure to Operate (failure to close ( OO )+ failure to open ( CC )) CCF events Summarized for Plant Specific MOV CCF evaluation for a Reference Plants When needed, the original event descriptions in the NRC CCF Analysis and Database System were retrieved & reviewed to figure out more details of failure events. 148 MOV CCF events after removing duplicate event One of the event due to internal flooding during outage was excluded from the analysis A total of 147 MOV CCF failure events were remained for evaluation. (*): Experience events providers Proprietary Data 14

15 II.C. Determination of the Recoverability of MOV CCF by Manual Handwheel Operation MOV CCFs which may be recovered by a local manual handwheel operation: Loss of power or signal to the motor or motor control circuit Failure of the motor itself Failure of valve actuator internals which can be bypassed by declutching (for example, damaged motor pinion gear or the worm shaft gear) Failure involved premature motor operation stop before the valve properly was positioned due to incorrect limit switch setting/or torque switch setting/spring pack preload. Failure involved the valve stem over travel due to incorrect limit switch setting or torque switch setting (except for the case where valve damage occurred before motor stop either by thermal overload or motor burn out occurred) 15

16 II.C. Determination of the Recoverability of MOV CCF by Manual Handwheel Operation MOV CCFs which may be recovered by a local manual handwheel operation (continued): Excessive spring pack preload caused insufficient displacement of the worm to trigger torque switch even if the motor was experiencing excessive resistance and consequently motor thermal overload trip or motor burn out occurred. Considering that handwheel operation can generate more torque than the motor, this kind of MOV failures may be recoverable by manual handwheel operation (except for the case where valve damage occurred before motor stop either by thermal overload or motor burn out occurred) The MOV failures which involved either the damage to the actuator sub-parts or valve sub-parts which are needed both in electrical power train and manual power train or the damage to the declutch mechanism may not be recovered by a manual handwheel operation. 16

17 MOV CCF Events and Their Recoverability by Manual Handwheel Operation (examples) 17

18 MOV CCF Events and Their Recoverability by Manual Handwheel Operation (examples) 18

19 II.D. Calculation of the Manually Recoverable Fraction of MOV CCF Among Total of 147 MOV CCF events, 102 event were determined as being recoverable by manual handwheel operation. Therefore, 70.0% of the MOV CCF events are recoverable by manual handwheel operation. 19

20 III. CONCLUSIONS In this paper, MOV CCF (failure to operate) experience events were analyzed and the fraction of MOV CCF events which may be recovered by a local manual handwheel operation was estimated. Based on the examination of MOV CCF experience events during 1980 and 1995, about 70.0 % of MOV CCFs were determined as recoverable by local manual handwheel operation. Crediting local manual recovery of MOV CCFs should also consider the feasibility of operator recovery action. 20