Prevention and improvement of negative operational margin for motor-operated flexible wedge gate valves evaluated via performance prediction in nuclear power plants

Operational margin is calculated using design values in Design Basis Review(DBR) report. After that, static diagnostic test and dynamic diagnostic test are performed in order at opening and closing stroke, to measure actual values of some parameters for calculating operational margin. Then a report distinct from the DBR report, called final evaluation report, is issued. In the final evaluation report, operational margin is calculated by applying actual values instead of design values. For some MOVs which are not applicable to diagnostic test under dynamic condition, an analytical method called performance prediction is used as an alternative. For performance prediction, a type of analysis program, EPRI MOV Performance Prediction Methodology (EPRI MOV PPM) is used. This study describes a kind of measures for improving or preventing negative operational margin for motor-operated flexible wedge gate valves which performance prediction is applied to. This study describes the measure for improving operational margin at opening stroke for motor-operated flexible wedge gate valves which performance prediction is applied to. With the review of equation (3) and (4), and comparison of valve evaluation result in table 2 and table 3, it is concluded that maximum thrust achieved during static closure stroke(FC) and measured static unwedging thrust(Fstat) are critical parameters determining required stem thrust at opening stroke.