Session: Session 3. Integrity Management of Critical Systems

Paper Number: 119693

119693 - Reliability Analysis of Steam Generator Level Control Valves 

The feedwater level in steam generators of a nuclear reactor is controlled by a group of air-operated level control valves (LCVs). In the air operated valves, aging related degradation of diaphragm causing the air leak is a dominant mode of malfunction of the valve actuator. The preventive maintenance (PM) through valve and actuator overhauls (OH) are undertaken to avoid such degradation failures. At present, a time-based approach is used to carry out PM of LCVs.  The main objective of this paper is to develop a risk-informed approach to the maintenance of the LCV population at a nuclear station. The first step in this direction is to quantify the lifetime distribution and reliability of the LCV.

The paper summarizes the operating experience of a group of 32 large LCVs (10” size) that includes occurrences of various types of malfunctions, repair actions to mitigate the effect of degradation, and valve overhauls over a period of 20 years.  Typical malfunctions of an LCV include air leaks from the actuator, defective operation of the positioner and accessories, , and packing leaks. The operating history of each valve is translated into lifetime data. The Weibuill distribution is fitted to this lifetime data using the maximum likelihood and probability paper plotting methods. Using the Weibull model, the mean time to failure and the valve reliability over the OH interval is computed for each malfunction mode, i.e., actuator, positioner, and valve packing. 

The Weibull analysis leads to a conclusion that the aging is absent from these three major components of the valve. The mean lifetimes of actuator, control and packing are estimated as 18, 19, and 30 years, respectively. The reliability of the valve actuator over a 4-year OH interval is estimated as 0.8.

Presenting Author: Mahesh Pandey University of Waterloo

Presenting Author Biography: Professor and Research Chair in the area of risk assessment and life cycle management of nuclear power plant systems for last 20 years.