The effect of operational loading on the failure characteristics of mechanical valves

Reliability data from two nuclear power stations are considered in an attempt to determine the effect of operational loading on the failure characteristics of mechanical valves. Failure rate are shown to be a function of operating pressure and temperature, and failure models have been tentatively suggested for estimating stress factors for application to generic data. The rates of failure were markedly different for water and steam valves. Generally valves operating in water are shown to be about a factor of 10 lower than the generic average values estimates from statistically-weighted data in the NCSR Data Base. Steam value failure rates for typical low-pressure steam conditions (say 125 lb/in2 and 1750C) correspond closely to the average values derived from the data base for 'normal industrial conditions'. Although there is insufficient data to establish correlation for particular failure models the tendency of one failure mode (external leakage) to become dominant at higher duties was noted. Analysis of repair times indicates a log normal distribution with a mean of 4 hours. (author)