ANALYSIS OF A 2-PHASE MODEL FOR OPTIMIZATION OF CONDITION-MONITORING INTERVALS

Conclusions - Condition monitoring is a maintenance strategy where dec isions are made depending on either continuously or regularly measured equipment states. It is often an efficient tool for cost-effective ma intenance, since compared with time-based preventive maintenance, it r educes uncertainty with respect to actual states of equipment, and can thus avoid unnecessary repair or replacement. However, it involves ca pital expenditure and/or operational costs to perform measurements. Th is paper presents a basic model for the economic evaluation and optimi zation of the interval between successive condition measurements (also called inspections), where measurements are expensive and cannot be m ade continuously. It assumes that the technique can detect an intermed iate state to failure for a failure mode of interest, The influence of competing risks is analyzed, leading to the conclusion that once the cost-effectiveness of the condition-monitoring has been established, c ompeting risks need not be considered in determining the optimum condi tion monitoring interval. Inspection is cost-effective if the intermed iate state has a: 1) non-decreasing hazard rate, and 2) shorter mean r esidence time than the good state (good-as-new condition), while costs of failure are high enough compared with inspection and repair costs in the intermediate state. Assuming that the distribution of the resid ence time in the second state is unimodal, estimation of the mean (or scale parameter) and standard deviation of this state, in many cases, provides enough information to make a good decision on the inspection interval. The most important model parameters are identified by sensit ivity analyses; it is shown that the model can be simplified without s eriously affecting optimal decision making.