Research On Preventive Maintenance Policy For Plant Systems Based On Delay Time Theory

Plant systems are common in daily life and production industry. The occurrence of failure will endanger person safety and production process, thus preventive maintenance (PM) policy has been widely carried out to avoid failure in various industries. Numerous PM policy studies have been proposed for plant systems; however, there are some problems in theoretical methods and practical applications that need to be addressed. Based on the delay time modeling technique, this paper analyzes the practical scenarios of performing and planning PM policy in industry, then proposes the following topics as follows.(1)An inspection and replacement policy with a three-stage failure process is proposed for a single-unit system, based on which inspection is periodically and perfectly carried out to check the system and the system is immediately replaced once its age reaches to a limit. It is assumed that there are two options if the minor defective state is identified by inspection:namely immediately renew or do nothing. However, immediate renewal is always to be done at the time of either the severe defective identification or failure. An inspection and replacement policy model using the renewal award theorem is established by taking all renewal scenarios into account, which aims to optimize the inspection and replacement intervals by minimizing the expected cost per unit time. Numerical example is given to illustrate the applicability of the proposed model.(2)Considering different levels of inspection in industry, a two-level inspection policy subject to minor and major inspections is further modeled for a single-unit system. The minor inspection is assumed to be imperfect to identify the minor defective state but can always reveal the severe defective state, and the major inspection can however reveal all states. The system is immediately renewed once it is identified to be in the defective state or a failure occurs. A single inspection policy including only major inspections is also given to illustrate the applicability of two-level inspection policy, and then the threshold for the minor inspection cost is derived to say whether the two-level inspection is better than the single inspection policy. Finally, some discussions are made in the numerical example.(3)A separated PM window is usually set in the single-unit system for removing the defects identified at previous inspection, so we model a two-level inspection policy with PM. There are two delay renewal scenarios involved in the proposed model:the minor inspection interval is shortened to increase the frequency to check the system if the minor defective state is found by inspection, and once the severe defective state is revealed by inspection, renewal is left to the next PM if the time interval to the next PM is less than a threshold level; otherwise renewal is done immediately. The comparison of the proposed policy with a single inspection policy with PM and a two-level inspection policy shows that the expected cost per unit time decreases based on the proposed policy with given distribution and cost parameters.(4) The above studies focus on a single-unit system, but complex system with numerous units is more common in industries. A case study for a complex system-the steel converter system in a mill is researched, where a periodical PM policy is executed for the system and the system is renewed with an interval of a year by an overhaul. Since production stoppages caused by waiting for raw materials provide windows to inspect and maintain the system, these production stoppages are incorporated into the PM model. Considering the nature of different defects in the system, two types of defects are presented:small and large defects. Through analyzing the collected data on maintenance actions and downtime record, the maximum-likelihood function based on two-stage delay time theory is constructed and the modeling parameters are estimated. Then it is verified by simulation test and goodness of fit test, finally the PM model is established to minimize the total downtime over a renewal cycle and find the optimal PM interval. The results show the current PM policy (2or3months) is not optimal and once the production stoppage process changes, managers should reconsider the PM policy.