Research On Joint Decision Of Maintenance And Spare Parts Inventory For A Single-unit System Based On Delay Time Theory

To guarantee equipment running safely and reliably,enterprises usually need to execute preventive maintenance(PM)and store the required spare parts in advance to improve the PM efficiency.However,uncertainty of plant systems degradation state results in the uncertainty of the maintenance demands,which in turn leads to the uncertainty of spare parts requirements.Therefore,blindly for maintenance and spare parts stock is likely to bring huge economic losses to the enterprise.It is a very important subject for the enterprise to develop an optimal maintenance policy,especially a joint optimal policy of maintenance and spare parts inventory according to the function degradation characteristics of equipment,improve the reliability level while reducing the long-run operational costs of equipment.In this paper,the delay time theory is used to describe the deterioration characteristics of plant systems.The joint decision of maintenance and spare parts inventory for the single-unit system is studied.The main research works are as follows:(1)Considering the frequent occurrence of false negative events in actual inspection activities,a joint policy of two-phase inspection and postponed ordering for spare parts is proposed,based on which the first phase inspection is carried out at time T,then the subsequent inspection interval is shortened,and spare ordering is placed at time ? rather than time 0.There exist two types of renewal events:(a)preventive replacement when the defective state is detected;(b)corrective replacement when the failure is arrived.When the replacement is needed we should consider the state of spare parts.If spare is not ordered,an immediate order is placed and replacement is delayed until the arrival time of spare parts.If the spare has been ordered but not on delivery,keeping the system operate or retaining its failed state until the spare arrival.Clearly,if the spare is available,the replacement can be performed immediately.A joint optimization model using the renewal reward theorem is established,which aims to minimize the expected cost per unit time,and artificial bee colony algorithm is used to obtain the optimal inspection and spare parts ordering policy.A numerical example fully demonstrates the effectiveness of the proposed model.(2)A two-phase inspection schedule and spare parts ordering policy with consideration of imperfect maintenance is proposed for the repairable equipment whose degradation process is subject to delay time theory.An order for spare is placed at the Mth inspection of the first phase inspection,after which the subsequent inspection interval is halved.Moreover,a combination maintenance policy of inspection-based and age-based is adopted in the proposed model,that is,imperfect maintenance is carried out when the defective state is found;when the service life of the equipment reaches a certain threshold level,a PM activity should be performed.The state of spare parts should also be considered when the replacement is needed for the equipment and,accordingly,all renewal scenarios can be determined.The proposed policy is compared with a periodical inspection schedule,and the result shows that the expected cost per unit time decreases significantly based on the proposed model with given parameters.Meanwhile,the optimal decision under the availability constraint is compared with the optimal decision which no constraint,and the result shows that the optimal decision under the availability constraint increases the expected cost per unit time,but the increase is small,while the device availability is significantly improved.(3)Considering that the quality of products begins to decrease when the production equipment in a defective state,an advanced replacement policy is proposed for a four-state single-unit manufacturing system.In terms of spare parts ordering,two modes of ordering,that is,a regular order and an emergency order are considered.Meanwhile,whether an emergency order is needed after the regular order has been placed is also concerned.Specifically,the model adopts a condition-based two-phase inspection and spare parts ordering policy,that is,when a minor defective state is identified the second phase inspection starts,meanwhile,a regular order is placed.There are two possible PR scenarios:an AR of the minor defective system is carried out at the Jth second phase inspection;a PR of the severe defective system.When the system needs to be replaced,whether the spare has been ordered should be firstly concerned.If it has not,an emergency order is placed;if the regular order has already arrived,an immediate replacement can be carried out;if the spare from a regular order has not delivered,a threshold level is introduced to decide whether to place an emergency order or wait for the arrival of the regular ordered spare.On this basis,a simulation model minimizing expected cost per unit time is established,and a numerical experiment is conducted using the refractory lining of the blast furnace for verification.(4)Aiming at the problem that the existing maintenance decision model and joint decision of maintenance and spare parts inventory cannot balance the reliability,availability and expected cost per unit time well,taking a maintenance decision as example,a multi-attribute utility theory is proposed to balance the reliability,availability and cost rate.Meanwhile,considering that imperfect maintenance activities are widely used in practical production,a periodical inspection policy model with three-stage failure process is developed.When a minor defect is detected imperfect maintenance should be performed;once a severe defect is found or a failure occurs the system should be replaced.The long-run availability and expected cost per unit time is established using the renewal reward theorem,and the long-run reliability level is adjusted by the reliability at the first inspection.The reliability,availability and expected cost per unit time are regarded as the attributes of the decision problem,and the multi-attribute utility theory is used to determine the inspection interval which maximizes the utility value of the decision-maker.