A. H. Christer

A state space condition monitoring model for furnace erosion prediction and replacement

The paper develops a replacement action decision aid for a key furnace component subject to condition monitoring. A state space model is used to predict the erosion condition of the inductors in an induction furnace in which a measure of the conductance ratio (CR) is used to indirectly assess the relative condition of the inductors, and to guide replacement decisions. This study seeks to improve on this decision process by establishing the relationship between CR and the erosion condition of the inductors. To establish such a relationship, a state space model has been established and the system parameters estimated from CR data. A replacement cost model to balance at any time costly replacements with possible catastrophic failure is also proposed based upon the predicted probability of inductor erosion conditional upon all available information. The well known Kalman filter is employed to derive the predicted and updated probability of inductor erosion level conditional upon CR data to date. This is the first time the condition monitoring decision process has been modelled for real plant based upon filtering theory. The model fits the data well, gives a sensible answer to the actual problem, and is transferable to other condition monitoring contexts. Possible extensions are discussed in the paper.

Towards a general condition based maintenance model for a stochastic dynamic system

This paper considers a stochastic dynamic system subject to random deterioration, with regular condition monitoring and preventive maintenance. A model is presented to advise at a monitoring check what maintenance action to take based upon the condition monitoring and preventive maintenance information obtained to date. A general assumption adopted in the paper is that the performance of the system concerned can not be described directly by the monitored information, but is correlated with it stochastically. The model is relevant to a large class of condition monitoring techniques currently employed in industry including vibration and oil analysis. The model is constructed under fairly general conditions and includes two novel developments. Firstly, the concept of the conditional residual time is used to measure the condition of the monitored system at the time of a monitoring check, and secondly, contrary to previous practice, the monitored observation is now assumed to be a function of the system condition. Relationships between the observed history of condition monitoring, preventive maintenance actions, and the condition of the system are established. Methods for estimating model parameters are discussed. Since the model presented is generally beyond the scope for an analytical solution, a numerical approximation method is also proposed. Finally, a case example is presented to illustrate the modelling concepts in the case of non-maintained plant.

A simple condition monitoring model for a direct monitoring process

This paper addresses the problem of condition monitoring of a component which has available a measure of condition called wear. Wear accumulates over time and monitoring inspections are performed at chosen times to monitor and measure the cumulative wear. If past measurements of wear are available up to the present, and the component is still active, the decision problem is to choose an appropriate time for the next inspection based upon the condition information obtained to date. A simple model which minimizes the expected cost per unit time over the time interval between the current inspection and the next inspection time is derived, and numerical examples are given to demonstrate the solution method.

Review of delay-time OR modelling of engineering aspects of maintenance

This review paper discusses the development of delay-time analysis as a means of modelling engineering aspects of maintenance problems. The main concern of the paper is with the philosophy, underlying principles, assumptions and experiences in practice, with references being given to more detailed accounts elsewhere. The current state of knowledge and research in the area will be outlined, and future trends in modelling applications and research predicted. A question that will be addressed is whether the application of delay-time analysis could and should be de-skilled to the extent that it could be effectively utilised by engineers in the absence of a modelling analyst.

A model of condition monitoring of a production plant

Abstract This paper addresses the modelling of a problem of growing interest to production engineers, namely the appropriate and cost-effective way of condition monitoring of a production plant. Here a condition monitoring test is assumed which checks the state of wear of a component and records a (0,1) signal depending upon all being well or that wear is below a critical level with the possibility of imminent failure. Decision variables are the critical warning level and frequency of monitoring inspections. Consequence variables are the cost of maintaining plant and the availability of plant for production.

Solution algorithms for a nonhomogeneous multi-component inspection model

In this paper, we present three solution algorithms for an established multi-component inspection system model based upon the delay time concept. First, Algorithm 1 is developed for obtaining the system replacement time if the defect arrival process is non-homogeneous. Algorithm 2 is presented as an extension to Algorithm 1 in which the non-constant optimal inspection intervals are also determined. Algorithm 3 is a numerical algorithm for solving an integral equation arising within the model in the case of opportunistic inspection at failures. Finally, an example is given to demonstrate the algorithm set in practice. The proof of the existence and uniqueness of solutions are presented.

Problem identification in maintenance modelling: a case study

This paper is concerned with a problem identification and problem focus process in maintenance modelling. It endeavours to describe the process of moving from vague problem understanding towards more specific problem formulation and problem focus in the pursuit of practical decision making. This process was conducted using several analytical tools that complemented each other such as regression analyses, snapshot modelling and delay time modelling. As in many case studies related to maintenance modelling, this study also makes use of the experience of experts. It can be seen from the paper that subjective data estimates can prove to be a useful input for modelling. The analysis shows how simple modelling of maintenance problems can provide useful insights and better understanding of the problem in hand.

A Review of Delay Time Analysis for Modelling Plant Maintenance

Delay time analysis is a pragmatic mathematical concept readily embraced by engineers, which has been developed as a means to model maintenance decision problems. Attention is focused upon the maintenance engineering decisions of what to do, as opposed to the logistical decisions of how to do it. This paper reviews the cumulative knowledge and experience of delay time modelling. The decision environment within which Delay time models are intended as decision aids is briefly reviewed, and the initial development of simple DT models for a repairable component and a complex plant presented. Variations on the basic model are outlined and discussed, including perfect and nonperfect inspection, steady state and non-steady state conditions, and homogeneous and non-homogeneous Poisson arrival rate of defects. Attention is given to the parameter estimation process, and both subjective and objective estimation techniques are outlined. Case sketches present practical experience in using the DT concept to model actual plant, to assess the benefits obtained, and to validate modelling and parameter assessment. References are given throughout to related work as well as to future developments.

Maintenance manpower modelling for a hospital building complex

A building maintenance manpower simulation model is developed within the case context of a specific hospital complex to investigate the potential gain to be realised using different manpower management and operational procedures. Allowance is made for daily variation in demand for maintenance, sickness and holiday patterns, and the characteristic of different trades. Numerical results are presented for the plumbing trade as a particular example of modelling output.

Modelling in the absence of data: a case study of fleet maintenance in a developing country

Adequate and relevant objective data for modelling maintenance decision problems are often incomplete or not readily accessible. This is particularly true in developing countries. In this paper the experience gained between 1991–95 in conducting a maintenance study of an inter-city express bus fleet in a developing country is presented. The lack of available maintenance records and operating data rendered the study the most data-starved maintenance modelling exercise the authors have met before or since. The study required the use of subjective methods to both define the problem and to estimate parameters, and the application of recently developed concepts in maintenance modelling along with snapshot analysis and delay time modelling. This imposed a structured approach to problem recognition and problem solution. The study contributed both directly and indirectly to a change in work culture and to a reduction in bus breakdown rate. The company was re-visited 5 years later specifically to seek evidence of lasting impact. Some evidence existed and is reported in the paper.