The influence of practical factors on the benefits of condition-based maintenance over time-based maintenance
Introduction
Due to ongoing automation of production processes and increasing reliance on expensive production equipment, the importance of effectively planned and performed maintenance activities is growing, and both the portion of employees working in maintenance and the maintenance costs are increasing [78]. As an illustration, over a quarter of the total workforce in the process industry, and up to 30% in the chemical industry, deal with maintenance operations [71]. In refineries, the maintenance and operations departments are usually the largest [17]. Furthermore, maintenance costs typically account for 15–70% of the total value of the end product [8], [44], the amount of money spent on maintenance of engineering structures and infrastructures is increasing continuously [68], and medical equipment maintenance nowadays demands large sums from hospital budgets [14].
Many firms still apply ‘traditional’ time-based maintenance (TBM) strategies, which are easy to implement as only the time that a unit is in service has to be recorded. However, substantial remaining useful life is wasted if the machine is still in reasonable condition when preventive maintenance is performed, and a breakdown might occur if it happens to deteriorate faster than expected. Due to the increasing technical possibilities to monitor, store, and analyze conditions, condition-based maintenance (CBM) strategies are gaining popularity [10], [18], [28], [59], [64]. Condition-based maintenance generally results in more effectively scheduled preventive maintenance, and, in the ideal case, preventive maintenance that is performed just before failure.
The relative benefit of CBM, however, strongly depends on the behavior of the deterioration process and the severity of failures. Furthermore, it is affected by various practical factors that are often present in practice, viz., required planning time, imperfect condition monitoring, and variation in the deterioration level at which failure occurs. CBM should only be applied if this relative benefit outweighs the efforts and costs during the entire life cycle that are required to apply CBM [22], [50], [60], [69]. The requisites to switch from time-based to condition-based maintenance include condition monitoring equipment and software to store, analyze, and initiate maintenance actions [3], [59]. Companies that are interested in implementing condition-based maintenance must also consider the risks related to the lack of experience [78]. Furthermore, they should realize that CBM requires a dynamic scheduling of maintenance activities, whereas they might not have the capability for such flexible planning.
The first contribution of this paper is to review studies that compare condition-based and time-based maintenance, as well as studies that consider the above practicals factor in a CBM model. Although both CBM and TBM have received ample attention in the scientific literature, few studies compare them. Moreover, existing comparative studies confine themselves to a few examples. Insights on how the various characteristics influence the performance of condition-based and time-based maintenance are lacking. Therefore, our second contribution is to derive insights on the effects of the various characteristics on the relative benefit of CBM from a numerical investigation. We start with the effects of the behavior of the deterioration process and the severity of failures. Thereafter, we extend our model and analyze the effects of the practical factors on the relative performance of CBM. The obtained insights are useful in practice to decide what factors are most important when considering to switch from TBM to CBM, and for avoiding the risk of switching from TBM to CBM in situations where benefits do not outweigh costs.
The remainder of this paper is organized as follows. In Section 2 we review existing studies that compare condition-based maintenance with time-based maintenance, and studies that consider planning time, accuracy of condition measurements, and predictability of the failure deterioration level. The approach that we use to compare the two maintenance strategies is discussed in Section 3. This section also contains formal definitions of the condition-based maintenance and the time-based maintenance strategy that we adopt. In Section 4 we consider the effect of the behavior of the deterioration process and of the severity of failures on the relative performance of CBM. In Section 5 we point out how this performance is influenced by required planning time, imperfect condition information, and predictability of the deterioration level at which failure occurs. We end with conclusions and suggestions for future research in Section 6.
Section snippets
Literature review
We start this section with a review of studies that compare condition-based maintenance with time-based maintenance. Thereafter, in Section 2.2, we review studies that consider various practical factors that influence the relative performance of condition-based maintenance.
Approach
We consider a single-unit system, i.e., a unit that can only be maintained in its entirety. The unit is monitored continuously and its deterioration is modeled using a gamma process. If the unit fails, a corrective maintenance action has to be performed. Preventive maintenance can be performed before failure of the unit. Both maintenance types make the unit as-good-as-new, maintaining and replacing the unit are thus interchangeable notions.
Corrective maintenance is assumed to be more expensive
A comparison of CBM and TBM
We start our comparison of the performances of condition-based maintenance and time-based maintenance with an initial setting of the parameters, the so-called base case. Thereafter, we will assess the effect of changing the (values of the parameters that drive the) behavior of the deterioration process and the cost structure.
In our base case, we select parameter values rather arbitrarily. Other values reveal similar patterns, and those are of interest to us rather than specific outcomes. We set
Practical factors affecting the benefits of CBM
So far, we assumed that maintenance can be performed immediately when a certain level of deterioration is reached, that the exact level of deterioration can be observed without any errors, and that failure always occurs at the exact same level of deterioration. In this section, we relax these assumptions and assess the influence on the relative performance of CBM.
Conclusions and future extensions
We have considered the benefit of condition-based maintenance compared with time-based maintenance. We started with a literature review of studies that compare CBM with TBM, and of studies that consider required planning time, imperfect condition monitoring, and variation in the deterioration level at which failure occurs in a CBM model. These practical factors affect the relative benefit of CBM. It turned out that existing studies confine themselves to a few examples and that general insights
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