Condition based maintenance versus Time based maintenance

As we will see, time based maintenance can be a better choice than condition based maintenance for certain cases.

Time based maintenance is easier to implement, once that the time that a unit has been on service it the only thing that must be recorded. However, substantial remaining useful life may be wasted if the asset is discarded still in reasonable condition, or breakdown might occur if it deteriorates faster than expected.

The benefits and success of condition based maintenance strongly depends on the behaviour of the deterioration process and the severity of failures. Some practical factors that affect the accuracy and precision of the collected data will severely affect the results of this maintenance technique.

Condition based maintenance should only be applied if the relative benefit (cost-savings) outweighs the efforts and costs of its application during the entire life cycle. To switch from time-based to condition-based maintenance is necessary to acquire and implement equipment and software that will store, analyse and initiate maintenance actions.

So, it may be important to understand if it justifies to switch from time based maintenance to condition based maintenance. Considering that both methods are working in optimal con- ditions, the performance of condition-based maintenance turns out to be much better than the time-based one, but in real case scenarios, it is no longer so. In some cases, a combination of this two techniques assures the best results.

De Jonge et al [23] studied how the influence o practical factors (planning time, imperfect condi- tion information and uncertain failure level) can affect the benefit of condition based maintenance over time based maintenance. A succinct resume of the overall analysis will be presented.

There are some pre-established studies, [24] and [25], that support that the relative benefit of condition based strategy turns out to increase in the relative replacement cost and that cost savings are more significant for short times between inspections. So, there is a explicit relation between this technique benefits and the replacement cost and time between inspections.

Other factors, as the behaviour of deterioration process and the cost of preventive mainte- nance, affect the overall benefit of one technique over the other. It turns out that the behaviour of the deterioration process is much more important than the cost of performing preventive maintenance. The cost differences between condition-based and time-based maintenance are substantial for small levels of variation in the deterioration process, but as this level gets bigger, the cost difference diminishes. Only for extremely small or extremely large preventive main- tenance costs, the benefit of condition-based maintenance is limited; but for a wide range of preventive maintenance cost there are substantial cost saving.

The efficiency of both methods depend on various factors as, for example, the deviation of the deterioration process (the failure may occur sooner than expected/predicted). For condition based monitoring, the accuracy of the prediction is severely influenced by the quality of mea- sured data. Vibration and oil analysis techniques are especially prone to inaccuracies. To study the influence of practical factor on the benefits of condition based monitoring over time based maintenance, we will consider three parameters at different ”weights”, input them in a mathe- matical model and draw results. The three parameters considered are the following: planning time, imperfect condition information and uncertain failure level.

Planning time represent the ”delay” between the time when it is realized that maintenance must be done (when condition-based monitoring detects a fault) and the time when mainte- nance is actually done (which is not always immediate, as the maintenance may depend on the availability of repair man or spare parts). It is considered that, for time based maintenance, it is always possible to perform the maintenance on the pre-defined data. Imperfect condition infor- mation represents the uncertainty in triggering the maintenance based on the acquire data. As this factor gets bigger, worse will be the results from the monitoring. As the measurement errors only influence the observed information, this factor doesn’t affect time-based maintenance. 42

4.1. MAINTENANCE TECHNIQUES

(a) Cost rate as a function of planning time s. (b) Cost rate for a varying level of uncertainty σp in the obtained deterioration information.

(c) Cost rate for a varying level of uncertainty σf in the failure level.

Figure 4.1: Cost rate under optimal TBM strategy and CBM strategy.

Uncertainty failure level represents the deviation in the deterioration process. As it gets larger, the variance of the time until failure will also get bigger. It affect both condition-based and time-based maintenance.

Required planning time and imperfect condition information only affect condition-based maintenance whereas uncertainty in the failure level affect both of the techniques (although it has a stronger effect on condition-based than on time-based monitoring).

The cost benefit of condition-base over time-based maintenance decreases linearly in the plan- ning time, until it equal zero when this value reach its limit (when planning time is equal the maintenance age of the optimal time-based maintenance strategy).

With imperfect condition monitoring, although for small values the cost benefit is substantial, for larger values there is a certain time where condition-based maintenance performs worse than time-based maintenance, turning out that the last one is more costly effective.

With uncertainty in the failure level, the relative cost benefit decreases, but it continues to be always positive.

All the factor can significantly affect the benefit of condition-based monitoring over time- based monitoring, although in the overall condition-based maintenance present a better perfor- mance than time-based maintenance.