point in the future—maintenance activities will be appropriate and cost-effective.
Operator-Based Maintenance
Operator-based maintenance uses the fact that operators are often the first line of defense against unplanned asset downtime. OBM assumes that the operators who are in daily contact with the assets can use their knowledge and skills to predict and prevent breakdowns and other losses. OBM is synonymous with autonomous maintenance, one of the basic pillars of total productive maintenance. TPM is a Japanese maintenance philosophy that involves operators performing some basic maintenance activities. The operators learn the maintenance skills they need through a training program and use those skills on a daily basis during operations.
Corrective Maintenance
CM, sometimes called repair, is performed to correct the deficiencies found during PM and CBM assessment; it restores the asset to good working condition after it has failed or stopped working. CM is also an action initiated as a result of an asset’s observed or measured condition before or after the functional failure. The CM work can be further classified into three categories.
CM—Scheduled
Scheduled CM is a repair activity performed to mitigate potential asset failure or correct deficiencies found during PM and CBM tasks. It brings an asset to its designed capacity or to an acceptable level in a planned way. This work should be planned and scheduled.
CM—Major Repairs/Projects (Planned and Scheduled)
In many organizations, all major repairs or improvement work valued over a certain threshold—e.g., overhauls and turnaround projects—are treated as capital projects for tax purposes. If these projects are to bring the asset back to the designed capacity, not to add additional capabilities, they should be treated as corrective maintenance. In that case, these projects should always be planned and scheduled.
CM—Reactive (Unscheduled), Aka Breakdowns/Emergency
Reactive (unscheduled) corrective maintenance basically repairs assets after they fail. This work is also known as breakdown or failure repair work. Most of the time, completing this work interferes with the regular weekly schedule. Unscheduled work costs much more than planned and scheduled work.
Some maintenance professionals classify maintenance in the following categories: PM, CBM/PdM, proactive work resulting from PM and CBM/PdM, and reactive CM (breakdowns/emergency). As we discussed in the previous chapter, it really does not matter how we classify them as long as maintenance management systems can provide us data in the desired format to help us to make the right decisions. Our objective is to reduce reactive breakdowns and then adjust or increase PM and CBM work accordingly.
Sometimes we try to mix maintenance work types with how we respond to get the work done. For example, is emergency work really unplanned/unscheduled CM, or is it reactive work that needs to be done now? In some organizations, the breakdown work is called urgent maintenance but could be done within 48 hours. Some regular work, also sometimes called routine work, may need to be completed in 5 or 7 days. These examples are not the work type, but just how we respond to get it done.
Sometimes a decision is made to take no actions or make no efforts to maintain the asset as the original equipment manufacturer (OEM)originally intended. Therefore, no PM program is established for that particular asset. This maintenance strategy, called run-to-failure (RTF),should be applied only after a risk to the business has been analyzed and its cost-effectiveness determined. In reality, this work should not be considered a failure or reactive work because we made the decision in advance not to perform any PM or CBM based on economic justification.
All maintenance work needs to be documented and classified in a CMMS/EAM system. Then, to optimize resources, all jobs should be prioritized per the organization’s priority system for execution. Every organization should, if it doesn’t have one, establish a jobs/work order priority system. Asset criticality is one of the factors in calculating job priority. Asset criticality is a ranking of factory/plant assets according to potential operational impact. Criticality supports prioritization of assets, which are important to monitor and should be maintained at a determined level of maintenance. Criticality helps to optimize resource utilization.
Asset Criticality Analysis
Asset criticality analysis (ACA) is performed to evaluate how an asset failure can impact organizational performance. ACA provides the basis for determining the level of rigor for each asset with respect to the maintenance strategy development processes.
ACA is based on a risk analysis including established criteria such as safety, environmental, and quality risks; operational impact (supply chain, demand, profitability, etc.); maintenance and reliability impact (consequence and severity of failure); single point of failure; asset replacement cost; and spares lead time. The purpose is to help us perform the appropriate level of maintenance to minimize risk and to optimize resource utilization, performance, and reliability.
The asset criticality number (ACN) is calculated by the following equation:
ACN = consequence of failure ×
(sum of severity of failure factors)
The asset’s criticality can be established, ranked, and named based on failure impact and its consequences, as shown below for each asset in the plant:
• Catastrophic or very high (VH), 4 out of 4, 8–9 out of 9
• Severe, high (H), 3 out of 4, 6–7 out of 9
• Moderate, medium (M), 2 out of 4, 3–5 out of 9
• Minor, low (L), 1 out of 4, 1–2 out of 9
This is just one example of establishing and ranking asset criticality. Some professionals rank just in 3 (A, B, C) or 1–9 or as shown above. Establishing criticality in a smaller group of 3 or 4 is a little easier,but higher grouping in 5, 9, or other amounts enhances the granularity. Grouping in 3, 4, or 5 is very common.
Figure 4.7 provides an example of establishing a criticality number or ranking based on established criteria. In this example, five criteria elements on the left are evaluated and assessed based on the consequences of failure on the right. The criteria elements could each have the same weighting factor or different ones. An aggregate of all factors divided by the number of factors produces the asset’s criticality number.
Suppose all five factors have the same weighting and the assessment criteria indicate the following:
| 1. Safety | Severe | 3 |
| 2. Environment | Minor | 1 |
| 3. Production/operation | Moderate | 2 |
| 4. Maintenance | Severe | 3 |
| 4. Other considerations | Minor | 1 |
| Total = 10 | ||
| Criticality = 10/5 = 2 (moderate) |
Therefore, in this example, the asset has a criticality of 2, or moderate.
Generally, the criticality ranking number is stored in a separate field against each asset in a CMMS/EAM system.
FIGURE 4.7 Asset Criticality and Consequences
Some benefits of having an asset criticality rating are:
• It supports the overall priority structure and importance of performing maintenance tasks.
• It determines