What is the difference between criticality and risk analysis with regards to maintenance and reliability?

Criticality is a measure of how important an asset is to your process. The more critical the asset, the more of an impact it will have if it fails.

Risk analysis, on the other hand, refers to the total amount of risk posed by specific failure modes, taking both probability and severity into account.

Assessing Asset Criticality

There are many ways to perform criticality analysis, one of which has to do with rating an asset based on its impact on various categories. These categories may include:

• Health and safety
• Natural environment
• Operations
• Customers

Give each asset a rating (often 1-6) on how severe its failure would be for each category. For instance, a failure in your conveyor system might rank a 5 on operations, but only a 1 for the environment.

Once you’ve ranked an asset in each of these categories, you might either multiply them together or leave them as individual criticality ratings. In either case, the higher the number is, the more critical the asset will be.

Data that informs asset criticality

Since an asset’s criticality is based on what could happen if it fails, the asset’s history can be a valuable resource when evaluating it. Work orders and maintenance reports provide insight into what can happen with a given piece of equipment, and they can also help you figure out how much downtime is associated with specific failure modes.

In addition to your maintenance records, other sources can help you determine how severe a given breakdown can be. For example, when evaluating the impact of a mechanical failure in your facility, you might look at records detailing defects, lost production time, and so forth.

Overall, the types of metrics you might look at include:

• Number and types of failures over the life of the asset
• Unplanned downtime associated with specific failure modes
• Number of defects
• Work order completion times or mean time to repair (MTTR)
• Maintenance costs associated with failures

Using asset criticality

Typically, the more critical an asset is, the more important it is to keep it from breaking down. As such, critical assets are often prioritized for preventive maintenance.

That said, the fact that an asset is critical doesn’t mean it should be the focus of all of your maintenance efforts. A common mistake maintenance teams make is to over-maintain critical equipment. In those cases, more time and resources are poured into keeping an asset healthy than it actually needs.

What criticality can be used for is to assist with maintenance scheduling. When creating the schedule for the week, maintenance planners can use the criticality of an asset to prioritize which tasks should be completed first.

In addition, critical assets may warrant more thorough maintenance planning and analysis. Knowing which machines are most important to keep running can give you a better idea of where to spend time on analysis and condition monitoring.

Risk Analysis

Risk analysis is a way to determine how much risk an asset poses to your facility, such as in safety or production. As such, it incorporates both the potential consequences of a failure, as well as how probable it might be.

The process of risk analysis takes the odds of a failure occurring (rated from 1-6) and multiplies it by the asset’s criticality. The end result is a ranking of how much risk is posed by the asset’s operations to your facility.

To get an idea of how likely a given failure mode would be, you can use data tracked through your CMMS. By tracking data logged into your system, you’re able to determine how frequently a given failure mode occurs and see what the risk would be for that failure in the future.

Data that supports risk analysis

Since risk deals with probability as well as consequence, it benefits from assessing the frequency of given failure modes as well as their severity. Mean time between failures (MTBF) can be useful when performing risk analysis, as can other historical data related to the asset.

Information can also be derived from manufacturer information provided for each asset. For instance, a manufacturer may rate a bearing system you use in rotating equipment for a set number of cycles. That number can be useful when determining the potential frequency of a given failure mode.

Using risk analysis

When done properly and thoroughly, risk analysis can give you insight into what shape your maintenance plan should take. It helps you know which failure modes are likely to occur, and therefore helps you determine what types of maintenance work are most worth including for each asset.

The primary benefit is it allows you to streamline your preventive maintenance plan. You’re better able to target PMs on keeping likely (and potentially detrimental) breakdowns from happening, while spending less time on tasks that yield little to no benefit.

Risk-based maintenance (RBM) relies heavily on risk analysis. The steps to creating a risk-based maintenance plan include:

1. Collect data from your machines and systems
2. Determine the likelihood that an asset will fail
3. Determine the consequences of failure
4. Rank the risks and their potential consequences
5. Plan inspections and PMs based on mitigating risks
6. Put the plan into action
7. Assess the results and repeat the process

In addition to creating the groundwork for RBM, risk analysis can also help with some of the more nuanced components of reliability planning, such as making sure you have the right spares in stock and handling plant redundancy.

Using Risk and Criticality Together

Criticality and risk go hand in hand when performing maintenance planning. The more critical the asset is, the more you’ll want to make sure it keeps running, but if it’s also low risk, it may not be as high of a priority in your maintenance planning unless a failure would be particularly severe.

Likewise, an asset that’s likely to fail but is not critical to your operations won’t necessarily be high risk because a failure would have a negligible impact.

On the other hand, equipment that’s both highly critical and failure-prone would take higher priority, so you’ll want to schedule preventive maintenance accordingly.

Criticality and risk in criticality analysis

While criticality and risk are technically two separate concepts, they are frequently used together in maintenance planning. Criticality analysis often takes more than severity into account by combining it with the likelihood of that asset failing.

Using both criticality and probability in criticality analysis helps improve maintenance planning by creating a more complete picture of the potential risks posed by each asset. Ratings for both criticality and risk are multiplied together to create a rating of how that asset should be prioritized, known as a risk priority number. The higher the number, the more of a priority it usually is for maintenance planning purposes.

Limitations of criticality and risk

While criticality and risk together often create a more robust assessment of how to prioritize maintenance tasks, they aren’t the only consideration that should be made.

Highly severe failure modes

For instance, if the failure mode of an asset would have catastrophic results in terms of safety and health (getting a rating of 6), it’s generally worth preventing, even if it’s not very likely (such as with a rating of 1). As such, PMs, condition monitoring, and other measures that prevent that failure mode may take priority over equipment faults that technically have a higher risk priority number but aren’t particularly severe.

Cost-effectiveness

Additionally, performing preventive maintenance on some pieces of equipment may not necessarily be cost-effective, even if that equipment is important to your process and tends to fail.

A prime example is equipment that is old or outdated. Older equipment tends to fail more often, causing more downtime than other factors, so while that may drive up the frequency of breakdowns, keeping up with some PMs may not be worthwhile if the machine is just going to be replaced in the near future.

Detection

In some cases, it may be clear that a given failure mode is both likely and potentially severe, but the odds of detecting that failure are low. As such, risk and criticality are not enough. Accounting for limited visibility over the equipment’s current condition is also important.

For this reason, maintenance teams may want to add detectability as a third rating into their criticality analysis, with 1 being easy to detect and 6 being hard or impossible.

Summary and Conclusion

To sum everything up:

1. Criticality is a representation of how important an asset is to your facility and how severe the results would be if it were to fail.
2. Risk deals with the severity of a failure as well as its probability.
3. Both risk and criticality play a role in criticality analysis.

When used properly, criticality and risk analysis can help when prioritizing maintenance and performing reliability planning.