Awhile back we published a quick “how to” on route building. We mentioned criticality assessment and mentioned that there were a few ways to do it. Now we’d like to discuss some of the various ways that equipment criticality can be assessed.
The largest misconception when it comes to criticality is what criticality actually means. Many maintenance and reliability professionals confuse “criticality” with “importance”. When you ask what the most critical pieces of equipment in a facility are, they will instinctively list the equipment that is most important. Here’s an item you want to remember, now and always; Criticality is based of probability and consequence of failure. Importance (or alternately “criticalness”) is based on the need for the asset to perform the functions of the system in which it is installed. We can’t alter the importance of an asset, unless redundancies are installed, be we can alter criticality, which is one of the basic functions of reliability centered maintenance. We want to reduce the probability of failure, consequences of failure, or both, for assets by prioritizing maintenance efforts. Having said that, how do we arrive at this assessment of actual criticality, versus a perceived level criticality that is in fact importance?
Let’s take a high-level view. There are basically two approaches, qualitative and quantitative. There is a military standard, MIL-STD-1629A, Procedures for Performing A Failure Mode, Effects and Criticality Analysis. The two methods are described in this standard. What’s the difference? Essentially numbers. Can you measure something and assign it a value? If so, that is a quantitative analysis. Actually, the basic methodology for both is quite similar. Certain factors must be identified.
- Items to be assessed
- Function of the items
- Failure modes
- Failure effects
- Failure causes
- Current controls
- Recommended actions
After identifying these factors, what must come next is a method to evaluate the risk associated with failure. Here’s where we split off between Risk Priority Number (RPN) approach (quantitative) and Criticality Analysis method (qualitative).
The RPN approach calculates a value for each asset that is assessed. Ratings are assigned to three basic factors; severity of failure effect, likelihood of occurrence, and likelihood of detection before the failure reaches the customer. The formula is RPN= Severity X Occurrence X Detection. The logic then being that assets with a higher RPN are inspected at a higher frequency than assets with lower RPNs. The metric typically used to determine periodicity of inspection is Mean Time Between Failures (MTBF). The general rule of thumb is to inspect an asset twice within its MTBF.; For example, if the MTBF for a motor is 120 days, you want to apply CM methodologies bimonthly at a minimum. This increases the likelihood of detection before catastrophic failure.
The qualitative approach views criticality from a couple of views. There’s the Mode Criticality, which assigns criticality to each potential failure modes of an asset by calculating the product (multiplying) of three factors. Those are Item Unreliability, Mode Ratio of Unreliability (aspects on unreliability that can be attributed to a particular failure mode) and Probability of Loss (severity). Once the Mode Criticality is calculated, Item Criticality is determined. This is accomplished by adding the individual Mode Criticalities for each asset.
The methods used for criticality assessment can be modified to meet the goals of your program. The discoveries made during this type of assessment can be used to apply maintenance strategies to reduce downtime, or even used to drive design changes to systems to reduce or even eliminate discovered failure modes.
The key thing in your journey to determine the actual criticality of your various assets is to partner with those who have experience in this area. If reliability is new to you or your organization, reach out to us. There’s no need for you to blaze a new trail, reinvent the wheel, anything like that when we have the expertise and experience to help you down this path.