Priority vs Criticality

by Bill Keeter

Priority something that is more important than other things and needs to be done or dealt with first criticality a relative ranking of equipment based on the probability of its failure and the consequences of the failure.

One of the most misused terms in the maintenance reliability world is criticality. Organizations often use the word criticality when what they are really talking about is criticalness. The fact that an item of equipment is critical to an organization’s success doesn’t define its criticality. This article will clarify the difference between critical items and criticality of items.

Why It Matters

Maintenance reliability efforts consist of two different types of work. There is the daily work of keeping equipment running by managing work orders and responding to emergencies, and the long-range work of activities designed to improve overall performance. The daily work requires a set of priorities to ensure the most important work is done first. The long-range work also requires a set of priorities that ensure improvement efforts are focused on where the most benefit can be gained for the least cost and effort.

Criticality is a function of probability and consequence of failure. It, therefore, can change based on efforts that either reduce probability, consequence, or both. Criticalness is a singular property and does not change unless another, more essential, item of equipment is installed in the system. It is possible for an extremely critical item to have a low criticality. In other words, criticality is useful for setting strategic priorities so long-term reliability efforts are focused on the right things; criticalness is useful for setting daily work priorities so the most important daily work is accomplished on the most essential items.

An Example

Everyone would agree that a turbine compressor in a primary gas compression train is essential to successfully compress and export natural gas. If it fails and there is no backup, the system will be unable to produce output. Its criticalness is high and the priority of daily work scheduled for it should be high as well. However, it may be well maintained by an effective combination of preventive maintenance (PM) and condition monitoring activities that reduce its probability of failure to an extremely low number, which means its criticality can be low compared to other items of equipment. Focusing reliability improvement efforts on the unit would likely be a waste of valuable resources that could better be used to improve reliability in other items of equipment.

Using Criticalness to Set Daily Priorities

Criticalness and work type are often used to create a ranking index for maintenance expenditures (RIME). The first step is to understand how essential certain types of equipment are in order to determine their criticalness. A starting point might be rankings, such as those in Table 1. These are only suggestions. An organization will want to set their own definitions for their system.

Table 1: Equipment Criticalness Ranking

The next step is to determine work order type priorities, such as those in Table 2.

Table 2: Work Order Type Priorities

These rankings are multiplied together to produce a priority number that can be managed based on local policy. This should be determined by a cross-functional team consisting of operations, maintenance and appropriate health, safety and environment (HSE) personnel.

Daily work now can be prioritized so the critical few work orders are accomplished first. It is important to not let work languish at the bottom of the index. This work can be managed be either designating a certain percentage of planned jobs to be done on work that has a priority number below a certain value, or by adding some specified number to a work order’s priority ranking each week so it floats upward (Figure 1).

Figure 1: Sample RIME chart

Using Criticality to Set Strategic Priorities

Determining criticality is a more complex process because it involves the interaction of probability of failure and consequence of failure. Criticality requires a level of granularity that allows the organization to recognize the few high criticality items that can lead to major improvements in performance. Applying the power law to the Pareto principle shows that it is possible for as little as five percent of the equipment in a facility to cause more than 50 percent of its losses. Criticality ranking systems with a granularity that only allows the identification of the top 25 percent of criticality are not strong enough to accomplish the goal.

Criticality is added from layer to layer in the hierarchy, so a good method is to do criticality at a high level with just a few questions (Figure 2). For instance, it is possible to divide a large facility into areas and determine area criticality first. The team can then drill down into the most critical areas by identifying system level criticality. The most critical systems or items can be then focused on using tools, such as reliability centered maintenance (RCM), to determine the best improvement strategy.

Figure 2: The additive property of criticality

The starting point for criticality analysis is to produce severity rankings for various impacts. A table similar to Table 3 would be a good starting point.

Table 3: Sample Severity Ranking Table

The severity rankings used should be in alignment with corporate guidelines for safety and environmental performance. Again, a cross-functional team of appropriate personnel is required to achieve consensus on rankings.

It is a simple process to rank each failure by maximum severity in each area and add them together to get an overall severity. The organization can then sort from highest to lowest to help prioritize performance improvement initiatives.

Conclusion

Prioritizing daily and strategic work efforts ensures that the limited resources in the organization are used as efficiently and effectively as possible. Using separate systems to perform prioritization ensures that a constant property is used to manage daily work and that long-range efforts are adjusted based on the reduction in criticality that comes from those efforts.