Nov-2024

Improving preventive maintenance using equipment criticality

Case study demonstrates the importance of rating equipment criticality in an effective preventive maintenance plan while reducing overall cost.

E. Charles Maier
Becht

Article Summary

Maintenance has changed over time. It started as a reactive need to fix equipment when it broke. As equipment became more complex, repairs became more expensive. This drove the idea of ‘PM’ or maintenance that would enable equipment to last longer or fail less frequently. Preventive maintenance (PM) programmes expanded and the cost of the programmes grew as the focus was on improving their effectiveness. The focus on PM is now on maintaining its effectiveness while reducing the overall cost.

Refinery maintenance is a complex process with many elements that interact with each other. For example, equipment reliability has a direct impact on maintenance cost. If equipment runs longer between failures, the total repair cost goes down. The run length can be a result of design, repair quality, and how the equipment is operated. Repair costs are impacted by the quality of repair plans, competence of the crafts executing the repair, and material availability.

When analysing the elements that impact maintenance value, we can group them into three main categories: demand, which are the elements that result in the need for maintenance; efficiency, which are the elements that affect the efficiency of maintenance process; and support, which are the elements that support maintenance. Figure 1 shows the categories and examples of elements within each category.

From benchmarking within the petrochemical industry, the contributions of the various drivers to maintenance value (measured by total cost) are:
· Demand – approximately 60%
· Support – approximately 20-30%
· Efficiency – approximately 10-20%.

The demand driver has the largest contribution to maintenance value. It is composed of elements that result in the need for maintenance. The quality of the PM programme is a key enabler for equipment reliability, which is a key element within the demand driver. The size of the programme has a direct correlation with maintenance value, and optimising it can greatly impact the demand driver. This is done through PM optimisation (PMO).

PMO optimisation tools
There are three main tools utilised for PMO:
· Failure Mode and Effects Analysis (FMEA)
· PM Library
· Failure Reporting, Analysis and Corrective Action System (FRACAS).

FMEA evaluates equipment at the component level, identifying failure modes for each component. The consequences of failure are quantified along with the probability of failure. Scenarios are considered for financial, health, safety, and environmental consequences. Mitigation strategies are developed to reduce risk to acceptable levels for each scenario. The mitigation strategies are typically a combination of PM, predictive maintenance, and spare parts stocking strategies.

A PM Library is a collection of PM by equipment type. It contains company or industry recommendations for PM. For a given piece of equipment, the recommended PM activity is adapted to local conditions (for example, climate or process) and implemented as the PM activity for that equipment.

FRACAS was developed by the US Department of Defense and published in DOD MIL-STD-2155. It contains a three-step methodology. In step 1, the failure is reported, and initial data is gathered. In step 2, the failure is analysed to identify the root cause of failure. In step 3, a corrective action is implemented and tracked.

The FMEA approach is the most proactive of the three approaches and requires the most resources to implement. It considers all failure modes and results in the highest equipment availability. However, this approach does not account for the fact that considering all failure modes provides reduced benefits for lower criticality equipment.

The bulk of the effort in the PM Library comes from establishing the database of recommended PM. This can be done at a corporate level with only some effort at the site level to localise the activities.

This approach reflects either corporate or industry-accepted practices. Without considering failure modes, the mitigation strategy can be too conservative as they occur more often than required for the risk or do not address a failure mode and are, therefore, unnecessary. It is also possible that a hidden defect is not addressed, as its failure mode was never considered.

The FRACAS approach is the most reactive and the slowest of the three as you wait for failures to occur before taking steps to mitigate future failures.

PMO using equipment criticality
Using only reliability-centred maintenance (RCM) or FMEA in a PMO implementation for all equipment has proven to be highly resource-intensive. Even streamlined approaches that only look at dominant failure modes have not proven to significantly reduce the resource effort. There is a need for an approach that reduces the resource effort while maintaining the overall benefit. The following approach uses equipment criticality to differentiate the specific approaches used for equipment.

Equipment criticality is a consequence-based ranking process that considers the safety and business consequences of failure for individual equipment items. It is based on both the frequency and consequence of failure. If a piece of equipment in a low criticality process fails, it will have a lower consequence compared to an identical piece of equipment in higher criticality process. As an example, the interior lights in your car failing to operate has a lower potential consequence than your headlights or brake lights failing to operate.

In a downstream refining environment, equipment downtime has varied consequences depending upon the product stream and configuration of the refinery. Therefore, it is recommended that a risk matrix is utilised to determine equipment criticality. Risk is a combination of consequence and frequency. A sample risk matrix is shown in Figure 2, where 5 is the highest frequency, and 5 is the highest consequence:

Step one of a PMO is the determination of equipment criticality, typically in a workshop format.
The workshop can be made more efficient if some prework is conducted beforehand. This consists of a review of site data and pre-assignment of criticalities to selected equipment. Instrumentation equipment lends itself to pre-assignment as it can typically be categorised based on specific criticalities. For example, emergency shutdown systems can be designated as safety-critical. Safety-critical equipment refers to equipment that serves as a barrier to prevent, detect, control, mitigate, or recover from a major incident such as fire or explosion.