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Make Better Maintenance Decisions

April 14, 2021
A simplified approach for asset criticality ranking can provide important insights

Plants rely on a wide variety of assets. Some are mission-critical, i.e., required for production, safety or environmental compliance, while others handle non-crucial functions. This difference in criticality affords an opportunity to prioritize maintenance strategy and optimize maintenance costs.

Thus, many manufacturers perform asset criticality ranking (ACR) or equipment criticality ranking (ECR) and often mark the criticality class in the equipment master of the enterprise resource planning (ERP) system. However, most organizations carry out this exercise as a sort of mechanical drill and don’t give it proper importance, time and effort. No wonder such companies fail to leverage the exercise to formulate maintenance strategies based on equipment criticality.

ACR involves ranking assets based on a well-structured procedure. An asset gets marks for various parameters, with the consolidated score leading to its classification into a criticality category. Usually, for simplicity, the ranking consists only of A, B and C categories.

There’s no particular right way of doing ACR. The ISO standards on asset management (ISO 55000, 55001 and 55002) don’t prescribe any methodology. So, manufacturers employ disparate methods and approaches to suit their needs, with varying levels of effectiveness.

The classical ACR method is similar to failure-modes-and-effects analysis (FMEA) and risk-based inspection (RBI). Each asset gets a risk priority number (RPN) that’s computed based on the history (frequency) or probability of failures and their consequence or impact. Then, the actions required to minimize the risks are listed. (Completion of the prescribed actions may lead to a revised RPN number the following year.) This methodology not only makes the ACR process very complicated and time consuming but also very subjective — which can lead to erroneous classifications. For instance, a rubber-lined storage tank for weak phosphoric acid that ruptured in the past may get categorized as a critical asset because of that failure while a production-critical high-speed compressor with no failures during the last 10–15 years of operation may get classified as semi-critical. The objectives of FMEA and RBI differ from those of ACR; tying them together dilutes FMEA and RBI and complicates the ACR process.

A Simplified Approach

The traditional methodology requires up-to-date data on anticipated or actual asset performance that many organizations lack. In addition, arriving at a consensus by the cross-functional team usually takes a lot of time. Moreover, the analysis is subjective and not repetitive. Due to these complexities, most organizations don’t go beyond ACR. However, ACR itself isn’t important; what you do with ACR is. ACR only is a means to an end — which is improved reliability and increased uptime as well as better policy decisions on maintenance prioritization, preventive and predictive maintenance strategies, capital and operational maintenance budgeting, stocking policy, etc.

This article introduces a simplified methodology that’s based on the function of the asset and its importance in maintaining safety, environmental compliance and production. It also considers the severity of operating parameters and complexity of maintainability but excludes the failure history of the asset. Thus, in this method, the asset’s criticality class remains more or less static. Such consistency is desirable. For instance, in the classical approach, you might downgrade a truly critical asset to semi-critical or non-critical status based on its history of lack of failures or low cost of maintenance — and that could be dangerous.

Applying The Approach

You must have a consolidated and up-to-date asset register with full details for each plant or unit within the manufacturing location before starting ACR. For a greenfield project, your engineering/procurement/construction contractor should provide you with this list; it should include tag numbers for all critical control valves, flow meters, transmitters and electrical drives. For an operating plant, you can extract the equipment master report from your ERP system. Asset criticality analysis should include rotating equipment; static equipment; mobile equipment; pipelines; electrical assets like transformers and switchgear; and process control assets, such as the distributed control system (DCS), instrumentation and control valves. Other necessary documents are process flow diagrams, equipment data sheets, etc.

Three Tables Mentioned In The Article

Parameters And Their Scores
Table 1. Scoring criteria should reflect the particular plant, ones given here are purely illustrative.

Criticality Analysis Worksheet
Table 2. Safety- and compliance-critical assets automatically get a 40 and A rating while the total for other assets determines their class.

Maintenance Rules Based On Criticality Class
Table 3. Rankings enable differentiating maintenance rules based on an asset’s criticality.

Then, you should follow eight steps:

1. Establish a cross-functional team consisting of process, mechanical, electrical and instrument personnel who have a thorough understanding of plant and equipment functions as well as constructional features of assets. This team will brainstorm to assign each asset appropriate scores for a number of key parameters.

2. Identify assets required for site safety. Label these assets as safety critical and put them into the critical class (A). They must be operational 24/7. (A spare nearby or an assembly in a warehouse don’t ensure uninterrupted protection.) Such assets typically include fire pumps, fire tender, fire alarm system, flare stack systems, etc. If you have multiple fire tenders, all should be available in healthy condition 24/7. You need not provide further details. Assign these assets a score of 40. (This is the maximum, as detailed later.)

3. Similarly, automatically classify assets required for legal, environmental and statutory compliance as Class A. Such assets include analyzers on stacks for online monitoring of emissions, flow meters on effluent discharge, the ambient air-monitoring system, etc. Give them a score of 40.

4. For each of the remaining assets, the cross-functional team must perform a more-detailed review of its role in production, media handled, operating conditions and maintainability (see Table 1). The team should review all documents and assign a score of 1 to 5, with 5 for most-severe, for each parameter based on specific criteria. The criteria listed in Table 1 are only illustrative; the team should alter them as appropriate.

5. Add the scores of all parameters from Table 1. The maximum score for any asset is 40 (8×5). The cross-functional team must decide the boundary lines for categorizing assets into Class A, B and C. In Table 2, assets scoring 25–40 are Class A, those scoring 12–24 are B, and ones scoring below 12 are C.

6. Populate the equipment master with the rankings and scores against each characteristic as shown in Table 2. Put all auxiliaries or child assets into the same class as the parent equipment. So, for instance, if you categorize a centrifugal compressor as Class A, also automatically classify auxiliaries such as its drive motor or steam turbine, lube oil and auxiliary pumps, etc., as Class A because the objective is to make the entire system reliable.

7. After completing classification, review the count of Class A, B and C assets within a plant area and perform any fine-tuning required. (Unlike the A, B and C classification of inventory where you can apply Pareto’s Principle to demarcate between the classes, in ACR most assets will go into Classes A and B with only about 10–20% in Class C.) The team then should sign off on the ACR; it should be treated as a controlled document and circulated per the distribution list. All process, maintenance and reliability engineers as well as planners and schedulers should get a copy.

8. After equipment class sign-off, update the criticality class in the equipment master of the ERP system and affix classification signs in prominent places on the equipment. In addition, cite the equipment classification in all maintenance reports such as preventive maintenance schedules, predictive maintenance reports, equipment history records, failure analysis studies and work orders as well as capital-expenditure and operating-expenditure budget proposals. Don’t allow the equipment classification to remain only on computers and the ERP system. Everyone, including operators, who must deal with the asset should fully understand its criticality. Visual display of equipment class is very important.

Derive Full Benefits

Every maintenance activity should add value to the business. ACR helps focus attention and resources in the right areas, thereby optimizing maintenance costs without compromising plant availability. If asset criticality analysis reveals that a Class A bad actor asset lacks redundancy, then you should budget capital to procure and install a suitable spare. If available space near the asset is a constraint, plan on keeping the assembly in a warehouse. ACR also tells you how often to perform preventive and corrective maintenance.

As stated earlier, many organizations use ACR but still too often treat all assets equally, applying their maintenance philosophies and strategies uniformly for all plant assets. This “one size fits all” approach isn’t optimal. For instance, sites usually set up their vibration-monitoring schedule and route plan plantwide, not based on ACR class. Thus, critical and non-critical rotating equipment get checked at the same frequency.

Table 3 illustrates how you can devise different maintenance rules for each class. Of course, you must tailor that table to suit your specific needs and then must follow it. Best-in-class companies neither overdo nor underdo any maintenance activity and aim to maximize uptime and reliability.

Remember that ACR is not just a reliability tool but also a business performance improvement tool. ACR helps:

• develop complete maintenance strategies;
• create training and development strategies;
• improve asset reliability and maximize plant uptime and availability;
• decrease maintenance costs;
• visibly reduce health, safety and environmental incidents;
• determine the proper budget for capital and operational expenditures;
• enhance document management, e.g., of equipment history, failure investigation, lifecycle cost and maintenance key performance indicators;
• cut turnaround costs;
• achieve ISO 55001 certification;
• devise a long-term asset plan;
• trim product manufacturing cost; and
• formulate inventory stocking policy and optimize inventory holding.

KUMAR R. IYER is head of manufacturing excellence for Indorama India Pvt. Ltd., Durgachak, India. Email him at [email protected].


1. ISO Asset Management Standards, 55000, 55001 and 55002, Intl. Org. for Standard’n., Geneva, Switz. (2014, 2014 and 2018).
2. Dunn, S., “Equipment Criticality Analysis — Is It a Waste of Time?,” Assetivity, South Perth, Australia.
3. Gorsuch, S., “Asset Criticality Is Not Just a Reliability Tool,” Viziya, Hamilton, Ont.
4. Wikoff, D., “How to Effectively Manage Assets by Criticality.”
5. Zardynezhad, S., “Equipment Critical Analysis: The Need for an Effective Maintenance Program,” Hydrocarbon Proc., May 2017.

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