May 2017

Special Focus: Maintenance and Reliability

Equipment critical analysis: The need for an effective maintenance program

Oil refineries and large petrochemical plants contain thousands of pieces of process and utilities equipment that are subject to wear, erosion, deterioration, aging, etc., resulting in increasing breakdowns and outages. Imagine being a maintenance engineer and receiving 50 work orders during an overhaul with a limited budget, time, labor, spare parts, tools, machines, etc. How does that engineer prioritize the work?

Zardynezhad, S., Toyo Engineering Canada Ltd.

Oil refineries and large petrochemical plants contain thousands of pieces of process and utilities equipment that are subject to wear, erosion, deterioration, aging, etc., resulting in increasing breakdowns and outages. Imagine being a maintenance engineer and receiving 50 work orders during an overhaul with a limited budget, time, labor, spare parts, tools, machines, etc. How does that engineer prioritize the work?

An important daily challenge at all hydrocarbon processing plants is equipment failure, which can have many causes and consequences. End users not only investigate causes, but also determine the best strategy to mitigate or avoid consequences. The consequences of an equipment failure include risks related to:

  • Safety
  • Environment
  • Production loss
  • Maintenance cost.

Each type of equipment has a unique role with a different criticality index. Criticality is the failure’s consequence in relation to health, safety, environment, loss of production and maintenance cost (TABLE 1). Selecting the equipment criticality index is explained in the following sections.


Equipment critical analysis is a quantitative analysis of equipment faults, and ranking them in order of serious consequences on safety, environment, production loss and maintenance cost. The key benefit of this analysis is to provide the means to recognize high-criticality vs. low-criticality equipment, reduce the level of uncertainty and focus on high-priority maintenance tasks. The analysis also helps select the best and most economic maintenance strategy, prioritize work orders and decide on insurance and the demand on spare parts. The inputs, tools, techniques and outputs of this analysis are depicted in TABLE 2.


The first step is to clarify the main systems inside a plant with unit boundaries. This action is normally accomplished by marking up piping and instrumentation diagrams (P&IDs) into main and sub-systems. For example, a gas compression unit in a gas plant can be divided as:

  • Main system—scrubbing, compression, cooling and other auxiliary systems
  • Sub-systems—includes components of the main system. For example, scrubbing would include scrubbers, pressure safety valves (PSVs), shutdowns, alarms, etc. as sub-systems.

For criticality analysis, the following drawings and documentation should be available: detailed plant/system description or control narratives, datasheets, P&IDs, process flow diagrams (PFDs), single-line diagrams, cause-and-effect diagrams, shutdown logic, etc.

To conduct the analysis, which mainly assesses the consequences of equipment failures and the degree of sparing and redundancy, the consequence classes must be properly defined prior to beginning the analysis. This helps assess the consequences of equipment failures, and the degree of sparing and redundancy. The definition of the consequence classes should be conducted in accordance with the company’s criteria for safety and the environment, and reflect actual plant operations for determining economic losses, such as costs related to lost operation. Decision criteria normally includes four main indexes for safety, environment, production, and operation and maintenance costs. Typically, each index contains a table with three different levels (A, B, C), as shown in TABLES 3 and 4.

Hidden faults that are not evident to the operator during normal operation should also be considered as input to the assessment.

Tools and techniques

Expert judgment should apply to the inputs used to develop the equipment criticality list/classification, and to all technical details during this assessment. Such expertise is provided by any group or individual with specialized knowledge or training in safety, cost estimation, operation, maintenance, environment, health, design, etc. Document analysis is used as another tool. A wide range of documents and drawings may be analyzed to help provide a more effective and efficient study.

Several group creativity activities, such as brainstorming, nominal group techniques, etc., can be organized to assess equipment. The analytical hierarchy process (AHP) method can be used as a powerful tool to prioritize assets according to their criticality. The AHP is built on three basic principles: decompositions, comparative judgment and hierarchy composition of synthesis priorities. Decomposition is another technique for dividing and subdividing systems and sub-systems into smaller components. A criticality assessment and respective plan are developed based on a set of hypotheses and assumptions.

  • An assumptions analysis validates all assumptions used during the study, and identifies risks to the assessments due to inaccuracy or incompleteness of the assumptions.
  • A risk probability analysis covers the probability that specific failure risk will occur. Risk impact analysis includes potential consequences and effects on safety, health, environment, production, and operational and maintenance costs.
  • Risk data quality assessment is a technique to evaluate the degree to which the data about risks is useful for risk management (i.e., the degree to which the risk is understood and the accuracy, quality, reliability and integrity of the data about the risk). Using quantitative risk analysis (QRA), the effect of identified failure risks on operational and maintenance costs, safety, health and environments can be quantitatively analyzed.

Application of equipment criticality index

The result of the assessment will be an equipment criticality list or classification. Advantages of the criticality list are that it:

  • Determines the most efficient, effective and economic maintenance strategy for each piece of equipment (e.g., predictive maintenance, preventive maintenance, run to failure, corrective maintenance, total productive maintenance, etc.)
  • Is a valuable input report to determine the optimum and economic spare parts inventory needed, and to decide which piece of equipment needs insurance or capital spare parts
  • Helps determine the overall priority for performing maintenance tasks when many maintenance activities, or “work order priorities” exist
  • Determines, at a high level, the risk mitigation strategy to be applied to equipment (i.e., condition monitoring and defect elimination on high-criticality items)
  • Helps operators decide conceptual and design evaluations of the high-critical equipment, and prepare the corrective actions
  • Helps reliability engineers focus on reliability improvement efforts on the most “critical” equipment.

Mean time between failures (MTBF)

To analyze and determine the equipment criticality index, personnel must define a maintenance index, an index of probability of failure or an MTBF index (TABLE 5).

Using evaluation tables for assessment

First, to define and classify failure for the equipment, the following questions should be addressed:

  • What are the consequences if the equipment works below the requirements?
  • What are the consequences if the equipment is completely out of service?

Next, consider the most serious and actual equipment failure scenario. Performance degradation due to equipment failure should also be considered. The effect of failure on safety, environment, production, and operational and maintenance costs is determined and integrated into the MTBF index (TABLE 6).


Typically, naphtha hydrotreaters and octanizer units in oil refineries contain 13 hydrogen (H2) gas compressors to boost, recycle or export H2 gas. In this case study, the main 5-MW, API 618 standard H2 recycle gas compressors were considered. Each reciprocating compressor had a 100% spare machine with two stages and four throws.

Possible compressor failures include:

  • Valve failure
  • Coupling failure
  • Piston rings failure
  • Cross head shoes failure
  • Bearing failures.

Coupling bearing and cross failures are the most serious failures, and can result in the compressor being put out of service.

Step 1: Safety index evaluation. TABLE 3 results to Index C, because there is no potential for injury and/or fire after failure.

Step 2: Environment index evaluation. TABLE 3 results to Index B, because compressor cylinders’ gas should be sent to the flare to allow workers to begin maintenance activity on the failed compressor.

Step 3: Production index evaluation. TABLES 3 and 4 result to Index C, because there is a spare machine, which causes no impact on production.

Step 4: Operation and maintenance cost index. TABLE 3 results to Index B. The cost of spare parts, gas sent to the flare, manpower cost, tools, etc. are assumed to be less than $30,000.

Note: MTBF index. TABLE 5 results to Index 2, with reference to maintenance experience, type of bearing and vendor’s recommendation, it was learned that the bearing failure was more than 5 yr old.

Based on TABLES 3, 4, 5 and 6, the criticality indexes for this case study are shown in Table 7. 

As a result, the higher equipment criticality index should be selected as the final equipment index for this compressor (i.e., M for medium).

Criticality index evaluation

The flow chart shown in FIG. 1 facilitates the process of criticality index evaluation for equipment.

Fig. 1. Criticality index evaluation for equipment.
Fig. 1. Criticality index evaluation for equipment.

This critical analysis can help the maintenance team prioritize equipment, select the best economic maintenance program, prioritize work orders despite resource constraints, plan for spare parts, reduce maintenance costs, plan for design modifications and upgrades, and increase productivity. The criticality analysis shown in this article clearly identifies the advantages of having selected equipment that is suitable for successful operations, with respect to production, business costs, safety and the environment. HP


The author is grateful to all of the people who supported his efforts and provided input for gathering data and finalizing the findings for this article.


  1. British Standard, BS 3811, “Glossary of maintenance management terms in terotechnology,” March 1984.
  2. Norsok Standard, Z-008, Rev-2, “Criticality analysis for maintenance purposes,” November 2001.
  3. Project Management Institute, “A guide to the project management body of knowledge (PMBOK Guide),” 5th Ed., January 2013.

The Author

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