By John E. Skog P.E., Maintenance and Test Engineering LLC
Over the past several decades, the role of maintenance in electric utilities has made a transition from being a supportive activity in the shadows of capital expansion initiatives to a central activity responsible for ensuring full functional life from expensive assets. The impacts maintenance has on both customer reliability and the utility bottom line are very significant. Every additional year of reliable operating life an asset provides results in a measurable reduction in the pressure to increase rates. The challenge for maintenance and asset managers today is to find the triangular balance between expenditures, reliability and pre-emptive replacement.
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Maintenance is more than just performing the right tasks and keeping equipment reliable. Maintenance now must include a deep understanding of failure mechanisms, economic analysis, end-of-life prediction, risk analysis, process measurement and stakeholder involvement, and a constant reminder to all that–while the current strategy is adaptive–it’s built on solid engineering principals that stand the test of time.
Is my Current Strategy Adequate?
Over the past 30 years, utility maintenance strategies have evolved from a loose set of guidelines to a complex and highly managed set of processes. In some utilities, the evolution has been constant and well-orchestrated; in others, the changes have been revolutionary. Most of these strategies are built upon experience and have been adapted to meet the needs of the time. The change in names generally implies a new emphasis on specific aspects of maintenance and not a wholesale change in philosophy.
While many of our utilities have a history of reliable and economic delivery of energy to their customers, this does not imply that our approach to maintenance is optimal or adequate. System reliability is a function of design, and equipment reliability is more of a function of maintenance. Due to a failure-tolerant design, many of our substations and transmission networks operate at a higher reliability level than the individual components. Time is still the dominant method for scheduling maintenance and planning equipment replacement within our industry. Time is an appropriate trigger for many maintenance activities, but it can also imply a lack of understanding of fundamental equipment mechanisms.
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Time-based maintenance (TBM) employs rigid maintenance schedules that are pre-set in advance and carried out at predetermined intervals. Intended to reduce the probability of failure or degradation, time-based maintenance activities may be triggered by calendar time, operating time or event count and include overhaul, replacement and lubrication. Time-based maintenance has a historical popularity based on manufacturer’s recommendations and ease of scheduling. Unfortunately, many of the recommendations do not address random wear patterns or actual aging mechanisms.
From TBM to RCM
Beyond TBM, a utility could use reliability-centered maintenance (RCM). This is an improved approach focused on identifying and establishing the operational, maintenance and capital improvement policies that will manage the risks of equipment failure most effectively. (It is defined by the technical standard SAE JA1011, Evaluation Criteria for RCM Processes.) RCM can be used to create a cost-effective maintenance strategy to address dominant causes of equipment failure. It is a systematic approach to defining a routine maintenance program composed of cost-effective tasks that preserve important functions. RCM was initially developed by the airline industry in the ’70s and applied to the T&D sector in the mid ’80s.
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RCM enables the definition of a complete maintenance program. It regards maintenance as the means to maintain required functions. As a discipline, it enables stakeholders to monitor, assess, predict and generally understand the working of their physical assets. This is embodied in the initial part of the RCM process: To identify the operating context of the machinery and write a “Failure Mode Effects and Criticality Analysis,” or FMECA. The important functions to preserve with routine maintenance are identified, their dominant failure modes and causes determined, and the consequences of failure ascertained with a FMECA.
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The second part of the RCM analysis is to apply the “Task Selection Logic,” which helps determine the appropriate maintenance tasks for the identified failure modes in the FMECA. Once the logic is complete for all elements in the FMECA, the resulting list of maintenance is “packaged” for systematic completion through maintenance tasks. The result is a maintenance program that focuses scarce economic resources on those items that would cause the most disruption if they were to fail.
Building on RCM
RCM emphasizes the use of predictive maintenance (PdM) techniques in addition to traditional preventive measures. Since about 80 percent to 85 percent of corrective maintenance (CM) happens randomly, you cannot predict a date when they occur, but you can detect that equipment has started to fail. The effective use of condition monitoring techniques to identify the initiation of a failure mechanism led to employment of a maintenance strategy known as condition-based maintenance (CBM). CBM is not a departure from the principals of RCM but rather a reinforcement of the value of condition monitoring tasks.
With time-random failures, the simplest (but not the only) management strategy is to inspect equipment and look for evidence of degraded conditions. These are generally go/no-go type inspections. Trending an equipment’s performance graphically (e.g. power factor over time) or periodic inspections of equipment condition through observation and data measurement (e.g. oil sampling, DGA, temperature measurement, etc.) provides a degradation continuum.
Condition-based maintenance attempts to evaluate the condition of equipment by performing periodic or continuous (on-line) equipment monitoring. The ultimate goal of PdM is to perform maintenance at a scheduled point in time when the maintenance activity is most cost effective and before the equipment fails. This is in contrast to time and/or operation count based maintenance where a piece of equipment gets maintained whether it needs it or not.
Additionally, total productive maintenance (TPM) is a maintenance program with a goal to markedly increase production while, at the same time, increasing employee morale and job satisfaction. TPM brings maintenance into focus as a necessary and vitally important part of the business. It is no longer regarded as a non-profit activity. Down-time for maintenance is scheduled as a part of the operating forecast. The goal is to hold emergency and unscheduled maintenance to a minimum.
The TPM program closely resembles total quality management (TQM) programs. Many of the tools (such as employee empowerment, benchmarking, documentation, etc.) used in TQM are used to implement and optimize TPM.
Additional Options
Risk is a concept that denotes a potential negative impact to an asset or some characteristic of value that may arise from some present process or future event. In everyday usage, risk is often used synonymously with the probability of a known loss.
In engineering and maintenance, the quantitative engineering definition of risk is: risk = (probability of failure) X (total cost of a failure).
Risk-based maintenance (RBM) extends the concepts of RCM with further refinement and expansion in the areas of failure consequence and failure mechanism analysis. RBM strategies use the concept of risk to direct inspection and maintenance resources to the parts of the system where they may have the greatest effect. This results in more efficient and cost-effective inspection and maintenance programs. Depending on the starting point, this may reduce both risk and direct operating costs. Or, it may result in an increase in direct costs with a corresponding benefit of reduced downtime.
Additionally, value-based maintenance is an extension of RCM and a specific application of risk-based maintenance. It builds off of classical RCM concepts and accepted concepts of risk assessment. VBM allows for a monetary comparison of maintenance with its benefit.
All the previously discussed maintenance strategies have merit and their benefits and value are not to be understated. The problem with many of these strategies is that they are not full-spectrum approaches to maintenance. In order to be a full spectrum approach they must include sustaining goals and objectives, a technical basis, documentation and knowledge transfer, a data collection and mining plan, risk assessment, economic linkages, measurement and feedback processes, and continuous improvement plans.
One maintenance system answers that call: Performance-focused maintenance (PFM) includes all the above elements and is flexible enough to incorporate previous efforts so not to expend precious resources “re-inventing the wheel.”
Performance-based Maintenance
PFM has a firm RCM technical foundation and incorporates the elements of CBM and RBM–taking a broad view of maintenance. It looks at the technical, financial, business, customer and regulatory aspect of maintenance. PFM is suitable for both utilities needing an in-depth analysis of their present maintenance approach and those who only need to perform some surgical corrections.
The PFM process does not require one to scrap their existing maintenance strategy; its flexible approach is designed to allow the asset or maintenance manager to apply PFM concepts to only those areas in need of improvement. If the current approach to maintenance does not have an appropriate balance between technology, business drivers, long-term reliability and a strong use of data, then PFM will provide the utility and maintenance organization with value.
While the PFM process is an excellent methodology for developing a new maintenance program, it is also effective in enhancing those portions of an existing maintenance program not meeting stakeholder expectations. Many times maintenance programs are “technically correct” but are continually the target of scrutiny and budget cutting exercises because they don’t appear to align with the overall needs of the utility.
The PFM Decision Tree (see Figure 2, pg. 74) is designed to allow managers to quickly see the potential value and application of PFM. It allows the manager to take advantage of previous research and hold fast to those previous efforts that have provided the utility with great maintenance value. It also identifies areas where collaborative activities can be beneficial to numerous contributors.
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The tree allows the manager to quickly determine the status of an existing maintenance program and identify how which PFM templates may be applied to correct any shortcomings. It asks the manager to evaluate specific aspects of the current maintenance program. If the evaluation indicates strengthening is necessary, application of a specific PFM template is recommended.
No matter what the forces are that are driving a change in your current maintenance program, realistic goals must be set so that targeted actions can take place. These goals must be specific, achievable and address the requirements of all major utility stakeholders. Without goals, it is difficult to determine if overall improvement really takes place and impossible to know when success is achieved. For maintenance, these goals can be generalized and quantified in terms of safety, functional reliability, equipment or system availability, equipment maintainability, economics and quality of service. These goals should form the foundation of any maintenance program and serve as both the starting point and final destination for PFM.
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If goals have not been correctly established, it is difficult to choose the appropriate course of action. Many times the goals of the maintenance organization, while valiant, do not meet and sometimes conflict with the expectations of the various stakeholders. Developing realistic goals (that can be embraced by all stakeholders) provides a framework for building maintenance tasks. Sometimes all the stakeholder desires can be met and sometimes they can’t. In the latter case, re-evaluation of the goals, system design or both may be required.
John Skog is president and principal consulting engineer of Maintenance and Test Engineering LLC. He received Bachelors and Masters Degrees in Electrical Engineering from Washington State University. John was a past Chairman of the Doble Advisory, Insulating Fluids and Cable Committees and presented numerous papers at the Doble Conference. He has spent 31 years in the utility industry.
