Choosing the right strategy to stay on the safe side

There’s no ‘one-size-fits-all’ approach on maintenance.

08/04/2016


Figure 1: Loss of material in the metal-sheet components of a superheater caused by sigma phase embrittlement, which resulted from using the wrong material. Image courtesy: TÜV SÜDIn recent years, the testing techniques and models for calculating and forecasting component behavior have grown in numbers and improved in reliability. Nevertheless, power stations repeatedly suffer failures and defects, sometimes of major proportions.

One of the leading causes is inadequate maintenance. Buzzwords and acronyms are no solution in these cases; only strategies tailored to the special features of the plants will be able to balance safety, costs, and benefits.

Whether leakage, cracks, wear, or operation and design errors, early identification of weak points in plants and systems and appropriate assessment can prevent many defects and unscheduled downtime and thus unnecessary costs. A requirement for this is the right maintenance strategy.

However, as the operating performance of each plant is unique, and particularly as complexity increases, there is no "one-size-fits-all" in maintenance strategies. What is important is that the strategy is customised to fit the plant and that the maintenance experts ask the right questions. Which maintenance strategies fit the company, the plant, and the maintenance organization? And which strategy is suitable at what times and for which service parameters?

Future-oriented...

Maintenance has long evolved from purely reactive failure recovery and remediation into a future-oriented service. Maintenance objectives include a high level of plant availability and operational safety while ensuring profitability. Obstacles to these objectives are increasing cost pressure and a lack of future strategies. Fundamental approaches are preventive, condition-based, predictive, and risk-based maintenance.

Preventive maintenance refers to anticipatory servicing without a conceptual framework, in which scheduled and standardized maintenance activities are carried out at fixed intervals. This form of maintenance reduces unexpected shutdowns but also replaces many components before they actually reach the wear limit. Preventive maintenance thus results in "excessive maintenance," which drives maintenance costs.

... and condition-based...

Condition-based maintenance monitors component condition to ensure timely identification of potential defects. This approach allows for timely planning of the necessary maintenance work. However, monitoring causes higher efforts and costs.

Predictive maintenance is a proactive form of maintenance which includes a conceptual framework. Possible defects are localised at an early stage and corrected as soon as possible. However, the strategy and its implementation are time-intensive and make high demands on the maintenance team.

...or risk-based?

Risk-based strategies, such as risk-based maintenance (RBM) or reliability-centered maintenance (RCM), focus on failure probabilities. Risk-based maintenance identifies the potential risks that may lead to plant breakdown and ranks them in order. Plant units or components that involve the highest risk of failure are addressed with priority. As complex technical structures, plants require systematic and practice-focused analysis.

For reliability-centered maintenance, all plant components must be analyzed for potential malfunctions and the consequences of these malfunctions defined. This approach enables the experts to select and implement the necessary maintenance measures and thus allows the best possible use to be made of different strategies while reducing costs.

Figure 2: Slag formation in the combustion chamber of a wood-fired boiler caused by temperatures above the deformation temperature. Image courtesy: TÜV SÜDContinuous improvement

Total productive maintenance (TPM) makes sensible use of production personnel. By carrying out routine maintenance and servicing of "their" equipment, they can identify possible wear or other weak points at an early stage. Preventive maintenance measures then avoid breakdowns and quality losses. This results in a continual improvement process and eases the workload of the maintenance team.

Deterministic or probabilistic?

The question of whether deterministic or probabilistic approaches are better is one of the fundamental questions in maintenance. Deterministic approaches focus on the cause and follow the "if-then principle": If x happens, y will result. Maintenance following this principle has proved effective from the point of safety technology, and deterministic methods form the basis of Germany's technology laws. On the downside, these approaches result time and again in excessive maintenance measures because of the safety factors that must be observed in design.

Probabilistic strategies, by contrast, are based on probabilities. They quantify potential risks according to their frequencies of occurrence and the severity of their consequences to prioritise maintenance measures. This reduces the costs of maintenance. However, the input data determined may be fraught with lack of precision, and in case of complex events, the approaches involve high efforts and fault-tree analyses.

Systematic and practice-oriented...

When choosing the overall maintenance strategy, the responsible parties must take into consideration the special features of the company and of the plant in question. The maintenance professionals should be aware of the conditions of operations and of the plant and include them in all their decisions. Practice in general engineering, experience, and in-depth knowledge of plants and systems are therefore important. The role of a maintenance professional today is that of a system analyst who keeps an eye on the full picture.

A carefully coordinated combination of individual strategies and tools creates the basis for practice-focused hazard assessment. This also means that "defects" need not be repaired in every case. Inhomogeneities in materials or incipient cracking that are detected with the help of non-destructive testing may not necessarily have to be repaired if fracture mechanics and risk assessments show such findings to be tolerable under the defined service parameters. Within the scope of high-quality analysis of the total system, the principle of "living with defects" is quite acceptable. Likewise, safe operation of plants of an advanced age can be continued with the help of a dedicated asset analysis and customized refurbishment and maintenance measures. This also applies to process control systems and, in particular, process control systems that are no longer produced. As long as these are functioning and their use can be continued with the help of repair strategies, these systems do not have to be replaced.

...as well as creative and solution-focused

To deliver maximum benefits for the company and ensuring profitability, qualified maintenance needs to find creative solutions on any given day. Recruitment of committed and highly qualified maintenance personnel is therefore of crucial importance.

Management needs to be able to motivate their personnel. This is only possible in an organization based on division of labor, which leaves staff scope for action and for taking on an appropriate level of responsibility. Acceptance of tolerable risks is also part of the deal. Given this, the working atmosphere should be based on clarity, consistency, trust, and appreciation.

In the future, maintenance will increasingly focus on the prevention of failure and continual improvement of plants and systems. Systematic knowledge and practice-focused approaches will come together to ensure the best possible plant management, offering long-term operational safety and reliability as well as maximum availability.

- Hans Christian Schröder is a senior power plants expert for TÜV SÜD Industrie Service in Mannheim, Germany.



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