Power Plant Maintenance: Which Strategy Is Best?

Power Plant Maintenance: Which Strategy Is Best?

Hans Christian Schröder 

Power Plant Maintenance

Lack of investment in maintenance could potentially have costly consequences. Besides safety risks, defects in power stations often result in cost-intensive remediation measures and temporary loss of power production. The ideal maintenance strategy is about applying the right measures at the right times. However, there are no silver bullets for power plant maintenance.

In complex systems, the causes that may result in defects and hazards are many and varied. They include: constant exposure to thermal loads, faults in control systems, leakage caused by manufacturing defects like leaking valves, incorrect operation and faulty design. Some years ago, for example, a steam explosion in a coal-fired power station caused millions of dollars in costs, showing how severe the consequences can be.

As a general rule, the timely identification and correct assessment of existing weaknesses in plants can prevent many defects. However, which maintenance strategy is best in the individual case on hand? The members of a company’s maintenance team frequently need to engage in a delicate balancing act, trying to achieve maximum benefits at minimum costs and efforts. So what is the actual level of risk, and what risks can be tolerated for how long? Which measures make good economic sense? Which strategies match the plant and the company’s structure?

Every maintenance measure is aimed at bringing a long-term increase in the plant’s availability. However, maintenance is not about remedying defects; it should not be equated with “remediation” and “repair”. Maintenance starts at a much earlier stage, aiming at the prevention of defects and breakdowns in operation by means of suitable measures. As plants are complex technological structures and as each plant has very specific operating performance, maintenance strategies cannot be transferred “as is” from one plant to another. Instead, they always need to be individually aligned to the plant in question.

Traditional Approaches

These approaches are based on various maintenance strategies, with the more common forms being failure recovery and preventive, condition-based or predictive maintenance. In failure recovery, an installation or plant is operated without any servicing until it fails. The maintenance costs involved in this strategy are very low, but the risk of unforeseeable machine failure and downtime is high.

Preventive maintenance involves precautionary servicing at defined intervals and the replacement of parts before they reach the wear limit. This reduces the risk of unplanned breakdown, but may lead to excessive maintenance. Condition-based maintenance monitors the condition of components for timely identification of possible defects. This form of maintenance requires efficient planning of maintenance activities and major monitoring efforts. Finally, predictive maintenance is a proactive maintenance approach in which experts actively look for defects. This form of maintenance is time-consuming and imposes high demands on the expertise and experience of maintenance personnel.

Risk-based Maintenance

Risk-based approaches are currently on the rise. The main forms are risk-based maintenance (RBM) and reliability-centred maintenance (RCM). Risk-based maintenance classifies plant units and components according to their failure probability and the severity of the possible consequences. For this purpose, the experts identify and prioritise the risks involved in an expected breakdown of the plant. As plants are complex structures, the interfaces and interactions between the individual plant units require systematic and practice-focused examination, which, however, is not always put into practice.

In reliability-centered maintenance, the individual components of a plant or system are inspected for possible malfunctions and the impacts of these malfunctions defined. An analysis then shows which maintenance strategy is best in which situation. Targeted use of the appropriate methods will increase plant reliability while simultaneously reducing costs.

Continuous Improvement

Another common approach is total productive maintenance (TPM). In this approach, the company actively and continuously involves its staff in maintenance measures. The members of staff carry out routine maintenance of their installations and systems, ensuring regular care and servicing and thus identifying possible wear at an early stage. Preventive maintenance measures then enable individual employees to ensure timely identification and prevention of malfunctions and quality losses. TPM is based on the idea of a continuous improvement process (CIP) and eases the workload of the actual maintenance team. Companies wishing to implement TPM must do so in a targeted change management process tailored to the plant and/or system in question.

Systematic Approach to Power Plant Maintenance

Generally, experts examining the various approaches must bear in mind that the overall maintenance strategy must fit the special features of both the organisation and the plants in question. The condition of both the operations and the plant, as well as any possible defects, must be known and included in the considerations. In this approach, defects such as a crack in a component expressly need not be repaired in every case. Risk analysis and modern testing methods may well show that the crack will not grow under the existing load (defined service parameters) and that periodic monitoring of the affected component may be sufficient. As a general rule, it can be said that a zero-defect plant or system is virtually non-existent. Given this, application of a qualified overall assessment under the principle of “living with imperfections” may indeed be an option.

Staff Are Important

Even the best maintenance strategy is only as good as the people realizing it, so employment of qualified maintenance staff is imperative. Maintenance today is no longer a support service, but a modern system-related service ensuring functional safety and/or a value-adding sub-process, which requires high standards of expertise and experience and involves a great deal of responsibility. General engineering practice, experience and sound plant knowledge play key roles. Maintenance professionals need to show a high level of creativity and solution orientation.

Managers and executives must be able to motivate their teams, accept risks and take over responsibility. In this context, an integrated approach to the plant must be given priority. To develop maintenance strategies tailored to the individual case on hand which ensure maximum long-term safety and availability at acceptable costs, the parties responsible need to have precise knowledge of the condition of a plant or system and reliable data.

Deterministic vs. Probabilistic

A frequent question in maintenance is whether the approach should be deterministic or probabilistic. A deterministic approach is based on the assumption that there are clear causal, “if-then” relationships: if x happens, y will result. These approaches, while having proved their worth from the perspective of safety technology, have repeatedly resulted in excessive maintenance measures because of the safety factors that must be observed in design.

Probabilistic approaches, by contrast, are based on probabilities, and quantify risks on the basis of their frequency of occurrence and their consequences. These approaches allow for prioritization of measures and reduction of consequences. However, the downside includes uncertainties concerning the quality of the input data, and the high costs and efforts involved in complex events and fault-tree analyses.

Hans Christian Schröder is head of power station and energy services for TÜV SÜD Industrie Service, Mannheim. Contact him via email athanschristian.schrö[email protected].

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