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Not every crack needs to be repaired

Maintenance strategies
Not every crack needs to be repaired

For plant managers, the objective is to prevent loss and at the same time operate chemical and process plants profitably and with a high level of availability. Yet which strategy is best to achieve this goal? Modern maintenance trends or a mix of new approaches are rarely effective. What is critical is understanding the plant’s specific cyclic loads, temperature parameters and material defects – and knowing which of these defects are tolerable. TÜV SÜD experts support plant operators in finding and implementing the right maintenance strategy.

Maintenance strategies

Not every crack needs to be repaired
For plant managers, the objective is to prevent loss and at the same time operate chemical and process plants profitably and with a high level of availability. Yet which strategy is best to achieve this goal? Modern maintenance trends or a mix of new approaches are rarely effective. What is critical is understanding the plant’s specific cyclic loads, temperature parameters and material defects – and knowing which of these defects are tolerable. TÜV SÜD experts support plant operators in finding and implementing the right maintenance strategy.
An explosion with a clearly visible impact: in a power station a pipe had ruptured, resulting in steam escaping at high pressure and penetrating the outer wall of the building. The incident caused damage to property and months of downtime for the power generation unit. The investigators found out that the incident had been caused by cracking in a boiler circulating pump, which in turn had been due to high cyclic loads. However, this result does not mean that every crack needs to be repaired immediately. The challenge lies in correctly assessing each individual irregularity. Depending on the function of a component and the stresses to which it is exposed, defects can sometimes be tolerated for a certain period – or even permanently if risk analyses and modern test methods have shown that no further propagation of the crack need be expected provided the load remains unchanged. As zero-defect plants are a myth, the principle of “having to live with faults” forms part of a qualified overall assessment. However, this approach requires an in-depth understanding of the condition and operation of a plant as well as its possible defects.
Also a question of approach
Maintenance is not so much remediation and repair as early risk assessment and targeted action. Professionals can choose between various strategies and approaches for this purpose – from the regular replacement of components to permanent monitoring and probability calculations. However, in practice the one approach that fits all a company’s requirements is virtually non-existent. Instead, experienced experts need to align the measures taken to the plant’s state of repair and operational strategy.
Failure recovery is the approach that involves the least effort, though it can result in unforeseeable consequences. In this case, the plant is operated without any servicing and mainte-nance until it fails. More strategic approaches involve preventive, condition-based and predictive maintenance. With preventive maintenance, the plant is serviced and maintained at pre-defined intervals. Components are regularly replaced even if they have not yet reached their wear limits. This strategy, while promising in terms of risk minimisation, may result in “excessive maintenance” based on the premature replacement of parts.
Condition-based maintenance continuously monitors the state of repair of the plant’s individual components. While requiring elaborate planning and execution, this strategy allows potential defects to be identified early, so that early action can be taken. Predictive mainte-nance, finally, actively seeks out faults and therefore takes up a lot of time and manpower.
What if …?
If x happens, y will result. This is the principle of any deterministic maintenance approach. Although deterministic approaches have proved their worth in practice, they frequently result in excessive maintenance measures as various safety coefficients must be complied with at the design stage, yet may not be needed to the same extent when related to the plant’s mode of operation later on. The probabilistic approach goes beyond the deterministic approach by additionally quantifying risks according to their frequency of occurrence and their severity. If x happens, y will result with a probability of z.
In practice, maintenance strategies today are characterised increasingly frequently by dedicated risk-focused approaches, such as risk based maintenance (RBM) or reliability centred maintenance (RCM). Risk based maintenance determines the probability of failure of plant components and units as well as the severity of the consequences. As plants are complex technical systems, maintenance experts who pursue this approach must not look at components as isolated, mutually inde-pendent parts but instead consider their reciprocal interactions. Unfortunately, this is not always the case in practice.
So what are the consequences of a specific malfunction of individual components? This is the lead question asked and analysed within the scope of reliability centred maintenance (RCM). Depending on the outcome, the experts can define targeted measures to be taken in a specific situation. This flexibility is intended to improve plant reliability, while the dedicated measures aim at reducing mainte-nance costs.
Keeping an eye on the entire plant
Total productive maintenance (TPM), which is currently on the rise, pursues an even more integrated approach. This strategy proactively involves the company’s employees in mainte-nance tasks, even if these tasks do not form part of their core responsibilities. The basic idea behind TPM, which is based on the idea of the continuous improvement process (CIP), is that each individual employee knows and services “their” part of the plant and is thus able to identify wear and deterioration at an early stage. Staff hence share in the responsibility for routine remediation measures at “their” plant. TPM presupposes strategic implementation within the company, as it requires the support of significantly more people than conventional maintenance strategies.
In any case, however, the workforce involved in plant operation plays a far greater role in maintenance today than in the past. Mainte-nance has long since evolved from a mere support service into a modern system-related service. It makes high demands on general engi-neering practices, calling for experience and an in-depth understanding of the plant as well as the creativity and ability to focus on solutions of all stakeholders. In other words, maintenance depends not only on the experience of the colleagues on site, but also on their understanding of the plant’s changing operational performance. Maintenance simultaneously places special demands on management staff, who need to accept risks and be prepared to take on responsibility.
TÜV SÜD experts support plant managers from the initial planning phase onwards, be it with new material concepts, process control systems or optimised designs. Non-destructive test methods, such as ultrasonic or dye penetrant, are important tools for providing information on a component’s state of repair during operation. Fracture mechanics and the finite element method (FEM) allow more detailed insights into how long certain components can continue to be used. Based on qualified data and statements, TÜV SÜD helps plant operators find and implement the right maintenance strategy, always keeping an eye on the entire plant.
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Hans Christian Schröder
Hans Christian Schröder
Senior Expert Power Plants,TÜV SÜD Industrie Service
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