Cost effective plant life management

30 October 2000

The COMSY software has been developed as a tool for use in ageing and service life management of mechanical components in power plants. This software helps to reduce costs of plant operation.

The purpose of a systematic ageing and plant life management programme is to allow the lifetime of plant components to be planned and to indicate when a component has reached the end of its effective lifetime before it fails. Another important function of such programmes is to increase the availability of power plants as they age, and to enable implementation of a targeted maintenance strategy in terms of its economic and technical effect.

The concept

Siemens Nuclear Power developed the Comsy (Condition Oriented Ageing and Plant Life Monitoring System) software. This system allows the overall lifetime of mechanical components to be tracked, with emphasis placed on economic operation.

The Comsy software acquires, manages and evaluates component and operating parameters relevant to service life. Plant data pertaining to individual vessel elements, piping elements and systems are stored in a virtual power plant data model. Based on this model, the program conducts a condition-oriented lifetime analysis for various damage mechanisms which typically occur in power plants, like strain-induced cracking, material fatigue, erosion-corrosion, cavitation erosion, and droplet impingement erosion. This process is supported via an intelligent user interface, powerful analysis functions (stress analysis, thermal-hydraulic and flow analysis functions, water chemistry cycle analysis), material libraries, and a module for management and evaluation of examination results.

The goal of the software system is to provide the following:

•Transparent display of the design and manufacturing data, as well as the continually updated as-is condition of the plant.

•Concentration of examination and maintenance activities in relevant system areas.

•Assessment of the effects of refitting work prior to its application via simulation calculations.

Service life limitation

The service life of mechanical components is limited by ageing and wear mechanisms – in particular corrosion and fatigue. To assess the service life of a component, the following questions must be asked:

•Which damage mechanisms are relevant to the material under its intended conditions of use?

•What rate of component damage progression can be expected under these circumstances?

•Which limiting condition caused by the progression of the damage places a restriction on the service life of the component?

The properties of the material, the ambient water chemistry and thermal hydraulic conditions, and the mechanical load on the component must be evaluated to assess the type of corrosion to be expected as well as the rate of damage progression. The limit on the service life of the component is reached, for example, when the maximum allowable stress in the pressure-retaining boundary is reached,

the maximum allowable utilisation factor is reached with respect to material fatigue, or if the toughness of the material drops below the required values.

Holistic life management

A suitable programme for ageing and plant-life management requires an overall view of the plant, because individual components cannot be viewed in isolation. The information that is used in making such an assessment must be stored in a component-specific manner (such as geometry and material) and it must be valid across components (such as design criteria, thermal-hydraulic and water chemistry conditions) and across systems (such as flow rates and availability times).

The Comsy application strategy emphasises the following:

•Determining the plant areas affected by the relevant ageing and damage. This step makes it possible to focus activities on prioritised system areas.

•Identifying at-risk components and systems. This allows examination and maintenance programmes to be structured economically to prevent premature failure of components.

•Evaluating nondestructive examination results which document the as-is condition of the component.

•Maintenance of an updated and transparent database which identifies the as-is condition of the plant.

If the type of damage and the rate of damage progression are known, then suitable remedies and preventative measures can be implemented to extend the service life of investment intensive programmes. Experience has shown that a maintenance management programme based on reliable service life predictions enables costs to be minimised and plant availability to be increased.

Power plant model

The corrosion models used to make analytical service life predictions require the use of a number of physical and chemical parameters which cannot always be taken directly from the plant documentation. To enable economic application of service life prediction despite this limitation, Comsy includes analysis functions which can determine corrosion-relevant parameters based on available documentation.

The program processes components such as piping or vessel elements individually. If a piping element is to be generated in Comsy for a specific piping run, for example, the user selects the corresponding system area, component type and base dimensions of the component. The program then generates a component data sheet covering all relevant aspects. Knowledge of specific material properties of components is indispensible in evaluating specific damage mechanisms. For this, Comsy relies on an 'as-built' database which includes the material acceptance documentation prepared when the material or component was manufactured.

In addition to material acceptance procedures, details concerning examination results such as examination records and pictures taken during visual inspections can be accessed for the component in question. The component data sheet makes it possible to access static documents – drawings, parts lists, reports and acceptance certificates which exist as files – a in power plant specific documentation systems.

Plant diagnosis

To determine the areas of a plant affected by the ageing and damage mechanisms in question, a first cost-effective step is the performance of a so-called rough analysis. In the rough analysis, the heat balance diagram of the water/steam cycle in the power plant is modelled using graphical tools, and the system parameters are specified for each sub-area. This model establishes the basic data structure of the virtual power plant, and allows an analysis of the water chemistry cycle to be performed based on the thermal-hydraulic parameters. Taking into consideration the materials used in each case, the sub-areas are then examined with respect to the potential risk posed by damage mechanisms. The results indicate which power plant systems have a limited service life based on their design and operating parameters. Systems which are definitively not at risk as indicated by the rough analysis need not be examined in future analyses.

In system areas in which the occurrence of a damage mechanism is a possibility, the existing risk must be examined by a 'detailed analysis', including a service life prediction for each component. This second step, which is based on the information used in the performance of the rough analysis, requires additional information about the relevant component or system parts in question and physical and chemical parameters for the implemented damage models. Subsequently, the automatic generation of service life predictions for each component is provided.

Service life prediction is the key function of a software system for ageing and plant life management. Only on this basis can maintenance management and plant availability be optimised and the service life of investment-intensive components be extended. An efficient service life management programme builds on these damage predictions, which are validated and optimised through the performance of a small number of examinations at critical points.

The preparation of damage models presumes a detailed understanding of the type of damage concerned, as well as the functional interactions of the relevant parameters which influence the rate of damage progression. Studies and damage analyses have been conducted in this area for 25 years by Siemens. The experience gained from these activities has been brought together in analytical and semi-empirical corrosion models for each damage mechanism. To date, damage models have been developed for the following types of corrosion: strain-induced corrosion cracking; material fatigue; erosion-corrosion; droplet impingement corrosion and cavitation erosion. A damage model for stress corrosion cracking is currently under development.

The rate of damage progression is determined for the relevant damage mechanism in each case using these damage models, whereby a corresponding safety factor is used to allow for uncertainties. The calculated rate of damage progression and the strength boundary conditions calaculated for the component are used by Comys to determine the minimum service life.

Based on the predicted service life, components can be prioritised for examination programmes and condition-based inspection plans can be prepared. The results of component examinations are fed back into the program, and are used to further optimise service life predictions over the life cycle of the component. Overall, this systematic, closed-loop process enables up-to-date maintenance of a database with quantifiable information relative to the as-is state of the plant.

Examination evaluation

Comsy acquires and assesses measurement results from non-destructive component examinations and visual inspections. The examination and inspection results are linked to the examined component for documentation of its as-is condition at that specific time, and are integrated into the virtual power plant model.

The evaluation of component examinations is supported by interactive analysis functions which greatly simplify the geometry-dependent evaluation of measurement results. A calibration function supports the comparison of the as-measured condition with the predicted progression of the damage, while making allowance for measurement tolerances. The results of this comparison are used to improve the accuracy of future service life predictions.

This process ensures that experience gained from evaluating examination data is fed back into the performance of analytical service life predictors. Examination data resulting from in-service inspections are thus consistently used in preparing a reliable, up-to-date database.

Application of Comys

In the course of using Comys on a BWR, it was found that the service life of the feedwater system was limited by three potential damage mechanisms: erosion corrosion, strain-induced corrosion cracking, and material fatigue. Using the service life predictions, the program was used to examine specific welds, piping elements and vessel nozzles. These examinations showed a high correlation with the calculated degree of damage. In the next step, examination results and predictions were correlated to enable a more precise determination of weak points in subsequent major inspections, and thus long-term maintenance planning capability.

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