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Projektinfo – Detailed information on energy research

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Scientists have developed a new concept for flexible and load-dependent inspection intervals of power plant components under alternating loads.
© TÜV Nord
Power plant technology
Projektinfo 01/2018

The damage tolerance diagram shows an example of how the residual life of a component can be extended if, instead of the purely mathematically calculated fatigue analysis (red), the actual operational profile of the power plant is used (blue) and incorporated in the fracture mechanics analysis.
© TÜV Nord

Service life of a component and cracking phases. Until the occurrence of a technical crack (about 2 mm), components should be tested according to the fatigue analysis. After it has occurred the fracture mechanical method should then be used.
© University of Rostock, Prof. M. Sander
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Material loading in flexible power plants

The conditions for operating conventional coal and natural gas power plants are changing considerably as a result of their combination with renewable energies. In future they will run more frequently under partial loads, have considerably more start-up and shut-down cycles, and the components will be subjected to greater and more diverse loads. Scientists have investigated the thermal and mechanical loads on thick-walled components in power plants under the new conditions. At any given safety level, the aim is to more precisely calculate the stability of components in regard to their damage and thus economically optimise their deployment times.

Steam power plants based on coal or natural gas have previously been designed for operating phases that remain as constant as possible with stable temperature and internal pressure conditions. Normally temperatures between 540 and 600 °C and pressures between 240 and 290 bar prevail in the steam turbine area. Power plants should reach this target range for full-load operation in a manner that is as system-friendly as possible and maintain it for a sustained period.

However, Germany‘s Energiewende – its energy transition – requires that these power plants become considerably more flexible. They work in close combination with wind farms and photovoltaic systems in the electricity grid and must meet not only the remaining power requirements that are not covered by renewables but also most of the system services for ensuring grid stability. As a result, these power plants are subjected to frequent starts and transitions into the partial load range. This entails a temperature change of several hundred degrees for the components every time in comparatively short intervals. These new conditions are placing a heavier burden on the components.

In the THERRI research project, scientists have therefore investigated the effects of cyclic temperature changes under the changed load profiles in power plants and have experimentally and numerically developed and tested a new method for conducting fracture mechanics-based analyses of the damage tolerance. Thanks to the project results, the residual lives of key plant components can be calculated more precisely for the first time without compromising on technical safety. These findings are important for the power plant operators‘ business calculations because, on the one hand, the plants achieve lower revenues owing to the frequent partial load operation and numerous shut-downs than in full load operation and because, on the other hand, these load changes entail higher maintenance and repair costs. The results of the investigations have been incorporated into a new draft guideline for the fracture mechanics concept. TÜV Nord carried out the project together with the University of Rostock and Research Centre Jülich. The practical tests took place at KNG Kraftwerk Rostock. Load-dependent inspection intervals were determined for boiler circulation pumps and superheater collectors.

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New method for assessing damage

Under partial load, even conventional power plants operate for a long time in the temperature range between 300 and 550 °C. This leads to other forms of material stress: creep cracks dominate at consistently high temperatures above approximately 550 °C, while fatigue cracks are the main risk below this value as a result of the more frequent changes in temperature and pressure. In practice, both forms overlap and the transition point is around 500 °C.

The more frequent partial load operation means that components are loaded differently in practice than the standard assumptions envisaged by the regulations. The project partners therefore developed a fracture mechanics-based assessment method that takes operational practice into account. It is suitable for lengthening the residual life of the components while maintaining a high level of safety. For this purpose, the partners have developed the necessary basic principles that were previously lacking through conducting experimental, numerical and material scientific investigations as well as practical tests.

The focus is on thick-walled power plant components such as ball fittings, high-pressure diverter stations and boiler circulation pumps. For the most part they consist of particularly resistant power steels, for example ferritic-martensitic steel. The cracking takes place in different phases . The scientist‘s methodology was based on using a hypothetical crack that can be detected as a technical crack using non-destructive testing methods. They investigated how fast this crack would spread, which parameters would influence this, and the length of time with which the component could still be used safely. They developed the damage tolerance analysis for this purely virtual crack. The THERRI methodology can also be used, however, on components with real cracks, which would then be damage analysis.

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Addresses

Project management
TÜV NORD AG

Experimental, numerical and analytical investigations
Universität Rostock, STM

Simulation of the power plant operation
Universität Rostock, LTT

Experimental investigations of atmospheric and creeping influence
Forschungszentrum Jülich GmbH, IEK-2

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