Power Plant Construction Holistic Measures to Counteract Quality Deviations in Components
Practical experiences show increasing quality deviations in delivered power plant components. Especially for components for the new generation of power plants with steam temperatures of up to 700 degrees, old integrity checks are to be complemented by holistic system considerations.
Globalised, deregulated markets increased competitive and cost pressure in power plant construction. Despite the increasing technical requirements on components, in regards to their resistance to increasing operating pressures and temperatures, they are expected to be as cheap as possible. Many contracts are awarded abroad under special conditions. The manufacturers of components are not always able to meet the requirements agreed upon. Quality deviations in delivered components such as boilers, turbines and generators often occur. In order to maintain a high level of safety for thermal power plants, the tester has to go from being purely an inspector to being an all-out operations analyst.
Modern power plants are subject to higher operational stress and have lower operating reserves. For power plant construction, the components' integrity is tested and periodically reviewed by the manufacturer on the basis of the existing rules and standards. However, the components will function reliably in later plant operation, only when the quality requirements are aligned with the specific conditions of the future plant operation in advance. In some cases, we also take warranty or guarantee measurements. Important points to be taken into consideration in addition to the process and plant design are the structural design and future operational stress. How is the power plant operated? What strategies are provided for its maintenance and repair?
Components for 700-degree power plants
With increased operating temperatures, coal plants are expected to continue to attain higher efficiencies of over 50%. In order to drive up the required temperatures of up to 700 degrees, new high temperature material concepts are needed. The use of, for example, nickel-alloyed steels places special demands on the manufacturing, construction, further processing and testing of components. At temperatures above 6000
C, the material changes are more difficult to recognize and can only be detected using advanced testing methods. In addition, for example, the service life of welds and basic materials is shortened. So far, there are still no reliable creep strain measurements for new materials and components over a longer period of operation.
Testing of High-temperature components
Special test runs are to provide information on the behaviour of high-temperature materials and components in operation. In this regard, the large-scale power plant in Germany started a research and development project in 2008, during which the operational integrity of components at 725 degrees are to be tested. The first results are expected in the coming years. But it is certain now that the new materials and components are much more sensitive to additional loads during operation. During the test run, the specific quality requirements for the components will be accurately designated.
New Testing Requirements
For the testing of high-temperature power plant components, various influencing factors must be determined: the resistance of the new materials, the applied manufacturing and processing methods (welding) as well as the operational load. Because of the special material properties, the application of conventional testing methods such as ambulatory Metallography (for example fabric prints) should be limited. A holistic testing concept is oriented on the factors influencing the service life of a component and ensures that it is not determined according to the tolerance level specified in the component's design. The actual testing activity no longer serves to detect defects. Rather, the plausibility of the overall design is evaluated in the context of operational stress and determination of the remaining service life.
Hitherto traditional testing according to current regulatory requirements is ad hoc and reflects only the current state of a power plant. A forecast of the long-term behaviour of systems and their components is in this way virtually impossible. In a prognostic activity, however, lies the key to minimising costs by more accurate data on the actual condition of components. From this, the remaining service life can be derived. The „gAte4optimisation" concept of TÜV SÜD make such a forecast possible based on an integral consideration of components, and appropriate non-destructive testing methods such an advanced eddy current testing, ultrasound testing with ultra modern equipment such as phased array or a simplified procedure for measuring local creep strain (TCR – TÜV-Creep-Replica).
Risk Oriented Implementation
For economic reasons, the quality requirements of individual components can be matched to their safety relevance in the overall system; without having to sacrifice security. Based on national and international standards and regulations well-thought-out power plant concepts can also cut costs of related components. To this end, planners should clarify with operators in advance what specific product quality is actually required for the respective components. The second step is to assess, which suppliers can fulfill the defined quality requirements.
Certification is not in Itself a Guarantee
Certification of companies, for example with DIN EN ISO 9001, provides no guarantee for the contractually agreed quality of their products. For example, the DIN EN ISO 9000 does not certify the product quality, but primarily the organizational processes of a company. For the quality management of power plant components, it is critical how quality requirements, cost efficiency, system availability and risks of downtime are conciliated in the future and in the long-term.
Quality deviations in components require holistic solution strategies to enable a high level of operational integrity. The integrated approach also allows a risk oriented implementation of the component requirements, which cuts costs. To assess the quality of components exactly, the component design, manufacturing technologies, and planned maintenance strategies as well as the operation mode of the power plant and operational load have to all be incorporated. This is particularly true for components that are exposed to elevated operating parameters.