A feed-in tariff (FIT) introduces feed-in priority for power generated from photovoltaic or wind farms, with far reaching consequences that also impact power stations operated by fossil fuels. Thermal Power Plants must be flexibly operated as energy from renewable sources is not generated at a constant rate. To bridge in the gap between supply and demand caused by volatile feed-in, the conventional power stations must step in. On the one hand, their operations are increasingly moving towards medium and peak loads and on the other hand, they must run at minimum loads significantly more often.
Germany has a FIT for power from renewable energy sources. India has inaugurated its solar program about a decade ago in January 2010 and aimed to install 20 GW solar power by 2022. Indonesia encouraged independent power producers to invest in the electric power sector and has made a different range of FIT regulations for different forms of renewable electric energy. Under the Philippines Renewable Energy Act of 2008, FIT system can be implemented to harness energy.
Risk of growing damage and downtime:
Fossil-fired power plants are increasingly required to react to fluctuating feed-in and consumption situations on the electricity market with volatile operation. This is associated with increasing demands on operational flexibility, which generally leads to a sharper expression and an increasing number of operational transients. This development leads to higher thermal stresses and more frequently alternating stresses on thick-walled components – especially in steam generator and pipeline components such as e.g. collectors, coolers and fittings.
The determination of the current state of progressive accumulative damage mechanisms as fatigue and creeping is thus given greater weight. By precisely determining the current state of exhaustion, the operator can utilize the whole lifetime potential of the components and systems that is still available. This opens the way for safe but also economical optimized plant operation, since only the necessary non-destructive test and maintenance measures need to be carried out and furthermore these can also be planned well. This reduces the risks of damage as illustrated in figure 1.
Figure 1: Comparisons of the risks involved in operating the power plant without monitoring the components and with monitoring the components with the help of TSE
The degree of damage can be immense and results in longer downtimes. An example of such a damage was caused in Staudinger Power Plant, Germany on the 12th of May 2014. A steam explosion in the boiler resulting in several holes in the facade. The reason for such an accident was due to the existing crack in the boiler circulation pump. Due to the frequent load changes, the pump was subjected to higher thermal shocks and pressure changes, hence resulting in material fatigue and crack propagation in the housing. This damage cost the operator E. ON around 25 million Euros. This value excludes the costs incurred from the downtime.
Monitoring with the help of TSE
To determine the degree of exhaustion of power plant components, at TÜV SÜD the computer program TSE (Temperature-Stress-Exhaustion) was developed. The publication shows the basic challenges and the procedure for the detailed determination of exhaustion.
In 2017, a pilot project was carried out together with the Stadtwerke München GmbH (Munich City Utilities) and was thereon extended to 14 other customers. This shows that the software and hence the service is based on the actual customer needs.
The TSE evaluates the load data from the entire past operation. Based on the pressure and temperature profiles, the program calculates the total exhaustion of the component, accounting for both creep exhaustion and alternating exhaustion (fatigue damage). TSE can also evaluate the temperatures measured at the outside of a component. This eliminates the need for internal temperature measurements. The data analysis is based on the procedure that complies with the regulations. The implementation of extensive online monitoring is therefore not necessary. The evaluation times of the offline lifespan calculation should be based on the most recently determined levels of exhaustion and the way the system is operated. In many cases, an annual offline evaluation cycle has proven itself, in which even new stress phenomena can be evaluated early enough. Based on this evaluation, TSE can be used to determine to what extent further flexibilization of plant operation is possible. Additionally, Non-Destructive Tests (NDT) can be carried out on selected components. The lifetime consumption can thus be controlled, which is associated to an economic optimization.
Variations in calculation
The calculations can be carried out conservatively either with the assumption of quasi-steady-state conditions or with the assumption of real unsteady-state conditions. The temporal development of the temperature distribution in the wall is determined based on the temperature profile measured on the inside or outside of the component. The temporal development of the component stresses/exhaustion can then be calculated from this. The basic approach at TSE is to be as realistic as possible and to avoid additional collateral that is not physically justified and is not required by the regulations.
For example, when calculating the stresses according to DIN EN 12952, TSE uses the integral mean wall temperature instead of the mid wall temperature. The reason for this is that there is a significant difference between mean wall and mid wall temperature and from a physical point of view, with the mean wall temperature the state of exhaustion can be calculated more precisely. TSE shows that in certain cases, when mid wall temperatures are calculated in comparison to mean wall temperatures, up to fifty percent higher levels of exhaustion are determined. This represents an exaggeration of the determined exhaustion compared to the real situation and can be avoided.
Benefits to the plant operators
The TSE monitors and predicts the remaining lifespan of critical components, hence aiding in the extension and optimisation of the power plants operations and being legally compliant with the regulations and standards as shown in figure 2. It forms the basis for targeted and efficient inspection measures, ensures the component availability and is cost-efficient.
Figure 2: State of exhaustion and permissible exhaustion levels in accordance with TRD/DIN/ASME regulations
The service can be universally applicated, irrespective of the manufacturer of the components and can be extended to other industries such as process industries, chemical industries etc.
TÜV SÜD Industrie Service GmbH
For More information, please visit: https://www.tuvsud.com/en-in/industries/energy/conventional-power/tse-power-plant-lifetime-extension