Reliable Monitoring of Pressurised Components Creep-fatigue life prediction in flexible plant operation
Metal components in chemical plants are often exposed to high temperatures or cyclic pressure and temperature loads, making them increasingly vulnerable to material fatigue. Consequently, the longer the service life of a component, the higher the risk of its failure and of production losses. What is needed, therefore, is a method for quick and realistic analysis of the component condition to enable timely intervention and optimum planning of maintenance measures to be effected.
Many chemical plants need to be operated at high temperatures under rapidly changing load parameters. These modes of operation may expose plant components to higher stresses, resulting in creep-fatigue degradation caused by pressure and/or thermal loads. In view of these developments, monitoring of component exhaustion is becoming increasingly vital. On the one hand, realistic knowledge of the state of creep fatigue plays an important role in comprehensive damage prevention; on the other, managers and operators must be able to make use of the full-service life offered by their plant components. And, monitoring also makes good sense because the results provide reliable information for condition-based maintenance.
Important degradation mechanisms acting on pressurised components include creep and fatigue, which are also referred to in technical literature as “high-temperature progressive deformation” and “progressive and localised structural damage under cyclic loading” respectively. The combination of these two degradation mechanisms is known as creep fatigue and are referred to in this article as “exhaustion” or the “state of exhaustion”. While creep is caused by static loads and depends on temperature and internal pressure, fatigue is caused by cyclic loads – in other words, changes in pressure and temperature. In the case of changes in temperature, fatigue increases in parallel to the rate of temperature changes. This is due to the following mechanism: as the thermal conductivity of the material is limited, steep temperature transients result in increasingly high temperature differences inside the component wall, which in turn give rise to mechanical stresses.
Initially, the impacts of these stresses are hardly detectable because they first become apparent at the component’s inner surface, where they are difficult to identify by non-destructive test methods (NDT). Identification is only possible if the component is already affected by incipient cracking. Defects and failures are the result, even though the responsible parties assumed they had taken reliable precautions in the form of NDT. Many of these defects and failures could have been prevented by needs-based calculation of component life.
Stress-strain analysis using TSE
The TSE (temperature stress exhaustion) software developed by TÜV SÜD calculates component exhaustion (fatigue and creep) on the basis of pressure and temperature curves. The program uses algorithms aligned to the specific regulations and standards in encapsulated functions and is fully in conformity with the applicable codes and standards. The reports of the results thus also provide a reliable legal compliance report.
Input data required by the software include data on component geometry and materials in addition to the measured temperature and pressure loads. Within the scope of the analysis, the program uses integrated non-stationary calculation of the temperature field to model the non-linear distribution of temperature across the component wall for a close network of interpolation points at any point in time throughout the analysis period. The software is also able to process temperatures measured on the component exterior. In other words, the TSE solution eliminates the need to perform complex temperature measurements on the interior wall of the component or the fluid, or even measurements of the temperature difference including determination of the temperature inside the wall. Even temporary measurements can be included in the calculation.
For plants in Germany, data analysis is aligned to technical regulations and standards harmonised across the EU. Using offline evaluation of the measured and saved data, the solution delivers proof of whether component exhaustion is below the critical thresholds defined in DIN EN standards. The intervals of this type of creep-fatigue life analysis should be based on the state of exhaustion determined in the most recent previous analysis and the mode of plant operation. In many cases, TÜV SÜD recommends annual evaluation cycles to ensure timely inclusion of new load phenomena in the evaluation.
Offline evaluation offers significant benefits, particularly in the as-is analysis of the stresses acting on the component which is part of the first step of the evaluation, because monitoring over longer continuous periods enables stress patterns to be identified. To this end, appropriate pattern recognition routines have been implemented in the TSE program. They enable critical load events to be allocated reliably and – where necessary – changes to be initiated in the mode of operation for the purpose of reducing loads. In step two of the evaluation, the program determines the actual state of component exhaustion (Fig. 1) and the fatigue reserves of the components available at this stage. The TSE software thus provides information about the remaining number of load cycles and the extent to which further changes in the mode of operation can be realised.
In a possible third step, the program models the fatigue progress resulting from new transient configurations and accumulates it with the degree of fatigue from previous operation. This approach allows experts to define optimised transients that ensure consistent use of fatigue reserves while keeping component exhaustion below critical thresholds. The software is thus designed to determine values that are as realistic as possible, while preventing the introduction of additional safety margins that are not justified from a physical perspective and not required by standards and regulations.
Presenting creep-fatigue as realistically as possible
The calculations performed by the TSE software do not use the temperature inside the component wall, but the physically correct integral mean wall temperature. If the program used the temperature inside the component wall, the calculated Delta T would be excessive, resulting in 50% higher fatigue values. This is one example demonstrating that the phenomenon of “fake fatigue” can be avoided by consistently relying on realistic analyses instead of more conservative methods – a necessary step for further improving the flexibility of plant operation (Figure 2).
The objective should not be to keep component exhaustion to a minimum, but to achieve controlled component exhaustion in line with the available fatigue reserves. To this end, experts could also carry out predictive “what-if” analyses calculating the respective progress of component exhaustion for various differing operating transients.
Evaluation of the condition of steam crackers
The following case study shows how TSE can be used for monitoring and calculating the remaining service life of components. TÜV SÜD was commissioned to evaluate the condition of the reactor pipes of a steam cracker. A steam cracker contains pipe coils with inside diameters of 90 to 120 mm and lengths of 60 to 80 m which are installed in a furnace-heated combustion chamber. Steam crackers use process steam to break down long-chain hydrocarbons into smaller unsaturated hydrocarbons which are needed for industrial processes such as PVC production. Process temperatures are extremely high, at around 850°C. In other words, the components of a steam cracker are exposed to very high thermal loads.
The experts started by evaluating the available information about component temperatures in the past. These sources were evaluated to build a basis of reliable data providing information about components’ lengths of exposure to specific temperatures.
Other essential data concerned the geometry and materials of the components. These data were found in the installation documentation maintained by the plant manager. Next, the experts carried out a preliminary sensitivity analysis to identify which components were inherently affected by progressive degradation caused by temperature and pressure. For these components, the actual progress of degradation was then calculated by the TSE software based on specific evaluation of temperature and pressure data and considering the specific data of component geometry and material. The results delivered basic information not only about the state of exhaustion of individual components, but also about the state of the system and the transients that are particularly significant in fatigue progress. The experts also provided the plant manager with suggestions on how to reduce component load, and gave recommendations for condition-based NDT measures that are in conformity with the applicable standards and regulations.
TEXT AND IMAGES: Franz Binder, Dipl.-Ing. TÜV SÜD Industrie Service GmbH
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