PC-FEB22-PG32-33.1_Layout 1 31/01/2022 14:01 Page 32
CHEMICALS & PHARMACEUTICALS
ASSESSING MATERIAL FATIGUE
Franz Binder, TÜV SÜD Industrie Service, discusses the benefits of TSE software, a monitoring tool which is used to calculate plant component exhaustion
etal components in chemical plants are exposed to high temperatures or cyclic pressure and temperature loads, making them vulnerable to material fatigue. The longer the service life of a component, the higher the risk of its failure. It is thus vital for the condition of components to be determined quickly and realistically, enabling timely actions to be taken where necessary. TSE software – a monitoring tool that can be used for all components exposed to cyclic loads – closes this gap. 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. Thus, 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, operators must be able to make use of the full-service life offered by their plant components. And, monitoring also makes sense because the results provide reliable information for maintenance works. Degradation mechanisms acting on components include creep and fatigue. The combination is known as creep fatigue and referred to in this article as “exhaustion”. While creep is caused by static loads and depends on temperature and internal pressure, fatigue is caused by cyclic loads (changes in pressure and temperature). The impacts of fatigue 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
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needs-based calculation of component life. The TSE (temperature stress exhaustion) software calculates component exhaustion (fatigue and creep) based on 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. 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. For plants in Germany, data analysis is aligned to technical regulations and standards harmonised across the EU. Using
Fig. 1: Workflow of offline calculation of service life (Image credits: TÜV SÜD)
32 FEBRUARY 2022 | PROCESS & CONTROL
Franz Binder, Team Lead Piping Systems and Lifecycle Analyses, TÜV SÜD Industrie Service (Image credits: TÜV SÜD)
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. TÜV SÜD recommends annual evaluation cycles to ensure timely inclusion of new load phenomena in the evaluation. Offline evaluation offers 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. 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. TSE is designed to determine values
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