Degradation Mechanisms Control of Mechanical Components During Operation

Abstract

There are important mechanical systems, structures and components (SSC) in industrial equipments and nuclear power plants. These SSC are decisive for safety and economical operation and they mustn’t fail. There are principles used for these components to ensure their integrity during operation. Their fundamentals are as follows: – achievement of required quality during design, manufacturing and assembly, – quality assurance in following operation, – appropriate quality certificate in operation. The design should include all degradation mechanisms but some of them are very difficult to specify exactly and analyze before operation. The hardly specified degradation mechanisms have to be excluded from this concept. Appropriate operation measures have to be determined. Typical examples are operation vibration, material corrosion phenomena, thermal stratification, striping and dynamic loading during valve operation, etc. Design specification can determine basic design temperature, pressure, sustained loads and time history loadings during normal, abnormal and emergency operation only. Efficiency of the measures during operation has to be verified. Necessary measures have to be determined during design for verification and control of the real reasons during the SSC service. Both certificates and design of structure properties (material properties, dimensions, shapes, etc.) shall allow exclusion of systematic mistakes during design, manufacturing and assembly. Control of specified and unspecified degradation reasons during operation is the first redundant measure for insurance of required quality during operation and also timely assessment of inspection results regarding their influence on components quality. Appropriate measures shall be determined in time to minimize this influence if necessary. Check of degradation consequences is the second important measure. Relevant consequences have to be discovered in time before SSC failure. These measures shall be periodically updated in accordance with latest knowledge and their evaluation regarding the equipment. If relevant consequences are detected necessary measures shall be undertaken to exclude these consequences during following operation. Certificate of sufficient quality in operation have to be made before SSC is put in operation or after long term operation. Basis for this certificate are actual design and relevant load, real geometry, performed attachments, piping hanging and relevant operation loadings including media and specified postulated rupture according to current knowledge. Potential degradation mechanisms in SSC operation are necessary to determine and to prove sufficient insurance of their reasons. There are three levels assigned in the concept regarding SSC quality in future operation: – quality has to be guaranteed (prevent failure - proactive approach) by monitoring of causes of damage; – quality has to be maintained (preventative maintenance - proactive approach) by monitoring of damage results; – no specific demands on quality (maintenance triggered by damage - reactive approach) statistical approach for damage. The paper deals with theoretical background of the integrity concept on Czech NPPs and example of practical application is presented.