BS IEC 62765-2:2019 pdf free download – Nuclear powers plants – Instrumentation and control important to safety – Management of ageing of sensors and transmitters
5.1 General
IAEA SSR-2/1 describes the requirements of ageing management in the design for safe operation over a lifetime. It is also necessary that a design margin be provided to take account of the ageing mechanism, neutron embrittlement, and wear out, as well as the potential for ageing degradation. SSR-2/2 also requires an effective ageing management program over the operating lifetime of the plant. Subclauses 6.1 35 to 6.1 52 of SSG-39:201 6 recommend design consideration for ageing management specific to I&C systems to meet the requirements of SSR-2/1 . The baseline of requirements of ageing management is to understand and identify the ageing phenomena, which cover the ageing mechanism, causes and potential effects of temperature sensors. Annex A summarises various kinds of tests for performance verification in order to identify a degradation in lifetime and to assess an ageing mechanism. IEC 62342 guides the general aspect of ageing management of instruments including temperature sensors, whereas IEC 60737 and others listed in Annex B cover characteristics and test methods of temperature sensors. Existing IAEA and IEC standards for ageing management standards are summarised in Annex B.
Performance tests in Annex A are important because they can detect ageing effects. Acceptance criteria for tests of temperature sensors are a critical element of an ageing management program (AMP) and they should be understood to identify the ageing effects, to evaluate the ageing problem, and to take a corrective action. The specified temperature tolerances of temperature sensors used as a basis for acceptance criteria for calibration tests are summarised in Annex C. Key steps and an example of AMP sheet are shown in Annex D.
5.2.2 RTDs in IEC standards
An RTD consists of a wired coil or deposited film of pure metal, commonly platinum, whose resistance changes with temperature. IEC 60737 describes the requirements of qualification tests and other design type tests of the temperature sensors in NPPs. IEC 60751 specifies temperature tolerances for each class of RTD, temperature/resistance relationship (i.e. a reference calibration diagram), and acceptance criteria for type tests and routine tests. IEC 60751 has been used widely as an industrial RTD standard. IEC 62397 specifies environmental qualification and other requirements for RTDs in NPPs.
5.2.3 Thermocouples in IEC standards
Thermocouples are temperature-sensitive elements that convert thermal difference into electric voltage. The principle is to measure an EMF voltage between two dissimilar conductors. Metal A and Metal B in Figure 1 have a contact potential; i.e. EMF between them, and the EMF changes as the temperature changes. The cold junction (T c ) is maintained at a reference temperature, while the hot junction (T h ) is attached to a probe to the process to be measured as shown in Figure 1 .