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Motor Control Centers (MCC) are used in many complex processes in the (petro)chemical industries. When these processes get out of control the effects may have a great impact on personal safety and the environment. The installations that control such processes require a SIL-rating to safeguard a controlled shut down. As one may know the required SIL-rating depends on the risk of the particular process. The SIL-rating is determined by means of a risk assessment. Introducing SIL to a MCC has an impact on the operating principles and the circuit design and the choice of components. For example requirements for auxiliary contacts to be mechanically linked to the main contacts for reliable feedback on the status. For the high Safety Integrity Levels require redundancy for interrupting a circuit, where a reliable status feedback is of high importance with respect to process safety. In terms of availability or controlled failure rate of applied components, the coordination with protection devices is essential. In addition to typical SIL-classification, parts of the installation (e.g. machinery) may also be defined by its Performance level (PL). This sets requirements for redundancy and MTBF study on components. This paper is a study of the requirements of the user, the risk analysis and required SIL-rating versus the impact on the MCC operating principle and design. Reliability data, MTBF and field experience, of components like contactors, relays and control and protection devices will be taken in account. A realistic case with control concepts and circuits will be analyzed with respect to reliability for process safety. |
