Abstract

The paper provides guidelines for the definition of reliability requirements for computerized safety shutdown systems in the process industries. The main question discussed is how to derive safety system requirements which ensure that the reliability of field devices and control logic modules is balanced from a safety point of view.Reliability figures of example safety systems are presented for various configurations of sensors, input/output cards. Central Processing Units (CPUs), and actuating elements. The figures are based on quantitative reliability analyses using a model and methodology for probabilistic safety assessment developed by SINTEF. The main new feature of this model compared to other models is that the effect of all types of failures occurring during field operation is considered in an integrated manner. Thus, failures due to excessive environmental stresses and human-made mistakes during engineering and operation are included in addition to failures due to natural aging of components (inherent failures). The effect of self-test is also included in the model.

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