NSIL Evaluation and Sensitivity Study of Diverse Redundant Structure

Abstract

In order to evaluate the safety integrity level (SIL) of safety instrumented systems (SIS) with diverse redundancy structures including more realistic assumptions, the average probability of failure on demand (PFDavg) models for the common diverse redundancy structures (1oo2, 2oo2, and 2oo3) have been established based on the fault tree analysis (FTA) method. These models take into account the common cause failures (CCF) and diagnostic coverage (DC) parameters with the correction coefficient. The model is applied to the SIL assessment of the emergency shutdown system (ESD) with diverse redundancy in gas storage and gathering. The proof test interval T is used as an indicator to compare and verify the sensitivity of both the new model and the previous models. The results can be concluded as follows: 1) The new model achieves higher consistency than the previous models; 2) The proposed model considers that different redundancy structures have CCF, which are more realistic and have higher applicability to be used to solve practical engineering problems.