M Omidvar Eshkalak - ۱Department of Chemical and Petroleum Engieering, Sharif University of Technology, Tehran, Iran
D Rashtchian - ۱Department of Chemical and Petroleum Engieering, Sharif University of Technology, Tehran, Iran

Safety Instrumented System (SIS) are systems or devices that take the process to a safe state when the process operates outside normal defined limits. The ANSI/ISA S84.01-1996 (1) and IEC 61508 (2) safety standards provide guidelines for the design, installation, operation, maintenance and test of SIS. These standards are intended that SISs achieve certain minimum performance levels to help ensure safety and environmental protection as well as provide economic benefits (3). SIL is a measure of safety system performance, It is not a directmeasure of process risk. Four integrity levels are defined by the standards (SIL 1, SIL 2, SIL 3, SIL 4); the higher the SIL, the more available the safety function (4). SIS standards cover a variety of techniques for determining Safety integrity level (5). The 3-dimensional Risk Matrix and the Risk Graph are two qualitative methods for determination of SIL. The risk matrix method has a number of disadvantages which are delineated in this paper. A modified risk matrix method has been developed to overcome these disadvantages.

SIL, SIS, PFD, SIF