Safety and availability optimization of safety instrumented system

With the presentation of IEC 61508 and IEC 61511 standards, the safety instrumented system (SIS) has played important roles in process industries. Safety and availability are both important aspects with interactions to affect the integrated performance of SIS. Safety is the premise, which is evaluated by probability of failures on demand (PFD); availability is the basis, which is decided by PFD and probability of safe failures (PFS) simultaneously. In this paper, firstly, factors contribute to safety and availability are analyzed and the relationship between the two aspects is discussed; secondly, optimization theories are proposed for the design process of SIS; thirdly, design variables, objective functions and boundary restrictions are decided; finally, global optimizations of objective functions according with the restrictions are achieved by searching methods. By this method, balance between safety and availability is reached and design optimization of SIS is achieved.