TABLE OF CONTENTS A working definition of the Safety Lifecycle is that it is an engineering process utilizing specific steps to ensure that Safety Instrumented Systems (SIS) are effective in their key mission of risk reduction as well as being cost effective over the life of the system. Activities associated with the Safety Lifecycle start when the conceptual design of facilities is complete and stop when the facilities are entirely decommissioned. Key activities associated with a Safety Lifecycle are outlined below.
Analyzing risks
Assessing the need for risk reduction
Establishing system performance requirements
Implementing the system according to the required performance criteria
Assuring that the system is always correctly operated & maintained
Safety Lifecycle analyses heavily involve probabilistic calculations to verify the integrity of the safety design.
The IEC 61508 standard defines safety as "freedom from unacceptable risk" (Ref. [1]). Functional safety has been defined as "part of the overall safety relating to the process and the Basic Process Control System (BPCS) which depends on the correct functioning of the SIS and other protection layers." The phrase "correct functioning of the SIS" identifies the key concern. A high level of functional safety means that a safety instrumented system (SIS) will work correctly and with a high probability of success.
Functional safety is thus the primary objective in designing a safety instrumented system (SIS). To achieve an acceptable level of functional safety, several issues must be considered that may not be part of the normal design process for automation...
