TABLE OF CONTENTS The conceptual design process for a safety instrumented function begins with a Safety Requirements Specification (SRS). The SRS is an important document. It should contain complete specifications for designing all the safety instrumented functions. For each SIF the following information should be included:
the hazard and its consequences
the demand frequency for the hazard
a reference to the appropriate P&ID drawing
a definition of the process safe state
a description of the safety instrumented function
a description of the process measurements and trip points
a description of the output response required for both primary equipment and auxiliary/secondary equipment
the relationship between process measurements and outputs, including logic, mathematical functions and any required permissive - this must be specified for all modes of operation, e.g., startup, normal, abnormal, emergency, shutdown, etc.
the required safety integrity level
target proof test intervals
maximum allowable spurious trip rate
maximum response time requirement for the SIF
requirements for manual activation of the SIF
requirements for reset of the SIF (latching or automatic reset)
SIF response to diagnostic faults (automatic shutdown, alarm only or other)
requirements for human interface - What variables must be displayed? What variables must be input?
maintenance override capability requirements
estimates for mean time to restore, startup time after a trip, etc.
expected environmental conditions during normal operation and emergency situations
Other items may be required for specific applications including any local regulatory requirements (must meet NFPA85 [Ref. [1]] for example), references to company specific requirements or...
