A free translation of "Safety Requirement Specification" (SRS) is Safety Requirement Specification and includes high-level description of the safety functions and requirements for each required safety function. For example, it may include the description "Measure the temperature in vessel V-1202 and if it ex ceeds 60 °C, stop the pump P-103 to the reactor within 2 seconds with a Safety Integrity Level SIL-3."
This description contains all the basic essential information to design a security function:
- The location and function of sensors and actuators related to a P&ID;
- The required SIL level, determined from a HAZOP/ZIL excercise;
- The response time required, determined from a HAZOP/ZIL excercise.
A number of choices can now be made for the security function:
- the required SIL level determines the maximum PFD ("Probability of Failure on Demand") for the complete loop;
- the required SIL level also determines the minimum SFF ("Safe Failure Fraction") for all components in the complete loop;
- if one of the selected components does not meet the minimum SFF requirement, it may be necessary to apply voting. For example, two transmitters may be applied in a 1-out-of-2 voting (1oo2) because a single transmitter has too low an SFF value and the voting configuration does meet the required SIL level.
A complete specification for a safety system includes, in addition to the above:
- redundancy requirements, to improve availability;
- maintenance override functions;
- communication with other systems;
- any preferred brands and models of the components, e.g. the safety PLC, power supplies, isolators, barriers, etc.;
- panel layout requirements such as, for example, location, dimensions, spare capacity;
- hardware requirements: ATEX classification, materials to be used, wire colors;
- user interface: graphical screens, philosophy on alarm handling, buttons and lights.
We are happy to help with setting up an SRS and all other design choices related to a safety system.