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A Safety Requirement Specification (SRS) contains basic high-level descriptions (“safety functions”) of the requirements on each safeguarding loop. For instance, it may read “Measure the temperature in vessel V-1202 and if it exceeds 60 degC then stop pump P-103 to the reactor within 2 seconds with Safety Integrity Level SIL-3”.

This description contains all the key information to design a safeguarding loop:

  1. The location and function of the sensors and actuators, from a P&ID
  2. The required SIL level, from a HAZOP/ SIL exercise
  3. The required response time, from a HAZOP/ SIL exercise

For the safeguarding loop, a number of choices can now be made:

  • The required SIL level determines the maximum PFD (Probability of Failure on Demand) for the loop as a whole
  • The required SIL level also determines the minimum SFF (Safe Failure Fraction, an indicator for the failure rate) for each of the components in the loop
  • If any of the selected components does not meet the SFF requirements, voting may be required. For instance, dual transmitters with 1-out-of-2 voting may achieve the required SIL level in spite of a low SFF for each transmitter

A full specification for a Safety Instrumented System includes all of the above, plus:

  • Redundancy requirements, to improve system availability
  • User interfaces: graphics, alarm philosophy, buttons and lamps
  • Maintenance override functions
  • Communication with other systems
  • Panel lay-out requirements: location, dimensions, spare capacity
  • Hardware requirements: ATEX classification, use of materials, wiring colours
  • Preferred makes and models of the Safety PLC, power supplies, isolators, barriers, etc

We can support the development of a Safety Requirement Specification and all the other design choices that define the Safety Instrumented System.

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