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K. Operation and Maintenance

IEC 61511 requires that the SIS be operated and maintained so that the designed safety function is preserved. The SIL of each SIF must be maintained throughout the lifecycle of the plant.

This function is usually performed by the user/operator and/or a maintenance contractor. However, the responsibility resides with the owner.

The operation and maintenance plan should address, at minimum, the following:

  • Proof testing, preventive and breakdown maintenance activities.
  • Verification of adherence to operation and maintenance procedures.
  • Designation and competence of persons, departments and organizations responsible.
  • Schedule adherence to all activities.
  • Additional mitigation actions necessary during bypass and/or testing.
  • Recording of actual process demand rate on the SIS.
  • Identification of the cause of process demands.
  • Recording of actual failure rates of SIS devices, including field equipment.
  • Identification of the cause of false trips.
  • Correct operation of each field sensor and final element.
  • Correct logic action of the SIS.
  • Correct alarms and indicators.
  • Verification and Validation of actual SIL of each SIF and confirmation of equipment failure rate assumptions during the design phase, as well as adequacy of the proof test interval necessary to maintain the designed safety function.

Note: COSIL® Safety System management tool-set for on-line / real time continuous SIL monitoring of all the Safety Instrumented Functions (SIF) in a process plant’s SIS is an excellent tool that provides the mechanism for SIS operation and maintenance validation.

COSIL® additionally provides the functionality to perform continuous on-line calculations of the Safety Instrumented Function’s (SIF) instantaneous probability to fail on demand (PFD). This measurement provides plant engineers with real time data for evaluating the actual instantaneous Risk Reduction Factor (RRF), conducive to better decision making in the area of improvements in plant safety. Knowledge of the instantaneous PFD provides a wealth of information over and above the PFDavg based SIL.

COSIL® is applicable to both "Demand mode of operation" and "Continuous mode of operation" as defined in IEC 61511-1 paragraph 3.2.43.2