Process industry is exposed to risks like fire, explosion, injuries and accidents etc causing fatalities and monetary losses. Safety Instrument System (SIS) is one of the most important layer of protection against accidents & hazards, in a chemical process industry. Occupational Safety and Health Administration, USA warrants that the design and implementation of safety system meet good engineering practice. Safety Performance criteria for SIS should be defined by Safety Integrity Levels (SIL). The determination of Safety Integrity Level required for the SIS will help verification of configuration of SIS to meet or exceed the required SIL and in turn the reliability of the system.


During the design stage of the project or modification or revamp, the safeguarding systems or mergency shut down systems (mechanical or instrumented) are identified. The requirement for an Instrumented Protective Functions (instrumented systems) is decided based on the design practices which are checked by technical disk HSE review or the HAZOP study. Consequence of failure in terms of safety, environment and economical losses are assessed along
with the probability of occurrence. This risk graph generated helps to assess the risk levels. The SIL assessment can be qualitative and quantitative.

The appropriate SIL is simply selected from the matrix once the demand rate and highest consequence class of all consequence categories have been determined.

Standards adopted for the study:

  • IEC 61508

  • IEC 61511

  • ANSIISA 84.01

Documents required for the study:

  • Process flow schemes

  • P & ID diagrams

  • Standards adopted for the instrument selection

  • Process safety study reports

  • Cause and effect matrices

Role of Circles Safety:

SIS study will be team exercise and competent personnel responsible for the areas of process technology, process safety, operations and process control should be part of the team. The Circles Safety expert will carry out the role of facilitator. The main task of the facilitator is to guide the team through the classification steps and to ensure that every step is recorded to achieve the objective.


  • Helps improve overall safety of the facility.

  • Prevents (or) mitigates consequences which can result in – loss of life, personnel injury,
    equipment damage, loss of production.

  •  Helps in complying with present (or) future government directives on Health, Safety and

  • Provides a better corporate image and helps in boosting employee morale.

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