process risk and safety specialists

nuclear and radiation consultants

Safety Integrity Level (SIL) Determination

The concept of safety integrity levels (SILs) was introduced to measure the quality or dependability of a system which has a safety function – a measure of the confidence with which the system can be expected to perform that function. Safety Instrumented Systems (SIS) are used in many systems for the prevention and mitigation of safety-related hazards.  A SIS may be made up of one or more Safety-Instrumented Functions (SIF).  The target Safety Integrity Level (SIL) of each safety-instrumented function (SIF) dictates the required performance of each SIF.  That performance criterion is determined via a SIL assessment.

SIL study is a structured and systematic examination by a multi-disciplinary team of design, operations, maintenance and management professionals. This team reviews each identified hazard and assigns a target SIL for each hazard. An approved technique is used, as defined in IEC 61508 / IEC 61511, and applied to all parts of a system to establish the minimum allowable SIL for each safety instrumented function in terms of Probability of Failure on Demand (PFD) so that safety and operability problems on the complete system are eventually identified and mitigated.

Bow-Tie Consulting will use the most appropriate methodology from IEC61511 in consultation with your requirements and have experience using all the following methods:

  • Quantitative method
  • Qualitative risk graph method
  • Qualitative hazardous event severity matrix method
  • Semi-quantitative method
  • Safety layer matrix method
  • Calibrated risk graph
  • Layer of protection analysis (LOPA) method

Our analysts have process knowledge as well as knowledge of instrumentation used in the development of SIL rated protection systems. This will ensure that the SIL allocation is done in awareness of the process hazards and the practicalities of the design of SIL protection of various ratings.

We hold a licence for exSILentia ™ which is one of the world recognised software systems with an integrated database of failure data. Our clients often are required to use this software when verifying that the SIL requirements are met by a particular design. We assist our clients in the design process by ensuring at an early stage in the design that the basic design can meet the SIL requirements.