What is SIL in process industry? how to calculate SIL level?

What is SIL?

SIL stands for Safety Integrity Level, IEC 61511/61508 standards describe four levels that indicate the measure of safety level or risk expected for a system. This a relative unit for the probability that the safety system will supply the safety function correctly for certain time period.

Higher the SIL value greater the reduction of risk.

Classification of SIL:

SIL is classified into four levels, SIL1, SIL2, SIL3, SIL4.


For the higher SIL standard number increases the risk is reduced. Each level represents an order of magnitude of risk reduction. The higher the SIL level, the greater the impact of a failure and the lower the failure rate that is acceptable. Which is shown below, as per IEC 61508:

How to determine SIL level?

The SIL level of a product is determined by three things:

  1. The Systematic Capability Rating: If the QMS meets the requirements of 61508 a SIL Capability rating is issued. The rating achieved depends on the effectiveness of your QMS. The certificate is for the systematic capability of a product
  2. The Architectural Constraints for the element: Architectural constraints are established by following Route 1H or Route 2H. Route 1H involves calculating the Safe Failure Fraction for the element. A valve is typically one component of the final element of a safety instrumented function (SIF).
  3. The PFDavg calculation for the product: The PFDavg is based on the dangerous failure rate, system diagnostics, proof test coverage and test intervals. Typically, a final element assembly will have a PFDavg the only meets SIL