SIL ( Safety Integrity Level )

                    SIL is defined as a relative level of risk reduction provided by a safety function or to specify a target level of risk reduction. In simple terms SIL is a measurement of performance required for a instrumental function.

                     SIL is the functional safety standards based on IEC 61508, SILs are defined,with SIL 4 is the most dependable and SIL 1 is being the least.

                      There are several methods are used to assign a SIL, these are normally used in combination, and may include,
 1. Risk matrices
 2. Risk graphs
 3. Layers of production analysis(LOPA)                
   this 3 are normally used

SIL level chart :

PFD(probability of failure demand ) and RRF(Risk reduction factor )

For Low demand operation:
SIL         PFD                                   PFD(power)                   RRF
  1          0.1-0.01                            10-1 to 10-2                   10-100
  2          0.01-0.001                        10-2 to 10-3                   100-1000
  3          0.001-0.0001                    10-3 to 10-4                   1000-1000
  4          0.0001-0.00001                10-4 to 10-5                   1000-10000

For Continuous  Operation 
SIL         PFD                                                 PFD(power)                    RRF
  1          0.00001-0.0000001                          10-5 to 10-6                  100,000-1,000,0000
  2          0.000001-0.00000001                      10-6 to 10-7                  100,000-10,000,0000
  3          0.0000001-0.000000001                  10-7 to 10-8                  10,000,000-100,000,000
  4          0.00000001-0.0000000001              10-8 to 10-9                  100,000,000-1,000,000,000