What is SIL Level in Instrumentation and Control

SIL stands for safety integrity level. Basically it is measurement of performance of safety instrumented system and relative level of risk reduction of safety function.

SIL Level Summary

Following table summarizes the required PFDs and availability compared to the required SIL level.

SIL	IMPLEMENTATION	Required PFD	Safety Availability
1	Initiator : Separate from control Logic Solver : Separate from control Relay / PLC/solid state/magnetic core hardware Final Element : Separate from control	10-1 >PFD≥ 10-2	0.9 to 0.99
2	Initiator : Separate from control Logic Solver : Separate from control Relay / PLC/solid state/magnetic core hardware Final Element : Separate from control, unrevealed failure robust	10-2 > PFD ≥ 10-3	0.99 to 0.999
3	Initiator: Separate from control, unrevealed failure robust Logic Solver : Separate from control Relay / PLC/solid state/magnetic core hardware Final Element : Separate from control, unrevealed failure robust	10-3 > PFD ≥ 10-4	0.999 to 0.9999

1. Required PFD is indicated on the basis of ‘Low Demand Mode of Operation.’ i.e. average PFD to perform its design
function on demand is evaluated with the required PFD criteria

2. The initiator is the measuring device and is the source of the signal acted upon by the SIS.

3. The Logic Solver is the operating technology of the logic solving element of the SIS.

4. The Final Element is the equipment that acts upon the process to prevent the hazardous event.

5. When the item is said to be separate from control then the implementation of the SIS must not be combined with the
process controls.

6. The proportion of the PFD made up by the logic solver is not generally large in comparison to the actuator or
initiator. The PFD figures refer to the PFD for the complete loop including initiator, logic solver, and final element.

What is SIL Level in Instrumentation and Control