What is a Safety Instrumented Function

Safety Instrumented Function

The Safety Instrumented Function, whose abbreviation is SIF, is a protection layer whose objective is to achieve or maintain a safe state of the process when a specific dangerous event occurs. The SIF is implemented in the SIS (Safety Instrumented System) which is normally composed of several Safety Functions.

Each Safety Instrumented Function is assigned a certain level of protection, defined by the SIL level (1, 2, 3 or 4), using one of the methodologies defined in IEC 61508/61511 as the “Risk Graph” , the “Risk Matrix” or the LOPA.

The following table shows the Risk Reduction Factor (RRF) of SIL levels in the case of a low demand SIF that is common in the process industry.

For example, in a SIF with SIL-1 we will reduce the risk a minimum of 10 times, with SIL-2 100 times and with SIL-3 1000 times.

The Safety Instrumented Function is composed of any combination of sensor, logic solver (PLC), final element and all necessary interfaces (cables, tubing, process connection, etc.).

We should remember the following:

SIS is composed of several Safety Functions (SIF).
Each SIF consists of a SENSOR subsystem, a LOGIC SOLVER subsystem and an ACTUATOR subsystem. Each of these subsystems has one or more devices with a certain architecture (1oo1, 1oo2, 2oo3, etc.). The logic of the sensor and actuator subsystems is programmed in the safety PLC.
A SENSOR or ACTUATOR subsystem can be part of one or more SIFs, there can even be two SIFs with the same elements. For example, a pressure transmitter with high and low pressure trip. We have two SIFs that must close the gas shut-off valves of the burner in case of both high and low pressure. In this case both SIFs use the same sensor, the same PLC and the same valves. The difference between the two is at the trip setpoint (and how the SIF behaves when a dangerous failure is detected based on the transmitter configuration “over / under range”).
Each SIF has a different assigned SIL level.
The parameters to calculate the Probability of Failure of each of the SIF subsystems can be different, although it is usual to use the same LT (Life Time or Mission Time) and the same SIF Start-up Time (for MTTFS calculation).