Safety Integrity Level (SIL)
A Safety Instrumented Function (SIF) is an independent Functional safety that actively performs an action when a measured value is exceeded. For example, stopping a pump or closing a valve if the pressure exceeds a set value. The LOPA (Layer Of Protection Analysis) indicates the reliability that the SIF must have in order to reach the safe area.
A SIF consists of a Sensor (e.g. the pressure transducer), a Logic Solver (e.g. a Safety PLC) and a Final Element (e.g. valve or pump).
– SIL (Safety Integrity Level) is the degree of reliability that a SIF (complete loop) must meet. What is the probability that the SIF will fail if it is called upon and has to do its work (PFD, Propability of Failure on Demand). Or the reverse: how much risk is reduced by applying a SIF (RRF, Risk Reduction Factor).
The higher the SIL level is, the smaller the chance that the security fails when it is called upon.
SIL
Suppose a process safety study shows that due to joint events a vessel will overflow once in 10 years, then a SIL 1 High level safety system will ensure that the overflow will not happen more than once in 100 years.
Now it depends on what the consequences of the overflow of this vessel are. If the overflow of this vessel causes a process disruption, a SIL 1 may be sufficient. This means that the probability of the vessel overflowing is no more than once every 100 years.
If, for example, it concerns a Knock Out vessel of a compressor, which has the consequence that there is a major damage to the compressor, a SIL 2 may be required. The probability of the vessel overflowing is then reduced to no more than once every 1000 years.
The PFD values are averages. It may happen tomorrow, but the probability of it happening again within the timeframe is reduced.
To determine whether the SIF meets the required RRF or PFDavg, you need failure rates of the sensor, logic solver and final element, and you can use aeShield’s SIL verification tool to determine whether with the chosen instrumentation and architecture this SIF meets the requirements.
