The term SIL stands for Safety Integrity Level. SIL is a measure of the effectiveness of a Safety Instrumented System (SIS) in reducing the risk associated with a particular hazard within a process industry. The concept of SIL is widely used in the field of process safety to ensure that Safety Instrumented Functions (SIFs) meet specified performance requirements.
SIL and Risk Reduction
The primary objective of assigning an SIL is to quantify the level of risk reduction that a safety system must achieve to bring the overall risk of a process to an acceptable risk
SIL is typically expressed on a scale from SIL 1 to SIL 4, with SIL 4 representing the highest level of risk reduction. As the higher required level of risk reduction, the SIL level increases.
The performance or effectiveness of SIL is presented by the Probability of Failure on Demand (PFD). On the other hand, PFD is a metric used to quantify the likelihood that a Safety Instrumented Function (SIF) will fail to perform its intended function upon demand.
| SIL Level | PFDavg (Low Demand) | RRF | PFHavg (High Demand) |
| 1 | <10-1 to < 10-2 | >10 to 100 | <10-5 to < 10-6 |
| 2 | <10-2 to < 10-3 | >100 to 1,000 | <10-6 to <10-7 |
| 3 | <10-3 to < 10-4 | >1,000 to 10,000 | <10-7 to <10-8 |
| 4 | <10-4 to <10-5 | >10,000 to 100,000 | <10-8 to <10-9 |
From the table, the higher the SIL, the lower the acceptable level of risk and, consequently, the lower the allowed Probability of Failure on Demand (PFD).
“Low Demand” refers to the situation where the safety function or safety instrumented system is called upon infrequently. It meant the system is typically in a passive state most of the time and is activated only when needed.
“High Demand and Continuous Demand” refers to situations where the safety function or safety instrumented system is called upon frequently or continuously. High demand scenarios imply that the system is active or demanded much more often, and it might be continuously providing its safety function.
In high demand scenarios, the Probability of Failure per hour (PFH) is used to assess the reliability of safety functions.
SIL, SIF, and SIS
In summary, a Safety Instrumented System (SIS) is the overarching safety system that encompasses multiple Safety Instrumented Functions (SIFs). Each SIF, in turn, is designed to address a specific hazard and is assigned a Safety Integrity Level (SIL) based on the required risk reduction. The relationships can be summarized as SIS > SIF > SIL, where SIFs are components of an SIS, and SIL is a measure assigned to each SIF.
| SIS | SIF | SIL | |
| Scope | Entire system | Specific function | – |
| Component | Overach SIF | Sensor+Logic solver+Final Element | – |
| Function | Manage overall | Implement a specific function | – |
| Measurement | – | – | 1, 2, 3, or 4 |
SIS and BPCS
BPCS or Basic Process Control System is focused on normal process control, while SIS is dedicated to ensuring the safety and integrity of the process by responding to abnormal or hazardous conditions. Hence, BPCS and SIS are two distinct systems that serve different purposes.
| BPCS | SIS | |
| Purpose | Control and optimize normal operation | Manage safety-critical functions |
| Functionality | Routine process control | Response to only abnormal situations |
| Control Loop | Aims to maintain the process within normal range | Design to bring the process to a safe state |
| Response Time | Faster response time | Lower response time but more in reliability |
The concept of BPCS and SIS is presented below. The ATM storage tank controls the liquid level incoming flow by using the level control loop (LIC-01) and in case of high- liquid level than the design set point, the high liquid level switch will isolate the inflow liquid via LIS-02 by closing the shut-off valve.
PROcess Safety TASK 