What is SIL and Do I Need It?
When looking for different instruments to place in your application, it is always important to understand the safety requirements of your environment.
You will often hear of the requirement to use an explosion-proof or an intrinsically safe device. Another requirement often thrown into the ring is the term “SIL”, an acronym for “Safety Integrity level”.
Before discussing the meaning of SIL, it is important to mention the safety standard IEC 61508 for designing SIL equipment. IEC 61511 is an international standard that lays out how to set up and maintain a safe automatic protection system for your environment. This system goes by the name of a Safety Instrumented System, also known as an SIS. The ultimate goal of SIS is to govern all potentially dangerous processes in your environment.
Each SIS is broken down into numerous Safety Instrumented Functions, also known as SIFs.
While the SIS simultaneously governs all processes, each SIF governs an independent process. A SIF will usually consist of several different instruments, all coordinated to bring the independent process back to a safe state. For example, a SIF for a high-pressure vent valve may consist of a pressure-sensing switch that sends a signal to a logic solver. That will then open a vent valve to prevent an overpressure scenario. All three of these devices will work in coordination to form the SIF. Before designing a SIF, you must first determine the Safety Integrity Level (SIL) required for that specific function. This is where the SIL requirement finally comes into play!
To determine the necessary SIL rating for a SIF, you first must perform a hazard and risk analysis of your specific function. Hazard analysis will determine the severity of a potential dangerous event caused by your application. Risk analysis will determine the frequency or the likelihood of a dangerous event occurring.
Depending on the hazard and risk analysis ratings, the SIF will receive a SIL requirement ranking from 1 through 4. The application may also not be dangerous enough to require SIL or may be too dangerous to be protected using just SIL requirements. Each of the different SIL ratings are designed to reduce possible risk within an SIF to an acceptable level. The greater the hazard and risk of the application, the higher the SIL rating will be.
A SIL 1 rated SIF will have a risk reduction factor of 10-100, while a SIL 2 rated SIF will have a 100-1,000. SIL 3 rated SIF will have a 1,000-10,000 risk reduction factor. Lastly, a SIL 4 rated SIF will have a 10,000-100,000 risk reduction factor.
Any devices or instruments used in a SIF must meet or exceed the SIL requirement.
Here at United Electric Controls, we have several options whenever someone is designing their application. First, we have our 100 Series mechanical switch with a general purpose enclosure. The 100 Series is certified for SIL 2 applications and, if used with redundancy, is certified for SIL 3 applications.
Next, we have our 120 Series mechanical switch with an explosion-proof enclosure. The 120 Series is also certified for SIL 2 applications and, if used with redundancy, is certified for SIL 3 applications.
Last, we have our One Series Smart Electronic Safety Transmitter, which was specifically designed for use in Safety Instrumented Systems. Some of the One Series Safety Transmitter features include: Full digital display, a 4-20 mA transmitter output, both high and low capacity switch relays, full self-diagnostics and fully adjustable set point and deadband values.
About the Author: Mark McCabe, Sales Applications Engineer
Mark McCabe has been with UE for just over 2 years. He is the Sales Applications Engineer focusing mainly on the One Series Smart Electronic Switch and Transmitter.