The Seal of Safety: Understanding SILs

If your operation involves the use of safety-critical systems, you will have come across the term “safety integrity levels,” or SILs. But what exactly are they? Why do they exist? And how do they support functional safety? This article answers some FAQs and helps you to understand what SILs mean for your organization.

Industrial safety goes deeper than ever before. Extending to the plant’s bottom line, it penetrates complex infrastructure and is embedded in all processes. Components that make up a safety instrumented system, in conjunction with other risk reduction measures help to minimize the risks of accidents caused by abnormal operating conditions. And since 1998, a benchmark has existed by which operators can rate the efficiency of safety instrumentation. The original concepts of safety integrity levels originated with the British Health and Safety Executive (HSE) and soon became the standard for safety-related systems, IEC 61508.

IEC 61511 stipulates the reliability requirements of safety instrumented systems and these can be defined by SILs. While the SIL can be determined by authorities, self-certification is also possible. This can be achieved with formal checks and tests during the approval process and at regular intervals throughout a plant’s lifecycle.

What is a SIL?

In short, a safety integrity level is a way to determine risk reduction. Four levels exist: SIL 1 represents the least amount of risk reduction and SIL 4 the highest. The greater the required risk reduction, the higher the necessary SIL for the safety instrumented function (SIF).

There is a common misconception that opting for a specific SIL-certified solution automatically provides the corresponding risk reduction level.

In most cases, all aspects of the SIF and safety instrumented system (SIS) should adhere to the same SIL. That also means that if all components of a SIS were rated SIL 3 except for one SIL 2-rated aspect, then the SIF cannot mitigate SIL 3 risks.

How Can I Achieve Optimal Risk Reduction?

There is a common misconception that opting for a specific SIL-certified solution automatically provides the corresponding risk reduction level. Rather, components are installed with failure rates that can be translated into a SIL rating when the entire SIS is considered.
But your safety processes are unique. A tolerable risk level to one plant operator might be unacceptable to another. The risk is determined by expert consultants during HAZOP where all appropriate layers of protection are analyzed. From that, the risk reduction factor is calculated through the number of times that a SIF reduced a risk. The risk reduction factor is the most accurate way to determine whether risk is effectively reduced.

Safety integrity levels reflect the amount of risk reduction provided by a safety instrumented function. Risk reduction factor is calculated during HAZOP.

In a competitive market, achieving and maintaining a greater risk reduction factor and SIL can be expensive, but is a necessary step in ensuring compliance. Generally speaking, the price of a SIL 2 certified sensor is higher than a SIL 1 certified sensor. This means that it is only worth investing in SIL 2, 3, or 4 where a high level of risk exists. SILs 3 and 4 are only a recommended target under specific critical circumstances, such as in control systems for rail transportation or in floating roof tanks storing toxic crude oil.

How Do I Ensure a SIL Is Maintained?

Regular proof testing is fundamental in managing your SIS. Tests should be performed periodically to reveal faults that could comprise operation or, in the worst case, place plant staff at risk.
However, proof testing could be considered a burden. Personnel might not understand all aspects of a system, or they might focus only on the necessary objectives to achieve compliance. HIMA recommends removing this burden by allowing expert consultants to take care of this process. HIMA safety experts perform a complete visual and functional check of an SIS, test all contact points, and take care of any relevant documentation. In this way, you can maintain the mandatory safety integrity level.