SIL 4 Safety Level Certification Guide for Control Valves

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As the recognition of industrial accidents and their impact on society becomes widespread, more and more companies are realizing the importance of safety.
In modern process industry production, due to complex processes and numerous equipment, any failure can have severe consequences for employees, the public, equipment, or the environment.
The SIL certification process involves helping businesses fully utilize their excellent engineering practices and safety technologies (IEC61508 and IEC61511) to prevent industrial accidents from recurring. This is because these experiences and technologies are based on a wealth of actual experience and lessons learned.
With the issuance of the IEC 61508 and IEC 61511 series standards, a systematic functional safety assessment and testing process can effectively address this issue.

Safety Integrity Level (SIL), abbreviated as SIL, is distinguished by the probability of dangerous failure occurring per hour.
There are four SIL levels specified in the international IEC standards, with SIL 4 indicating a high degree of integrity and SIL 1 representing a low level. For each safety integrity level, there are design specifications that can reduce design errors.
The required safety level for the production process is assessed and determined by a specialized production technology company. Generally, processes with high safety requirements require a safety level of SIL 3.

What is the failure rate of SIL certification?
During the SIL certification process, a concept we often mention is the equipment failure rate, which is a core term in SIL certification. It can be said that without determining the failure rate, the SIL certification is incomplete and inadequate. So, what exactly is the concept of failure rate?
Firstly, to understand failure rate, we must know what is considered as failure. According to GB/T 20438.4 and also the definition in IEC 61508, failure is "the termination of the ability of a functional unit to perform a required function, or the operation of the functional unit in a non-required manner." A more intuitive understanding would be: normally, y = f(x), but after failure, it becomes y ≠ f(x), where f() represents the pre-set function, and y stands for our safety expectations. This also explains why "function" is the same word used for both function and function in English.
This statement might be too formal and not easily understood by the general public. So, speaking in layman's terms, let's liken it to a lighting system. If the lighting system doesn't light up or doesn't light up according to our specified requirements, it's considered to be malfunctioning.
So said, you might know that when a product doesn't function according to our specified requirements, we can say it's failed. Then, what's the probability of this failure? That's the failure rate.
After understanding what failure is, we can start categorizing types of failure. In the concept of functional safety, failures can be divided into two main categories based on their generation cause: systematic failure and random hardware failure. Among them, random hardware failure is quantifiable. The calculation process of PFDavg is based on a probabilistic analysis of random hardware failure.
In the SIL certification, we analyze the theoretical model of the product and utilize the empirical data from its market sales to determine the failure rate. This, combined with other aspects of the SIL certification, forms a comprehensive system to ensure the reliable and safe execution of safety functions.

The standard for SIL Safety Integrity Level certification primarily includes:
1. IEC 61508—sets fundamental safety requirements for both normal system operation and fault prediction capabilities. These requirements cover general safety management systems, specific product design, and process design that meets safety requirements, aiming to prevent both systematic design failures and random hardware failures.
2. IEC61511 – A functional safety standard specifically for safety instrumented systems in the process industry, which is a domain standard introduced by the International Electrotechnical Commission (IEC) following the foundational functional safety standard IEC61508. In China, the harmonized standard for IEC61511 is GB/T 21109.
3. ISO 13849-1 – Comprehensive safety assessment from components to the entire system. This standard also offers designers more quantifiable methods for design implementation.
4. IEC62061 - Standard in the field of electrical systems.
5. IEC61326-3-2 – Specifies additional requirements for the immunity levels of safety-related equipment, including low probability end-of-site occurrences.
6. ISO 26262 – Primarily focused on specific electrical devices, electronic equipment, and programmable electronic devices in the automotive industry, these are specialized components exclusively for automotive use, aimed at enhancing the functional safety of automotive electronics and electrical products.
SIL certification is a third-party assessment, verification, and certification based on standards such as IEC 61508 (GB/T 20438), IEC 61511 (GB/T 21109), IEC 61513, IEC 13849-1, IEC 62061, and IEC 61800-5-2, which evaluate and confirm the Safety Integrity Level (SIL) or Performance Level (PL) of safety equipment. Functional safety certification primarily involves document management (FSM) assessments for the development process of safety equipment, hardware reliability calculations and evaluations, software assessments, environmental testing, and EMC electromagnetic compatibility testing. SIL certification is divided into four levels: SIL1, SIL2, SIL3, and SIL4, encompassing both product and system levels, with SIL4 being the most stringent.