Proof tests in functional safety

Functional safety verification tests are essential to ensure proper operation of various protection systems. This analysis is vital at all stages of the safety lifecycle, but what exactly do they entail?

What Is a Verification Test?

To better understand functional safety verification tests, think of them as similar to quality controls in any production chain. However, in this case, they apply to the lifecycle of SIS (Safety Instrumented Systems) and involve analyzing each element’s response to its specific requirements.

Types of Functional Safety Verification Tests

These tests can be classified into two types based on their focus. It’s essential to differentiate between them, as they determine the entire analysis process, although their ultimate purpose is the same. These are:

Comprehensive Verification Tests

As the name suggests, these tests aim to review the entire safety instrumented system, from detection and logical devices to output circuits and responses. It can be conducted directly or through the consolidation of various partial analyses.

Partial Verification Tests

In contrast, partial verification tests focus exclusively on one or two specific elements of the system. The goal is to ensure that the safety instrumented functions of each device can operate flawlessly. It is advisable to apply both verification methodologies periodically.

Quality Controls Throughout the Safety Lifecycle

As mentioned, one of the main characteristics of functional safety verification tests is their application across all phases of the process. It’s not an isolated procedure but rather a continuous presence in each phase of the safety lifecycle, which includes planning, design, implementation, evaluation, and the use and maintenance of systems.

System Planning

Planning safety instrumented systems requires analyzing the situation and context of the working environment. It involves identifying risks, their potential impact, and studying the safety needs and requirements of each process to gain essential insights before proceeding to the subsequent stages.

Design and Implementation

The design of safety instrumented systems must follow the earlier analysis. At this point, safety verification tests become even more critical, as implementing new SIS requires specific attention to ensure that the protection is adequate and reaches the required safety levels for each case.

System Evaluation

While evaluation is an inherent process in functional safety systems, this phase also requires verification. In this case, it’s not a review of the process itself (which is advisable but outside the scope of verification tests), but a complement to the usual analysis methodologies, such as HAZOP evaluation.

Use and Maintenance of Elements

In the final phase of the safety lifecycle, which involves the use and maintenance of systems, verification tests become even more relevant than in previous stages. The goal is to study the day-to-day operation of SIS and identify potential process errors.

The Significance of SIL Classification in Verification Tests

Several crucial aspects are considered during functional safety verification tests, such as the probability of failure, systematic capabilities, and architectural constraints within an installation. To perform a comprehensive review, understanding the Safety Integrity Levels (SIL) in each element and all protection layers, as well as the tools used in the analysis, is essential.

Defining Failures in Safety Verification Tests

When conducting a functional safety verification test, it’s possible to classify failures into four types based on their hazard level and the risk detection capability. This classification plays a vital role in prioritizing the treatment of failures when necessary, and it includes:

  • Hazard Undetected
  • Safe Undetected
  • Hazard Detected
  • Safe Detected

Key Factors in Functional Safety Verification Tests

When establishing safety integrity levels, verification tests consider various values. Two key concepts used to determine where each system should be placed on the SIL scale (which ranges from SIL 1 to SIL 4) are the Risk Reduction Factor (RRF) and the Average Probability of Failure on Demand (PFDavg).

Risk Reduction Factor (RRF)

The RRF is determined through verification tests and assigns a value based on the minimization of hazards when applying a safety instrumented system. It also considers the mitigation of impact in case of a failure.

Average Probability of Failure on Demand (PFDavg)

On the other hand, PFDavg assesses hazard levels based on the frequency of demand in a safety system. It translates into an indication ranging from 0.001 to 1000, depending on the estimated time between two failures.

Functional Safety Verification Tests: An Essential Step

Just as HAZOP analyses are crucial in some industrial sectors, functional safety verification tests are unavoidable. Through these controls, Positive Engineering ensures the protection of installations, processes, and employees as a top priority in any company’s vicinity.