Functional Safety Course

Functional safety is the part of overall safety that depends on a system performing a required safety function correctly. In the process industry, this means that safety-related control and protection systems must detect defined hazardous conditions and take the process to, or maintain it in, a safe state.

A Basic Process Control System (BPCS) controls the process during normal operation. A Safety Instrumented Function (SIF) is a specific safety action required for a defined hazardous condition. A Safety Instrumented System (SIS) is the complete system that performs one or more SIFs, normally using sensors, a logic solver and final elements.

The starting point is the risk created by the process and the level of risk considered tolerable. The difference between them defines the necessary risk reduction. Protection layers are then assigned, and the Safety Integrity Level specifies the integrity required from a Safety Instrumented Function. IEC 61508 defines SIL 1 as the lowest and SIL 4 as the highest integrity level.

PFD is the probability that a safety function will fail when demanded and is used for low-demand applications. PFH is the average frequency of a dangerous failure per hour and is used for high-demand or continuous-demand applications. The correct measure depends on how often the safety function is expected to operate.

A protection layer is an independent mechanism that reduces risk through control, prevention or mitigation. Examples can include process design measures, alarms with operator response, pressure relief, mechanical protection and Safety Instrumented Functions. The course explains how protection layers relate to risk reduction and SIL allocation.

Proof testing is used to reveal hidden faults that may prevent a Safety Instrumented System from performing its intended function. When faults are found, the system can be restored to a condition in which the safety function is available. Proof-test interval, coverage and quality directly influence the achieved safety performance.

The Safety Requirements Specification records all requirements that apply to the safety functions performed by the SIS. It should define the hazardous condition to be detected, the required safe action, set points, response time, operating modes, integrity target, testing needs, interfaces and other information needed for design and validation.

A random failure occurs unpredictably because of a hardware failure mechanism. A systematic failure has an identifiable cause in design, manufacture, operation or documentation and is prevented by improving the relevant process. A common-cause failure can cause two or more separate channels to fail from the same event, potentially defeating redundancy.

Hardware fault tolerance describes the ability of a system to continue performing its required function when faults are present. In an MooN architecture, N independent channels are provided and M channels are sufficient to perform the safety function. Examples include 1oo2, 2oo2, 1oo3 and 2oo3 arrangements.

HAZOP uses structured guidewords such as no, more, less, reverse and other than to identify credible process deviations. The resulting hazardous scenarios can be used to define required risk reduction, identify potential Safety Instrumented Functions and support later SIL determination or LOPA.

The lifecycle begins with hazard and risk assessment and continues through allocation of safety functions, the Safety Requirements Specification, design, verification, installation, commissioning, validation, operation, maintenance, modification and eventual decommissioning. Functional safety management, assessment and audit support these activities throughout.