2.2
Risk Reduction
Functional safety relates to proper equipment operation, as well as other risk reduction facilities. Layers of protection (LOP)
concepts are as follows:
Control and Monitoring
Prevention
Mitigation
Plant Emergency Response
BPCS
Process Alarms
Operator Supervision
Mechanical Protection
Alarms with Operator Action
Safety Instrumented Systems
Plant Evacuation Procedures
Relief Valve
Barrier
Process
The LOP around a process can be used to introduce risk reduction. Failure to carefully analyze the available LOP and the
likelihood-consequence relationship of the risks involved with process control failure can lead to an expensive over-design of
the system. The goal is to reduce the risk to a level that is as low as reasonably practicable (ALARP).
Residual
Risk
Tolerable
Risk
Inherent
Process
Risk
Necessary Risk Reduction
Actual Risk Reduction
Increasing Risk
Residual
Risk
Tolerable
Risk
Inherent
Process
Risk
Necessary Risk Reduction
Actual Risk Reduction
To achieve functional safety, it is necessary to analyze the potential hazards to personnel and property, including any
environmental impact, that could occur when the control of equipment is lost.
Requirements for safety function and integrity must be met to achieve functional safety. Safety function requirements describe
what the safety function does and is derived from the hazard analysis. The safety integrity requirement is a quantitative
measure of the likelihood that a safety function will perform its assigned task adequately. For safety functions to be
effectively identified and implemented, the system as a whole must be considered.
A primary parameter used in determining the risk reduction in a safety controller is the Average Probability of Failure on
Demand (PFD
avg
). The inverse of the PFD
avg
is the Risk Reduction Factor (RRF).
RR-
F =
1
PFD
avg
Functional Safety
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