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Safety Considerations Guide for Trident v2 Systems - TUV ...

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Protection Layers<br />

Methods that provide layers of protection should be:<br />

• Independent<br />

• Verifiable<br />

• Dependable<br />

• Designed <strong>for</strong> the specific safety risk<br />

Overview 3<br />

This figure shows how layers of protection can be used to reduce unacceptable risk to an<br />

acceptable level. The amount of risk reduction <strong>for</strong> each layer is dependent on the specific nature<br />

of the safety risk and the impact of the layer on the risk. Economic analysis should be used to<br />

determine the appropriate combination of layers <strong>for</strong> mitigating safety risks.<br />

0<br />

SV<br />

Acceptable Risk Level<br />

SIS<br />

Process<br />

Lower Risk Higher Risk<br />

* BPCS–Basic process control system<br />

SIS–<strong>Safety</strong>-instrumented system<br />

SV–<strong>Safety</strong> (relief) valve<br />

BPCS*<br />

Mechanical Integrity<br />

Inherent Process Risk<br />

Figure 1 Effect of Protection Layers on Process Risk<br />

When an SIS is required, one of the following should be determined:<br />

• Level of risk reduction assigned to the SIS<br />

• <strong>Safety</strong> integrity level capability (SIL capability) of the SIS<br />

Typically, a determination is made according to the requirements of the ANSI/ISA S84.01 or<br />

IEC 61508 standards during a process hazard analysis (PHA).<br />

<strong>Safety</strong> <strong>Considerations</strong> <strong>Guide</strong> <strong>for</strong> <strong>Trident</strong> <strong>v2</strong> <strong>Systems</strong>

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