4. Safety instrumented system and safety functions
4.1. Safety instrumented system including an actuator
Typically, a safety instrumented system including an actuator is composed of the
components as shown in the figure.
Figure 2: Typical safety instrumented system
[1] Sensor
[2] Controls (standard and safety PLC)
[3] Actuator with actuator controls and FQM
[4] Valve
[5] DCS
The safety integrity level is always assigned to an overall safety instrumented function
and not to an individual component.
For an individual component (e.g. fail safe unit), safety figures are determined.These
figures are used to assign the devices to a potential safety integrity level (SIL).The
final classification of the safety instrumented function can only be made after
assessing and calculating all subsystems.
4.2. Safety functions
In calculating the safety figures of the actuator system, the following safety functions
are taken into account:
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with non safety-related actuators Safety instrumented system and safety functions
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