10
Competence in Functional Safety
Functional safety of globe valves, rotary plug valves, ball valves and butterfly valves
5 Intended use of control valves in
safety-instrumented systems
Responsibility
The reliability of mechanical components is significantly affected by the operating condi-
tions and, as a result, by systematic failures. This needs to be taken into account when
selecting and sizing devices.
Safety-instrumented function
During normal operation, the signal pressure is applied to the pneumatic actuator. To
meet the requirements of the safety-instrumented function, the actuator is usually vented
by a solenoid valve. The force of the actuator springs moves the valve to its end position,
i.e. the valve is either completely opened or closed.
The end position of the valve must not be impeded by mechanical equip-
ment, such as travel stops or handwheels, under any circumstances.
When the signal pressure is applied to the actuator again, the valve moves to the corre-
sponding position. If the actuator is to be locked after a case of demand, this must be
ensured by the operator using suitable means.
Operator
Features of final elements
Contact with the process media may cause systematic failure and, as a result, affect the
safety-related availability of the safety-instrumented systems. The influence of specific
process conditions must be analyzed and taken into account during sizing and main-
tenance.
These conditions arise from process requirements. To rule out systematic failure, we re-
commend creating a loop data sheet according to the German standard VDI 2180-5,
section 4 (recommendations for final elements).
In case of doubt, consult the manufacturer for valve sizing.
To reduce systematic failure, diverse redundancies may be advantageous (e.g. globe
valve and ball valve).
Operator
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