11
Competence in Functional Safety
Functional safety of globe valves, rotary plug valves, ball valves and butterfly valves
Responsibility
The joint use of a control valve within the safety loop by a control loop of a basic
process control system makes it possible to increase the diagnostic coverage of the
safety-instrumented system. This joint use can lead to additional risk. This aspect must
be taken into account in the risk analysis.
Online tests, such as the partial stroke test and other diagnostic processes integrated
into the valve positioner, can be regarded as state-of-the-art test methods. They can
be used to lengthen the proof test interval or to improve the safety margin (discovery
of undetected systematic failures).
VDI 2180-5, section 4 provides special instructions.
Avoiding systematic failure
To avoid systematic failure, the user must take into account the following application-
specific factors besides the manufacturer specifications:
Corrosion (destruction primarily of metals due to chemical and physical processes)
Material fatigue, e.g. in bellows seals
Wear induced by the process medium
Abrasion (material removed by solids contained in the process medium)
Medium deposits
Aging (damage caused to organic materials, e.g. plastics or elastomers, by exposure
to light and heat)
Chemical attack (organic materials, e.g. plastics or elastomers, which swell, leach out
or decompose due to exposure to chemicals)
If no experience data exist for the devices used, a visual inspection of the safety equip-
ment must be performed after a short time in operation.
Operator
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