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SIL Safety Manual
DOC.SILM.EF.EN Rev. 1
September 2018
Periodic Test and Maintenance Requirements
NOTE:
• The ones listed above are the major failure modes of the main components
• In case of manual procedure, the detectable failure modes are the same as per
automatic procedure. The test coverage is lower than the one for automatic mode,
due to the human factor.
• The manual procedure cannot be considered as diagnostic.
8.3 Partial-Stroke Test
The “Partial Stroke Test” (“On-line”) can be performed to improve the PFD
AVG
value.
For the execution of a partial stoke test the actuator performs a partial open-close cycle
operation, typically a 15-25 degrees rotation of the valve, in order to check the correct
functioning of the actuator and the correct movement of the valve (not sticking and able
to move).
The “Partial Stroke Test” (“On line”) can be performed to satisfy PFD
AVG
(average probability
of failure on demand) value.
• Recommended Test Interval = 1 to 3 months.
• Procedure:
– Operate the actuator to perform N°2 partial open/close cycles, to verify
the correct functioning of the actuator/valve assembly;
– Verify that the partial stroke manoeuvre was performed correctly and
within the expected time;
– Verify the Correct performing of partial stroke operation and within the
expected time; (eg. check locally, or automatically via Logic solver, or via
the Partial stroke testing system the correct movement of the actuator/
valve assembly)
– Inspect the actuator components for any leakages (internal and external).
• The procedure can be performed manually or automatically. In the following, for
both cases, the following points are listed:
– Parameters to be measured
– Instruments to be used
– Failure modes detected
– Diagnostic Coverage/Proof Test Coverage
The above parameters to check will depend from the partial stroke test system available.
Considering the application of the above described Partial Stroke Test procedure,
the “Diagnostic Coverage” is >90 %.
In case of manual procedure the “Test Coverage” shall take into account also the test
imperfection and the reliability/competence of the operator.
Section 8: Periodic Test and Maintenance Requirements
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