November 2017
SIL Safety Manual
SM-025-GVO Rev. 0
16
Section 8: Periodic Test and Maintenance Requirements
Periodic Test and Maintenance Requirements
8.4
Partial Stroke Test
The “Partial Stroke Test” (“On line”) can be performed to improve the PFDAVG 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).
In case of GVO-CLP-SR/FS, a specific operating diagram can be used, to allow the
performing of PST without testing the pneumatic quick exhaust valve.
The PFDAVG values quoted in SIL certification document are relevant to indicated
“Partial Stroke Test” intervals.
Partial Stroke Test procedure:
1.
Operate the actuator to perform N°2 partial open/close cycles, to verify the correct
functioning of the actuator/valve assembly;
2.
Verify that the partial stroke manoeuvre was performed correctly and within the
expected time;
3.
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
Parameters to be measured:
The following parameters have to be measured for an effective Partial Stroke Test:
•
Linear position of the shaft;
•
Time necessary to reach to final position;
•
Output torque (as indirect measure, by means of the of pressure measurement in
the cylinder chamber)
NOTE:
Not all parameters are needed, but the following combinations can be used:
1.
Measurement of linear position as function of time (pressure measurement is optional)
2.
Measurement of cylinder chamber pressure as function of time
3.
Verification of final position achievement in the established maximum time.
