In Safety Instrumented System (SIS), Proof testing is very critical to ensure all SIS Sensors. Logic solver & Final control elements are functioning healthily as designed.
Various International Standards such as IEC, ISA & ANSI govern the requirements of SIS Proof testing.
Proof testing is a test performed to reveal undetected faults in a Safety
Instrumented System. So in case of any instrument/system deterioration, the system can be restored to its designed functionality as per IEC-61511 standards.
Proof testing is crucial for the plant’s safe operation without any potential hazard due to the lack of all SIS instruments’ performance as per the SIS design intent.
Proof testing supports early detection of failure, so that failure can be attended to as quickly as possible and maintain the SIS functionality much more reliable.
In the event of not performing a Proof test on the SIS instruments as per the SIS calculations, it may lead to Dangerous failure which may eventually end up in catastrophic events.
Below project upgrade/modification activities involving Safety Instrumented system requires to perform proof testing.
During SIS Design proper care to be taken after finalizing the SIS configuration based on the PHA/LOPA outcome which may end up in
Performing SIS proof tests & method of the test varies with the configuration of SIS sensors, Final control elements (viz. 1oo2, 2oo3, 2oo2, etc), and their model details (Ex. Orifice based pressure, Differential pressure/level, Radar level, Inline Mass/Vortex flowmeters, etc) and Pneumatic operated/Electrically operated valves.
However, below basic procedures can be performed on the sensors at calculated frequency. This may be as early as 2.5 to 3 years even if the proof test is calculated as 5 or 6 years.
S.no | Type of test | Outcome |
1 | Visual inspection of field transmitters | This will reveal problems that are not identified by the instrument self-diagnostic tests. The instrument will remain available for its function. |
2 | Wetted part inspection (process lines) | The instrument will remain available for its function. |
3 | Visual inspection of Flowmeter configurations | Time constants and/or K factors may get disturbed in Intelligent flow instruments if the proper soft lock is not programmed. |
4 | Visual inspection of Control valves | If there are leaks in the Air filter regulator, booster relays can be corrected If there are solenoid valve coil damage, air leak from the spools, and end connections the same can be conducted. |
Proof test plan to be developed cautiously taking into consideration
Details of records shall be
a) Description of the tests and inspections performed including identification of the test procedure used
b) Date of the individual tests and inspections performed on the SIS instrument/valves.
c) Name of the technician who performed the tests and inspections
d) Serial number of the instrument or Plant identifier of the system tested (e.g., loop number, tag number, equipment number, and SIF number)
e) Results of the tests and inspection including the “as-found” condition, all faults found (including the failure mode), and the “as-left” condition.
Reference:
IEC 61511: “Functional safety – Safety instrumented systems for the process industry sector”
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