In Safety Instrumented System (SIS) – When it is designed, decisions and assumptions are made which factor into establishing the proof test interval.
SIS Proof Testing is a routine maintenance activity to be performed for SIS instruments at a periodical interval.
At times the proof test can’t be done as per the schedule and in such cases ‘Deferral’ has to be made according to IEC standards. This topic describes what is required when the proof test interval cannot be met.
There may be a possibility of postponement of plant shutdown or Turnaround.
The instrument/valve which is scheduled for Proof testing may not be possible to do it online and require equipment, and unit shutdown.
Plant operations are not in a position to shut down the plant either due to high market demand for the product or availability of raw material in huge quantities.
When the SIS Proof testing is not performed on time as per schedule, there is a possibility to take ‘Deferral’ to a certain extent.
Deferral process initiation is done based on evaluation & confirmation that it is safe to allow the deferral. This is done by first identifying and involving key Production, Maintenance, and Design personnel including the Safety experts.
Then by reviewing the equipment history, conducting a visual inspection, performing Probability of Failure on Demand (PFD) calculations, and gaining the acceptance and approval of plant management, the operation of the unit may continue, up to the limits of the deferral period.
However, the deferral process defines the approval roles in plants and functions which are required for deferring a test or repair of an SIS.
The SIS shall be designed in accordance with the maintenance and testing requirements defined in the Safety Requirement Specification. The design shall allow for testing of the SIS either for the full system or in parts.
Where the interval between scheduled process downtime is greater than the proof test interval, then online test facilities are required to perform testing.
The maximum time the SIS is allowed to be in bypass (repair or testing) while the safe operation of the process is continued shall be defined.
Deferring tests without proper evaluation and approval is not acceptable. The SIS Test Deferral Process is to be followed when delays in excess of the defined schedule date:
Remember, 1 year is the maximum duration allowed for SIS deferral.
Even though Deferral is an alternate option while not able to perform the SIS proof testing as per schedule, precautions are to be taken before issuing/approving SIS Deferral.
Approval must be obtained from the Business Leader to put alternative protection in place & document in SIS records.
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