SIS Testing and Repair Deferral

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.

Probable Reasons for Not Performing SIS Testing

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.

What is Deferral?

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.

SIS Testing and Repair Deferral
Image Courtesy: ABB

Maintenance or Testing Design Requirements

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.

Deferral Process

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:

  1. If a proof test is not completed by the end of the month in which it was due and an approved deferral is not obtained, the test will be considered “overdue” and a Test recovery plan shall be developed.
  2. A test deferral of up to 50% of the prescribed inspection frequency, beyond the scheduled test date, requires approval by:
    • Production Leader,
    • Safety Leader and
    • Technology Leader (as applicable)
  3. Test deferrals beyond 50%, but not exceeding 100% (maximum 1 year) require additional approval by:
    • Technology Center In-charge,
    • Plant Operations Leader and
    • Business unit Operation Leader

Remember, 1 year is the maximum duration allowed for SIS deferral.

Verifying 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.

  • Review the equipment history
  • Identify what other verification actions may be appropriate for Probability of Failure on Demand (PFD) calculations.
  • Gain the concurrence of the qualified expert to the deferral.
  • Define the communication plan.
  • Obtain approval signatures for the required leadership levels.
  • Update required information in the maintenance management system
  • File the Deferral form in the SIS File and applicable LOPA documentation.

Repair Deferrals

  1. When repair is needed:
    • At the time a failure or malfunction is noted, an assessment on the repair is made by the owner, SIS Instrument Coach, and Maintenance personnel.
    • Repairs not complete within the time it was due and an approved deferral is not obtained, the SIS loop is considered impaired
  2. Deferring repairs without proper evaluation and approval is not acceptable. The SIS Test Deferral Process is to be followed when delays in excess of defined proof testing are expected.

Approval must be obtained from the Business Leader to put alternative protection in place & document in SIS records.

  • If the essential operation of the process must be discontinued.
  1. Repair deferral applies only to those SIS components which are redundant

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