Institute of Measurement and Control Functional Safety Conference 2016
Challenges in Achieving Safety Instrumented Function Response Time for a Fast-Acting Process
Page 13
performance before implementation, and that the corresponding test conditions are clearly
understood.
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Focus should be placed on response time requirements particularly for instruments with
mechanical elements and for valves, which generally have slower response times.
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The end-to-end response time performance must be demonstrated in validation testing during
installation.
e.
Operations and Maintenance
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Diagnostic and discrepancy alarms should be considered in the design of the SIF.
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Periodic testing of SIFRT should also be included in the overall periodic testing of the SIF.
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SIFRT targets should be clearly documented in the SRS and project design documentation to
ensure that any future modifications to the SIF (for example set-point change, device
replacement) do not compromise the ability of the SIF to satisfy SIFRT requirements.
Conclusion
To achieve functional safety and truly qualify a SIF as adequate and effective in mitigating a hazard, both
the safety integrity levels based on PFD, HFT and SC, in addition to the SIFRT are critical factors that
should be evaluated. The importance of SIFRT is even more evident for rapidly developing hazardous
scenarios.
Whereas the functional safety standards are detailed on the requirements for achieving SIL targets, the
requirements for SIFRT are not as clearly defined. As such, it is not an area of emphasis and is often
overlooked. However, this paper has highlighted that there are clear advantages to making consideration
of PST and SIFRT throughout the lifecycle of a SIF.
The relationship between PST and SIFRT should be defined early in the design and documented. The
ideal time for determining the margin between PST and SIFRT was identified as prior to the HAZOP
workshop. However, it is understood that determination of this margin is an iterative process and may be
revisited during the design stage.
The method of measurement and technology selection was shown to greatly influence the response time
of devices. Guarantee of device response time performance from suppliers was identified as one of the
main challenges but is an important step in confirming the design of the SIF and ensuring that SIFRT
targets would be satisfied. End-to-end validation testing of response time performance during
implementation is essential to confirm that target SIFRT is satisfied. In addition, periodic testing during
operation should be performed to ensure that the SIF continues to meet the SIF response time
requirements throughout its lifetime.
As a final point, SIFRT target and performance results should be clearly documented in the design
documents and SRS throughout the safety lifecyle.