Functional Safety 2016

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.

Focus should be placed on response time requirements particularly for instruments with

mechanical elements and for valves, which generally have slower response times.

The end-to-end response time performance must be demonstrated in validation testing during

installation.

Operations and Maintenance

Diagnostic and discrepancy alarms should be considered in the design of the SIF.

Periodic testing of SIFRT should also be included in the overall periodic testing of the SIF.

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.