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Institute of Measurement and Control Functional Safety Conference 2016
Challenges in Achieving Safety Instrumented Function Response Time for a Fast-Acting Process
Page 11
important to truly qualify an IPL as effective. In the example, one of the initiating events was a rapidly
developing case resulting in a short PST. Investigations into temperature sensing technology showed that
the response time was too slow for this rapidly developing case. As a result, although there were existing
temperature SIFs and alarms, these could not be credited as IPLs.
Alarm List and Operator Response Manual
The PST is also an important consideration in determining SIF trip set-points. ISA TR84.00.04 (2015)
Part 1, Annex Q4 states that “Set-points for SIFs should be selected such that actions can be taken to
achieve or maintain the safe state of the process within the allocated process safety time.” Annex Q
provides guidance on determining the trip set-point. Trip set-points are normally governed by a controlled
procedure. This becomes even more important for fast acting SIFs, because set-point change has the
potential to render the SIF unable to respond within the PST margin. Therefore, these critical cases should
be documented on the Alarm and Set-point List.
The SIFRT should also be documented in Alarm Rationalization exercises as it can influence the priority
of the alarm, the priority of the associated diagnostic alarm and the required operator response.
Instrument Specification
The required response time for sensor and end devices must be stated on the instrument specification
sheets and be thoroughly evaluated during the procurement process. This can influence sensor selection,
actuator sizing and the requirement for additional hardware such as quick exhaust valves. Clearly stating
the response time requirement on the instrument specification sheet would also assist in ensuring that
future device replacement, upgrade or change-out would conform to the response time requirements.
Safety Requirement Specification
The IEC functional safety standards set out requirements for documenting aspects of the SIFRT within
the safety requirement specifications (SRS). The SRS should be developed or modified to document the
specification targets and the performance results for each new or modified SIF as per the following:
a.
Margin between PST and SIFRT
This should be established early in the project phase and clearly documented in the SRS.
b.
SIFRT for each SIF
The IEC-61511-1 (2016) sections 10.3.1 and 10.3.2 state that the response time requirement for each SIF
to bring the process to a safe state within the process safety time should be documented in the SRS. This
clause identifies the requirements for each SIF and not just the fast acting SIFs.
The IEC-61511-1 (2016) section 9.2.9 states that the Process Safety Requirements or Process Description
can be sufficiently detailed in its description of the SIS and can include information such as the set-points
for sensors, the time available for response and the valve closure time elements.
c.
Performance Results
The SRS should also include confirmation that the target SIFRT is being achieved based on the selected
design: