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CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Demonstrating Prior Use v4
Page 4 of 30
1.
EXECUTIVE SUMMARY
The final report of the Process Safety Leadership Groups (PSLG) safety and
environmental standards for fuel storage sites was published in December 2009.
Appendix 4 of that report provides guidance on the architecture and design of automatic
overfill protection systems for bulk gasoline storage tanks, one of the systems (or layers
of protection) necessary to achieve the target Safety Integrity Level (SIL) level identified
through the risk assessment.
The PSLG report provides supplementary guidance to the British Standard on the
design, operation and maintenance of safety instrumented systems (for example an
automatic overfill protection system) BS EN 61511, Functional safety – Safety
instrumented systems for the process industry sector.
For a safety instrumented function designed to achieve a specific safety integrity level,
BS EN 61511 has architectural requirements for the subsystems that comprise that
safety instrumented function (sensors, logic solver and final elements). These
architectural requirements are in addition to the failure measure requirements for the
intended safety integrity level - BS EN 61511 Clause 11.4. The architectural
requirements are expressed in terms of hardware fault tolerance (the number of
dangerous failures that a subsystem can tolerate and still perform its function as
intended).
If the end user wishes to reduce the hardware fault tolerance requirements for a specific
safety instrumented function, the end user can gather evidence to meet the "Prior Use"
requirements described in BS EN 61511 Clause 11.5.3. This allows the end user to
reduce the hardware fault tolerance requirements by 1 - see BS EN 61511 Clause
11.4.4. The demonstration of "Prior Use" in BS EN 61511 is solely related to allowing a
modification of the hardware fault tolerance needed for a specific safety integrity level.
A working group was commissioned under CDOIF to develop this guideline to assist
users in preparing a case for demonstration of prior use. This is not intended to be
prescriptive in defining the mechanism by which prior use should be demonstrated, but
aims to highlight key factors that should be considered.