(91b) Analysis of a Complex Protection Scheme with Component Sharing between Protection Layers
- Conference: AIChE Spring Meeting and Global Congress on Process Safety
- Year: 2018
- Proceeding: 2018 Spring Meeting and 14th Global Congress on Process Safety
- Group: Global Congress on Process Safety
- Time: Tuesday, April 24, 2018 - 10:45am-11:15am
were found to be difficult to properly evaluate with simple analysis tools. These complex systems have been
proven designs but had not been evaluated with a LOPA/SIS work process, or a HIPS work process.
The challenge of shared components, longer test intervals and complex protection layer design led to using
better modeling techniques and tools to close risk gaps and also often reduce overall protection scheme cost.
This work process can be used for LOPA scenarios that are difficult to close and/or for alternative risk assessment
of HIPS or QRA scenarios.
This paper uses an example of a batch reactor system with BPCS and SIS instrumented systems and component data
to discuss the issues with shared instrumentation, complex protection scheme design and test interval optimization
for meeting required safety criteria with low on-going maintenance costs. Starting with a LOPA analysis that
identifies shared components, a logic model is developed including shared components and common cause failures.
This scenario and protection scheme is evaluated using fault tree software with the capability to correctly model
shared components, beta factors, IEC 61508 averaging and functional and proof testing of the components .
The Isograph RWB13.0.2 software is used to evaluate the meta-function (BPCS loop and SIS loop that have some
shared components). The impacts of functional testing for the final elements and voting logic on the inputs,
and beta factors for common cause are evaluated and discussed. The PFD of the SIS and meta-function are
calculated as are the false trip rate for the SIS Loop, BPCS loop, and meta-function. The beta factors are calculated per
IEC guidelines with the M out of N beta factor corrections. An illustration of the RWB IEC 61508 FT model, and use the
IEC 61508 EXT averaging is shown with beta factor color coding in the fault tree diagrams. Results show the options
available for test intervals to meet the PFD targets and false trip targets.
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