(91b) Analysis of a Complex Protection Scheme with Component Sharing between Protection Layers

Stack, R. J., Dow Chemical Company
After many years of implementation of a LOPA/SIS work process, many scenarios and protection schemes

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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