94-95 / 973
Hybrid applications of IEC61511 – Laurie Blackmore
Disadvantages include:
§
SIS is
much
bigger than the non-61511 (API) design leading to increased complexity of
operation, maintenance. This increases the risk of systematic faults - the primary
cause of major incidents
§
Testing frequency is still based on API RP14C (3 months transmitters, 1 year SDV‘s)
rather than (longer) IEC61511 intervals. This reduces ability to focus on the high-risk
functions
§
The engineering cost and potential schedule increase associated with IEC61511 is
incurred without the benefit of equipment rationalisation or reduced testing costs.
These increases are considerably less than in the previous case.
Lifecycle/contract issues:
§
Since there is no SIL assessment make sure it is clear in the contract to what functions
these pre-defined SIL‘s apply. What is
Safety
? This can have big impact on
engineering hours and equipment costs.
AVOID THIS DESIGN BASIS WHEREVER POSSIBLE
15
API design but with pre-specified integrity levels for safety functions