by Paul Gruhn
A chain is only as strong as its weakest link. Detector coverage (will the right number of detectors actually see the release) and mitigation effectiveness (the probability that the system will actually mitigate to consequences of the event) dominate overall system performance much more than the hardware alone. Just as it is misleading to focus on the logic solver alone in a safety instrumented system (where field devices dominate overall system performance), it is inappropriate to focus on the performance or certification of fire and gas devices. For example, if the detector coverage is 90% (an optimistic figure), and the mitigation effectiveness is also 90% (also an optimistic figure), it does not matter what hardware is used, the overall system effectiveness cannot be greater than 81% (.9 x .9), which is below SIL 1.
However, it is possible to apply performance-based concepts to fire and gas systems. It is possible to assign risk reduction targets for fire and gas systems and apply quantitative techniques in system verification. An ISA 84 committee working group spent more than five years working on a technical report covering ways to account for detector coverage, mitigation effectiveness and other factors, thus allowing a quantitative, performance-based approach to fire and gas system design.
To learn more about the difficulties of assigning SIL rating to fire and gas systems and certifying individual devices, read the full paper “SIL ratings and certification for fire & gas system hardware; Is industry barking up the wrong tree?”