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©ABBGroup
October23,2016|Slide15
And then Instruments got clever
§
Modern ‘intelligent’ instruments and programmable
systems have built in diagnostics
§
The diagnostics may detect potentially dangerous failures
§
We now have four failure modes
l
Failure rate
Undetected
Detected
l
S
Safe
l
SU
It triggers the safe function putting
the plant in the safe state or it does
not have effect on the output.
l
SD
It triggers the safe function putting the
plant in the safe state, but it also
provides a diagnostic output
l
D
Dangerous
l
DU
It prevents the safe function from
being triggered, even in case of
demand (i.e. a process dangerous
condition)
l
DD
It prevents the safe function from
being triggered, but it provides a
diagnostic output
©ABBGroup
October23,2016|Slide16
Safe Failure Fraction (SFF)
The measure of how effective diagnostics are is called the
Safe Failure Fraction (SFF)
λ
SU
+ λ
SD
+ λ
DD
SFF =
λ
SD
+ λ
SU
+ λ
DD
+ λ
DU