6
Competence in Functional Safety
Functional safety of globe valves, rotary plug valves, ball valves and butterfly valves
4 General aspects of functional safety
4.1 Standards, terms and abbreviations
Abbreviation Designation Description
SIL Safety Integrity Level One of four discrete levels for specifying the safety integrity requirements
of the safety functions to be allocated to the E/E/PE safety-related systems,
where SIL 4 has the highest level of safety integrity and SIL 1 has the lowest.
MTBF Mean Time Between Failures Mean time between two failures
MTTR Mean Time To Restoration Mean time between the occurrence of a failure in a device or system and its
repair
HFT Hardware Fault Tolerance Capability of a functional unit to continue executing the demanded function
in case of faults or deviations.
λ
sd
Failure rate for all safe detected
failures
λ
su
Failure rate for all safe undetected failures
λ
dd
Failure rate for all dangerous detected failures
λ
du
Failure rate for all dangerous undetected
failures
SFF Safe Failure Fraction Fraction of non-hazardous failures, i.e. the fraction of failures without the
potential to set the safety-related system to a dangerous or impermissible
state.
PFD
avg
Average Probability of Failure on
Demand
Average likelihood that a dangerous safety function failures occurs on
demand.
T
I
Test Interval between life testing of the safety
function
Average likelihood that a dangerous safety function failures occurs on
demand.
Low demand
mode
Low demand mode of operation Low demand mode is where the frequency of demands for operation made
on a safety-related system is no greater than one per year and no greater
than twice the proof test frequency.
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