8
Competence in Functional Safety
Functional safety of globe valves, rotary plug valves, ball valves and butterfly valves
4.2 Determining the safety integrity level
The achievable safety integrity level (SIL) is determined by the following safety-related data:
Average probability of failure on demand (PFD
avg
)
Hardware fault tolerance (HFT)
Safe failure fraction (SFF)
The following table in accordance with IEC 61508 and IEC 61511 shows how the safety integrity level (SIL)
depends on the average probability of failure on demand (PFD
avg
). It is based on low demand mode of
operation, i.e. the frequency of demands on a safety-related system is no greater than once per year.
Safety integrity level (SIL)
PFDavg (low demand mode)
4
≥
10
-5
to
<
10
-4
3
≥
10
-4
to
<
10
-3
2
≥
10
-3
to
<
10
-2
1
≥
10
-2
to
<
10
-1
PFD
avg
in low demand mode of operation according to IEC 61508-1, Table 2
The sensor, logic unit and final element form a safety-related system that performs a safety function.
Sensor
e.g. pressure
measuring instrument
PFD
avg
≤
X %
≤
X %
≤
X %
Logic unit
e.g. PLC
Final element
e.g. valve
The average probability of failure on demand (PFD
avg
= sum of sensor, logic unit and final element failures) must be within
the range of the demanded safety integrity level (SIL) in case of demand as listed in the above table.
