Siemens CALOMAT 6 - Safety Integrity Level (SIL)

Siemens AG
SIL Safety Manual CALOMAT 6 Gas Analyzers 8
A5E00729184-02, 07/2010
2.2 Safety Integrity Level (SIL)
Definition: SIL
The international standard IEC 61508 defines four discrete Safety Integrity Levels
(SIL) from SIL 1 to SIL 4. Each level corresponds to the probability range for the
failure in a safety function. The higher the SIL of the safety-instrumented system,
the higher the probability that the required safety function will work.
The achievable SIL is determined by the following safety characteristics:
 Average probability of dangerous failure of a safety function in case of
demand (PFD
AVG)
 Hardware fault tolerance (HFT)
 Safe failure fraction (SFF)
Description
The following table shows the dependency of the SIL on the average
probability of dangerous failures of a safety function of the entire safety-
instrumented system (PFD
AVG). The table deals with “Low demand mode”, i.e.
the safety function is required a maximum of once per year on average.
SIL PFD
AVG
4 ≥ 10
-5
… < 10
-4
3 ≥ 10
-4
… < 10
-3
2 ≥ 10
-3
… < 10
-2
1 ≥ 10
-2
… < 10
-1
Table 2-1 Safety Integrity Level
The “average probability of dangerous failures of the entire safety
instrumented system” (PFD
AVG) is normally spilt between the three subsystems
in the following figure.
Figure 2-2 PFD distribution
The following table shows the achievable Safety Integrity Level (SIL) for the
entire safety-instrumented system for type B systems depending on the
proportion of safe failures (SFF) and the hardware fault tolerance (HFT). Type
B systems include sensors and positioners actuators with complex
components, e.g. microprocessors (see also IEC 61508, Section 2).
012
<60% Not allowed SIL1 SIL2
60 to 90% SIL1 SIL2 SIL3
90 to 99% SIL2 SIL3 SIL4
>99% SIL3 SIL4 SIL4
HFT
SFF
Sensor
e.g. Gas
Analyzer
Final control-
ling element
e.g. valve
Control
system or
logic unit
e.g. PLC