Safety Integrity Level (Sil) - Siemens SITRANS P DS III Operating Instructions Manual

The combination of transmitter, automation system and final controlling element forms a
safety-instrumented system that performs a safety function. The emphasis of this description
is on the transmitter. For information on requirements for the automation system or final
controlling element, please refer to the corresponding standards.
The transmitter generates process-related measured values that are transferred to the
automation system. The automation system monitors these measured values. In the event of
a fault, the automation system generates shutdown signals for connected final controlling
elements that set the associated valve to the defined safety position. Faults are:
● Violations of the preset high or low limits
● Deviations between the two measured values
The automation system program must monitor the measured values of both SITRANS
P DS III devices. As soon as the measured values differ by e.g. 2% or more, the system
must be brought into the safe state and the fault must be located.
Two SITRANS P DS III devices are required for multi-channel operation for SIL 3. Operation
with one DS III is not permitted.
Note
Switching-off of system at high monitoring accuracy
The two transmitters are connected to the process at different positions. Actual differences in
pressure ≥ the total tolerance (safety function) can occur when the process is started up or if
there are other pressure variations. A difference in pressure ≥ the total tolerance (safety
function) will shut down the system.
• Match the monitoring accuracy of the automation system to the process.
• Mount the two transmitters exposed to equal conditions.
8.2

Safety Integrity Level (SIL)

The international standard IEC 61508 defines four discrete Safety Integrity Levels (SIL) from
SIL 1 to SIL 4. Every level corresponds to a probability range for the failure of a safety
function.
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
The table deals with "Low demand mode", i.e. the safety function is required a maximum of
once per year on average.
Table 8- 1
SITRANS P DS III with HART
Operating Instructions, 06/2013, A5E00047092-09
Safety Integrity Level
SIL Interval
4 10 ≤ PFD
-5
AVG
3 10 ≤ PFD
-4
AVG
< 10
-4
< 10
-3
Functional safety
8.2 Safety Integrity Level (SIL)
).
AVG
147