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Functional Safety K-LB-*.*, F*-LB-I, P-LB-*.*.*
Planning
3.2
Assumptions
The following assumptions have been made during the FMEDA:
•
The stress levels are average for an industrial environment and the environment is similar
to IEC/EN 60654-1 Class C (sheltered location) with temperature limits in the range
of the manufacturer's specifications and an average temperature of 40 ºC over a long
period. The humidity level is within manufacturer's rating.
•
The listed failure rates are valid for operating stress conditions typical of an industrial field
environment similar to IEC/EN 60654-1 Class C with an average temperature over a long
period of time of 40 ºC. For a higher average temperature of 60 ºC, the failure rates must
be multiplied by a factor of 2.5 based on experience. A similar factor must be used
if frequent temperature fluctuations are expected.
•
Failure rate based on the Siemens standard SN 29500.
•
The control loop has a hardware fault tolerance of 0 and it is a type A device.
A SFF value for this device is not given, since this value has to be calculated
in conjunction with the connected field device, as described in the following section.
Application Information
The surge protection barrier and the connected device (field device, isolator or actuator) have
to be considered together. The PFD
safety loop is, see Figure 5.1:
•
Actuator (valve) 40 %
•
Transmitter (sensor) 25 %
•
Isolator 10 %
As an overview for SIL 2/SIL 3 safety loop this means:
Total
Actuator (40 %)
Transmitter
(25 %)
Isolator (10 %)
Table 3.1
avg
SIL 2
PFH
-6
10
-7
4 x 10
-7
2.5 x 10
-7
10
Overview PFD
/PFH budget
avg
/PFH budget of the device categories in the entire
SIL 3
PFD
PFH
avg
-2
-7
10
10
-3
-8
4 x 10
4 x 10
-3
2.5 x 10
2.5 x 10
-3
-8
10
10
PFD
avg
-3
10
-4
4 x 10
-8
-4
2.5 x 10
-4
10
11
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