© exida.com GmbH TURCK 04-07-14 R001 V2R0.doc; February 8, 2013
Stephan Aschenbrenner Page 14 of 25
5 Results of the assessment
exida.com
did the FMEDAs together with Werner Turck GmbH & Co. KG.
For the calculation of the Safe Failure Fraction (SFF) the following has to be noted:
λ
total
consists of the sum of all component failure rates. This means:
λ
total
= λ
safe
+ λ
dangerous
+ λ
no effect
.
SFF = 1 – λ
du
/ λ
total
For the FMEDAs failure modes and distributions were used based on information gained from
[N3] to [N5].
For the calculation of the PFD
AVG
the following Markov models for a 1oo1D and a 1oo1 system
were used. As after a complete proof test all states are going back to the OK state no proof test
rate is shown in the Markov models but included in the calculation.
The proof test time was changed using the Microsoft® Excel 2000 based FMEDA tool of
exida.com as a simulation tool. The results are documented in the following sections.
Abbreviations:
du The system has failed dangerous
undetected
dd The system has failed dangerous detected
s The system has failed safe
λ
du
Failure rate of dangerous undetected
failures
λ
dd
Failure rate of dangerous detected failures
λ
s
Failure rate of safe failures
T
Test
Test time
τ
Test
Test rate (1 / T
Test
)
T
Repair
Repair time
τ
Repair
Repair rate (1 / T
Repair
)
Figure 3: Markov model for a 1oo1D structure
λ
λλ
λ
d
λ
λλ
λ
s
d
ok
s
τ
ττ
τ
Repair
Abbreviations:
d The system has failed dangerous
s The system has failed safe
λ
d
Failure rate of dangerous failures
λ
s
Failure rate of safe failures
T
Repair
Repair time
τ
Repair
Repair rate (1 / T
Repair
)
Figure 4: Markov model for a 1oo1 structure
λ
λλ
λ
du
λ
λλ
λ
s
du
dd
ok
s
λ
λλ
λ
dd
τ
ττ
τ
Repair
τ
ττ
τ
Test
To Next Page
Loading...