Statox 503 Control Module
Page
42
of
46
Issue 10 / 2018
Average Probability of Failure on Demand
The Statox 503 Control Module is classified to operate in the low – demand mode.
These PFD
avg
calculations for a 1oo1-System with a hardware failure tolerance of 0 are based on the
simplified equation
PFD
avg
= 0,5
*
T
Proof
*
DU
. It bases on the assumption that the time for repair is much
shorter than the prooftest interval T
Proof
.
Safety function
T
Proof
= 1 Year
T
Proof
= 2 Years
T
Proof
= 5 Years
PFD
avg
for Statox 503,
operating as
Control Module
SF1
2,2 x 10
-4
4,3 x 10
-4
1,05 x 10
-3
SF2
2,2 x 10
-4
4,2 x 10
-4
1,02 x 10
-3
SF3
2,5 x 10
-4
4,7 x 10
-4
1,16 x 10
-3
SF4
2,4 x 10
-4
4,6 x 10
-4
1,13 x 10
-3
PFD
avg
= Average Probability of Dangerous Failure on Demand
Classification of the Safety Integrity Level (SIL)
The Statox 503 is classified as a complex system following IEC 61508-2 type B. At a hardware failure
tolerance of 0 and a safe failure fraction > 90% at these PFD values results a SIL 2 compliance for all safety
functions (see IEC 61508-2, Table 3).
Livetime
The livetime of Statox 503 is expected to be 10 years. Above failure rates are applicable within this time
interval. After 10 years these data might be too optimistic.
