Safety-related characteristics, 2 specific safety-related characteristics – KROHNE TT 51 SERIES EN User Manual
Page 14
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SAFETY-RELATED CHARACTERISTICS
14
TT 51 SERIES
www.krohne.com
09/2010 - 4000869801 - AD TT 51 SIL R01 en
7.2 Specific safety-related characteristics
According to table 2 of IEC 61508-1 the average PFD for systems operating in low demand mode
has to be ≥10
-3
to ≤10
-2
for SIL 2 Safety Instrumented Functions (SIFs). For systems operating in
high demand mode of operation the PFH value has to be ≥10
-7
to ≤10
-6
for SIL 2 SIFs according to
table 3 of IEC 61508-1. A generally accepted distribution of PFD
avg
and PFH values of a SIF over
the sensor part, logic solver part, and final element part assumes that 35% of the total SIF
PFD
avg
value is caused by the sensor part (including the transmitter).
For a SIL 2 application operating in low demand
low demand
low demand
low demand mode the total PFD
avg
value of the SIF should be
smaller than 1.00E-02, hence the maximum allowable PFD
avg
value for the sensor part would
then be 3.50E-03.
For a SIL 2 application operating in high demand
high demand
high demand
high demand mode the total PFH value for the SIF should be
smaller than 1.00E-06 1/h, hence the maximum allowable PFH value for the sensor part would
be 3.50E-07 1/h.
For type B components with a hardware fault tolerance of 0 the SFF shall be > 90% for SIL 2 SIFs
according to table 3 of IEC 61508-2.
Under the assumptions described in the chapter before and the definitions given in chapter
"Desription of the failure categories" the following table show the failure rates according to
IEC 61508.
λ
SD
:
Fail safe detected
λ
SU
:
Fail safe undetected
λ
DD
:
Fail dangerous detected
λ
DU
:
Fail dangerous undetected
FIT:
Failure rate [1/h]
SFF:
The number listed is for reference only. The SFF, PFD
avg
and PFH must be determined for
the complete subsystem.
PFD
avg
:
The PFD
avg
was calculated for profile 2 using Markov modeling. The results must be
considered in combination with PFD
avg
values of other devices of the Safety Instrumented
Function (SIF) in order to determine suitability for a specific Safety Integrity Level (SIL)
For SIL 1 applications, the PFD
avg
value needs to be < 10
-1
for the SIF.
For SIL 2 applications, the PFD
avg
value needs to be < 10
-2
for the SIF.
T[Proof]:
It is assumed that proof testing is performed with a proof test coverage of 99%.
PFH:
= λ
DU
(Fail dangerous undetected)
SIL AC:
SIL AC (architectural constraints) means that the calculated values are within the range for
hardware architectural constraints for the corresponding SIL level
.book Page 14 Thursday, September 9, 2010 4:19 PM