UNDERSTANDING
SAFETY INTEGRITY LEVELS
IEC
61508
SIL LEVELS ACCORDING IEC 61508 / IEC 61511
TOLERABLE RISKS AND ALARP (ANNEX ‘B’)
SAFETY:
SIL
PFDavg
RRF
PFDavg
Safety
Integrity
Level
Average probability of
failure on
demand per year
(low demand)
Risk
Reduction
Factor
Average probability of
failure on
demand per hour
(high demand)
SIL 4
≥ 10-5 and < 10-4
100000 to 10000
≥ 10-9 and < 10-8
SIL 3
≥ 10-4 and < 10-3
10000 to 1000
≥ 10-8 and < 10-7
SIL 2
≥ 10-3 and < 10-2
1000 to 100
≥ 10-7 and < 10-6
SIL 1
≥ 10-2 and < 10-1
100 to 10
≥ 10-6 and < 10-5
IEC
61511
Risk cannot be justified except
in extraordinary circumstances
Intolerable Region
FREEDOM FROM
UNACCEPTABLE
RISK
Tolerable only if further
risk reduction
is impracticable or if its cost are
grossly disproportional to the gained
improvement.
As the risk is reduced, the less
proportionately, it is necessary to
spend to reduce it further,
to satisfy ALARP.
The concept of diminishing
proportion is shown by the triangle.
The ALARP or
tolerability Region
Risk is undertaken only if
a benefit is desired
AVERAGE PROBABILITY OF FAILURE ON DEMAND (PFDAVG)
PFDavg
λ DU ×
1oo1
1oo2
1oo2D
( λDU
1
(
)
+ λ DU2
Vapor cloud explosion (BLEVE)
⎡⎣(1-β ) × ( λDU × TI) ⎤⎦
(β × λDU × TI)
+
3
2
TI
3
⎡⎣ (1-β ) × ( λ DU × TI) ⎤⎦ +
)⎤⎥ × TI
2
2
⎡⎣(1-β ) × ( λ DU × TI) ⎤⎦ +
3
MEAN TIME TO FAILURE SPURIOUS
2oo2
2oo3
Actual risk reduction
Partial risk covered by
other technology
safety-related systems
2
2
AVAILABILITY AND RELIABILITY
Flash Fire
λ=
< 60%
60% - < 90%
90% - < 99%
> 99%
< 60%
60% - < 90%
90% - < 99%
> 99%
Failures per unit time
Components exposed to functional failure
A
2
2λ S × MTTR
B
B
C
1
2
6λ S × MTTR
2oo2
∑λ
+
DD
+
∑λ
Hardware fault
tolerance
0
∑λ
DU
+
SD
+
MTTF = MTBF - MTTR =
Availability
V
o
t
i
n
g
∑λ
SD
+
SU
∑λ
Hardware fault
tolerance
1
TYPE A Components
SIL 1
SIL 2
SIL 2
SIL 3
SIL 3
SIL 4
SIL 3
SIL 4
TYPE B Components
Not allowed
SIL 1
SIL 1
SIL 2
SIL 2
SIL 3
SIL 3
SIL 4
SU
∑λ
= 1∑λ
=
=
Jet Fire
=
Operating Time + Repair Time
MTTF
MTTF + MTTR
Hardware fault
tolerance
2
t
Failure time
=
MTTF
MTBF
=
μ
μ+ λ
=
TTF
MTTR
MTBF
Repair
time
(failure)
=
Success
MTBM
MTBM + MSD
λ
μ
RELIABILITY
Acronyms:
MTBF: Mean Time Between Failures
MTTF: Mean Time To Failure
MTTR: Mean Time To Repair
MTBM: Mean Time Between Maintenance
MSD: Expected Mean System Downtime
DU
Time
MTTF
Operating Time
TOT
AVAILABILITY
UNRELIABILITY
UNAVAILABILITY
Success
Failure
MTTF
MTTR
SAFETY INTEGRITY LEVEL CALCULATION
Pool Fire
Safety integrity of non-SIS prevention/
mitigation protection layers, other protection layers, and SIS matched to the
necessary risk reduction
Process and
process
control system
SIL3
SIL 4
SIL 4
SIL 4
Necessary Risk Reduction
Consequence
of Hazardous
Event
Process
Risk
SIL2
SIL 3
SIL 4
SIL 4
Failure rates categories: λDD: dangerous detected; λDU: dangerous undetected
λSD: safe detected;
λSU: safe undetected
0
1
λ
Unavailability = 1- Availability =
2oo3
∑λ
Operating time
MTBF = MTTF + MTTR
SAFE FAILURE FRACTION (SFF) AND SIL LEVELS
SFF
1
Failure Rate :
1oo2
A
1
DD
Reliability
Basic Concepts:
B
1oo1
Partial risk covered by
external risk
reduction facilities
Risk reduction obtained by all safety-related systems and external risk reduction systems
( β × λDU × TI)
A
A
1
2λS
∑λ
Partial risk covered by
E/E/PE
safety-related system
1 FIT = 1 × 10-9 Failures per hour
1
λS
1oo2
Necessary risk reduction
SYSTEM ARCHITECTURES
MTTFs
1oo1
INCREASING RISK
(β × λDU × TI)
TI: Proof Test time interval
Et: Test Effectiveness
λDU: dangerous undetected failures
⎡⎛
TI ⎞
SL ⎤
λDU ⎢⎜ Et × ⎟ + (1-Et ) ⎥
2⎠
2⎦
⎣⎝
1oo1
(Et ≠ 100%)
EUC Risk
3
TI
×
2
⎥
⎥⎦
Tolerable
Risk
⎡⎣(1-β ) × ( λ DU × TI) ⎤⎦
(β × λDU × TI)
+
4
2
TI3
×
4
) (
)
Residual
Risk
2
2
⎡ λ DU × λ DU + λ DU × λ DU
1
2
1
3
⎢
⎢+ λ
DU 2 × λ DU3
⎢⎣
(
RISK REDUCTION
-
λ DU1 × λ DU2 × λ DU3
2oo2
RISK IS
NEGLIGIBLE
With common causes (Beta factor)
TI
2
λ DU1 × λ DU2 ×
1oo3
It is necessary to maintain
assurance that risk remains
at this level
No need for detailed working
to demonstrate ALARP
Simplified equations
Without common causes
2oo3
Broadly Acceptable
Region
Tolerable accident frequency
1
=
Frequency of accidents w ithout protections RRF
Frequency
of Hazardous
Event
Fireball
Non-SIS
prevention /
mitigation
protection
layers
SIS
Quantitative Method for SIL level determination
As found in IEC 61508 Annex ‘C’
Other
protection
layers
Tolerable
Risk Target
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