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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