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Chemical Technology • July 2015
is categorised into four levels, known as safety integrity
levels, abbreviated as SIL. Safety integrity levels can be
determined by either basic calculations or a risk graph or
by simply using a matrix.
Once the required level of safety is known, the safety
instrumented systemdesign can be specified in terms of the
proof test period, component reliability and the redundancy
of components comprising the safety instrumented system.
It was further shown that another use of layer of protec-
tion analysis is that the selected safety integrity level for the
safety instrumented system can be reduced by taking credit
for other layers of protection, which may include design
integrity, control, indications and alarms, physical protec-
tion devices such as relief valves and emergency response.
Acknowledgement
Content was reviewed by David Macdonald, SIS Specialist
who provided valuable input.
References
International Electro-technical Commission standard
IEC 61508 – Functional safety of electrical/electronic/
programmable electronic safety-related systems.
HSE, UK Health and Safety Executive, Reducing Risks,
Protecting People (R2P2), 2001 – ISBN 07176 21 51 0.
IEC 61511-1, clause 11, " Functional safety - Safety instru-
mented systems for the process industry sector - Part 1:
Framework, definitions, system, hardware and software
requirements", 2003-01.
z
Table 3 SIL MATRIX
Severity
Catastrophic
Critical
Marginal
Negligible
Financial effect
R100m
R50m
R1m
R100 000
Environmental damage
Permanent
Long term
Medium
Short
Health effect
Fatal
Irreversible
Major
Minor
Event Safety
Frequency
> 1 death and injuries
1 death / injuries
Disabling injuries
Minor injuries
10 per year
Too high
Too high
Too high
Too high
1 per year
SIL 4
SIL 3
SIL 2
SIL 1
1 per 10 years
SIL 3
SIL 2
SIL 1
ALARP
1 per 100 years
SIL 2
SIL 1
ALARP
ALARP
1 per 1000 years
SIL 1
ALARP
ALARP
ALARP
1 per 10 000 years
SIL 1
ALARP
ALARP
NONE
1 per 100 000 years
ALARP
ALARP
NONE
NONE
Figure 8: Description of inputs into the SIL risk graph
Figure 6: SIL 3 instrumented protection configuration
Figure 7: SIL Risk graph
Consequences
Exposure time
C1 Minor injury
F1 Rare to more often
C2 Permanent injury > 1 persons; 1 death
F2 Frequent to permanent
C3 Death of several persons
C4 Very many people killed
Avoidance of hazard
Probability of unwanted occurrence
P1 Possible under certain circumstances
W1 Very slight
P2 Almost impossible
W2 Slight
W3 Relatively high
PLANT MAINTENANCE, SAFETY,
HEALTH & QUALITY
Safety instrumented systems are
normally incorporated in hazardous
installations as a first choice of a
layer of protection.