Chemical Technology July 2015
PLANT MAINTENANCE, SAFETY, HEALTH & QUALITY
Figure 6: SIL 3 instrumented protection configuration
Figure 7: SIL Risk graph
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
Consequences
Exposure time
C1 Minor injury
F1 Rare to more often
C2 Permanent injury > 1 persons; 1 death
F2 Frequent to permanent
Safety instrumented systems are normally incorporated in hazardous installations as a first choice of a layer of protection.
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
Figure 8: Description of inputs into the SIL risk graph
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
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.
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Chemical Technology • July 2015
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