26 Process Fluid Control
SIL and IEC 61508 - The international
functional safety standard
1. Risk and functional safety 2. Risk assessment
» Safety can be defined as "freedom from unacceptable risk".
» Industrial processes are never completely free of
risks. Functional safety can be used to evaluate and
reduce residual risk to an acceptable level.
» The risk of an industrial process is evaluated by the plant
operator using a risk graph (see picture 1) for example.
» The risk graph rates the possible threat for people and
environment and determines the required level of risk reduction.
» This required level of risk reduction is called safety
integrity level (SIL). An industrial process with a demand
of SIL4 represents the highest hazard level.
C= Extend of damage
C1: slight injury of a person
C2: severe, irreversible injury of one or several persons
C3: death of several persons
C4: catastrophic implication with many death persons
F= Duration of stay
F1: rarely or more oen
F2: frequently up to permanent
P= Hazard prevention
P1: possible under certain conditions
P2: rarely possible
W= Probability with which undesirable incidents happen
W1: very low
W2: low
W3: relatively high
= No safety requirement
Risk-graph and requirement classes
according to: IEC 61 508 / 61 511
Start
C1
F1
F1
F2
F2
C2
C3
C4
P1
P1
P2
P2
W3 W2 W1
SIL 1
SIL 1 SIL 1
SIL2 SIL 1 SIL 1
SIL3 SIL2 SIL 1
SIL3 SIL3 SIL2
SIL4 SIL3 SIL3
SIL4 SIL4 SIL3
