CFSE Exam Preparation: Section I-8
Section 8:
SIL Assignment
• SIL Selection
• Hazard Matrix Procedure
• Risk Graph
• Numerical Risk Targets
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CFSE Exam Preparation: Section I-8
Safety Integrity Levels
Safety Integrity Probability of failure Risk Reduction
Level on demand per year Factor
(Demand mode of operation)
SIL 4 10-4 > PFD >= 10-5 10000 to 100000
SIL 3 10-3 > PFD >= 10-4 1000 to 10000
SIL 2 10-2 > PFD >= 10-3 100 to 1000
SIL 1 10-1 > PFD >= 10-2 10 to 100
CFSE Exam Preparation: Section I-8
How do I assign SIL?
• Process for assigning SIL (which will move
process risk to a tolerable region) must be
created
• Numerous techniques have been formulated
• Some are quantitative giving numerical
targets for risk
• Some are qualitative, obscuring numerical
targets in qualitative categories
• Be consistent!
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CFSE Exam Preparation: Section I-8
Hazard Matrix Procedure
• Categorize consequence
• Categorize likelihood
• Select SIL from matrix that corresponds to
selected consequence and likelihood
categories
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CFSE Exam Preparation: Section I-8
Hazard Matrix
Categorize Consequence
Severity Rating Impact
Minor Impact initially limited to local area of the event with potential for broader
consequence if corrective action is not taken.
Serious One that could cause: Any serious injury or fatality on-site or off-site, or
property damage of $1 MM on-site, or $5 MM off-site.
Extensive One that is five or more times worse than a SERIOUS accident.
Based on information found in Guidelines for the 5
Safe Automation of Chemical Process, AIChE
CFSE Exam Preparation: Section I-8
Hazard Matrix
Categorize Likelihood
Likelihood
Type of Events Frequency /Year Qualitative Ranking
A failure or series of failures with a very low f < 10-4 Low
probability of occurrence within the expected
lifetime of the plant.
A failure or series of failures with a low probability 10-4 < f< 10-2 Moderate
of occurrence within the expected lifetime of the
plant.
A failure can reasonably be expected within the 10-2 < f High
expected lifetime of the plant.
Based on information found in Guidelines for the 6
Safe Automation of Chemical Process, AIChE
CFSE Exam Preparation: Section I-8
Hazard Matrix
Assigning SIL
Hazardous Event Likelihood
High
2 3* 3*
• Use selected likelihood
Moderate
and consequence 1 2 3*
categories to determine
NR 1 3*
Low
the SIL required
Minor Serious Extensive
Hazardous Event Severity Rating
* One SIL 3 SIF may not provide enough
risk reduction
Based on information found in 7
IEC d61511 Section E
CFSE Exam Preparation: Section I-8
Examples
Hazard Matrix SIL Assignment
• Example 1 High 2 3 3
Likelihood
– A SIF was identified during a HAZOP Moderate 1 2 3
study. The HAZOP team determined Low NR 1 3
that the consequence is Serious and the
Extensive
Serious
Likelihood is High
Minor
• What is the SIL? Severity
• Example 2 High 2 3 3
Likelihood
Moderate 1 2 3
– Further analysis showed that this Low NR 1 3
scenario yielded a consequence of 0.21
Probable Loss of Life (PLL) and a
Extensive
Serious
Minor
likelihood of 1/576 incidents per year
Severity
• What is the SIL?
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CFSE Exam Preparation: Section I-8
Risk Graph
• Select categories for risk graph parameters
– Consequence
– Occupancy
– Probability of avoiding hazard
– Demand rate
• Follow path determined by selected
parameters
• Select SIL
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CFSE Exam Preparation: Section I-8
Risk Graph
Parameters
Parameters Description
Consequence C Average number of fatalities likely to result from the hazard. Determined
by calculating the average numbers in the exposed area when the area is
occupied, taking into account the vulnerability to the hazardous event.
Occupancy F Probability that the exposed area is occupied. Determined by calculating
the fraction of time the area is occupied.
Probability of P The probability that exposed persons are able to avoid the hazard if the
avoiding the hazard protection system fails on demand. This depends on there being
independent methods of alerting the exposed persons to the hazard and
manual methods of preventing the hazard or methods of escape.
The number of times per year that the hazardous event would occur if no
Demand Rate W SIS was fitted. This can be determined by considering all the failures
which can lead to one hazard and estimating the overall rate of occurrence.
Based on information found 10
IEC d1511-3 Section D
CFSE Exam Preparation: Section I-8
Risk Graph
Consequence
Parameters Classification Comments
Consequence (C) CA 1. The classification system has been
Minor Injury
Average number of fatalities. developed to deal with injury and death
to people.
CB Range 0.01 to 0.1 2. For the interpretation of CA, CB, CC,
This can be calculated by determining the average
number of people present when the area exposed to and CD, the consequences of the
the hazard is occupied and multiplying by the CC accident and normal healing shall be
Range > 0.1 to 1
vulnerability to the identified hazard. taken into account.
CD Range > 1
The vulnerability is determined by the nature of the
hazard being protected against. The following
factors can be used.
V=0.01 Small release of flammable or toxic
V = 0.1 Large release of flammable or toxic
V = 0.5 As above but highly toxic or flammable
V = 1 Rupture or explosion
Based on information found 11
IEC d1511-3 Section D
CFSE Exam Preparation: Section I-8
Risk Graph
Occupancy
Parameters Classification Comments
Occupancy (F) FA 3. See comment 1 above.
Rare to more often
This is calculated by determining the length of time exposure in the
the area exposed to the hazard is occupied during a hazardous zone.
normal working period. Occupancy less than 0.1.
NOTE – If the time in the hazardous area is FB Frequency to permanent
different depending on the shift being operated then exposure in the
the maximum should be selected. hazardous zone.
NOTE – It is only appropriate to use F A where it
can be shown that the demand rate is random and
not related to when occupancy could be higher than
normal. The latter is usually the case with demands
that occur at equipment start-up.
Based on information found 12
IEC d1511-3 Section D
CFSE Exam Preparation: Section I-8
Risk Graph
Probability of Avoidance
Parameters Classification Comments
Probability of avoiding the hazardous event PA 4. PA should only be selected if all the
Adopted if all conditions
(P) if the protection system fails to operate. following are TRUE:
in note 4 are satisfied.
• Facilities are provided to alert the
operator that the SIS has failed
PB Adopted if all the • Independent facilities are provided to
conditions are not shut down such that the hazard can be
satisfied. avoided or which enable all persons to
escape to a safe area.
• The time between the operator being
alerted and a hazardous event occurring
exceeds 1 hour.
Based on information found 13
IEC d1511-3 Section D
CFSE Exam Preparation: Section I-8
Risk Graph
Demand Rate
Parameters Classification Comments
Demand Rate (W) without protection system. W1 5. The purpose of the W factor is to
Demand rate less than
estimate the frequency of the hazard
0.03 per year.
To determine the demand rate it is necessary to taking place without the addition of the
consider all sources of failure that can lead to one W2 Demand rate between 0.3 SIS.
hazardous event. In determining thee demand and 0.03 per year.
rate, limited credit can be allowed for control 6. If the demand rate is very high (e.g., 10
system performance and intervention. The W3 Demand rate between 3 per year) the SIL has to be determined
performance which can be claimed if the control and 0.3 per year. by another method or the risk graph
system is not to be designed and maintained must be recalibrated. Then the
according to IEC61511, is limited to below the operation mode is high demand or
For demand rates higher
performance ranges associated with SIL 1. continuous (IEC, 61511-1, Clause
than 3 per year higher
3.1.48.2).
integrity shall be needed.
Based on information found 14
IEC d1511-3 Section D
CFSE Exam Preparation: Section I-8
Risk Graph
Assigning SIL
W3 W2 W1
CA X1
Starting point for
a --- ---
PA X2
risk reduction
CB
FA
1 a ---
estimation PB X3
CC
FB
PA 2 1 a
PB X4
FA
FB PA 3 2 1
PB X5
FA
CD PA 4 3 2
X6
FB
PB b 4 3
C = Consequence parameter
F = Exposure time parameter --- = No safety requirements
P = Possibility of failing to avoid hazard A = No special safety requirements
W = Demand rate assuming no protection B = A single E/E/PS is not sufficient
1,2,3,4 = Safety Integrity Level
Based on information found in 15
IEC d61511-3 Section D
CFSE Exam Preparation: Section I-8
Example
Risk Graph SIL Assignment
• Example 3
– A SIF was identified during a HAZOP study. The
HAZOP team determined that one or more
fatalities are likely, the area is normally occupied,
no possibility of avoiding the hazard and the
demand rate is 0.05 per year.
• What is the SIL?
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CFSE Exam Preparation: Section I-8
Example 3
Risk Graph Assigning SIL
W3 W2 W1
CA X1
Starting point for
a --- ---
PA X2
risk reduction
CB
FA
1 a ---
estimation PB X3
CC
FB
PA 2 1 a
PB X4
FA
FB PA 3 2 1
PB X5
FA
CD PA 4 3 2
X6
FB
PB b 4 3
C = Consequence parameter
F = Exposure time parameter --- = No safety requirements
P = Possibility of failing to avoid hazard A = No special safety requirements
W = Demand rate assuming no protection B = A single E/E/PS is not sufficient
1,2,3,4 = Safety Integrity Level
Based on information found in 17
IEC d61511-3 Section D
CFSE Exam Preparation: Section I-8
Example
Risk Graph SIL Assignment
• Example 4
– Further analysis showed that this scenario yielded
a consequence of 0.21 PLL and a likelihood of
1/576 incidents per year.
• What is the SIL given this new information?
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CFSE Exam Preparation: Section I-8
Example 4
Risk Graph Assigning SIL
W3 W2 W1
CA X1
Starting point for
a --- ---
PA X2
risk reduction
CB
FA
1 a ---
estimation PB X3
CC
FB
PA 2 1 a
PB X4
FA
FB PA 3 2 1
PB X5
FA
CD PA 4 3 2
X6
FB
PB b 4 3
C = Consequence parameter
F = Exposure time parameter --- = No safety requirements
P = Possibility of failing to avoid hazard A = No special safety requirements
W = Demand rate assuming no protection B = A single E/E/PS is not sufficient
1,2,3,4 = Safety Integrity Level
Based on information found in 19
IEC d61511-3 Section D
CFSE Exam Preparation: Section I-8
SIL selection using
Frequency Based Targets
• Select frequency target based on
consequence (consequence constant)
• Calculate required risk reduction
• Assign SIL based on required risk reduction
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CFSE Exam Preparation: Section I-8
Frequency Based Targets
Selecting the Target
• The frequency that is allowable for a
hazardous event depends on the
consequence
Impact Event Level Consequence Target Freq.
Impact initially limited to local area of event with potential 1.0 x 10-3
Minor (M) for broader consequence, if corrective action not taken.
One that could cause: Any serious injury or fatality on-site or off-
Serious (S) site, or property damage of $1 MM on-site, or $5 MM off-site.
1.0 x 10-4
One that is five or more times worse than a SERIOUS 1.0 x 10-6
Extensive (E) accident.
per year
Based on information found 21
IEC d1511-3 Section F
CFSE Exam Preparation: Section I-8
Frequency Based Targets
Calculate Risk Reduction
• Required risk reduction is a function of
unmitigated accident frequency and the
frequency target
FTarget
PFDSIF =
FUnmitigated event
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CFSE Exam Preparation: Section I-8
Frequency Based Targets
Assign SIL
• Select SIL based on
required PFD SIL PFD
• PFD target converted to
SIL based on table
4 10-4>PFD>10-5
specified in ISA S84 and 3 10-3>PFD>10-4
IEC 61511/61508 2 10-2>PFD>10-3
• Selected SIL should
give MORE risk
1 10-1>PFD>10-2
reduction than required
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CFSE Exam Preparation: Section I-8
Another target frequency method:
Individual Risk Targets
• Convert likelihood and consequence into
frequency target
• Calculate required risk reduction
• Assign SIL based on required risk reduction
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CFSE Exam Preparation: Section I-8
Individual Risk Targets
Calculate frequency target
• Calculate frequency target – a function of
tolerable individual risk and probable loss of
life
Findividual risk
Ftarget =
PLL
• Calculate require risk reduction and assign
SIL with the same method as the previous
method
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CFSE Exam Preparation: Section I-8
Summary:
SIL Assignment
• How do I use tolerable risk to assign SIL?
• Risk Matrix
• Risk Graph
• Numerical Risk Targets
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CFSE Exam Preparation: Section I-8
Exercise 1
SIL Assignment
An accident scenario yielded a consequence of 0.21
Probable Loss of Life (PLL) and a likelihood of 1/576
incidents per year. What SIL should be selected?
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CFSE Exam Preparation: Section I-8
Exercise 1 (Key)
SIL Assignment
An accident scenario yielded a consequence of 0.21
Probable Loss of Life (PLL) and a likelihood of 1/576
incidents per year. What SIL should be selected?
Step 1 - The frequency category associated with
PLL=0.21 is serious, which has an associated
frequency target of 1.0x10-4.
Step 2 - Applying the target PFD equation yields.
PFD = 1.0x10-4 / (1/576) = 0.0576 RRF = 17.4
Step 3 - In order to achieve RRF=17.4, SIL 2 should
be selected???
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CFSE Exam Preparation: Section I-8
Exercise 2
SIL Assignment
An accident scenario yielded a consequence of
0.21 Probable Loss of Life (PLL) and a
likelihood of 1/576 incidents per year – tolerable
individual risk of fatality at this facility is 1x10 -4
What SIL should be selected?
Step 1 – Determine the tolerable frequency of this event.
F(tol) = 1x10-4 / 0.21 = 4.76x10-4
Step 2 – Applying the target PFD equation yields.
PFD = 4.76x10-4 / (1/576) = 0.274 RRF = 3.64
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CFSE Exam Preparation: Section I-8
Exercise 2 (Key)
SIL Assignment
An accident scenario yielded a consequence of
0.21 Probable Loss of Life (PLL) and a
likelihood of 1/576 incidents per year – tolerable
individual risk of fatality at this facility is 1x10 -4
What SIL should be selected?
Step 1 – Determine the tolerable frequency of this event.
F(tol) = 1x10-4 / 0.21 = 4.76x10-4
Step 2 – Applying the target PFD equation yields.
PFD = 4.76x10-4 / (1/576) = 0.274 RRF = 3.64
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CFSE Exam Preparation: Section I-8
Exercise 2 (Key)
SIL Assignment
Step 3 – Select SIL based on RRF.
For, RRF = 3.64 SIL = 1???
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