Reference Document 2: Additional Risk Matrix Examples for
Management Practice 4
Included in this appendix are additional Risk Matrix Examples which support Management
Practice 4. The following are various strategies that companies can consider adopting to
fulfill this Management Practice.
Severity
Another way to define Tolerable Risk that includes quantitative criteria for likelihood as
shown in the following Risk Matrix Table.
0 1 2 3 4 5
Likelihood
(events/year)
> 10-1
10-1 to 10-2
10-2 to 10-3
10-3 to 10-4
10-4 to 10-5
< 10-5
Risk is Tolerable
Risk is NOT Tolerable
The scenario identified from a sample Process Hazard Analysis (PHA) is as follows (for
demonstration purpose only):
Cause: Failure of a storage vessel’s level control loop
Undesired Consequence: Overflow and spill of a flammable liquid from the vessels
conservation vent, fire if there is an ignition source, operator injury if there is an operator
in the vicinity of the fire
Consequence Severity assigned by the PHA based on the most appropriate definition in
Table 1 (found in main body of the guidance) = Safety 3
Layer of Protection: High Level interlock shuts down feeds to the process vessel.
An example Fault Tree Diagram for this scenario is shown below.
Operator
Injury
And
Fire
Operator in
vicinity
And
Spill
Source of
Ingition
Oxygen
Present
And
Level Control
Failure
High Level
Interlock not
availble
Assuming hypothetically that:
The probability of a source of ignition in the vicinity of the spill = 1.0
The probability of oxygen being present = 1.0
The probability of an operator being in the vicinity of the fire = 0.5
From published failure rate data:
The likelihood of the control loop failing = 5.36 x 10-2 failures per year
The Probability of Failure on Demand (PFD) of a high level interlock that is proof checked
once a year = 2.3 x 10-2
The Likelihood of a spill = Likelihood of control loop failure x interlock PFD = 5.36 x 10-2 x
2.3 x 10-2
= 1.24 x 10-3 events per year
The Likelihood of a fire = Likelihood of a spill x probability of a source of ignition x
probability of oxygen being present
= 1.24 x 10-3 x 1.0 x 1.0 = 1.24 x 10-3
The Likelihood of an operator injury = Likelihood of a fire x probability of an operator in
the vicinity of the fire
= 1.24 x 10-3 x 0.5 = 0.62 x 10-3 events per year
Severity
As shown below, the cell in the Risk Matrix for a Severity of 3 and a Likelihood of 0.62 x 10-3
events per year indicates a Tolerable Risk for this hypothetical scenario.
0 1 2 3 4 5
Likelihood
(events/year)
> 10-1
10-1 to 10-2
10-2 to 10-3
10-3 to 10-4
10-4 to 10-5
< 10-5
Therefore, under this hypothetical scenario and sample risk matrix, no additional
recommendations or safeguards are needed.