1. EVENT TREE ANALYSIS
Event tree analysis evaluates potential accident outcomes that might result following an equipment failure or process upset known as an initiating event. It is a “forward-thinking” process, i.e. the analyst begins with an initiating event and develops the following sequences of events that describes potential accidents, accounting for both the successes and failures of the safety functions as the accident progresses.

2. Guidelines 1. Identify an initiating event of interest.
2. Identify the safety functions designed to
deal with the initiating event.
3. Construct the event tree.
4. Describe the resulting accident event
sequences.

3. Step 1 Identify the initiating event
system or equipment failure
human error
process upset
[Example]
“Loss of Cooling Water”
to an Oxidation Reactor

4. Step 2 Identify the Safety Functions Designed to Deal with the Initiating Event
Safety system that automatically respond to the initiating event.
Alarms that alert the operator when the initiating event occurs and operator actions designed to be performed in response to alarms or required by procedures.
Barriers or Containment methods that are intended to limit the effects of the initiating event.

5. Example Oxidation reactor high temp. Alarm alerts operator at temp T1.
Operator reestablish cooling water flow to the oxidation reactor.
Automatic shutdown system stops reaction at temp. T2. T2 > T1
These safety functions are listed in the order in which they are intended to occur.

6. FIRST STEP IN CONSTRUCTING EVENT TREE
Step 3: Construct the Event Tree
a. Enter the initiating event and safety functions.
Oxidation reactor
high temperature
alarm alerts operator
at temperature T1
Operator
reestablishes
cooling water flow
to oxidation reactor
Automatic
shutdown system
stops reaction at
temperature T2
SAFETY
FUNCTION
INITIATING EVENT:
Loss of cooling water
to oxidation reactor
FIRST STEP IN CONSTRUCTING EVENT TREE

7. REPRESENTATION OF THE FIRST SAFETY FUNCTION
Step 3: Construct the Event Tree
b. Evaluate the safety functions.
Oxidation reactor
high temperature
alarm alerts operator
at temperature T1
Operator
reestablishes
cooling water flow
to oxidation reactor
Automatic
shutdown system
stops reaction at
temperature T2
SAFETY
FUNCTION
INITIATING EVENT:
Loss of cooling water
to oxidation reactor
Success
Failure
REPRESENTATION OF THE FIRST SAFETY FUNCTION

8. REPRESENTATION OF THE SECOND SAFETY FUNCTION
Step 3: Construct the Event Tree
b) Evaluate the safety functions.
Oxidation reactor
high temperature
alarm alerts operator
at temperature T1
Operator
reestablishes
cooling water flow
to oxidation reactor
Automatic
shutdown system
stops reaction at
temperature T2
SAFETY
FUNCTION
INITIATING EVENT:
Loss of cooling water
to oxidation reactor
Success
If the safety function does not affect the course of the accident, the accident path proceeds with no branch pt to the next safety function.
Failure
REPRESENTATION OF THE SECOND SAFETY FUNCTION

9. Step 3: b. Evaluate safety functions.
Oxidation reactor
high temperature
alarm alerts operator
at temperature T1
Operator
reestablishes
cooling water flow
to oxidation reactor
Automatic
shutdown system
stops reaction at
temperature T2
SAFETY
FUNCTION
INITIATING EVENT:
Loss of cooling water
to oxidation reactor
Success
Completed !
Failure
COMPLETED EVENT TREE

10. Step 4: Describe the Accident Sequence
Oxidation reactor
high temperature
alarm alerts operator
at temperature T1
Operator
reestablishes
cooling water flow
to oxidation reactor
Automatic
shutdown system
stops reaction at
temperature T2
SAFETY
FUNCTION B C D A
Safe condition,
return to normal
operation AC
Safe condition,
process shutdown
INITIATING EVENT:
Loss of cooling water
to oxidation reactor
ACD
Unsafe condition,
runaway reaction,
operator aware of
problem A AB
Unstable condition,
process shutdown
ABD
Unsafe condition,
runaway reaction,
operator unaware
of problem
Success
Failure
ACCIDENT SEQUENCES

11. Reactor TIC TIA Alarm at T > TA Cooling Coils Reactor Feed
Cooling Water Out
Cooling
Water In
Reactor
TIC
Temperature
Controller
TIA
Figure Reactor with high temperature alarm and temperature controller.
Alarm at
T > TA
Thermocouple
High Temperature Alarm

12. Safety Function: Identifier: B C D E
High Temp
Alarm Alerts
Operator
Operator
Notices
High Temp
Operator
Re-starts
Cooling
Operator
Shuts Down
Reactor
Safety Function:
Identifier: B C D E
Failures/Demand:
Result A
0.7425
Continue Operation
Shut Down
Runaway
0.99 AD
0.2227
0.2475
ADE A 1 AB
Initiating Event:
Loss of Cooling
1 Occurrence/yr.
ABD
0.0075
ABDE
0.01
ABC
0.0025
ABCD
ABCDE
Shutdown = = occurrences/yr.
Runaway = = occurrences/yr.
Figure Event tree for a loss of coolant accident for the reactor of Figure 11-8.

13. Success of Safety Function (1-0.01)*0.5 = 0.495 Occurrence/yr.
0.01 Failures/Demand
Initiating
Event
0.5 Occurrences/yr.
Success of Safety Function
(1-0.01)*0.5 = Occurrence/yr.
Failure of Safety Function
0.01*0.5 = Occurrence/yr.
Figure The computational sequence across a safety function in an event tree.

14. Safety Function: Result Identifier: B C D E F
High Temp
Alarm Alerts
Operator
Operator
Notices
High Temp
Operator
Re-starts
Cooling
Operator
Shuts Down
Operator
Shuts Down
Reactor
Safety Function:
Identifier: B C D E F
Failures/Demand:
Result A
0.7425
Continue Operation
Shut Down
Runaway AD
0.2450
0.99
ADE
0.2475
ADEF A 1 AB
Continue Operation
Shut Down
Runaway
Initiating Event:
Loss of Cooling
1 Occurrence/yr.
ABD
ABDE
ABDEF
0.01
ABC
Continue Operation
Shut Down
Runaway
ABCD
0.0025
ABCDE
ABCDEF
Shutdown = = occurrences/yr.
Runaway = = occurrences/yr.
Figure Event tree for the reactor of Figure This includes a high temperature shutdown system.