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EVENT TREE ANALYSIS Event tree analysis evaluates potential accident outcomes that might result following an equipment failure or process upset known as.

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Presentation on theme: "EVENT TREE ANALYSIS Event tree analysis evaluates potential accident outcomes that might result following an equipment failure or process upset known as."— Presentation transcript:

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 T 1. Operator reestablish cooling water flow to the oxidation reactor. Automatic shutdown system stops reaction at temp. T 2. T 2 > T 1 These safety functions are listed in the order in which they are intended to occur.

6 Step 3: Construct the Event Tree a. Enter the initiating event and safety functions. SAFETY FUNCTION 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 INITIATING EVENT : Loss of cooling water to oxidation reactor FIRST STEP IN CONSTRUCTING EVENT TREE

7 Step 3: Construct the Event Tree b. Evaluate the safety functions. SAFETY FUNCTION 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 INITIATING EVENT: Loss of cooling water to oxidation reactor REPRESENTATION OF THE FIRST SAFETY FUNCTION Success Failure

8 Step 3: Construct the Event Tree b) Evaluate the safety functions. SAFETY FUNCTION 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 INITIATING EVENT: Loss of cooling water to oxidation reactor REPRESENTATION OF THE SECOND SAFETY FUNCTION Success Failure 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.

9 Step 3: b. Evaluate safety functions. SAFETY FUNCTION 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 INITIATING EVENT: Loss of cooling water to oxidation reactor COMPLETED EVENT TREE Success Failure Completed !

10 Step 4: Describe the Accident Sequence SAFETY FUNCTION 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 INITIATING EVENT: Loss of cooling water to oxidation reactor ACCIDENT SEQUENCES Success Failure Safe condition, return to normal operation Safe condition, process shutdown Unsafe condition, runaway reaction, operator aware of problem Unstable condition, process shutdown Unsafe condition, runaway reaction, operator unaware of problem B A CD A AC ACD AB ABD

11 Reactor TIA TIC Alarm at T > T A Figure 11-8 Reactor with high temperature alarm and temperature controller. Cooling Coils Thermocouple High Temperature Alarm Temperature Controller Reactor Feed Cooling Water Out Cooling Water In

12 Safety Function: Identifier: B C D E Failures/Demand: High Temp Alarm Alerts Operator Notices High Temp Operator Re-starts Cooling Operator Shuts Down Reactor Result Shutdown = = occurrences/yr. Runaway = = occurrences/yr. Figure 11-9 Event tree for a loss of coolant accident for the reactor of Figure Initiating Event: Loss of Cooling 1 Occurrence/yr. A1A1 A AD ADE AB ABD ABDE ABC ABCD ABCDE Continue Operation Shut Down Runaway Continue Operation Shut Down Runaway Continue Operation Shut Down Runaway

13 Safety Function 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: Identifier: B C D E F Failures/Demand: High Temp Alarm Alerts Operator Notices High Temp Operator Re-starts Cooling Operator Shuts Down Result Shutdown = = occurrences/yr. Runaway = = occurrences/yr. Figure Event tree for the reactor of Figure This includes a high temperature shutdown system. Initiating Event: Loss of Cooling 1 Occurrence/yr. A1A1 A Continue Operation Shut Down Runaway Operator Shuts Down Reactor AD ADE ADEF AB ABD ABDE ABDEF ABC ABCD ABCDE ABCDEF Continue Operation Shut Down Runaway Continue Operation Shut Down Runaway


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