Risk Assessment Fault And Event Tree Analysis Biology Essay

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Identify the purpose and objectives of fault tree analysis and event tree analysis. Compare and contrast fault tree and event tree analysis and with other techniques for reliability and risk assessment such as reliability block diagrams, failure modes and effects analysis and bowtie assessment

Fault tree analysis makes use of Boolean logical functions and graphical methods to identify probable faults and likely failures of any given system to establish the associated hazard in order to institute corrective measures to the product to enhance is safety hence improve its reliability. This analysis adopts the top-down approach in breaking down the problem. The key purpose of fault tree analysis is identification of the shortcomings in a given product or service in order to come up with the appropriate solutions to the shortcomings. On the other hand, event tree analysis applies the bottom-up criterion in risk assessment for most management and decision systems. Just like fault tree analysis, the method makes use of Boolean combinational logic in developing the analysis. (Rechard 1999)

Event tree analysis stands out in the sense that a number of failures can be analysed simultaneously without need to foresee end events and weakness of the system can easily be identified for rectification. The weakness of this method is that anticipation of operation pathways is necessary whereby some successes and failures cannot be clearly distinguished as it the case for fault tree analysis. Fault tree analysis serves a number of purposes such as provision of qualitative and quantitative formats for evaluation as well as giving a vivid system function description leading to undesirable outcomes. Event and fault tree analysis helps identify potential failures in manufacture and processing sectors. (Hixenbaugh 1968)

Other forms of risk assessment include Failure Modes and Effects Analysis and Bowtie method and the use of reliability block diagrams. These forms are closely linked to the fault tree Analysis except that whereas the methods are inductive in nature, fault tree analysis assumes a top-down deductive approach capable of breaking down the complexity of any given system. (Hixenbaugh 1968) Failure Mode and Effects analysis applies the bottom up approach with specific focus on a single element (subsystem) of the entire system. This therefore indicates the Failure Modes and Effects Analysis (FMEA) and fault tree Analysis (FTA) are complimentary methods whereby FTA FMEA is used to analyze internal initial faults and FTA used for multiple external failures affecting the system.

Complete a fault tree analysis for a system or systems forming part of a typical plant design with which you are familiar. The system design can be from an actual design or can be assumed.

Problem: LOSING OF A COOLANT

A cooling system is a very integral component in an industrial setting for safety, protection and even maintenance of equipment. (Fayssal 1990) A typical example is the cooling system used in power plants such as the Pressurized water reactor and the Boiling water reactor. Major components of the system include:

The core: This is the central processing part of the plant

Pressurizer: Provides and control pressure for normal working of the system.

Steam Generator: Propagates steam in the system

Turbines: They have rotary motion providing mechanical power in the system

Condensers: Are the main locations for system cooling

Heaters: They supply heat in the system in prescribed locations

Valves: Are the main control points for the flow of water and steam

Diagrams

Fig. 1 Coolant

Fig.2 Typical power plant with coolant

Basic Event

Failure

Failure Rate Data

References

Failure Probability

Core

Overheating

22.456E-6

 

NPRD-95 2-217

2.458E-2

Pressurizer (PZR)

 Bursts

14.125E-6

 NPRD-95 2-221

 1.546E-2

Steam Generator (SG)

 Breaks down

0.8792E-6

 NPRD-95 2-224

 9.627E-4

Reactor coolant pump (RCP)

 Pump fails

0.1467E-6

 NPRD-95 2-163

 5.124E-4

Safety valve (SV)

 Blockage

1.0264E-6

 NPRD-95 2-157

 1.124E-3

Mainsteah isolation valve (MSIV)

 Blockage

0.0453E-6

 NPRD-95 2-157

 4.960E-5

Throttle valve (TV)

 Valve fails

0.2719E-6

 NPRD-95 2-157

 2.977E-4

Moisture Separator Reheater

 Fails

0.1181E-6

 NPRD-95 2-186

 1.293E-4

Main turbine (MT)

 Breaks down

0.0213E-6

 NPRD-95 2-169

 2.332E-5

Turbine LP (TLP)

 Breaks down

0.4475E-6

 NPRD-95 2-168

 4.900E-4

Main condenser (MC)

 Condenser fails

0.1124E-6

 NPRD-95 2-156

 1.231E-4

Condensate pump (CP)

Condenser fails

0.2245E-6

 NPRD-95 2-156

 2.458E-4

Clean up system (CUS)

Residue accumulation

0.1824E-6

 NPRD-95 2-114

 1.972E-4

LP heater (LPH)

Heater fails 

0.1246E-6

 NPRD-95 2-148

 1.364E-4

HP heater (HPH)

Heater fails

0.1476E-6

 NPRD-95 2-148

 1.616E-4

Condensate storage tank (CST)

Coil failure

0.1654E-6

 NPRD-95 2-156

 1.811E-4

Safety injection system (SIS)

System fails 

0.5713E-6

 NPRD-95 2-157

 6.255E-4

Safeguards pumps (SP)

Pump fails

0.6231E-6

 NPRD-95 2-163

 6.822E-4

Auxiliary feed water (AFW)

Supply cut off

0.7481E-6

 NPRD-95 2-152

 8.192E-4

Test interval = (365 x 24) x (3 / 12) = 2190 hours

FP= FRD x time in hours/2

Table 1

NOTE:

The test interval has been taken for three months.

The failure rate data is obtained from sources indicated in the Reference

Calculations were done based on the formula FP= FRD x time in hours/2

Time interval was taken as 2190 hours.

FAULT TREE ANALYSIS

Definition of gates used

OR GATE

AND GATE

The cut set table makes use of AND gates in computation of the probabilities

Cut set

Probability

Cut set

Probability

Core . SP

 1.678E-5

AFW . CS

1.6154E-7

Core .SIS

 1.538E-5

AFW . LPH1

1.1173E-7

PZR . SP

 1.055E-5

AFW . LPH2

1.1173E-6

PZR . SIS

 9.67E-6

CST . SG

1.7434E-6

SG . SP

 6.567E-7

CST, Condenser

2.2293E-6

SG . SIS

 5.928E-7

CST . MT

4.2232E-6

RCP . SP

 3.495E-6

CST . MSR

2.3416E-8

RCP . SIS

 3.205E-7

CST . CP

4.4514E-8

AFW . SG

 7.886E-7

CST . MFWP

4.2812E-7

AFW Condenser

 1.008E-8

CST . SV1

2.0355E-7

AFW . MT

 1.911E-7

CST . SV2

2.0355E-8

AFW . MSR

 1.059E-6

CST . SV3

2.0355E-8

AFW . CP

 2.014E-8

CST . MSIV

8.9825E-7

AFW . MFWP

 1.936E-7

CST . TV

5.3913E-7

AFW . SV1

 9.208E-6

CST . (T1.T3)

8.8739E-8

AFW . SV2

 9.208E-6

CST . HPH

2.9265E-8

AFW . SV3

 9.208E-6

CST . CS

3.5712E-7

AFW . MSIV

 4.063E-8

CST . LPH1

2.4702E-8

AFW . TV

 2.438E-7

CST . LPH2

2.4702E-8

AFW . (T1.T3)

 4.014E-7

AFW . HPH

 1.324E-8

 

Total Probability

4.675 E-5

Fussel Vessely and Birnbaum

Basic Event

Fussel Vessely

Birnbaum

Core

 0.687

0.483

Pressurizer (PZR)

 0.046

0.018

Steam Generator (SG)

0.094

0.06

Reactor coolant pump (RCP)

 0.016

0.014

Safety valve (SV)

0.024 

0.002

Main steam isolation valve (MSIV)

 0.021

 0.055

Throttle valve (TV)

 0.014

 0.092

Moisture separator reheater (MSR)

 0.045

 0.084

Main turbine (MTHP)

 0.062

 0.072

Turbine LP (TLP)

 0.076

 0.078

Main condenser (MC)

 0.038

 0.032

Condensate pump (CP)

 0.064

 0.008

Clean up system (CUS)

 0.087

 0.012

LP heater (LPH)

 0.026

 0.014

HP heater (HPH)

 0.042

 0.026

condensate storage tank (CST)

 0.065

 0.045

safety injection system (SIS)

 0.072

 0.033

safeguards pumps (SP)

 0.014

 0.017

auxiliary feed water

 0.541

 0.034

NOTE:

The Fussel Vessely is obtained by adding all the probabilities containing a specific component in table 2 then dividing by the total probability TP found in table 2. (Ericson 1999)

For example, (Core .SIS) + (Core . SP)/TP=(1.678+1.538)E-5/4.675E-5=0.678

Birnbaum values are obtained by taking the sum of probability in table 2 and dividing by the specific component probability.

For example, Core/TP= 22.46E-5/4.675E-5=0.483

EVENT TREE ANALYSIS OF PLANT HAZARD

DISCUSSION

The event tree analysis makes use of failure probabilities and determined failure frequencies. (Eckberg 1964) Each component is analysed in depth to evaluate the occurrences taking place in case of a failure. The above case sequentially analyses the events that follow the likely core failure due to melting and explosion. For instance failure rate data for the specific core is given as 1E-6. In case the core fails due to melting and explosion, the system may be affected in various ways. Pressure may be released prematurely and the valve may either fail to open or close. Further, the amount of pressure may increase or decrease disproportionately affecting other parts of the system. (DeLong1970) The control and protection system may fail to detect the failure in good time which is again dangerous to the entire system. Event tree analysis here can be used to highlight likely risks associated with core failure to the system.

As shown by the diagram above, the end probability for the given situation is obtained by getting the sum of individual probabilities leading to the ultimate consequence which is core heating the multiplied by the failure frequency of the core. (Begley 1968)

CALCULATION: (0.1+0.1+0.5) * 1E-6 = 7E-7

CONCLUSION

Fault and event tree analysis are key methods in risk assessment especially in identifying the most probable causes of failure giving details of the multiple failures (Acharya 1990). The methods are thus very important in formulating the possible remedies to the foreseen failures.

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