CE1400
ENVIRONMENT AND SAFETY ENGINEERING
Lecture 29
Safety Engineering
FTA
Dr. Mahendra Chinthala
Assistant Professor
Department of Chemical Engineering
National Institute of Technology Rourkela
Safety Engineering:
Introduction: Background, Benefits of safety in Industry, Safety Terms and
Definitions.
Workplace Accidents and Safety: Accident Causation Theories (Domino
Theory, Human Factor Theory), Accident Investigation and Reporting. Legal
Aspects of Safety: Factories Act 1948, Other International Codes (OSHA Laws &
Regulations).
Hazards Related to Various Industries: Chemical, Electrical, Mining, and
Construction Safety and Risk Management: Safety Management Principles,
Safety Program Plan, Safety Committees, Safety Performance Measures, Risk
Assessment, Risk Management.
Safety Analysis Methods: FMEA, FTA, Markov Method, HAZOP, JHA, Control
Charts.
Human Factors in Safety: Job Stress, Ergonomics, Human behaviour; Personal
Protective Equipment; Safety Costing; Safety Cost Estimation Methods, Safety
Cost Estimation.
Fault Tree Analysis (FTA)
❑ Fault Tree Analysis was originally developed in 1962 at Bell
Laboratories by H.A. Watson.
❑ FTA is a deductive analysis approach for resolving an undesired
event into its causes.
❑ Logic diagrams and Boolean Algebra are used to identify the
cause of the top event
❑ FTA is widely used in industry to perform reliability analysis of
engineering systems during design and development.
Objectives of FTA
❑ To identify critical areas
❑ To recognize cost-effective improvements
❑ To confirm the ability of a system to meet its imposed safety
requirements
❑ To confirm system reliability
❑ To understand the functional relationship of systems failure.
Fault Tress Analysis (FTA)
❑ Fault tree is the logical model of the relationship of the undesired event to
more basic events
❑ FTA begins by identifying an undesirable event, called top event,
associated with a system under consideration.
❑ The middle events are intermediate events and the basic events are at the
bottom.
❑ Fault events that can cause the top event are generated and
conducted by logic operators such as OR and AND.
Fault Tress Analysis (FTA)
Fault Tress Analysis (FTA)
Basic Event: A lower most event that can not be further
developed.
Intermediate Event: This can be a intermediate event
(or) a top event. They are a result of logical combination
of lower level events.
Undeveloped Event: An event which has scope for
further development but not done usually because of
insufficient data.
External Event: An event external to the system which
can cause failure
Fault Tress Analysis (FTA)
OR Gate: Either one of the bottom event results in the occurrence of
the top event.
AND Gate: For the top event to occur all the bottom events should
occur.
Inhibit Gate: The top event occurs only if the bottom
event occurs and the inhibit condition is true.
Procedure for Fault Tree Analysis
Define top
event
Define
overall
structure
Explore
each branch
in successive
level
Solve the
fault tree
Perform
corrections
if required
and make
decisions
Fault Tree analysis symbols
Example of FTA
Example of FTA
Example of FTA
Probability evaluation of Fault Trees
❑ Under certain circumstances, it may be desirable to predict the probability of
occurrence of certain unsafe events.
❑ The determination of the probability of occurrence of the logic gates output fault
even is absolutely necessary..
❑ The probability of occurrence of an AND gates output fault even tis expressed
by
▪
▪
▪
K is the total number of input fault events
𝑃𝐴 𝐸 is the occurrence probability of a AND gates output fault event , say E
𝑃(𝐸𝑖 ) is the occurrence probability of input fault event Ei for I = 1, 2,3….k
Similarly the probability of occurrence of an OR gate’s output fault
event is given by
Example of FTA
Advantages of FTA
▪ User could select the top event to be specific to the failure of
interest.
▪ The minimal cut sets provide enormous inside into the various
failure modes for top event to occur.
▪ Minimal cut sets with a product of 4 or more independent
failure will increase the reliability of the system.
▪ Provide a qualitative and quantitative reliability analysis.
▪ Software are available to construct fault tree, to determine cut
sets and to calculate the failure probabilities.
Disadvantages of FTA
▪
▪
▪
▪
Can be enormous (thousands gate and intermediate events)
Not necessarily all failure modes are considered.
Need experienced engineers
Assume hardware not to fail partially (such the possibility of
valve leak is not considered).
▪ Assume failure of one component does not put stress on the
other components (that could change component failure
probabilities).
▪ External events not correctly treated
References
1. Fault Tree Handbook with Aerospace Applications’, Version1.1, NASA Publication, August 2002.
http://www.conceptdraw.com/examples/fta