Hazard and Risk Analysis Flashcards
Define Risk in Hazard analysis:
Combination of the frequency or probability of a specified hazardous event and its consequence.
Define Risk when the consequence can be quantified:
Product of frequency of an event and its consequence.
Why do we need to quantify risks?
To identify high risk hazards for which particular care should be taken.
What is probability when talking about risk?
A measure for the likeliness that an event will occur
What does it mean when two events are independent?
If the occurrence of one does not affect the probability of the other.
What does it mean when two events are mutually exclusive?
If the both cannot occur at the same time.
What cant complete independence in safety systems be hard to achieve?
The reason for one system failing might be the reason for another failing.
What does FMEA stand for?
Failure Modes and Effects Analysis
What is FMEA?
A systematic method for identifying and preventing product and process problems before they occur.
What is a brief overview of FMEA?
- Divide system into components/subsystems
- Identify all ways a particular component of a subsystem can fail and the effects of the failure on the system
- Systematically analyse the failures
Define failure modes:
The ways a component of a system can fail.
Expand on “Assess each subsystem and determine whether the failure of the subsystem would affect the main system”:
- If the subsystem has no effect, ignore it
- Otherwise, break the subsystem into further subsystems and repeat the process until the component level is reached.
Expand on: “Assess the failure’s effects” in the FMEA proccess:
- Usually the worst-credible case is accessed
- Determine its mission phase (installation, operations, maintenance and repair)
- Identify whether the failure is a single-point failure
- Determine methods of corrective action.
What is a single-point failure?
Failure of a single component that could bring down the entire system
What are some limitations of FMEA?
- FMEA creates products which are correct as opposed to safe.
-Contributes but doesn’t guarantee safety. - Only investigates single point failures
- FMEA is primarily and engineering tool not a safety analysis tool
What does FMECA stand for?
Failure Modes, Effects and Criticality Analysis
How is FMECA different to FMEA?
FMECA analyses the risk ascociated with a failure and if the risk exceeds a certain value action must be taken.
For each failure it determines:
- the probability of its occurrence
- the probability of the occurrence of the consequences
- a number measuring the criticality
What are Risk Priority Numbers (RPN)?
Calculated by the product of a measure for severity, probability and detection (likelihood that cause of failure is detected before reaching customer).
What does HAZOP stand for?
Hazard and Operability Studies
Where is HAZOP mainly used?
Chemical industries
What is the general procedure of HAZOP?
- Define objectives and scop of the analysis
- Select a HAZOP team
- Dissect design into nodes and identify lines into those nodes
- Analyse deviations for each line and identify hazard control methods
- Document results
- Track hazard control implementation.
What is a node?
A location where process parameters can change
What is a line?
Interface between nodes.
What are some examples of guide words?
- No
- More
- Less
- As well as
- Part of
- Reverse
- Other than
- Early
- Late
- Before
- After
What does the ‘No’ guide word mean for HAZOP?
No part of intended result achieved.
What does the ‘No’ guid word mean for Computer-based systems?
No data or control signal exchanged.
What does the ‘More’ guide word mean for HAZOP?
Quantitative increase in the physical quantity
What does the ‘More’ guide word mean for Computer-Based Systems?
Signal magnitude or data rate too high
What does the ‘Less’ guide word mean for Hazop?
Quantitative decrease in the physical quantity
What does the ‘Less’ guide word mean for Computer-based Systems?
Signal magnitude or data rate too low.
What does the ‘As well as’ guide word mean for HAZOP?
Intended activity occurs but with additional results
What does the ‘As well as’ guide word mean for Computer-based Systems?
Redundant data sent in addition to intended value. Function has overlooked side effect.
What does the ‘Part of’ guide word mean for HAZOP?
Only part of intended activity occurs
What does the ‘Part of’ guide word mean for Computer-Based Systems?
Incomplete data transmitted.
What does the ‘Reverse’ guid word mean for HAZOP?
Opposite of what is intended occurs
What does the ‘Reverse’ guide word mean for Computer-Based Systems?
Polarity of magnitude changes reversed.
Because of overflow error integer becomes negative
What does the ‘Other Than’ guide word mean for HAZOP?
No part of intended activity occurs and something else happens instead
What does the ‘Other Than’ guide word mean for Computer-Based systems?
Data complete but incorrect
What does the ‘Early’ guide word mean for HAZOP?
Not used
What does the ‘Early’ guide word mean for Computer-based systems.
Signal arrives too early
What does the ‘Late’ guide word mean for HAZOP?
Not used
What does the ‘Late’ guide word mean for Computer-based Systems?
Signal arrives too late
What does the ‘Before’ guide word mean for HAZOP?
Not used
What does the ‘Before’ guide word mean for Computer-Based Systems?
Signal arrives earlier than intended within a sequence
What does the ‘After’ guide word mean for HAZOP?
Not used
What does the ‘After’ guide word mean for Computer-Based Systems?
Signal arrives later than intended withing a sequence
What do Event Tree Analysis (ETA) do?
Draws a tree of possible sequences of unintended events and determines possible accident as a result of these events
How does an ETA work?
- Trace sequences of events until they may or may not lead to an accident
- Draw a decision tree in order to identify sequences of events resulting in accidents
- For each such sequence determine its outcome
What is ETA good for?
Calculation of probability of events
How is Fault Tree Analysis (FTA) different to ETA?
FTA goes backwards from the accident and identifies the faults causing it, whereas ETA goes forward tracing sequence of events until the may or may not cause an accident
How does Fault Tree Analysis work?
Starts with possible accidents and determines using logical gates possible combination of events leading to this accident.
What is FTA drawn with?
Logic gates
What are the conditions in FTA?
Disjunctive (Conditions ascociated with OR) or Conjunctive (Conditions asociated with AND)
Define disjunctive:
If one condition is satisfied the event occurs.
Define conjunctive:
If all of the conditions are satisfied the event occurs
What are minimal cut sets?
Conjunctions that determine a minimal sequence of events resulting in an accident
What does FMEA and FMECA focus on?
Avoidance of failures
What type of failures does FMEA and FMECA locate?
Single-point failures
What type of systems does FMEA and FMECA produce?
Highly reliable systems, but they may not identify all hazards. Best for areas where high reliability is crucial
What is bad about ETA?
Event trees might grow too big.
What’s ETA used for in practise?
To obtain good estimates for accidents of nuclear power stations.
What is the most suitable hazard and risk analysis technique?
FTA in order to identify hazards, but best complement with HAZOP (And FMEA/FMECA if reliability is important)
