Managing Infrastructure and Systems Flashcards
Why do we want components and systems to be reliable?
- Safety reasons
- Financial reasons (maintenance and delays are costly)
What is the purpose of system reliability analysis?
To evaluate the performance of a system using known
information about the system components and structure.
Why perform reliability analyses?
To assess the adequacy of engineering systems
– at the design stage or to assess upgrades.
To satisfy regulatory requirements
– demonstrate that a system is fit for purpose.
To support decision making, e.g.:
– to find a balance between safety and cost,
– to determine optimal maintenance strategies.
What can happen without reliability analyses?
• Decisions can be subjective and based on biased information.
• Decisions can be inconsistent and based on qualitative
measures or prejudices.
• Available finances can be used inefficiently.
What is the definition for Reliability?
Reliability, Rsys(t) : the probability that the system failure mode does not occur from 0 to time t, given that the system worked at time 0.
What is the definition for Unreliability?
Unreliability, Fsys(t) : the probability that the system failure mode occurs at least once from 0 to time t given that it worked at time 0.
What is the definition for Availability?
Availability, Asys(t) : the probability that the system is operational at
time t, given that it was operational at time 0.
What is the definition for Unavailability?
Unavailability, Qsys(t) : the probability that the system failure mode
exists at time t
What is the definition for Failure rate?
Failure rate : the rate at which the system failure mode occurs.
What is the definition of a path set?
A path set is a list of components, such that if they all work then the system is also in the working state.
What is the definition of a minimal path set?
Is a path set, such that if any component is removed the system no longer functions
What is the definition of a cut set?
Is a list of components, such that if they all fail then the system is also in the failed state
What is the definition of a minimal cut set?
Is a cut set, such that if any item is removed the system will no longer fail
Describe what goes inside the connectivity matrix.
cij = k, where k is the number of edges from node i to node j.
(0 for diagonal)
Describe what goes inside the connection matrix.
1 for diagonal terms (certain to connect)
0 i f no connection
A where A is the component linking node to node
Which 3 methods could you use to calculate the unreliability of an RBD that cannot be represented as a combination of series and parallel networks?
Solution Methods:
– Key Element Method.
– Conversion from Deltas to Stars.
– Minimal Path Set / Cut Set Evaluation.
What is the expression for…
a) system success using key element method
b) system failure using key element method
…. given that E is the key element
a) RSYS = (RX) (RE) + (RY) (QE)
b) QSYS = (QX)(RE) + (QY)(QE)
Give three matrix methods you can use to find the minimal path sets and state which matrix each method relates to
General Algorithm - using the connectivity matrix
Node Removal - using the connection matrix
Matrix multiplication - using the connection matrix
Give two uses of RBD analysis
- Helps to find points of failure and identifies what is making the system unreliable
- Shows a visual representation of a system and therefore reliability can be assessed without numbers
How could you improve a system’s reliability? (Once completing RBD analysis)
- Reduce the amount of series systems
- Increase the redundancy
- Upgrade the components to ones with more reliability
- Improve the accuracy of the data on the reliability on for the components
How can you improve RBD analysis?
- RBDs are often simplified. To improve analysis, create a more accurate model
- Minimise assumptions and make sure any you do make are reasonable
Why might an RBD analysis not be accurate?
- RBDs don’t take into account the effect of not have all 3 in a 2/3 component working (i.e. if 1 fails it might still work but with less power)
- The analysis doesn’t take into account time variations in demand
What is FMEA?
FMEA is a powerful design tool that analyses each potential failure mode in the system to examine the
effects on the system
What is FMECA?
When FMEA is extended to classify each potential failure effect according to its severity the method is known as FMECA