Week 4 Flashcards
What are the steps of QRA?
- Defining the scopes and objectives (what are we try to put a number on?)
- Identifying the hazard that can occur in the plant (or part of the plant)
- Probability analysis (working out the
likelihood of the events taking place) - Consequence analysis (working out the effect of the events taking place)
- Calculating the risk from the above
- Is risk acceptable or not?
- If not, develop control measures
- If yes, operate, control, and review.
Hazard Identification
A source of potential harm (situation, process, substances)
Tangible, real and often phyisal and only relate to damage, loss or detrimental outcomes
Hazard is independent of likelihood ie. risk
Probability analysis
Step 1: specifying an undesired end event
Step 2: specifying the initiation events (IE) that could start the progress towards the end event
Step 3: Mapping the sequence of events that lead IEs to the the end event
Consequence Analysis Steps
Step 1: specifying an undesired end event
Step 2: identifying the results of end event
Step 3: identifying the final undesired outcomes
Logic Tree
A part of probability analysis.
First Level = IEs
Second level = event
Third level = incident
Has ‘or’ and ‘and’ logic gates
Protective Systems
In a protected system, the trip is a second-level detector, installed at some higher level, the so-called emergency level.
* If the trip detects liquid it sends a signal to ESD which triggers the relevant valve to immediately turn off. There is no control function.
D terminology
Demand rate; the rate (occasions/year) at
which a protective system is called on to act.
eg. overpressure occurs and valve is expected to open
H terminology
Hazard realisation rate: number of incidents per year
eg. the vessel explodes because the pressure
relive valve failed to open
F terminology
Failure rate: number of times per year that a protective system is in a failed state.
eg. The number of times per year that the valve is unable to open
fdT terminology
Fractional dead time: the fraction of total time
that the device is in a failed state and is
incapable of providing protection.
eg. The fraction of total time the pressure relive valve can’t open
determining F and fdT
Failure rate (F) is determined by quality of manufacture and age of device via “bath-tub” curve.
Early failures:
The first part is a decreasing failure rate, caused by manufacturing defects or installation mistakes
Random failures:
The second part is a constant failure rate, caused by all sorts of reason during the usual life.
Wear-out failures:
The third part is an increasing failure rate, as the product exceeds its design lifetime
Fractional dead time “fdt” determined by:
- how soon the failure is noticed
- how quickly it is then repaired
