![]() This is why other safety precautions would be taken, including the provision of some kind of containment system – such as an impermeable retaining wall or bund surrounding the tank – that would retain any spillage. In reality, failure could take much longer or, conversely, could happen the day after commissioning the installation. However, if the failure was due to (say) a breakdown in the power supply to the tank control system, or because the devices had both been over-insulated and overheated, or even because both devices came from a faulty batch, then adding the second device would have little if any effect in reducing the probability of an incident.Įven if you can assume independent failure of the two devices, this only implies a reduced probability of failure. The calculation you carried out in Activity 7 assumes that the two devices would fail independently (through some random fault, perhaps). This means the probability of two such failures simultaneously is 10−4 × 10 −4 = 10 −8 You need to go back to the original problem and look at the probability of failure for one safety device, which was 1 in 10 000 or 10−4. Since for any event, , the calculation that the level detector doesn’t fail on any single occasion is 1 minus the probability that it does fail – that is, That would give three possible outcomes: the level detector doesn’t fail, it fails once or it fails twice. To calculate this, consider what the probability would be if the tank were filled twice. It will be assumed here that regular maintenance checks ensure that the device does not deteriorate significantly, so that the probability of failure stays constant over time. For instance, you could assess the probability of a failure within 25 years. What you can say is that after a given interval, there is a particular probability of failure. This isn’t a question you can answer, since the device might never fail. If the tank is filled every day, how often is failure likely? An assessment of the device suggests that there is a 1 in 10 000 probability that it will fail each time the tank is filled. For instance, suppose that an oil storage tank is fitted with a level-detecting device that shuts off the valve that lets oil into the tank when it is full. ![]() However, it can be hard to assess what this probability means, in practical terms. The probability of a component failing is calculated in a similar way.
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