There are only two ways to get high system reliability if you want a highly reliable system. Equipment in an operation can be configured in series or in parallel. In series, one item connects sequentially to the other. In parallel, each item is arranged as a companion, where one duplicates the other.
A series arrangement needs exceptionally reliable individual equipment to get a highly reliable system. A parallel arrangement can form a highly reliable system even if individual equipment has poor reliability.
It takes just one item to fail in a series arrangement and the whole system is failed. Each individual item must be exceptional reliable when equipment is configured in series to get a highly reliable system. That is not the case when equipment is arranged in a parallel redundancy.
A system is formed when equipment is combined to do a duty. When the pump, coupling, and electric motor in the image above are connected they form a series system used to move liquid through pipes from one point to another. The reliability of the whole system depends on the reliability of each item. When one item fails—be it the pump, or coupling, or motor—the whole system stops and no liquid flows.
Series System Reliability
The Reliability Block Diagram for a series arrangement is shown at the top of the slide. The upper equation in the slide is used to calculate the series system reliability. The reliability of a series system is the multiplication of the reliability each individual item in the system. The upper table in the slide shows an example calculation for two items in series were each has a reliability of 0.95. Reliability is the chance of success, so 0.95 reliability means that in every hundred uses the item fails to complete its duty five times—a 5% chance of failure and 95% chance of success.
When both items of 0.95 reliability must work together as a system, the system reliability is only 0.9. When you put 10 items that are each 0.95 reliable in series the system reliability is but 0.6. That is an atrocious system reliability. Series system reliability falls when you include more equipment because each time you add an item into the series it brings with it more opportunities for things to go wrong that stop the system working.
To create a highly reliable system each item in it must be far more reliable. You can see the effect on system reliability as individual item reliability rises in the top table on the slide. The far right-hand column in the upper table shows the system reliability if each item was 0.9999 reliable, i.e. it fails once in every 10,000 uses. When 10 items of that reliability form a series system, the system reliability is 0.999, or one failure in every thousand times the system is used. Each item in a ten-item series system needs to be at least ten times more reliable, i.e. 0.9999, than the whole system to get a system that is 0.999 reliable.
Parallel System Reliability
The other way to combine equipment is as a parallel system arrangement. In the lower image on the slide you see two filter stations in parallel.
They can be configured to work in one of two ways. One is the duty circuit taking the full service, and the other is the standby doing no work, but ready to come online when the duty goes offline—this is known as a duty and standby configuration, or a standby redundancy.
The other arrangement is where both circuits operate together and share the service load, and if one circuit goes offline the remaining circuit can handle the full duty. This operating philosophy is called a fully active arrangement. The lower table shows when two systems of 0.6 reliability are paralleled in a fully active arrangement the system reliability is 0.84. When six separate systems of 0.6 reliability are paralleled, the system reliability is 0.996. By putting equipment in parallel you create a more reliable system because when one item fails the other item continues delivering the duty.
Getting a Highly Reliable System
When you use equipment there are only two ways to get high system reliability. If it’s a series system, then every piece of equipment in the arrangement must exceptionally reliable. The other way is to duplicate the system and have a complete second system in parallel ready to replace the first system when it fails.
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