In similar way to electronic systems, redundancy is extensively employed technique for enhancing the reliability of many industrial plants and processes. Because many process plants units such as pumps, valves and compressors involve moving parts, they are prone to wear and therefore require periodic inspection and maintenance. Where the plant is required to operate continuously, isolating valves are needed on either side of the unit so that it can be taken off-stream for inspection, maintenance or exchange. The arrangement depends upon the way in which redundancy is employed.
A redundancy arrangement in an industrial plant, for instance, will be used where a gas or liquid is pumped into a container and a failure of supply would be dangerous; two or more pumps may be installed in parallel as illustrated in Figure 1.0 below, with isolating valves which allow each pump to be disconnected.
To allow for failure, two or more valves can be installed in series, if they are motor operated, separate or stand-by power supplies may be desirable. In order to take a valve off stream, it is necessary to provide a bypass as well as isolating valves as shown below:
Other devices may also be connected in series for greater reliability; for instance, pressure regulators can be connected in series with their main failure mode being to open; that is, no regulation, so two regulators are used in series for better reliability.
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