Application of Reliability Block Diagram Analysis to Overall Plant Configuration

For quite some time manufacturers of durable goods have used Reliability Block Diagram techniques to forecast the reliability of new products before finalizing designs. When products are unlikely to deliver the desired reliability, the manufacturer can either select components with higher reliability or modify the configuration to deliver higher reliability. In either case, the resulting combination of component selection and configuration will determine the "inherent reliability" of the product.

In many cases, plant operators set goals for plant output and reliability based on simply addressing known problems with individual equipment items. They do not take into consideration the “inherent reliability” or the level of reliability “they have a right to expect” based on the cumulative reliability based in individual component reliability and configuration. Application of Reliability Block Diagram techniques provide plant operators with a new tool in understanding where they need to invest scarce resources to improve reliability.

Expert Analysis

The following example describes unexpected results from a study of a fairly modern fuel-efficient refinery. In this case, the individual units in the refinery were “close coupled” meaning that the products from one operating unit was fed directly to the next unit in the processing scheme. This feature eliminated the need to cool down the products then reheat them as would be required if intermediate products were sent to storage.

This design worked well while all the operating units were new, but as the units aged and became less reliable, the failure of one unit would result in the need to shutdown upstream and downstream units.

A number of studies had been conducted to reduce failures of individual components and improve reliability of individual operating units. These studies and implementation of their recommendations were resulting in only a small improvement.

When an RBD study of the complete system was conducted, it showed that reliability could be most improved by providing intermediate storage between units operating at approximately half full. If a downstream unit had a shutdown, the upstream unit could run down into the empty portion of the tank until repairs could be made. The same is true if an upstream unit failed. Then the down stream unit could draw from the partially full tank until the upstream unit came back on line. The study showed that most repairs and restarts could be achieved within a few hours. It was only the fact that the entire chain of units was being forced down by the failure of any individual unit.

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