Reliability Block Diagram (RBD) – System Reliability Analysis

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A Reliability Block Diagram (RBD) is a graphical representation used in reliability engineering and system analysis to model the reliability of complex systems.

The basic symbols used in a Reliability Block Diagram include:

  1. Block: Representing the components or subsystems.
  2. Series Connection (AND gate): Shown as a line connecting blocks in series. The system fails if any of the connected components fail.
  3. Parallel Connection (OR gate): Shown as lines converging to a point. The fails if all the connected components fail.

Example

Determine the pump set reliability where the centrifugal pump reliability is 0.85 and the electrical motor is 0.85.

Determine the reliability of the pumping operation where the operating mode is 2×100% (1 run and 1 stand-by)

Challenging of RBD

  • Simplification Assumption: RBDs often rely on simplifying assumptions, such as assuming components are statistically independent or that failure rates remain constant over time.
  • Limited to Static Systems: RBDs are most suitable for static systems where the reliability of components remains constant over time. They may not be as effective for dynamic systems with changing conditions or where the reliability of components is influenced by external factors. That means RBD is not able to capture the time-dependent behavior of components, such as wear and aging.
  • Complexity for Large System: As the size of the system increases, creating and analyzing an RBD can become complex.
  • Limited in Representing Common-Cause Failures: RBDs may not be the most suitable tool for representing and analyzing common-cause failures, where multiple components fail simultaneously due to a shared cause.

Reliability Data

Standard failure data is available from such as:

  • OREDA – Offshore Reliability Data
  • RAC – Reliability Analysis Centre
  • Process Equipment Reliability Data AICH

Reliability Formula

The below table expresses the reliability formula.

Code & Standard

IEC-61078: Analysis Techniques for dependability – Reliablity Block Diagram and boolean methods.