How To Evaluate The Reliability Of A System Or Process
60% of failures and safety issues can be prevented by ensuring there is a robust equipment design and that Maintenance & Reliability is taken into account during the design phase. Equipment should be designed with the following in mind;
- Designed for Fault Tolerance
- Designed to Fail Safely
- Designed with early warning of the failure to the user
- Designed with a built-in diagnostic system to identify fault location
- Designed to eliminate all or critical failure modes cost effectively, if possible.
To ensure that the equipment design is capable or yielding our requirements an analysis needs to be performed on the design to ensure the system or process can deliver. There is much different analysis available to perform the analysis, but a relatively simple and widely accepted approach is the Reliability Block Diagram.
Reliability Block Diagram
The Reliability Block Diagram (RBD) is used to identify potential areas of poor reliability and where improvements can be made to lower the failure rates for the equipment. This method can be used in both the design and operational phase to identify poor reliability and provide targeted improvements.
The RBD shows the logical connections of components within a piece of equipment. It is not necessarily the schematic diagram of the equipment, but the functional components of the system. The equipment is made up of multiple components/systems in series, parallel and a combination of the two. These components/systems and configuration of them provides us with the inherent reliability of the equipment. The RBD analysis consists of reducing the system to simple series and parallel blocks which can be analyzed using the appropriate Reliability formula. Reliability typically utilizes three main formulas;
Reliability(Mission Time) = e-λt
λ (lambda) = Failure rate = 1/MTBF
t = mission time in cycles, hours, miles, etc.
e = natural logarithm = 2.71828
- Reliability(System) = R1 x R2 x R3 x R4 x ….RN
- Reliability(Active Redundant Parallel System) = 1 – (1 – R1)(1 – R2)
Now that the Reliability formulas are understood, the RBD can be built. A simple series RBD is shown as;