Control redundancy is used to improve the availability of the plant's process. Its implementation varies with each application
and the criticality of the process to the plant's revenue.
The premise of redundancy is that all control equipment has a mean-time-between-failure (MTBF) that can be compensated
for with redundancy, so that the mean-time-between- forced-outage (MTBFO) of the entire system is better than the MTBF of
the individual components. Improvement in MTBFO depends on how the redundancy is applied and whether the inevitable
failures can be detected and repaired online without interrupting the process. Field components (for example, sensors,
actuators, and wiring) cause over half of forced outages. Therefore, redundancy of field components is an important
consideration in the overall control system.
Most discussions of redundancy focus on its contributions to starting and running reliability. However, tripping reliability is
another important safety aspect of all control systems, and there is usually some compromise between the two objectives. For
example, two hydraulic trip solenoids provide better tripping reliability than one (that is, either solenoid trips), but less
Basic reliability terminology:
Availability = [MTBFO / (MTBFO + MTTR)] x 100%