There are several ways to improve instrumentation systems :
- Use of redundancy;
- Use of de-rating for components;
- Use of self-checking (or periodic testing);
- Inclusion of field instruments in the analysis;
- Use of full automation and fool-proofing techniques;
- Use of preventive maintenance;
- Careful design of unit and system configuration.
Implementation of these principles will increase the cost of the system substantially, depending on how far each method is applied.
The use of redundancy in all systems is usually incorporated in the design.
The 1oo2 (one out of two) redundancy is suitable for systems in which safety is more critical than loss of production, whereas 2oo2 (two out of two) is the reverse.
A compromise is 2oo3 (two out of three), where both safety and production are improved.
To minimize operational and maintenance errors, full automation, fool-proofing techniques, and preventive/planned maintenance should be employed. De-rating and a well-designed system (units, configuration) will substantially reduce the effect of environmental stresses.
Of course, a well designed environment (control rooms) is always necessary. It is important to include field instruments (transmitters, valves) in the analysis because it is common knowledge that between 60 and 80 per cent of shutdowns is caused by the failure of these items.
The main advantage of 2oo3 systems is the inclusive superior availability. Availability depends on MTBF (mean time between failures) and mean time to repair (MTTR).
In a 1oo1 or 1oo2 configuration, a single failure causes a shutdown and requires immediate maintenance attendance.
In a well-designed 2oo3 safety system, however, a single failure can be tolerated for several days to several months, depending on the system´s robustness.