The importance of diagnostics in the SIS
The diagnostics in an Safety Instrumented System (SIS) are of crucial importance because they are the key to reduce the rates of undetected dangerous failures and, therefore, to reduce the probability of failure on demand (PFD / PFH) and increase the SIL.
We can see it in the following 2 equations (used in SILcet), for the 1oo1 and 1oo2 architectures, which are used to calculate the Probability of Failure and the SIL achieved.
The objective of the diagnostics is to detect any internal failure in a component. What they do is to monitor the correct operation of the devices that intervene in a SIF (safety instrumented function).
We can classify the diagnostics in 2 types:
Product diagnostics or self-diagnostics. They are those that come integrated from factory with the product (sensors, PLC, final elements). In the safety PLCs the self-diagnostics are very high, in certified 4-20 mA transmitters they are high. It is in the final elements of the SIF that, depending on the type, we can find certified products without product diagnostics (as in shut off valves) because they are usually purely mechanical elements.
Application diagnostics. They are additional diagnostics of each specific application. They are not always necessary because it depends on many factors, but for SIL-2 and 3 levels they can be essential to meet the required SIL. To implement them we will need to add some software routines in the PLC and, sometimes, also external wiring to the PLC and some additional hardware components (limit switch, valve positioner, line monitoring resistor, transmitter, DO feedback to DI, etc.)
Some practical examples of this type of diagnostic:
- 4-20 mA transmitter signal diagnostic (out of range, frozen signal, etc.)
- Diagnostics by comparison (IEC-61508 gives them a lot of credit).
- Use of the Hart protocol to diagnose problems in the transmitters or in their wiring from the PLC to the instrument (due to earth leakage, etc.)
- Diagnostic to detect the failure of the digital outputs in a PLC as it is not a standard feature in all PLCs.
- The partial stroke test (PST) in a safety valve.
- Diagnostic of valve failures by using transmitters. It is an interesting application that can be used only in certain designs.
- Other cases such as detection of cable break, etc.
These are some examples that we explained in the course SIFs Design and calculation of SIL.