Automatic diagnostic interval

Mode Demand Interval versus Automatic Diagnostic Interval Demand Interval versus Manual Proof Test Interval Probability Measure... 

Tia is the term used for the automatic diagnostic interval used in PES self checking systems. 

Three time intervals must be known to define what credit may be taken for automatic diagnostic testing and manual proof testing. These three time intervals are the average demand interval, the manual proof test interval and the automatic diagnostic test interval (usually the worst-case time is considered). The three modes and their relationsldps are shown in Table 7.2. 

Problem Layer of protection analysis has indicated that a demand would occur every 5 years on average for a particular process hazard. Although most automatic diagnostics execute every minute, the worst-case time period for automatic diagnostics within the equipment is once per week. A proof test interval of one year is proposed for a manual test and inspection. Would this SIF be classified as low demand ... 

SOLUTION Automatic diagnostics are performed many times within the expected average demand interval. The proof test is done at least two times within the expected average demand period so the SIF would be classified as low demand. 

Automatic diagnostics continuously check the PLC system functions at short intervals within the fault tolerant time of the process. 

Formula set 2 is used to calculate the PFDavg for any system with automatic diagnostics (or a portion thereof). Note that the test interval, Tia, is very short and should be carried out within the fault tolerant time of the process (process safety time, hence the need to define this period in the SRS) if the test itself is not going to affect the availability of the system. Typically Tia would be in the range 1 to 10 seconds. 

When the demand frequency is more than twice the periodic proof-test frequency, the application should be considered a high-demand mode application. Therefore the equations and techniques that use test interval as a key variable are not valid. In effect, one cannot take credit for periodic inspection unless it is done very frequently. Credit may be taken for diagnostics that cause the device to fail to the safe state (i.e. automatic process shutdown on any detected dangerous failure) in the high-demand case, as long as the diagnostic time period plus the time necessary to safely return the process to a safe state is less than the available process safety time (the time period between initiation of a demand and the hazard). 

The safety parameters listed in the safety manual, the SIF designer SIL achieved using the PFDavg considering the architecture, proof test interval, proof test coverage, automatic internal diagnostic, repair time, and failure rates of the entire equipment are included in the SIF. The HFT must be checked and taken into consideration by the SIF designer to ensure that each subsystem within the SIF is in compliance with the minimum HFT requirements. 

---