Covert failure

For example, failures which do not initially influence the course of the process might not be corrected without delay. If this is true then it cannot be ruled out that a second failure can occur before the first failure has been corrected. The same applies to covert failures. These types of failures should be treated as dependent or common cause failures. 

Some covert failures are not anticipated because they are unusual. For example, one chemical plant used a series of large, liquid-filled reactors. A noble metal compound catalyst, which was in solution, was fed continuously into the reactors. Over a period of years some of noble metal came out of solution, and slowly formed a metal lining that was steadily growing in thickness on the inside walls of the reactors. The process and economic consequences of this problem had been analyzed and accepted. However, no one had considered the civil and structural engineering implications of this uncontrolled change the fact that the reactors were slowly getting heavier, and possibly overloading their foundations. 

High risk of covert failures (undetected dangerous failure modes) through lack of diagnostics... 

Here s a simple example of the covert failure problem. The output stage of the PLC operates a fail-safe solenoid or motor trip relay. It may have to stay energized for weeks but we won t know if it is shorted until it has to trip the function. This is an unrevealed fail to danger condition or covert fault . The broken wire fault is an overt fault or revealed fault, which will fail to a safe (off) state but creates a nuisance trip . 

A covert failure is a dangerous failure imtil it is detected and rectified. Hence the PFDavg calculation is based on this mode. Typical of covert failures is the stuck relay contact or the PLC output hoard with a frozen status. These faults will not be self-revealing if the safety system is just dormant with a static logic condition for weeks at a time. 

It s helpful to be aware at this stage that fast scanning auto-diagnostics can effectively remove the first category of faults firom the covert failures list and place them into the overt failures list. This will apply if the SIS is always tripped out (and the plant) when a fault is detected. This would be the case in a single channel system. The alternative approach is to allow the plant to stay in operation without SIS protection until the repair has been completed. In this case the only contribution to failure probability is the portion of time spent repairing the equipment whilst the plant is still running. 

The influence of manual proof testing on covert failures... 

Supplier to provide MTTF data, failure mode listing, covert failure rates. Method of data/source to be stated. 

A failure that impairs the system safety, but remains undetected (see also under covert failure ). It is related to the risk (PFD) involved in various types of processes. These types of failures can accumulate in a safety system, causing a degradation of the safety performance (SIL), as a function of time. 

Written proof-test procedures are developed for every SIF to reveal covert dangerous failures. These procedures describe... 

The concept of device diagnostics to diagnose overt and covert component failures has undergone sea changes in the present days. Apart from monitoring own... 

---