6.3

Report - Lifetime of Critical Components

According to Section 7.4 of IEC 61508-2, a useful lifetime,

based on experience, should be assumed.

Although a constant failure rate is assumed by the proba-

bilistic estimation method, this only applies provided that

the useful lifetime of components is not exceeded. Beyond

their useful lifetime, the result of the probabilistic calcula-

tion method is therefore meaningless, as the probability of

failure significantly increases with time. The useful lifetime

is highly dependent on the subsystem itself and its operating

conditions.

This assumption of a constant failure rate is based on the

bathtub curve. Therefore it is obvious that the PFDavg

calculation is only valid for components that have this

constant domain and that the validity of the calculation is

limited to the useful lifetime of each component.

As there are no electrolytic capacitors used, there are no

electrical components that limit the useful lifetime of

the system.

Based on general field failure data, a useful life period of

approximately 50 years is expected for the Echotel Model

961/962 Ultrasonic Level Switches.

When plant experience indicates a shorter useful lifetime

than indicated, the number based on plant experience

should be used.

15

51-650 Echotel SIL Safety Manual