KROHNE H250 M9 Safet V1 EN - Manual (Page 8)

6
LIFE TIME / PROOF TESTS
8
H250
www.krohne.com 02/2012 - 4000656701 MA H250-M9 SIL R02
Life time / Proof tests
6.1 Life time
Although a constant failure rate is assumed by the probabilistic estimation method this only
applies provided that the useful lifetime of components is not exceeded.
Beyond their useful lifetime, the result of the probabilistic calculation method is meaningless, as
the probability of failure significantly increases with time.
The useful lifetime is highly dependent on the component itself and its operating conditions –
temperature in particular (for example, electrolyte capacitors can be very sensitive).
This assumption of a constant failure rate is based on the bathtub curve, which shows the typical
behaviour for electronic components. Therefore it is obvious that the PFD
AVG
calculation is only
valid for components which have this constant domain and that the validity of the calculation is
limited to the useful lifetime of each component.
It is assumed that early failures are detected to a huge percentage during the installation period
and therefore the assumption of a constant failure rate during the useful lifetime is valid.
According to section 7.4.7.4 of IEC 61508-2, a useful lifetime, based on experience, should be
assumed.
According to section 7.4.7.4 note 3 of IEC 61508-2 experience has shown that the useful lifetime
often lies within a range of 8 to 12 years.
KROHNE recommends an operational life time for variable area flowmeters no longer than 10
years in SIL rated applications. However, if the user is monitoring the instruments over their life
time demonstrating the required results (e.g. constant failure rate), this can allow safety
capability exceeding this period on the user’ s own responsibility.
The required cyclic proof test interval can be found in the table chapter 7.2.
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