© exida.com GmbH berthold 0408-10 r003 v1r3.doc, Apr. 12, 2007
Rainer Faller Page 3 of 18
The failure rates are valid for the useful life of the instrument. The circuits of the Level
Transmitter LB490 Uni-Probe evaluation unit do not contain any components with limited useful
lifetime which are contributing to the dangerous undetected failure rate. For typical applications,
the photomultiplier tube has a useful lifetime of more than 7,5 years with 60Co radiation source
and more than 21 years with 137Cs radiation source. When plant conditions and experience
indicate a shorter useful lifetime than indicated in this appendix, the number based on plant
experience shall be used.
The PFD
AVG
was calculated for three different proof test intervals.
Table 2 Summary for the Level Transmitter LB490 Uni-Probe incl. photomultiplier – PFD
AVG
values
T[Proof] = 1 year T[Proof] = 2 years T[Proof] = 5 years
PFD
AVG
= 3,2E-04 PFD
AVG
= 6,4E-04 PFD
AVG
= 1,6E-03
The boxes marked in green mean that the calculated PFD
AVG
values are within the allowed
range for SIL 2 according to table 2 of IEC 61508-1 and table 3.1 of ANSI/ISA–84.01–1996 and
do fulfill the requirement to not claim more than 35% of this range, i.e. to be better than or equal
to 3,5E-03.
© exida.com GmbH berthold 0408-10 r003 v1r3.doc, Apr. 12, 2007
Rainer Faller Page 4 of 18
Table of Contents
Management summary....................................................................................................2
1 Purpose and Scope ...................................................................................................5
2 Project management.................................................................................................. 6
2.1 exida.com .....................................................................................................................6
2.2 Roles of the parties involved..........................................................................................6
2.3 Standards / Literature used............................................................................................6
2.4 Reference documents....................................................................................................7
2.4.1 Documentation provided by the customer............................................................7
2.4.2 Documentation generated or reviewed by exida.com ..........................................7
3 Description of the analyzed modules .........................................................................8
3.1 System description.........................................................................................................8
3.2 Measuring principle........................................................................................................9
4 Failure Modes, Effects, and Diagnostics Analysis ...................................................10
4.1 Description of the failure categories.............................................................................10
4.2 Methodology – FMEDA, Failure rates..........................................................................11
4.2.1 FMEDA...............................................................................................................11
4.2.2 Failure rates .......................................................................................................11
4.2.3 Assumption ........................................................................................................11
5 Results of the assessment.......................................................................................13
5.1 Level Transmitter LB490 Uni-Probe.............................................................................13
6 Terms and Definitions .............................................................................................. 15
7 Status of the document ............................................................................................ 16
7.1 Liability .........................................................................................................................16
7.2 Releases ......................................................................................................................16
7.3 Release Signatures......................................................................................................16
Appendix 1: Possibilities to reveal dangerous undetected faults during proof test ........17
Appendix 2: Impact of lifetime of critical components on the failure rate .......................18
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