BERTHOLD TECHNOLOGIES Uni-Probe LB 490, Руководство пользователя

Страница: 81 / 730

Uni-Probe LB 490
BERTHOLD TECHNOLOGIES GmbH & Co. KG
1 – 81
Volume 1 9 Functional Safety
1
©
exida.com
GmbH
berthold 0408-10 r003 v1r3.doc, Apr. 12, 2007
Rainer Faller
Page 17 of 18
Appendix 1: Possibilities to reveal dangerous undetected faults during
proof test
According to section 7.4.3.2.2 f) of IEC 61508-2 proof tests shall be undertaken to reveal dangerous faults which are undetected by diagnostic tests. This
means
that
it
is necessary to specify how dangerous undetected faults which have been
noted during the FMEDA can be detected during proof testing. Table 3 shows an sensitivity analysis of the most critical dangerous undetected
faults
and
indicates how these faults can be detected during proof testing.
Table 3: Sensitiv
ity
Analy
sis of dangerous
undetected faults of the ev
aluation unit
Component
%
of total
O
du
Detection through
IC1 – CPU
21%
Functional test by closing the radiation source
IC31 – EEPROM
13%
Functional test by closing the radiation source
IC2 – OpAmp
11%
Functional test by closing the radiation source
Q1 7%
Calibration
IC32
3%
Functional test by comparing the measured pulse rate with the expected “empty” pulse rate.
IC39 – Pulse shaping
3%
Empty tank calibration
IC43
3%
Empty tank calibration
Table 4: Sensitiv
ity
Analy
sis of dangerous
undetected faults of the 4..20mA process output
Component
%
of total
O
du
Detection through
IC19 – Opto-coupler
35%
Functional test by comparing the measured pulse rate with the expected “empty” pulse rate.
IC32 – OpAmp
23%
Functional test, see above
IC14 – DAC
17%
Functional test, see above
IC23 – DAC driver
15%
Functional test, see above
R112 – Measurement
5%
Functional test, see above
The proof tests referenced in table 3 and 4 are described in detailed in the Safety Manual.
©
exida.com
GmbH
berthold 0408-10 r003 v1r3.doc, Apr. 12, 2007
Rainer Faller
Page 18 of 18
Appendix 2: Impact of lifetime of critical components on the failure rate
Although a constant failure rate is assumed by the probabilistic estimation method (see
section
4.2.3) this only applies provided that the useful lifetime of components is
not
exceeded.
Beyond
their
useful
lifetime
(i.e.
as
the
probability of failure significantly increases with time) the results
of the probabilistic calculation method is therefore meaningless. 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 is based on the bathtub curve, which shows the typical behavior 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. 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 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.