BERTHOLD TECHNOLOGIES Uni-Probe LB 490, Руководство пользователя

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25.5.09
9 Functional Safety Volume 1
©
exida.com
GmbH
berthold 0408-10 r003 v1r3.doc, Apr. 12, 2007
Rainer Faller
Page 11 of 18
4.2
Methodology
– FMEDA, Failure rates
4.2.1 FMEDA A Failure Modes and Effects Analysis (FMEA) is a systematic
way
to
identify
and
evaluate
the
effects
of different component failure modes, to determine what could eliminate or reduce the
chance of failure, and to document the system in consideration. An FMEDA (Failure Mode Effect and Diagnostic Analysis) is an FMEA extension.
It
combines
standard
FMEA
techniques
with
extension to identify online diagnostics techniques and the
failure modes relevant to safety instrumented system design. It is
a
technique
recommended
to
generate
failure rates for each important category (safe detected, safe undetected, dangerous
detected,
dangerous
undetected, fail high, fail low) in the safety models. The format for the
FMEDA is an extension of the standard FMEA format from MIL
STD
1629A,
Failure
Modes
and
Effects Analysis. 4.2.2 Failure
rates
The failure rate data used by
ex
ida
in this FMEDA are from the
Siemens
SN
29500
failure
rate
database. The rates were chosen in a way that
is
appropriate
for
safety
integrity
level
verification calculations. The rates were chosen to match operating
stress
conditions
typical
of
an industrial field environment similar to IEC 60654-1, class C. It is expected that
the
actual
number of field failures will be less than the number predicted by these failure rates. The
user
of these numbers is responsible for determining their applicability to any particular
environment. Accurate plant specific data may be used for this purpose.
If
a
user
has
data
collected
from
a good proof test reporting system that indicates higher failure rates, the higher
numbers
shall
be
used.
Some industrial plant sites have high levels of stress. Under those
conditions the failure rate data is adjusted to a higher value to account for
the
specific
conditions of the plant. 4.2.3 Assumption The
following
assumptions
have been made during the Failure Modes, Effects, and Diagnostic
Analysis (FMEDA) of the Level Transmitter LB490 Uni-Probe with 4..20mA
NAMUR
NE43
compliant current output.
x
Failure rates are constant, wear out mechanisms are not included.
x
Propagation of failures is not relevant.
x
The
HART™
communication
is not used for safety applications. The correct configuration
set up using HART™ shall be checked by functional testing.
x
Only the NAMUR NE43 compliant current output is used for safety application.
x
The
current
output
signal
is
fed
to
a
SIL compliant analog input of a safety PLC. The safety
PLC analog input and the application program are
configured
according
to
NAMUR
NE43
to
detect under-range and over-range failures.
x
The stress levels are average for an industrial environment and can be compared
to
the
Ground Fixed classification of MIL-HNBK-217F. Alternatively, the assumed environment is similar to:
o
IEC 60654-1, Class C (sheltered location) with temperature limits within
the
manufacturer’s
rating
and
an average temperature over a long period of time of 40ºC.
Humidity levels are assumed within manufacturer’s rating.
©
exida.com
GmbH
berthold 0408-10 r003 v1r3.doc, Apr. 12, 2007
Rainer Faller
Page 12 of 18
x
The
Level Transmitter LB490 Uni-Probe is operated in the low demand mode of operation or
is operated in high demand mode of operation with a demand rate of less than once per day.
x
Internal
power
supply
failure
rates are included in the FMEDA, but not external power supply
failures.