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with non safety-related actuators
9.
Safety-related figures
9.1.
Determination of the figures
9.2.
Specific figures for fail safe unit in SIL version with actuators of SQ .2 series
The calculation of the safety-related parameters is based on the indicated safety
functions. Hardware assessments are based on Failure Modes, Effects and
Diagnostic Analysis (FMEDA). FMEDA is a step to assess functional device
safety in compliance with IEC 61508. On the basis of FMEDA, the failure rates
and the fraction of safe failures of a device are determined.
The failure rates for mechanical parts are taken from the exida database for
mechanical components. The electronic failure rates as base failure rates are
taken from the SIEMENS Standard SN 29500.
In compliance with table 2 of IEC 61508-1, the average PFD value for systems
with low demand mode is:
-
SIL 2 safety functions: ≥ 10
-
SIL 3 safety functions: ≥ 10
Since actuators only represent a part of the overall safety function, the actuator
PFD value including the gearbox should not account for more than approx.
40 % of the permissible total value (PFD
in the following value:
-
PFD actuator + gearbox for SIL 2 applications: ≲ 4.0E-03
The fail safe unit is classified as type A component with a hardware fault toler-
ance of 0. The SFF for the type A subsystem should be between 60 % and <90
% according to table 2 of IEC 61508-2 for SIL 2 (subsystems with a hardware
fault tolerance of 0).
The safety-related parameters, in particular PFD, depend on the measures and
intervals to be defined by the operator (e.g. T
are not exclusively performed within FQM but have to be initiated and/or evaluated
by the safety PLC, the diagnostic test interval can be defined within certain limits by
the operator. For this, respect the following:
The test frequency should equal at least ten times the demand rate
The test interval for the ESD safety function must be at least 22 days
The defined test interval must be considered appropriately in the PFD calculation
The following key figure tables provide an example of safety-related figures for the
different versions based on the exemplary assumptions regarding Td and T
the relevant parameters with regard to the indications made, refer to the pertaining
order-related Declaration of Incorporation. If test intervals, MRT or other parameters
differ from the values specified here, this will have an impact on the PFD values.
They can neither be taken from this safety manual nor from the declaration of
incorporation, but must be recalculated.
When determining the PFD values, please note that the stipulated proof test cannot
fully restore the system. For this reason, the following data is used for calculation:
PTC = 95 % (proof test coverage rate [%])
(PTC for performing the proof test described in this manual)
T
= 1 year or as indicated (proof test interval [h])
proof
T
= 10 years (demand interval = lifetime [h])
mission
MRT = 72 hours (mean repair time [h])
Td
= 730 hours
ESD_PVST
Diagnostic test interval of actuator monitoring (when executing a PVST on
monthly basis [h])
MTTR
= 802 hours
PVST
The following formula can be used for the calculation of the PFD
3
2
to < 10
-4
-3
to < 10
) of a safety function. This results
avg
, MRT, ...). Since diagnostic functions
Proof
Safety-related figures
. For
proof
values:
avg
29
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