Functional Safety Manual
for the Memosens transmitter Liquiline M CM42 SIL
Version:
Page:
2.0
67
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The measuring chain is connected to a PCS (e.g. a PLC), which is itself connected to
some kind of actor to activate the safe state.
You can calculate the PFD value of the complete chain (PFD
avg
mc; mc means
measuring chain) by summing up the individual PFD values of all components in the
chain, including the communication protocol (here the Memosens protocol):
PFD
avg
mc = PFD
avg
sensor
+ PFD
avg
cable
+ PFD
avg
transmitter
+ PFD
avg
Memosens protocol
Then for a complete safety instrumented system (SIS) you get:
PFD
avg
sis = PFD
avg
mc
+ PFD
avg
PCS
+ PFD
avg
actor
As an example, the value of the complete (non-redundant) Hauser pH
measuring chain, described at the beginning of this section, we get
(The Memosens protocol has been taken into account with 1% of the PFD SIL-2 value =
1.0 E-4):
PFD
avg
mc
= 8.3 E-4 + 0.9 E-4 + 10.3 E-4 + 1.0 E-4 =
20.5 E-4
(Proof test intervals are chosen to be 1 year for all devices)
According to IEC 61508 you need a maximum PFD
avg
of 1E-2 to realize a SIL-2 SIS. So
the just calculated value accords to about 21% of the SIL-2 PFD
avg
value. That means
the PCS and actors can use the remaining 79% of the SIL-2 PFD
avg
value.
Of course, you also have to calculate and use the SFF given in the IEC 61508 to fulfil all
requirements of the standard.
