Functional Safety; Safety Function - Siemens Sitrans P500 Operating Instructions Manual

Functional safety

7.1

Safety function

The safety function of SITRANS P refers to the measurement of pressures. (Ordering option
-Z C20) Add an additional safety accuracy of 1.1% of the maximum measuring range to the
application-specific measurement error.
Take the following conditions into consideration for safety accuracy in flow measurements:
● Turn-Down: ≤10:1
● Root extraction characteristic with a root transition point of up to 15% (this corresponds to
2.25% of the measured differential pressure)
If taken into consideration, the following recommendations result for the safety accuracy in
flow applications:
Flow 50 ... 100%
Flow 25 ... 50%
Flow 0 ... 25%
Total tolerance (safety function) = ± [application-specific measurement error + safety
accuracy].
Pressure transmitter safety accuracy: the maximum effect of a random non-critical individual
error on the measured value.
The diagnostics function will respond within 2 seconds in the worst-case scenario.
The safety accuracy together with the application-specific measurement error allows the
system operator to include a backup for process monitoring. Even if a random individual
error occurs that is within the safety accuracy, the system can still be safely shut down.
As there is also always a possibility of dangerous faults occurring, these are categorized and
listed in the manufacturer declaration for the device (SIL declaration of conformity, functional
safety pursuant to IEC 61508).
Example:
A silo is to be securely monitored to check that the level does not exceed 10 meters.
Application-specific measurement error: 0,1 %
Safety accuracy: 1,1 %
Total tolerance: 1,2 %
SITRANS P500 with HART
Operating Instructions, 12/2014, A5E02344528-09
Safety accuracy = 1.1%
Safety accuracy = 2.2%
Safety accuracy = 8.0%
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