Definition - Siemens SITRANS F Additional Operating Instructions

SITRANS FX330
Homogeneous redundancy
The degree of a functional safe device is determined by the accomplished qualitative safety
integrity level and the quantitative hazard rate determined by the failure rate of the deployed
hardware.
If two or more identically constructed devices are used in parallel (device A is of the same type as
device A' like the "Dual Version"), the hazard rate for the combination (here: sensor subsystem)
can be reduced.
A precondition for this is a logic subsystem comparing the output of devices A and A': The
complete sensor subsystem fails if at least one of the two devices fails (availability is reduced to
50% of the value of a single device A!).
Figure 2-1: Homogeneous redundancy
1 Sensor subsystem
2 Measurement device A
3 Measurement device A'
4 Logic subsystem

2.4 Definition

The flowmeter provides a safety function according to the international standard IEC 61508 [N2].
It is defined as follows:
Output of the volume flow rate at the 4...20 mA current output with a maximum delay of Output of the volume flow rate at the 4...20 mA current output with a maximum delay of Output of the volume flow rate at the 4...20 mA current output with a maximum delay of Output of the volume flow rate at the 4...20 mA current output with a maximum delay of
30 seconds and a measurement accuracy better than 4%. 30 seconds and a measurement accuracy better than 4%.
30 seconds and a measurement accuracy better than 4%. 30 seconds and a measurement accuracy better than 4%.
Remarks
• The function is always and only executed in SIL mode.
• The measurement uncertainty of the flowmeter in safe operation equals to those in non-SIL
operation as described in [N1]. It consists of the uncertainty of measurement function and
current output.
• Concerning random errors in the device, the safety tolerance must be considered. The safety
tolerance is the tolerable error before setting the safe state of the device. A random fault can
cause an error of up to 2% of the present measurement value or output current before it is
signaled. For further information refer to
10/2017 - A5E40875009-AB EN
SPECIFICATION OF THE SAFETY FUNCTION
www.siemens.com/flow
Measuring accuracy
on page 28.
2
11