VEGA VEGAMET 391 With SIL qualification - Manual (Page 36)

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9 Application examples
VEGAMET 391 • With SIL qualification
38704-EN-130228
9 Application examples
9.1 Overfill protection according to SIL
The described setup of sensor and VEGAMET is specified for overfill
protection according to SIL2. Filling and emptying are realized via a
separate control (e.g. PLC).
The level is detected via a sensor and transfered by means of the
4 … 20 mA signal to the signal conditioning instrument. To prevent an
overfilling, the SIL relay integrated in the signal conditioning instru-
ment switches off the filling pump when the adjustable switching
threshold is exceeded.
Due to the geometrical form of the spherical tank, the vessel volume
does not increase linearly with the level. This can be compensated for
by selecting the linearization curve integrated in the sensor. It speci-
fies the relationship between percentage level and vessel volume. If
the level should be displayed on the sensor in litres, a scaling must
also be carried out. For this purpose, the linearized percentage value
is converted into a volume, e.g. in the unit "litre." If the signal condi-
tioning instrument should also display the level in litres, the scaling
must also be carried out in the signal conditioning instrument.
Note:
The settings for the indication of the signal conditioning instrument
(adjustment, linearization and scaling) do not influence the safety
function of the SIL relay.
For overfill protection, the relay mode "Overfill protection" is set for SIL
relay 3. The relay is thus switched off when the max. level is exceeded
(switching point High) (safe currentless status), and switched on
again when the min. level (switching point Low) is underrun.
0% = 4 mA
90% = 18,4 mA
100% = 20 mA
60% = 13,6 mA
1
2
3
Fig. 8: Overfill protection according to SIL
1 SIL relay 3
2 Upper switching point Hi (OFF) of SIL relay 3
3 Lower switching point Lo (ON) of SIL relay 3
A cylindrical tank has a capacity of 10000 litres. The measurement is
carried out with a level sensor operating according to the principle of
Functional principle
Example