VEGA VEGATROL 20 Ex - Manual (Page 4)

VEGATROL 20 Ex 4
2 Technical Information
2.1 Product description
General
VEGATROL 20 for liquid gas consists of:
- VEGATROL 20 system sensor
- signal conditioning instrument VEGATOR 526 W Ex.
System sensor VEGATROL 20 includes in the concentric
tube the vibrating probe VEGASWING as well as
additionally a capacitive measuring electrode either as rod
or cable electrode.
The oscillators for VEGASWING as well as the measuring
electrode are also integrated in the housing of the system
sensor.
The vibrating level switch in conjunction with VEGATOR
526 W Ex is used as overfill protection.
The capacitive measuring electrode can be used as
follows:
- in conjunction with a signal conditioning instrument
VEGAMET for continuous level measurement
- in conjunction with a signal conditioning instrument
VEGATOR for an additional switch point (e.g.
operational switch-off acc. to VdTÜV).
These signal conditioning instruments are not part of
VEGATROL 20 and have to be ordered separately.
The application range comprises stationary vessels acc. to
the pressure vessel regulations for storing liquid gas acc.
to DIN 51 622:
- in individual vessel plants
- in liquid gas plants
- in distribution plants
- in plants with several vessels
each of group A, B, C and D.
When use in distribution plants up from group C min. two
separate overfill protections must be installed acc. to the
requirement of TRB 801, no. 25. Even in these cases one
VEGATROL 20 is sufficient due to the VdTÜV-approval to
meet with the TRB-requirement.
In plants with several vessels VEGATROL 20 can be used
as continuous monitoring and locking systems.
Function
The level is detected by a vibrating level switch
VEGASWING. Its tuning fork is piezoelectrically energized
and vibrates at its mechanical resonance frequency. A
second piezoelectric element measures this resonance
frequency. If the tuning fork is covered by the liquid gas,
the resonance frequency changes proportional to the
degree of covering. The oscillator in VEGASWING detects
continuously the resonance frequency and converts the
value into a current to the signal conditioning instrument.
When the switch point is reached an erratic increase of the
current signal is triggered. The level relay in the signal
conditioning instrument de-energizes.
The continuous level measurement as well as the additio-
nal level detection are realized via the integral capacitive
measuring electrode. This electrode in conjunction with
the concentric tube or the metallic container wall forms a
capacitor. The liquid gas acts as dielectricum and changes
therefore the capacitance of this capacitor. The respective
oscillator converts the capacitance into a level proportional
direct current. The measuring range includes the total
length of the rod or cable electrode, measured from the
end of the VEGASWING.
The detected level can be used to realize an additional
switch point, e.g. an operational switch off point.
Fail-safe / Self-monitoring
A special feature of the overfill protection of VEGATROL
20 is the integral self-monitoring. It is based on a redun-
dant microprocessor system in the signal conditioning
instrument and includes the whole measuring chain from
the tuning fork via the oscillator, the two-wire line as well
as the signal conditioning instrument.
The self-monitoring includes two routines:
-
the permanent monitoring
- the cyclical self-test.
During the permanent monitoring the signal conditioning
instrument causes the oscillator of the sensor to trigger a
test cycle in 960 msec. cycle. Defined test signals are
transmitted via the two-wire line to the signal conditioning
instrument. Possible failures in the sensor electronics as
well as line break, short-circuit and mechanical damage on
the tuning fork are signalled via these test signals.
During the cyclical selftest security relevant components
and procedures (e.g. level relay, CPU, memory, time
reaction) are checked in the signal conditioning instrument
in a very tough time pattern. The function test of the level
relays is made without interruption of the circuit of
connected systems.
If during the self-monitoring a failure is detected, the level
relays and the fail safe relay de-energize.
Due to the above mentioned measures a reliable and
correct function of the overfill protection is ensured without
any staff involved for control tasks.
2 Technical Information