African Petrochemicals Jan/Feb Edition 15_1 {2018}

6
Continued from page 5
When the echo curves of a 26-GHz
sensor and a VEGAPULS 64 are
compared in the same application,
even experts are impressed.
And with smaller, horizontal LPG
tanks, there is a further advantage:
bypass tubes are often used in
these round tanks. VEGAPULS 64
can be installed directly on the tank
without a bypass tube. This saves
additional costs.
VEGASWING 66 is a universal vibrating level
switch for all liquids and appropriate for
extreme process temperatures and pressures.
Better utilization of tank volume
80 GHz has a positive effect,
especially when measuring liquids
with low dielectric values at the
bottom of the vessel. Normally, radar signals are reflected at the
surface of the medium. In the case of media with small dielectric
values, however, some of the signals penetrate the medium and are
then reflected by the underlying container bottom. As a consequence,
two signals are received: one from the liquid surface and one from
the tank bottom. The lower the dielectric value of the medium and
the lower the level, the larger the signals from the tank bottom. With
the previous 26-GHz sensors, these two signals overlapped and mixed
together, especially at low levels (only a few cm of filling height). The
smaller level signal was practically swallowed by the larger bottom
signal. For the radar sensor, the level signal was no longer visible. Due
to the significantly shorter wavelength of the 80-GHz signals from
VEGAPULS 64, the signals that pass through the medium are more
strongly attenuated by the medium than those emitted by 26-GHz
sensors. As a result, the reflection from the bottom of the vessel
is significantly weaker. Even at low levels, the level signal is much
stronger than the bottom signal. The result is that even the lowest
filling levels (a few mm) can still be measured reliably and accurately.
Processes can thus be further optimized and liquids reliably measured
even at the bottom of large-volume containers. In other words, the
tank volume can be utilized even better.
Different antenna systems in different sizes are available for VEGAPULS
64. At the moment, the thread sizes on offer are ¾” (beam angle 14°), 1½”
(7°), DN50 (6°) and DN80 (3°). Until now, VEGAPULS 64 was only approved
for temperatures down to -40° C, but from the middle of 2017 onwards it
will be possible to use the sensor in applications down to -196° C.
Intelligent solutions for processes at sea
If the storage tanks are on a ship, one of the first questions a user
asks is where can the sensor be installed, in view of the many internal
installations in the tanks. In a typical tank there are pumps that require a
pipe for the hydraulic feed, another pipe for the return feed and a pipe
for the conveyed product. Often there are two pumps, one for the large
quantities, one for the residual liquid. All internal fixtures and equipment
(the pumps alone have six pipes) generate interfering signals. Trying
to get a good, clean measuring signal here is a difficult undertaking.
Nevertheless, radar level transmitters are the right choice because they
are robust and measure contactlessly. Measurement inside a standpipe is
usually preferred for this application. In times past, six-meter long pipes
(maximum available length) were welded together to make a standpipe;
however, the welding seams disturbed the measuring signal again and
again. VEGA was able to drastically reduce these interfering reflections
with its special patented weld for such pipes.
Difficult ambient conditions
Level measurement is necessary not only in the storage and cargo
tanks, but in the liquefaction and regasification plants as well. Some
liquefaction plants are located at sea and VEGA sensors are deployed
there as well. These plants are relatively complex and extend over
multiple process stages. In each stage, the level is an important
variable playing a major role in protecting the compressors. Measuring
instruments control the level and have to function reliably at high
pressures (16-18 bar) and very low temperatures (-163° C) in heavy seas.
Since the tanks used here are usually smaller and have extremely fast
filling or emptying operations, the surface of the product is often quite
turbulent. Differential pressure measurement, for example, could be
The radar sensor for liquids VEGAPULS 64 (flange
version) is appropriate for badly reflecting media,
such as low-dielectric liquids.
used here, but because of the low temperatures it is extremely difficult
in practice: the pressure detecting elements, the diaphragm seals, get
covered with layers of ice. Vegaflex 86 is being used more and more for
this particular application, the continuous measurement of levels in the
various process stages of the liquefaction plants. This robust guided
radar sensor is ideal for the task because it measures without being
affected by media properties such as density or dielectric constant. Its
rugged mechanical construction and second process seal, the so-called
“second line of defense”, protect the sensor. It measures down to -196°
C and is not bothered at all by turbulent surfaces. However, installing
the sensor on board a ship requires a lot of experience. Whereas in
applications on land the sensor can simply be mounted in the intended
tank opening or bypass and measurement started more or less
immediately, on ships the sensor has to be installed in a special way, in
order to compensate for heeling movement in heavy seas.
Reliable limit level detection
LNG processes, whether on land or on ships, require not only continuous
level measurement but also point level detection at many points.
VEGASWING 66 is often chosen for this task. In general, vibrating
level switches are very popular among users in the chemical and
petrochemical industry as well as in refineries because they are extremely
easy to operate and ensure safe, reliable measurement. In contrast to
float systems, VEGASWING 66 offers a much higher level of security,
as the sensor constantly performs self-diagnosis and monitors the
sensing element. That’s why VEGASWING 66 is preferred over flotation
devices in safety-critical applications. However, until two years ago it
wasn’t possible to use these devices at very low temperatures. For these
applications VEGA developed a patented inductive drive that excites
the tuning fork effortlessly even under extreme temperature conditions.
The user benefits greatly from the simple handling of this vibrating level
switch and its extended application range down to -196° C (normal
temperature in the LNG environment) and pressure range of -1 to +160
bar. Because the sensor measures so reliably irrespective of density, no
matter what the product, it is perfect for high and high-high alarms in
various stations on the ship as well as on land. It is used, for example, in
the pump pit of a liquefaction terminal. To keep the pump from running
dry, it must always be covered with liquid (LNG). VEGASWING 66 takes
over level control here, ensuring reliable pump operation.
Conclusion: Whether on land or at sea – there is hardly any process in
the LNG/LPG industry that does not require pressure measurement,
continuous level measurement or point level detection. Thanks to the
plics® concept, VEGA sensors have the advantage that the user interface
is the same for every measuring instrument, regardless of the measuring
method. Even users with little experience in instrumentation can quickly
find their way around. The sensors have become even easier to operate
through the combination of Plicscom and Bluetooth. Their wireless
Bluetooth communication is particularly useful for hard-to-reach areas,
harsh industrial environments and Ex hazardous areas. The modular
plics® concept, which VEGA introduced more than 14 years ago, really
shows its strengths here. Plicscom is backward compatible and can be
used for the entire installed base of plics® sensors – in pressure and level
applications, with 70 different types of instruments and the serviceproven
adjustment structure, with no software update required. This
allows the user to configure and parameterize plics® sensors conveniently
from a safe distance with a smartphone or tablet.