Level measurement under extreme conditions
On liquid gas tanks on land, it is mandatory for any radar sensor to be separated from the process during operation
Ultimately, level measurement in LNG/LPG processes, just like in other applications, is always about answering the simple question, How much product is in the tank? However, obtaining an exact value is not easy. The products are often under pressure, cryogenic and highly explosive. Sensors from Vega have been operating successfully in all stages of the LNG/LPG process chain for many years.
Clemens Hengstler, Product Manager Radar at Vega Grieshaber KG
Throughout the individual production stages, from liquefaction, storage, shipping, vaporiser and pipeline, to further processing in the petrochemical or chemical industry or consumption as an energy carrier, there are numerous measuring points where safe, reliable level measurement is required. But the installation situation and the process conditions present significant obstacles to reliable measurement. And the product itself, with its unusual properties, does not make the task any easier, either.
Challenging product
Let’s first look at the product: LNG has a low density and an extremely low dielectric constant. These physical properties also change quite often depending on the composition of the product or the supplier or the place of origin of the LNG/LPG. As a result, many measuring principles cannot ensure reliable, precise measurement. Then there are also the very low temperatures, which make the selection of workable measuring principles even smaller. For example, mechanical measuring techniques often malfunction and are maintenance intensive. That’s why sensors based on radar technology are the most preferred measuring device. And this is true not only for LNG/LPG applications: in other industrial areas as well, radar level transmitters have largely replaced previous measuring principles, such as electromechanical sounding, differential pressure and ultrasonics. For example, Vega has already installed more than 500,000 radar sensors worldwide, many of them in LNG/LPG applications.
Tricky internal fixtures
Since last year, the new VEGAPULS 64 offers a radar level measurement solution that works even better, particularly in difficult installation situations. The crucial difference from previous radar level instruments is its frequency: 80 GHz instead of the usual 26 GHz. This allows a focusing of the radar beam that is more than three times better, which in turn has a whole series of positive effects on the measurement. The most obvious advantage: the narrow measuring beam simply flies past internal tank components. Until now, standpipes were often installed in storage tanks/spherical tanks to facilitate measurement with radar. Yet they have several disadvantages – on the one hand, they reduce measurement accuracy, and on the other, they are extremely laborious and expensive to install. However, they were needed to guide the radar energy and help avoid disturbing reflections from the tank internals. Thanks to its very good focusing/signal bundling, the new 80-GHz radar sensor Vegapuls 64 can be mounted directly on a tank without a standpipe. This makes measurement more reliable and accurate, and the measuring point less expensive to plan and set up.
In addition, Vegapuls 64 is very suitable for poorly reflective media, i.e. liquids with low dielectric values, which are common in this industry. Vegapuls 64 has an exceptionally large dynamic range. The dynamic range of a radar sensor, i.e. the difference between the largest and the smallest signal, determines which applications the sensor can be used in. The greater the dynamics, the wider the application range of the sensors and the higher the measurement certainty. Larger signals automatically lead to better measurement. This means that media with poor reflective properties can now be measured much better than before.
Why this measuring instrument, although only a short time on the market, has already proven its prowess in the industry becomes clear when we take a closer look at the individual stations in a typical LNG/LPG process chain.
Problem: ball valve
Typical for industry is the strict requirement that every sensor must be separable
from the process by means of a fitting, without interrupting the process, i.e. without shutting down the production equipment, or part of it. This requirement is absolutely mandatory for liquid gas tanks on land. For a radar sensor, this means that it must be mounted on a ball valve. The purpose of this is to guarantee that the radar sensor can also be separated from the process during operation. The intention was to increase availability this way, but it often had the opposite effect. Production systems did indeed become more maintenance friendly, but also more maintenance intensive. Installation on a ball valve was never recommended for radar sensors, because the ball valve itself causes large interfering reflections in the close range. Due to reflection at sealing points and connection points in the ball valve and their multiplication within the throat of the ball valve, interfering signals found their way directly into the measuring path of the sensor. Reliable measurement was impossible, especially in liquids with low dielectric values. Particularly in the upper area of the tank, where the sensor was used to detect overfilling, the relatively small signals of the medium could not be optimally detected due to the strong noise in the close range. With Vegapuls 64, the influence of the ball valve is much smaller because of the considerably better signal focusing and, as a result, the ball valve generates almost no disturbing signals. Interfering signals in the close range are thus avoided and reliable measurement is ensured. Another advantage for the user: the new sensor can be installed on existing shut-off devices – this keeps rebuilding and retrofitting costs to a minimum. Critical applications, where the previous technology was problematic, can be easily upgraded to the new 80-GHz technology to increase measurement reliability, thereby increasing plant availability and reducing maintenance requirements. 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.
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 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 service-proven 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.
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