In the LNG terminal in the port of Zeebrugge, the temperature is freezing cold all year round. And it’s no wonder, since liquefied LNG reaches temperatures between -161 and -164° C, creating an environment that pushes not only measuring instruments, but many other system components as well, to their limits. Nevertheless, failure is not an option for the on-site level switch that monitors a special pump and protects it against dry run.
Sometimes one just happens to be in the right place at the right time. That was the case a few years ago, when Fluxys Belgium urgently needed a replacement for a worn-out level switch. At about the same time – a good 600 kilometres to the south – VEGA’s development team was looking for a pilot user. The Black Forest instrument maker in Schiltach wanted to know if its newly developed VEGASWING 66 level switch would prove successful in LNG applications.
The development of VEGASWING 66 was progressing very well at that time. Until then, vibrating level switches, which are typically used as point level switches for liquid media, could not be used at extremely low temperatures because the piezo crystals driving the vibrating elements no longer function at these temperatures. At the same time, however, this type of measuring instrument was very common and popular, above all because of its simplicity, for example with regard to commissioning and signal evaluation, as well as reliability. The sensor also monitors itself. For that reason
VEGA wanted to further develop this proven, simple measuring device for extremely demanding environments. It was a new patented inductive drive that brought about the breakthrough. The device easily manages to excite the tuning fork even under extreme temperature conditions. This means the user can still take advantage of the simple installation and handling of vibrating level switches, yet have an extended application range of -196° to +450° C and -1 to +160 bar.
VEGASWING 66 is equipped with a system that monitors the vibrating element and the electronics. What is more, the sensor is SIL2 qualified and a function test can be carried out by simply pressing a key. The instrument measures independently of the medium. Level, density, dielectric constant and foam do not affect the quality of the measurement.
Despite its market success at that time, it was not clear whether VEGASWING 66 could also cope with the conditions in an LNG terminal. For a successful application, one must know that LNG has an extremely low density and a very low dielectric constant. These physical properties even change often depending on the composition of the medium, the supplier and the place of origin of the LNG/LPG. Due to these factors alone, it’s not easy for many measuring techniques to guarantee a reliable and precise measurement. But here there are also the very low temperatures, which make the available selection of workable measuring principles even smaller.
European hub for LNG Fluxys Belgium is an independent operator of natural gas transmission networks and storage infrastructure in Belgium. The company is a pioneer in the area of LNG and has meanwhile expanded its network into a hub for international gas distribution in north-western Europe. The centre of its operations is the LNG terminal in the port of Zeebrugge. Liquefied LNG is unloaded there, typically being delivered by ship from the Gulf region. Natural gas is liquefied by cooling it to a temperature of -162° C at normal atmospheric pressure. This process makes the volume of natural gas 600 times smaller than its gaseous state and allows the transport of large amounts of energy by LNG ship over long distances.
In terminal facilities, LNG is temporarily stored as a buffer in storage tanks and fed into the gas network for transmission to other locations or reloading onto LNG ships. Since the plant was commissioned in 1987, more than 1,600 LNG tankers have docked and unloaded at the terminal. The LNG is pumped out, temporarily stored in the terminal if necessary, or immediately converted into gas and fed into the Fluxys Belgium gas network. From there it is forwarded to neighbouring countries as needed. The process can also function in reverse: natural gas can be liquefied at the Zeebrugge terminal and loaded onto LNG ships for export to the UK or Scandinavia.
In this process, some of the liquid LNG gets gasified. This gaseous part is liquefied by increasing the pressure in the so-called “reliquefier”. The gas is fed in from above. Through the decrease in temperature, it liquefies and collects in liquid form at the bottom of the reliquefier. And this is exactly where VEGASWING 66 is deployed.
Below the reliquefier there is a special pump capable of pumping cryogenic liquids.
This pump cannot be allowed to run dry. In other words: It must be reliably determined whether or not there is still liquid LNG in the tank, so that the pump can be stopped in time. A pump of this kind costs more than €10,000. What is more, the delivery time for a replacement is several months.
The measuring instrument used here thus carries a huge responsibility and, being of high quality, is accordingly in service for a long period of time. The ultrasonic sensor used at the LNG terminal was already old and had been discontinued. For that reason, the operator was desperately looking for an alternative and happened to come across the vibrating level switch VEGASWING 66. Although VEGA was convinced that the measurement would work in this constellation, it had never been tried before. The first on-site trials already revealed the biggest challenge: LNG has a very low density of 0.44 g/cm3. But VEGASWING 66 at that time could only detect a density upwards of 0.47 g/cm3.
Measurement at the limit
To find out if VEGASWING 66 would be able to detect the very low density of approx. 0.44 g/cm³, a test instrument was installed on site to empirically determine the actual frequency change by immersing the tuning fork in LNG. Up until then, LNG could not damp the vibration frequency of the fork enough to trigger a switching command. So the sensor was adapted once again by the VEGA engineers in Schiltach and optimized for these critical switching limits. A short time later, VEGASWING 66 was installed at the LNG terminal and has since been measuring the filling level in the sump very reliably. If the sensor detects that the fork is not immersed, the special cryogenic pump is switched off to prevent dry run damage.
VEGASWING 66 thus assumes not only the function of level control, but also the responsibility for the safe and reliable operation of the facility. The application was also a milestone for VEGA’s internal development. In the meantime, the changes to the switching limits have been adopted as the standard.
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