State-of-the-art measurement technology in LNG Plants
The main objectives of LNG plant automation are increased safety, availability and reliability. In order to ensure stable operation, the deployed measurement technology has to overcome multiple challenges. These are, on the one hand, troublesome product characteristics, such as: cryogenic temperatures, low dielectric values, fluctuating densities, and turbulent processes. And on the other hand, factors that make installation problematic and costly, such as: high tank mounting sockets, standpipes or stilling wells, shut-off valves and insulation systems.
For more than 60 years now, VEGA has been involved in the measurement of difficult media under atypical process conditions. An additional plus is the integrated self-monitoring system in the sensors that constantly provides information about the sensor status
and enables preventive maintenance before faults or breakdowns occur. The concept is rounded off by the option of plug & play solutions, such as the guided radar sensors, which can be delivered preassembled and adjusted with the type of bypass system required by the particular region or country.
For historical reasons, conventional differential pressure systems dominate in the LNG industry, especially in the area of level measurement. These are connected to the process via impulse lines and thus decoupled from the cryogenic liquids. But, alongside the classic differential pressure transmitters, state-of-the-art pressure sensors with oil-free, ceramic-capacitive measuring cells are now gaining ground in some applications. The so-called CERTEC® measuring cell of sapphire-ceramic® is a mechanically robust and dynamically resilient pressure measuring cell, characterised above all by notable long-term stability, chemical resistance, extensive overload and pressure shock resistance, and abrasion resistance.
Differential pressure and process pressure transmitters
The challenges associated with pressure transmitters lie in the implementation
of complex piping, the maintenance requirements, and the influence of density fluctuations on the measuring result. In these areas, technologies such as non-contact radar sensors or guided radar sensors have proven to be viable alternatives. Although they are often somewhat more expensive to acquire than pressure transmitters, these technologies are virtually maintenance free and can reduce the total operating costs, due to their reliability and high installation flexibility.
This article will now take a closer look at the advantages of a non-contact radar sensor. In order to avoid negative influence from internal installations or turbulence in the tank, standpipes or stilling wells are usually installed. When using the
new 80-GHz high-frequency sensors, such accessories can be dispensed with and costs saved. In fact, these sensors achieve their best performance when they can measure freely inside the vessel. Especially in cases where interrupting the production process means a lot
of work and costs, shut-off devices are mounted directly on tanks in order to separate the installed measurement technology from the ongoing process. The signal focusing of free-radiating 80-GHz radar sensors enables simple mounting on a ball valve and operation without causing interfering reflections that can negatively affect the measurement. Reliable measurement is thus guaranteed.
Various solutions for level measurement in cryogenic tanks.
As already mentioned, a typical requirement in LNG plants is for it to be possible to separate the sensors
from the main process, via a valve, without causing an interruption – i.e. without partially or fully shutting down the plant. In answer to this requirement, as an alternative to free-radiating radar sensors, guided radar sensors are available, which can be mounted in combination with bypass systems on the side of a tank. The valves installed between the tank and the bypass allow the sensors to be isolated from the process during operation.
Due to their low operating frequency, guided radar sensors are specialised for use in standpipes and stilling wells as well as external chamber systems. Different rod, cable, and coaxial versions are available, which are selected according to the desired installation configuration and process requirements. In the coaxial version, the radar signal runs exclusively along a rod in a perforated tube and is therefore completely unaffected by external influences.
The ambient conditions in some LNG applications are often unstable. Measuring instruments are exposed to rapid level changes, volatile atmospheres due to evaporation,
as well as significant temperature, pressure and density fluctuations. Know-how is crucial, and the continuous development of sensor software enables safe and reliable use of free-radiating and guided radar sensors even in critical applications.
Level measurement and high alarm in type-C tanks.
In addition to continuous level measurement, there is often a need for point level detection, whether as overfill protection in tanks or dry run protection for pumps. These point level sensors must also be able to meet the stringent demands of the industry.
Vibrating level switches are universally applicable and suitable for all liquids. Whether as compact version or
with tube extension, they detect the limit level reliably with millimetre precision. The wide range of applications is unique in terms of process temperature (-196 °C … +450 °C) and process pressure (-1 … +160 bar). Tuning forks can also be used for densities as low as 0.42 g/cm3.
Measurement technology from VEGA used worldwide in the most advanced LNG plants
The key to greater safety, higher availability and maximum reliability lies in the selection of the right technology,
the ideal configuration and the correct setup and commissioning of the sensors. Certain applications may
be more suitable for radar sensors, whereas others for pressure transmitters, and in some cases a simple point level detector may be sufficient. Our sensors are in use in the world’s largest and most advanced LNG plants, e.g. in the USA, Russia and Australia.
Author: Sebastian Harbig, Business Development Manager, VEGA Grieshaber KG
About The Author
