Liquefied natural gas is methane with some ethane possibly mixed in cooled down to -162 degC (-260 degF) to transition the gas into a liquid state. It is liquefied:
…for ease and safety of non-pressurized storage or transport. It takes up about 1/600th the volume of natural gas in the gaseous state…
In a Tank Storage article, Radar Provides Precise Level Measurement in LNG Tank Gauging Systems, Emerson’s Håkan Jubel describes various approaches to level measurement and their importance in safety, inventory management and custody transfer.
He opens describing the complexity of these LNG storage tanks. They have a:
…typical capacity of up to 200,000 m3. They include an inner liquid containment steel tank, an outer concrete (or steel) tank for secondary containment control, and thermal insulation between the two to minimise liquid boil-off.
Level and temperature are two key measurements in this application. From a safety standpoint, the measures can help prevent overfill conditions and provide early warning if any leaks develop.
Because these tanks are so large, with the required measurement range often over 40 metres (131 feet), this makes accuracy difficult to achieve. Also, these closed tanks are not opened during operation, and instruments are normally not accessed for maintenance purposes during their entire lifecycle.
And unlike a typical tank with only one vapor space, these LNG tanks have:
…two different vapour spaces – one outside the tank’s fixed suspended deck and one inside it. These large vapour spaces have different temperatures, and for inventory purposes, this needs to be considered when calculating the liquid equivalent within the spaces.
Traditional methods of level measurement for these tanks include float and tape or servo technology. There are limitations with these mechanical measurement approaches. For float and tape, the:
…accuracy of a float gauge is often poor. There are plenty of error sources, such as buoyancy differences, dead-band, backlash and hysteresis in the mechanisms.
…generally perform better than float gauges, but they also have many moving parts. Because the displacer and the wire are in contact with the tank liquid, servo gauges may require more attention for calibration, routine maintenance and repair.
Non-contacting radar technology, such as the Rosemount 5900S radar level gauge, can:
…deliver a 180% reduction in volume uncertainty over traditional methods.
From a reliability standpoint, these radar level gauges:
…can boast impressive reliability, with mean time between failures for critical parts measured in decades. In addition, their design minimises maintenance requirements because they have no moving parts and do not touch the liquid.
Read the article for more about how this technology works, along with temperature measurement detect stratification issues, how they can be used in safety instrumented functions, and how proof testing in these safety applications can be remotely performed.
Visit the Radar Level for Tank Gauging section on Emerson.com for more on this measurement technology. You can also connect and interact with other level and temperature measurement experts in the Measurement Instrumentation group in the Emerson Exchange 365 community.