Level and temperature measurement technology plays a key role in modern liquefied natural gas (LNG) tank gauging systems. In a Control Engineering Europe article, ‘Increasing safety and optimizing tank usage at LNG terminals’, Victoria Lund Mattsson, a solutions engineer with Emerson’s tank gauging business, explains how modern tank gauging systems not only provide accurate and reliable level and temperature measurements, but also prevent overfills and predict rollovers in LNG tanks. The article states that:
…Liquefied natural gas (LNG) is stored in full containment, cryogenic tanks that can hold up to 200,000 cubic metres. These vessels include an inner steel tank to contain the liquid, and an outer concrete or steel tank serving as a secondary containment measure. There is thermal insulation between the two tanks to minimise liquid boil-off during storage. Designing and constructing such large and technically complex structures is costly and obtaining a return on investment (ROI) can take a long time. One way to achieve faster payback is to implement a modern tank gauging system that will help to optimise tank usage, increase safety, and minimize operating and maintenance costs.
Victoria explains the level measurement challenges presented by this application:
…The size of the tanks means the required level measurement range can often be over 40m, making high levels of accuracy difficult to achieve. During their lifespan, the tanks are not opened during operation so instruments cannot be easily accessed for maintenance and calibration. Instrument reliability is therefore crucial, and redundant level measurements are often vital.
The inner structure of full containment cryogenic storage tanks presents a further challenge. Standard storage tanks have only one vapour space, but full containment tanks have two – one outside the tank’s fixed suspended deck and another inside it. These two spaces have different temperatures and for inventory purposes this needs to be considered when calculating the liquid equivalent within the spaces.
A look inside a typical full containment cryogenic storage tank.
As Victoria notes, the modern approach to providing precise level measurement involves using non-contacting radar gauges. These devices can boast impressive reliability, with the mean time between failures for critical parts measured in decades. In addition, they have no moving or wetted parts, which helps to minimise maintenance requirements and the frequency of parts replacement.
Non-contacting radar gauges use microwave signals that are emitted towards the surface of the liquid and reflected back to the transmitter, enabling the level to be measured accurately and reliably. To perform continuous level measurements in a full containment, cryogenic storage tank, a radar gauge needs a sufficiently strong reflected signal, known as an echo, from the liquefied gas surface. Radar gauges based on frequency modulated continuous wave (FMCW) technology have superior sensitivity, which maximises their signal strength and enables them to deliver excellent measurement accuracy and reliability. To combat the extreme temperatures within cryogenic tanks, a specific antenna option is used to enable a gauge to function correctly.
Instrumentation used in the Rosemount™ Tank Gauging System from Emerson.
Victoria goes on to explain how automatic tank gauging and overfill prevention can be provided in the same tank gauging system:
…The Rosemount™ Tank Gauging System from Emerson, for example, is certified according to the IEC 61511 standard as a system solution, meeting specifications for both process control and safety applications. It is common practice for systems to include three level gauges – primary and secondary gauges supporting the basic process control system, and a third providing information for the OPS, with alarms triggered on a two-out-of-three voting scheme.
A key consideration when storing LNG is the potential for rollovers. As Victoria explains, rollovers are:
…dangerous releases of boil-off vapour and pressure build-up that can occur when stratification (when two separate layers of LNG are formed within a tank) is left unchecked. The consequences of rollovers can be severe – causing tank integrity damage that can release large quantities of natural gas into the atmosphere. As a preventative action to detect stratification, LNG tank gauging systems can include software solutions to calculate when a rollover might occur, and level temperature density devices to monitor tank temperature and density profiles.
Organizations have traditionally kept their rollover prediction and inventory management software installed separately on different PCs. However, the Rosemount TankMaster™ Inventory Management Software package from Emerson offers both functions in a single solution, which helps to lower costs, improve ease-of-use and accelerate operator training.
To learn more about Emerson’s measurement solutions for full containment cryogenic storage tanks, visit Emerson.com/RosemountLNG.