In tank gauging systems for cryogenic vessels that contain liquefied gases such as LNG and LPG, reliable level measurement is vital for inventory management and custody transfer purposes, and to prevent overfills. Servo gauges have often been used to perform level measurement in these applications. However, in a Tank Storage magazine article, ‘The right level measurement for liquefied gas’, Johan Sandberg, business development manager for Rosemount™ Tank Gauging at Emerson, explains that:
…Servo technology has some major shortcomings. Servo gauges have many moving parts that are susceptible to mechanical wear, increasing their maintenance requirements and likelihood of failure. Because the displacer and the wire are in contact with the liquid, they can become contaminated, affecting the measurement accuracy, and can mean recalibration, maintenance and repairs are required. Additionally, servo gauging is sensitive to changes in liquid density, which affects its measurement accuracy.
Despite these limitations, some organizations continue using servo technology because of concerns that radar gauges might not perform well in these vessels due to high gas density in the vapor space. However, Johan explains that such concerns are unfounded:
…Radar level gauges have been proven to perform excellently on virtually all liquids stored in atmospheric tanks, and on liquefied gases in refrigerated tanks. They are used on more than 8,000 LNG and LPG tanks worldwide. During nearly 40 years of service, no Emerson radar installation has experienced any safety- or reliability-related problems due to vapour or high gas density.
Johan describes how modern top-down radar gauges use frequency modulated continuous wave (FMCW) technology to optimize their radar signal strength. This results in level measurement accuracy to within 0.5 millimeters and a 180% reduction in volume uncertainty when compared with servo gauges. Radar level gauges can also boast impressive reliability, with mean time between failures for critical parts measured in decades.
The article states that it is typical to install three level gauges – two supporting the tank gauging system and a third providing information for the overfill prevention system. There is a common misconception that different device technologies must be used for the tank gauging and overfill prevention systems, which is known as diverse separation. However, the global safety standard IEC 61511 confirms that it is legitimate to use the same technology for both systems, which is referred to as identical separation.
New installations can therefore use radar level gauges in both the tank gauging system and the overfill prevention system, but some tanks have practical limitations that make upgrades with two separate level gauges cost-prohibitive. For instance, an additional tank opening might not be available, and the necessary modifications would involve the tank being taken out of service. Johan explains that this problem is solved by Emerson’s Rosemount 5900S 2-in-1 Radar Level Gauge:
…which consists of two independent electrical units and a common antenna. When connected with its cables separated in different cable trays and with separate power sources, a single level gauge can be used for both tank gauging and separate overfill prevention purposes, thereby requiring only a single tank opening and minimal or no tank modifications.
Level gauges used in a safety instrumented system must be periodically proof-tested to ensure that they will function properly. These tests have traditionally been carried out by technicians in the field and verified by a worker in the control room. This time-consuming method can involve workers climbing tanks to access the instruments, putting their safety at risk. However, as Johan explains:
…the Rosemount 5900S has functionality that enables proof-tests to be performed remotely from the control room, thereby making the procedure safer, faster and more efficient.
The article then provides a great example of the use of radar technology at the Gasum LNG Terminal in Lysekil, Sweden, which stores and distributes LNG for production processes in nearby industries. The terminal has a 42 m high, above-ground concrete tank, with a 30,000 m3 capacity. Gasified product is distributed to a neighbouring refinery via a pipeline. If the refinery had to shut down because its energy supply stopped, it would cost several million dollars. It is therefore critical to maintain an uninterrupted energy supply. Johan explains that radar technology is Gasum’s preferred choice to provide level measurement:
…because it requires minimum maintenance. The tank uses three Rosemount 5900S Radar Level Gauges with a specific LNG antenna, suitable for cryogenic temperatures. The application is set up so that it requires alarms from at least two of the three gauges to shut down the process, thereby avoiding a false alarm that could interrupt production and affect profitability. In addition, having three gauges means that there is always one spare device in usage.
Since installing the radar-based solution, Gasum has been delighted with its accuracy and reliability. There have been no unplanned shutdowns, no maintenance requirements that involved having to open the cryogenic tank, and being able to proof-test the level gauges remotely from the control room has made the procedure quicker and safer.
Visit here to learn more about Emerson’s measurement solutions for cryogenic and refrigerated storage.
Read the LNG Storage Tank Radar Level Measurement blog post to learn about the reliability of radar level gauges for LNG storage tanks.