To optimize safety, it is essential to obtain accurate measurements of the liquid levels within LNG tanks.
It is essential to obtain very precise and reliable level measurements in the cryogenic or refrigerated tanks that store liquefied natural gas (LNG), as this helps to prevent overfills, optimize site and worker safety, and ensure compliance with environmental regulations. Level measurement technology forms an integral part of the overfill prevention systems (OPS) that minimize the risk of potentially disastrous spills and leaks from LNG tanks. However, as I explained in a Hazardex article, ‘How Advanced Radar Technology Increases Safety at LNG Terminals’, measuring the level of LNG is a challenging application.
LNG is a hazardous fuel, stored in large tanks with a capacity of up to 200,000 cubic meters. The required level measurement range often exceeds 40 meters (131 feet), making it difficult to achieve a high degree of accuracy. In addition, these tanks are not usually opened during operation or even for maintenance purposes during their entire lifecycle. This makes it important to select exceptionally reliable level measurement technology with minimal maintenance needs. The article explains that the modern approach to level measurement in LNG tanks is to use non-contacting radar gauges. It states:
…Radar level gauges work by emitting microwave signals towards the surface of the liquid, and a precise measurement is achieved by analysing the signals reflected back to the transmitter. A strong reflected signal, commonly referred to as an echo, is required to ensure a reliable measurement. To maximise signal strength, the latest radar gauges leverage frequency modulated continuous wave (FMCW) technology, boasting a sensitivity more than 30 times greater than devices using older pulse modulation techniques.
This heightened sensitivity enables FMCW devices, such as the Rosemount 5900S Radar Level Gauge from Emerson, to deliver superior measurement accuracy. To mitigate the fact that the device’s signal strength can be impacted by disturbances to the liquid surface, a still-pipe can be deployed to minimize such disturbances, resulting in a more robust echo.
The Rosemount 5900S Radar Level Gauge provides measurement accuracy to within an impressive 0.5 millimeters (0.020 inches) at distances in excess of 60 meters (196 feet). This level of precision facilitates a reduction in volume uncertainty of up to 180% compared to less sophisticated methods such as servo gauges. The radar device provides a further significant advantage through its ability to verify its measurements whilst a tank remains in operation. This is achieved by comparing its measured values to the known distance of a verification pin mounted in the still-pipe, along with a deflection plate at the end of the pipe.
The Rosemount 5900S Radar Level Gauge excels in measuring the levels of LNG in cryogenic or refrigerated tanks.
Despite the benefits of radar technology, some organizations have continued to use servo technology on LNG tanks because of concerns that high gas density in the vapor space might impact the performance of radar level gauges. However, as the article explains:
…these concerns are unfounded, as vapour and high gas density do not influence radar signals or compromise the measurement accuracy of radar gauges. In reality, radar gauges excel in measuring the levels of liquefied gases in cryogenic or refrigerated tanks. This is evident in their successful deployment on more than 8,000 liquefied gas tanks worldwide over almost 40 years.
In addition to their exceptional measurement accuracy, non-contacting radar gauges provide outstanding reliability. During their long lifespan, their availability is usually close to 100%, helping to ensure that the operational availability of tanks is maximized. The article continues:
…The design of non-contacting radar level gauges – devoid of moving parts and eliminating the need for contact with the liquid – minimises maintenance requirements. The mean time between failures for critical parts is measured in decades, and should a failure occur, diagnostic software will typically identify it and take the device to a safe state.
In LNG storage applications, it is common practice to install three level gauges per tank – one supporting the tank gauging system; a second, redundant gauge for inventory; and a third providing information for the OPS – with safety instrumented system (SIS) alarms triggered on a two-out-of-three voting scheme. The article explains that the International Electrotechnical Commission’s global functional safety standard IEC 61511 validates the same level measurement technology being used for both tank gauging and the OPS – an approach known as identical separation.
Whilst radar devices can easily be used for both tank gauging and OPS purposes on new LNG tanks, existing tanks may have practical limitations that make it cost-prohibitive to deploy two separate radar level gauges. The article states:
…Scenarios where only one tank opening is available, and modifications would involve taking the tank out of service, can result in additional costs and reduced throughput. Addressing this challenge, 2-in-1 radar level gauges provide an innovative solution.
The Rosemount 5900S Radar Level Gauge can serve as both an automatic tank gauge and an independent overfill prevention sensor, and requires just a single tank opening.
The Rosemount 5900S Radar Level Gauge consists of two separate and independent electrical units and a common antenna. By connecting the cables separately in different cable trays and utilizing separate power sources, a single level gauge can provide a ‘2-in-1’ option, serving as both an automatic tank gauge and an independent overfill prevention sensor. The article explains:
…The primary advantage of this 2-in-1 configuration is its efficiency in requiring only a single tank opening. This allows for cost-efficient upgrades of existing tanks by replacing a single automatic tank gauge or overfill prevention sensor with two continuous level measurements, necessitating minimal tank modifications. Often, a radar level gauge with 2-in-1 technology fits the antenna of earlier generations of devices, thereby requiring no tank modifications at all.
Visit here to learn more about Emerson’s measurement solutions for cryogenic and refrigerated tanks.
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