Diversifying level measurement technology in the LNG industry

Natural gas continues to be a part of the energy puzzle that powers our world, and reliable, accurate level measurement is a part of the solution.

Cryogenic processes and applications place high demands on level measurement instrumentation. Any sensor or transmitter must be robust enough to handle low dielectric fluids and volatile liquid surface conditions, as well as having the ability to withstand the high demands of potential thermal shock. The liquefied natural gas (LNG) industry experiences these challenges every day.

Considered to be a cleaner, more environmentally friendly alternative to oil, natural gas has seen a recent boom thanks to technological developments in production, storage, and transportation. As infrastructure and new innovations have made natural gas a more economically feasible energy source, worldwide demand and production have grown. Fortunately, level measurement technology has grown with it, enabling upstream, midstream, and downstream companies to safely maximise vessel usage and optimise their processes.

The LNG industry has traditionally relied on floats, displacer technologies, and differential pressure for level measurements. However, these devices are prone to measurement errors because of mechanical wear and tear as well as fluid density changes. As a result, newer facilities are being engineered and built with newer, more modern level measurement technologies and at the same time, older facilities are upgrading and adapting.

Since natural gas extraction, processing, and transportation use complex processes involving extreme temperatures, pressures, or both, level measurements are rarely a one-size-fits-all. A single facility may need to employ multiple different technologies for level measurements throughout its operation. This article highlights a few of those technologies, their uses, and advantages: 80 GHz radar, guided wave radar, and vibration point level switches.

Isolated, non-contact level measurements in storage tanks

Filling and emptying large LNG storage tanks are lengthy processes, so any downtime is time and money lost. Since mechanical level measurement instrumentation is often associated with high maintenance costs, they reduce the efficiency of any operation.

To further complicate matters, natural gas is stored in its liquid form at temperatures of –162°C or colder. Variations from this temperature alter the product’s density, and any change in density can result in measurement errors when using pressure measurements for level.

These obstacles can easily be overcome with an electronic 80 GHz radar sensor mounted on a ball valve for isolation. Radar sensors make accurate level measurements despite any changing densities, and radar sensors with 80 GHz transmission frequencies use a narrow-focused signal to deliver a level measurement in tight spaces, including ball valves. Such radar sensors also utilise more sensitive electronics, which enable the sensor to detect a wider range of signals, including those returning from a poorly reflective product like LNG.

Figure 1: 80 GHz non-contact radar instruments are useful for level measurement in storage tanks.

The ball valve on an LNG storage tank serves to isolate the instrument from an ongoing, potentially hazardous process. In the event that the radar sensor requires maintenance, operators can simply close the ball valve and remove the sensor — all while the storage tank continues to fill or empty without interruption.

Measuring sudden level changes in surge drums

Surge drums are scattered throughout any LNG processing facility, guaranteeing a steady flow between process units, all while separating liquid out of gas. A level measurement in this vital application ensures the surge drum continues to operate at peak capacity without any liquid carryover that could potentially damage downstream equipment.

The environment inside a surge drum is volatile, and rapid level changes are the norm. Heavy vapours and an array of liquids are constantly entering the vessel, causing temperature and pressure changes in addition to liquid density fluctuations. Guided wave radar technology has proved to be most effective at outputting accurate, reliable level measurements in this type of application.

Guided wave radar sensors have no moving parts, which eliminates the possibility of any mechanical failure. The technology has the added ability to measure interface applications if necessary. Measurements are immune to any vapours in the air as well as changing temperatures, pressures and densities. Installation is relatively simple, with flexible mounting options, either inside the vessel itself or inside a bypass chamber or bridle for an isolated measurement that doesn’t affect the process.

Figure 2: Guided wave radar can be used for level and interface measurements in surge drums.

A little preparation in selecting the right guided wave radar sensor can save time during start-up. First and foremost, technicians should confirm they select a sensor capable of withstanding the surge drum’s high pressures. If the required process data is available, some instrument vendors can supply these sensors preassembled, pre-adjusted, and ready for installation.

Pump control and overfill protection in the recondenser with point level vibration switches

In any LNG operation, boil-off gas (BOG) is inevitable, and efficient operations send this BOG to a recondenser. The recondenser liquefies the BOG and uses a special pump to recirculate the LNG back into storage.

Without any level measurement controls, overfills can result in severe safety risks to personnel, environmental damage and economic losses. Because of the risks associated with the recondenser, all instrumentation must comply with overfill safety rules and regulations.

When recondenser levels drop too low, the pump runs dry, resulting in damages. Repairing or replacing the pump are costly and time-consuming exercises. And without a pump on the recondenser, the complete processing unit could be forced to shut down.

Any point level sensor must be able to withstand the high pressures and cryogenic temperatures associated with the recondenser. The right type of vibrating fork point level switch that is robust enough for this task, and that complies with all API 2350 guidelines for overfill safety, should be selected.

The point level switch, when paired with the right controller, can provide overfill alarming along with proof testing that can provide online function tests, confirming the switch is still operating properly. Such controllers matched to the sensor can provide test functions to evaluate any damage to the vibrating forks, their frequency and amplitude, and even the incoming power. Testing functions like these eliminate the need for manual instrumentation inspections, giving operators peace of mind and keeping the recondenser — and by extension, the entire LNG terminal — operational.

Multiple level measurement technologies for safe, efficient operations

As the world continues to search for energy sources with fewer emissions, natural gas will continue to be a part of the energy puzzle that powers our world. It’s crucial for every operation in the supply chain — from extraction to processing to transportation — to be as efficient as possible to meet the world’s energy needs. Reliable, accurate level measurement instrumentation is a part of this solution.

With diverse processes and conditions, it only makes sense for operators and technicians to take advantage of the diverse level measurement technologies available today. Some level measurements may be better suited for non-contact radar, so the sensor can more easily be isolated from the process using a ball valve. Measurements in bypass chambers may require guided wave radar technology. And sometimes, robust vibrating point level switches can provide overfill protection and vital pump controls.

When choosing a measurement technology for a specific application, consulting level measurement experts can provide insight and experience to help operators make better decisions.

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