Icy Travels
Transporting helium at temperatures close to absolute zero is challenging. HELICS tanks from Linde Engineering provide the optimum transport solution.
Helium becomes a liquid at minus 269 degrees Celsius, making this inert gas ideal for (ultra-) low-temperature applications. The task of transporting helium from its source to the point of use at temperatures close to absolute zero poses many challenges, however. Linde Engineering’s solution is the HELICS™ tank for the cryogenic transport of liquid helium.
UN Portable Tank for Helium (HELICS)
- For more than 12 years, Linde has been specialising in UN portable tanks for helium (known as HELICS).
- HELICS-tanks optimize the transportation of liquid helium and ensure smooth delivery anywhere in the world.
The ability to remain liquid at extremely low temperatures is one of helium’s invaluable properties. That is what makes this rare gas indispensable in so many clinical settings. It is used to cool the magnetic coils of MRI machines. Research institutes like CERN also rely on liquid helium to operate their particle accelerators. Only the liquid form of the gas is capable of bringing the temperature of the magnetic coils down to 2 Kelvin, or minus 271 degrees Celsius, making them superconductive. Thanks to these extremely strong magnetic fields, accelerated atomic nuclei remain on a circular orbit and can be used for physics experiments. In hospital MRI scanners, the magnetic moments of the protons are used to visualize human tissue. In 2018, one quarter of all helium used worldwide was required in liquid form. MRI scanning is the main application. Looking beyond healthcare, the semiconductor industry harnesses other useful properties of helium, namely its inert nature and high thermal conductivity. Since helium forms no chemical compounds, it classifies as an inert gas. This makes it popular for other industrial applications such as welding.
Sourcing Helium
Global demand of helium – a name derived from “hélios”, the Greek word for sun – stood at 6.2 billion cubic feet (approximately 175 million cubic meters) in 2018. Most people know helium as the lightweight gas used to fill balloons. This is, however, rather a wasteful use of this rare gas because helium is currently a scarce and expensive commodity. This is mostly down to delays in the development of new sources. On top of this, 80 percent of worldwide demand is being met by only five sources of supply. “Our discussions with customers mostly now revolve around how much helium we can deliver. For them, the price is of secondary importance,” according to Kurt Richter, Head of Supply Chain Global Helium at Linde plc, explaining the current state of play in the helium market. It is expected that the situation will not ease. The reason is that helium deposits form in geological rock formations similar to those that are home to natural gas. It is produced as a result of the atomic decay of uranium or thorium nuclei. Some natural gas fields contain a helium component of up to 16 percent by volume. “That is why helium production by means of fractional distillation only takes place in fields where the natural gas extracted also contains helium,” explains Richter. Linde has long-term supply contracts with different helium sources and ranks as the world’s number one supplier of the gas.
Preparing Helium for Market Distribution
In order to transport this valuable raw material from its sourcing locations to customers all over the world, Linde has developed special cryogenic transportation tanks marketed under the name HELICS (formerly Helicon). Around 250 of these tanks are currently in use around the globe. Richter and his team are able to pinpoint the exact location of each HELICS tank, coordinate the retanking process, and arrange global transport routes. The 12-meter-long containers can be filled with approximately 40,000 liters of liquid helium, which corresponds to around 28,000 cubic meters of helium in gaseous form. At the core of every HELICS tank is a stainless steel vessel filled with liquid helium. This is suspended in a vacuum-insulated space by means of a sophisticated fixture. This helps to protect against vibration and shock, while also keeping ambient heat transfer to a minimum. The tanks feature an extremely effective vacuum, which is virtually free of molecules which would otherwise conduct heat,” says Richter. In addition, the vessels are surrounded by a super-insulation layer and an “icy shield” composed of over one metric ton of liquid nitrogen. This liquid nitrogen slowly evaporates and has to be topped up periodically. By refilling the liquid nitrogen, the Linde experts ensure that very little heat reaches the cryogenic helium. Richter explains the structure as follows: “The insulation layer combines materials that reflect the thermal influx with materials of low conductivity. This keeps the interior temperature as low as possible.”
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Robust Stainless Steel Design
Another advantage of the HELICS tanks is the fact that they are made entirely of stainless steel, which makes them much less susceptible to the corroding effect of salt water compared with the carbon steel versions of competitors. Carbon steel tanks have to undergo a general overhaul due to corrosion every eight years or so, which puts them out of action for a few weeks or even months at a time. Linde’s helium tanks avoid this lengthy downtime and only have to undergo the legally required inspection every two and a half years. Despite their somewhat intricate insulation technology and valuable cargo, Linde’s helium tanks are robust enough to withstand the typically rough handling at ports or rough transit over bumpy roads. “When designing the HELICS, we carefully positioned all valves, access valves and the phase separator in such way to avoid the risk of damage,” points out Richter.
The Special Properties of Helium – Liquid and Gaseous Form in the Same Tank
With their user-friendly design, the HELICS tanks double up as transport and storage vessels. “The helium tanks go to our regional subsidiaries – in Germany, for instance, this would be Lohhof near Munich,” says Richter, “where our colleagues either withdraw the inert gas in liquid form to fill dewars, which have a capacity of between 30 and 450 liters, or otherwise transfer the gaseous helium from the tanks into gas cylinders.” For this, the liquid has to be vaporized first or – in the case of cryogenic gas – preheated in a heat exchanger and then compressed. “If liquid helium is to be withdrawn, the pressure in the container must be as low as possible – 0.5 bar is ideal. If a HELICS tank has a pressure of 5.5 bar, for example, gaseous helium has to be withdrawn first in order to reduce the pressure. Only then can liquid helium be transferred.” These physical properties mean that liquid helium customers cannot use the full tank load of product as liquid. The actual amount available to them ultimately also depends on the route and duration of the transport. Linde guarantees a holding time of up to 40 days in HELICS tanks.
“With our HELICS tanks, we can guarantee liquid helium holding times of around 40 days.”
Icy Temperature Guaranteed Until Next Refill
As soon as the helium tank starts to run low, Richter and his team arrange for it to be refilled. Customers must ensure that around 200 kilograms of helium is left in the HELICS tank in order to keep the tank cold enough for a direct refill. The cooling-down procedure needed for brand new tanks can take up to 14 days to complete and consumes a lot of energy. This step is necessary, however, to ensure that the HELICS tanks are able to keep the inert gas at an ultra-low-temperature in liquid form for a couple of weeks. So, it is ready to be used in high-tech applications around the world.
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