Cryogenic liquids such as liquid oxygen (LOX), liquid argon (LAR), and liquid nitrogen (LIN) are typically transported and stored as liquids, because more product can be transported when the product is in liquid form.
Cryogenic tanks & trailers are used to transport and store the gas in its liquid form. Since the tanks are produced to store the liquefied gas, at -196ºC or below, they are designed like large, oversized ‘Thermos’ bottles, because even a small amount of heat inlet will return the products to their gaseous state.
The double wall tanks consist of two cylindrical vessels, a stainless steel inner tank (the pressure vessel) and an outer jacket in carbon steel. The air in the space between the inner and outer vessel is known as the annular space, which is evacuated by means of a pump to create a vacuum.
The annular space between inner and outer vessel is filled with an insulating material such as Perlite or Super insulation, and then evacuated to a high vacuum to achieve minimum heat inlet (and therefore avoid evaporation losses).
Various phases of the tests may be supervised by independent inspection institutions during the manufacturing process. After completing the piping of the inner tank (the pressure vessel), it is tested hydrostatically.
Cryogenic tanks are delivered under positive pressure to avoid contamination of the inner vessel during storage and shipment before its commissioning.
The technology: Avoiding tank heat inleak
Cryogenic liquid storage vessels are designed to minimise the transfer of heat into the tank. Heat transfer or ‘inleak’ is the transition of thermal energy from a hotter mass to a cooler one, a process that is inevitable and can only be slowed.
The heat which is transferred into the tank is absorbed by the cryogenic liquid in the tank and will cause some of the liquid to be vaporized into gas (‘boil off’). The vaporized gas will be either retained in the tank, used by the customer through the economiser circuit or in some cases, vented to the atmosphere.
Various items that help reduce heat inleak through design and by insulating the cold inner vessel from the warmer outer vessel, include:
Vacuum – The vacuum fights against conduction by removing the air particles between the inner and outer vessel to reduce transfer of heat by direct contact of the air particles.
The vacuum is arguably the most critical element in a cryogenic tank and its lifespan is certainly essential when choosing a tank manufacturer, since trace gas molecules begin to enter the vacuum space from the moment of manufacture.
To slow the entry of trace gas molecules, manufacturers use molecular sieve, that is used as an absorbent to capture such gas molecules.
Perlite – In the form of a fine white powder, Perlite is a volcanic glass used as an industrial insulation that fills the annular space completely. Because of its unique properties, perlite insulation has found wide acceptance in the insulating of cryogenic storage tanks.
Multilayer Super Insulation (MLSI) – The inner vessel is wrapped with multiple, alternating layers of insulating material typically made of inorganic glass fibre, separated by a reflective radiant barrier made of aluminum.
The aluminum helps to reflect the suns rays and re-emit the suns electromagnetic waves.
Paint – The white finishing coat also helps reduce the heat transfer by reflecting the Sun’s electromagnetic waves, since when light hits a white object, it gets re-emitted.
Cryogenic liquid storage vessels are designed to keep the cryogenic liquids as cold as possible to avoid ‘boil off’ by minimising heat transfer into the tank.
Each manufacturer uses the same basic recipes but only their knowledge and expertise in each domain can differentiate one tank or one tank manufacturer from another – in particular, when it comes to the lifespan of the vacuum, undeniably the most technical aspect of a cryogenic tank.