Cryogenic vessels are containers that are used to store products at temperatures lower than -128°C (-200°F).
One of the most important aspects of storing gases in the cryogenic (liquid) state is the saving in space. Gaseous oxygen requires 862 times more storage volume than liquid oxygen. Gaseous nitrogen requires 696 times more storage volume.
Cryogenic vessels must be fabricated of materials that retain good impact values at extremely low temperatures, and must have effective insulation around the inner vessels to protect them from atmospheric heat.
The main problem in cryogenics is to maintain the gases at temperatures ranging from -70 to -270°C (-100 to 452°F). The solution is in the correct selection and fabrication of materials for low temperature applications. Materials generally used are 9% nickel steel, 304 stainless steel, and various aluminum alloys, most notably 5083. All have good weldability and ductility, prime requirements for use in low-temperature service. While these are good selections, there is always a reasonable concern that these high-strength materials may be subject to brittle fracture under certain conditions. A notched specimen must be used in ductility tests. Smooth specimens may show amazing ductility at low temperatures, but a notched specimen may fail in brittle fracture, indicating its notch sensitivity. See CHARPY TEST.
Actually, copper was the most widely used materialfor early low-temperature work. One of the more successful alloys in such service is silicon bronze containing 3% silicon and 1% manganese.
9% Nickel Steel
Nine percent nickel steel is a low-carbon, high-nickel, steel plate material primarily intended for pressure vessel use at low temperatures. When quenched and tempered, or double-normalized and tempered, it has good notch toughness characteristics down to -195°C (-320°F).
Fabrication with 9% nickel steel has been done with a high-nickel, chromium-iron electrode. The composition of this electrode produces joints with strengths higher than the minimum specified for the A353 steel.
A quenched and tempered 9% nickel steel in the as welded condition exhibits low-temperature notch toughness equal to that of the double-normalized and tempered metal. A basic requirement in welding 9% nickel steel is extreme cleanliness. Before fabrication, components should be pickled or sandblasted.
Immediately prior to fitting the components together to close or restrict access to inner surfaces of a vessel, these surfaces should be cleaned again to ensure removal of all dirt and oil. A muriatic acid wash, followed by a water rinse, is suggested.
For stress relieving after welding, a furnace with a neutral or reducing atmosphere is recommended. Normally, a detrimental scale should not appear under these stress relief conditions. However, as a final precaution, another muriatic acid wash and water rinse after stress relief will remove any remaining scale and loose particles.
Stress relief should be conducted in accordance with American Society for Testing Materials (ASTM) and ASME code specifications. Nine percent nickel steel can meet and exceed these specifications in the as-welded condition.
The stainless steels, especially type 304, are the most widely used material for containers subject to temperatures lower than -195°C (-320°F). Although somewhat expensive, austenitic stainless steel has been a favored material for cryogenic containers. A disadvantage of austenitic stainless is that it may transform to brittle martensitic stainless when exposed to extremely low temperatures over a pro- longed period. Some of the first vessels fabricated with A363 steel were welded with 310 stainless steel electrodes. Weld joint strength, however, was somewhat below the minimum specified tensile strength. Thus, the designer could not make full use of the
strength of this steel. In all other aspects, the 25Cr, 20Ni stainless steel joint was satisfactory.
Fluorine, which is a super-cryogenic deoxidizer used as a rocket propellent, is highly corrosive, and must be stored in Monec vessels. Fluorine is the most powerful oxidizing agent known, reacting with practically all known organic and inorganic substances.
For cryogenic service in the range of -100 to -195°C (-150 to -320″F), two weldable aluminum alloys, 5083 and 5086, are frequently used for cryogenic vessels. These are both high-strength alloys of aluminum, magnesium and manganese, but they have the excellent weld ductility characteristic of other alloys in the 5XXX series.
One of the most popular of these aluminum-magnesium-manganese alloys, 5083, offers a combination of properties required for cryogenic service: good weldability and weld ductility, resistance to corrosion and stress concentration, and in addition, light weight.
Since high-strength materials may be subject to brittle fracture under certain conditions, ductility at low temperatures is a major concern. An extensive battery of tests, however, has proved that at temperatures as low as -195°C (-320″F), 5083-H113 aluminum alloy plate and welds made with 5183 alloy filler can be used without the occurrence of ductile-brittle
transition.
The 5083-H113 aluminum alloy plate was used for the study because its temper is much stronger than the annealed 5083-0, and since the latter is a softer, more ductile-tempered plate, it would be at least equivalent to the H113 in brittle fracture resistance.
In the unnotched tensile impact tests, the increased strain rate did not produce a ductile-brittle transition in either the 5083 plate or its welds. In the notched tensile impact tests, a ductile-brittle transition was not produced in either the 5083 plate or its heat-affected zone, or in the 5183 weld deposit. Plate properties were virtually insensitive to testing temperature.
In the Charpy keyhole impact tests (most likely to produce a ductile-brittle transition in fracture-susceptible material) the results were the same as in the tensile impact tests.
Vessel Construction
The basic construction of this type of vessel consists of two or more concentric tanks, one inside the other. The most common type of large volume storage or transport container consists of an inner vessel and an outer vessel, with the vacuum space between filledwith insulating powder such as pearlite, silica aerogel, phenolic spheres, or diatomaceous earth. The area between the vessels is evacuated to a high vacuum.
The outer shell is constructed to withstand rough treatment and to act as protection for the inner vessel. The inner shell is supported within the outer shell by rods, cables, or chains strong and flexible enough to withstand lateral or vertical jars and sudden forces of acceleration. Connection between the shells should have a minimum contact area consistent with adequate strength to minimize heat flow from the shell to the liquid in the inner vessel.