Cryogenic transfer lines Under vacuum lines

Cryogenic transfer lines, commonly referred to as vacuum lines, are designed to transfer a cryogenic fluid from the production or storage source to the point of use. Production sources are liquefiers for liquefied gas, or refrigerators for cold gas. Storage facilities are pressurized cryogenic liquid tanks.

Cryogenic fluids can be gaseous, typically at temperatures below – 80°C, or liquefied. Cryogenic fluids have a wide range of applications:

• transport and storage of liquefied gas to exploit the density of the liquid;

• cooling equipment such as superconducting magnets;

• rapid freezing.

A gas in liquid form can be stored or transported in large quantities at low pressures of less than 15 bar. The volume ratio between liquid and gas is around 700.

The most commonly encountered cryogenic fluids are nitrogen, oxygen, argon, hydrogen, liquefied natural gas and helium.

In recent years, the size of industrial and scientific cryogenic installations has increased significantly. This increase in power implies the construction of longer cryogenic fluid transfer lines, with more complex routing and larger pipe diameters. This evolution leads to complexity in design control and reliability. Controlling the design rules for these lines is becoming an essential factor in the safe operation of these facilities.

The first problem with vacuum lines is to compensate for the contraction of the internal piping during cooling. The thermal contraction of austenitic steels is 3 mm per meter from ambient temperature of 20°C to liquid nitrogen temperature – 196°C.

The second challenge facing vacuum lines is to find the best compromise between mechanical resistance to different loads and limitation of thermal losses. This compromise is achieved by a detailed study of the line's flexibility to compensate for thermal contraction, and by optimizing the line's internal supports. In addition to optimizing supports, transfer lines are vacuum-insulated.

Last but not least, maintaining the insulating vacuum throughout the entire period of use is achieved by a few simple design rules and, above all, by the high quality of welded joints to guarantee watertightness.

Some transfer lines with no thermal performance requirements or short lengths are insulated with an insulating material such as polyurethane or foamglass applied directly to the transfer pipe. These simple designs are not described in this article.