Maintaining lubricity at cryogenic temperatures

Graphite Metalizing Corp. last winter saw big news in the area of low temperatures. Yes, there were the record-setting temperatures in the northern U.S. and Canada caused by the shift in the polar vortex, but those temperatures were nothing compared with NASA’s announcement of a plan to create the coldest spot in the known universe.

(The components should be able to withstand the low temperatures without shrinking, becoming brittle, losing strength or cracking)

The Cold Atom Laboratory (CAL), scheduled for a launch to the International Space Station in 2016, will conduct experiments at temperatures below 100 picokelvins, or less than one ten-billionth of a degree above absolute zero (0° K, -273.15° C or -459.67°F). This is about three degrees Kelvin colder than interstellar space.

On the Earth, progress is also being made on the application of cryogenics, not just for quantum research projects, which the CAL will be engaged in, but in the turbocompressors and other equipment needed to support the booming LNG industry, Magnetic Resonance Imaging (MRI), which requires liquid helium cooling, and superconducting power transmission lines.

When designing pumps, compressors, valves and ancillary components for cryogenic applications, one is concerned with more than the ability of the components to withstand the low temperatures without shrinking, becoming brittle, losing strength or cracking. Whenever there are moving parts, there is the need to minimize friction and wear, and to prevent sticking or galling.

Dry lubrication

Unlike higher-temperature applications, the use of fluid lubricants in low temperatures is out of the question. This means that components need to be either coated with, or machined from, materials that provide a low coefficient of friction at the intended temperature range.

When taking the surface coating approach, it is assumed that there will be little or no wear, and so components must be manufactured with enough clearance to accommodate thermal expansion and contraction without causing contact, which wears out the surface. Similarly, precision manufacturing is needed to avoid any types of eccentricities or vibration that would lead to wear.

Using Graphalloy

Another approach is to build bearings and bushings out of a self-lubricating material that will wear in over time. This allows the components to be built with, and maintain, tighter tolerances. For example, a graphite and metal composite known as Graphalloy consists of a metal impregnated into a graphite substrate under high temperature and pressure. In doing so, the metal forms long, continuous filaments, giving the material its strength. A variety of metals and alloys can be used, including tin, lead, bronze, copper, iron and silver.

The graphite provides a low friction surface, does not have a melting point and does not soften at high temperatures, so it can be used in non-oxidizing temperatures up to 1,000° F and up to 750° F in air. It does not get brittle at temperatures as low as -450° F.

 

You can read the rest of this article in the May/June print edition of Turbomachinery International