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MATERIALS MAINTAINING LUBRICITY AT CRYOGENIC TEMPERATURES COMPONENTS USED IN PUMPS, COMPRESSORS AND VALVES FOR CRYOGENIC APPLICATIONS MUST WITHSTAND LOW TEMPERATURES WITHOUT SHRINKING, BECOMING BRITTLE, LOSING STRENGTH OR CRACKING

Figure 1: Cryogenic applications, such as LNG, demand bushings and bearings that can withstand low temperatures

power transmission lines. any types of eccentricities or vibration that EBEN WALKER When designing pumps, compressors, would to wear. valves and ancillary components for cryo- Another approach is to build bearings ast winter saw big news in the area of genic applications, one is concerned with and bushings out of a self-lubricating mater- low temperatures. Yes, there were the more than the ability of the components to ial that will wear in over time. This allows record-setting temperatures in the withstand the low temperatures without the components to be built with, and main- northern U.S. and Canada caused by shrinking, becoming brittle, losing tain, tighter tolerances. For example, the Lthe shift in the polar vortex, but those tem- strength or cracking. Whenever there are Graphalloy brand of graphite and peratures were nothing compared with moving parts, there is the need to mini- composite consists of a metal impregnated NASA’s announcement of a plan to create mize friction and wear, and to prevent into a graphite substrate under high temper- the coldest spot in the known universe. The sticking or galling (Figure 1). ature and pressure. In doing so, the metal Cold Atom Laboratory (CAL), scheduled forms long, continuous filaments, giving the for a launch to the International Space Dry lubrication material its strength. A variety of and Station in 2016, will conduct experiments Unlike higher-temperature applications, the alloys can be used, including , lead, at temperatures below 100 picokelvins, or use of fluid lubricants in low temperatures , , and . less than one ten-billionth of a degree above is out of the question. This means that com- The graphite provides a low friction sur- absolute zero (0°K, -273.15°C or - ponents need to be either coated with, or face, does not have a melting point and does 459.67°F). This is about three degrees machined from, materials that provide a not soften at high temperatures, so it can be Kelvin colder than interstellar space. low coefficient of friction at the intended used in non-oxidizing temperatures up to Here on Earth, progress is also being temperature range. 1,000° F and up to 750°F in air. It does not made on the application of cryogenics, When taking the surface coating get brittle at temperatures as low as -450°F. not just for quantum research projects, approach, one is assuming that there will Recently the Graphalloy material hit a which the CAL will be engaged in, but in be little or no wear, and so components new record low temperature application in a the turbocompressors and other equip- must be manufactured with enough clear- magnetometer. It was not quite as low as ment needed to support the booming ance to accommodate thermal expansion what the CAL will reach, since those can LNG industry, Magnetic Resonance and contraction without causing contact, only be achieved in space, but it was not Imaging (MRI), which requires liquid which wears out the surface. Similarly, pre- much warmer. helium cooling, and superconducting cision manufacturing is needed to avoid (Continued on p. 42)

40 Turbomachinery International • May/June 2014 www.turbomachinerymag.com Cryogenic 5-14:CAREB 05-09.qxd 4/24/2014 3:02 PM Page 42

Figure 2: Pumping fluids with low lubricity such as LNG and Natural Gas Liquids (NGLs) can be especially difficult for metallic bearings. The hydrodynamic film provided by these low lubricity liquids is unable to provide sufficient lubrication, which can lead to metal-on-metal contact, galling or seizing Magnetometers design of the bearings.” low as 4.2°K. Magnetometers are used in laboratories to The RSM consisted of a meter-long The RSM was found to achieve a resolu- measure the magnetic properties of samples, stainless steel tube that was 28.5 mm in tion exceeding that of typical VSM by a fac- often at temperatures near absolute zero. One diameter. A DC motor was installed out- tor of ten. “The magnetometer is thus ideal- type of magnetometer, called a SQUID side the tube to spin the 6.3 mm diameter ly suited for a precise and swift characteriza- (Superconducting quantum interference drive shaft. The drive shaft was coated with tion of superconducting and magnetic sam- device), is extremely sensitive, but as a result a carbon-fiber-reinforced polymer and sup- ples in a wide temperature and magnetic also picks up noise from other magnetic fields ported by three 25 mm long Grade 212 field range.” nearby. Graphalloy bearings. In this case, the graphite and metal Another is the Vibrating Sample bearings were operating at a minute load Magnetometer (VSM) which vibrates the compared to the service they would see in sample inside an inductive pickup coil or a “The bearings an LNG compressor, boil-off gas (BOG) SQUID coil. The vibration can damage frag- compressor or other similar low-tempera- ile samples, and generates heat, raising the performed in liq- ture application. The Atominstitut’s temperature. SQUIDs are more accurate, but research shows that the bearings worked VSMs have a faster measurement speed. uid helium at 4.2K without being replaced through six years of Researchers at the Vienna University of experiments being conducted at tempera- Technology’s Atominstitut decided to try ° tures nearly 200°F lower than the storage another approach, building a Rotating (-269 C) ” temperature of LNG, -260°F. TI Sample Magnetometer (RSM). They pub- Of those three, “The bottom bearing is lished the results of their project in the the most crucial component, because it has to Author American Institute of Physics’ Review of guarantee stable rotation (the main source of Eben Walker is President of Scientific Instruments. (Rotating sample noise are vibrations) and operate with low Graphite Metalizing Corp, a magnetometer for cryogenic temperatures friction to avoid heating.” According to the company specializing in self- and high magnetic fields; Review of paper, this material was chosen because “it lubricating Graphalloy bear- Scientific Instruments 82, 063902 (2011); maintains self-lubrication at cryogenic tem- ing materials suitable for tem- doi: 10.1063/1.3600454). peratures.” peratures from -450°F to According to the researchers, “The limit- The sample to be measured is placed in a +1000°F. For more informa- ed space and the cryogenic temperatures holder near the bottom of the drive shaft and tion, visit impose the most critical design parameters: rotated at 15 Hz 1 mm above a set of pickup www.Graphalloy.com the small bore size of the magnet requires a coils as the bottom of the tube. The tests very compact pick-up coil system and the were conducted using both gaseous and liq- low temperatures demand a very careful uid helium for cooling, with temperatures as

42 Turbomachinery International • May/June 2014 www.turbomachinerymag.com