38 n TRANSPORTATION July/August 2023 www.drivesncontrols.com Cryogenic bearings smooth the way to ‘greener’ marine power LNG (liquified natural gas) has long been regarded as a sustainable fuel and has gained significant traction as a viable alternative for a wide range of commercial transport applications, including shipping. It is estimated that 3% of global greenhouse gas emissions are generated by maritime traffic which is why the International Maritime Organisation (IMO) is looking to cut CO2 emissions. LNG meets all the current IMO and EU regulations as a marine fuel and switching to it results in the complete removal of SOx and particles, as well as reducing NOx emissions by up to 85%. The result is a reduction in CO2 emissions by at least 20%. While LNG is a much “greener” fuel than the viscous diesel fuels typically used to power container ships and cruise liners, it does present significant challenges to fuel systems and pumps, specifically dealing with the extremely low (cryogenic) temperatures. A vital component in LNG pumps are the bearings which have to withstand being submerged in LNG at temperatures of –196ºC, or liquid hydrogen at –253ºC. As you approach absolute zero (0K) the ability to use liquid lubrication becomes increasingly challenging, and dry-running bearings are mandatory. Standard steel bearings are ineffective, offering poor performance resulting in short service lives and unexpected breakdowns. In addition to the challenges of lubrication, the expansion and contraction of the different materials used in the bearing housings becomes even more critical for bearing efficiency. Balancing the pump system to account for the contraction of the materials, is essential. In addition to lubrication issues, special material selection and heat treatment processes are essential to help ensure that warping or non-linear contraction of the bearing components doesn’t occur. Material choices Bearing cage failure is common in cryogenic applications so material selection, in addition to the cage design, is critical to performance. Many common engineering plastics become too brittle at cryogenic temperatures. These include lower-cost options such as Peek and PTFE with custom additives, or even Durafilm or PCTFE can be specified if necessary. Additionally, materials become increasingly brittle at cryogenic temperatures so the cage geometry is also critical. Oxfordshire-based Carter has developed a proprietary two-piece riveted cryo-cage design for deep-groove ball bearings that it says increases performance dramatically. Traditional bearings used in lowtemperature applications operate effectively with liquid lubricants creating a thin film between the steel rolling elements which minimises metal-to-metal contact and the resulting risks of micro-welding adhesion. However, this is not possible for bearings operating in cryogenic temperatures. Making bearings from ceramic materials such as silicon nitride, not only removes the need for liquid lubricants but also reduces their mass significantly and increases the bearings’ speed capabilities. This, and the application of dry film lubricants such as tungsten disulphide, allow the bearings to operate at cryogenic temperatures. How do you know if a cryogenic bearing is going to be suitable to meet the demands of an application? Testing is crucial which is why manufacturers such as Carter have developed proprietary cryogenic testing procedures to minimise the risks before the bearings arrive on-site. For example, submerging bearings in liquid nitrogen at –196ºC represents an even more robust test than submerging in LNG. Analysing a bearing’s performance torque and dimensions also helps to ensure optimal performance. n Commercial ship designers are increasing turning to LNG as a ‘cleaner’ fuel than traditional diesel. But LNG pumps need to operate at cryogenic temperatures which means that they cannot use traditional bearings. Specialised low-temperature bearings are needed instead. LNG has to be transported in special vessels that can keep it at extremely low temperatures Carter Manufacturing has developed a two-piece riveted cage design for deep-groove cryogenic ball bearings that, it claims, increases performance dramatically.
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